acorde_core/model/

commands.rs

use super::change_hint::{ChangeHint, ChangeScope};
use super::duration::Duration;
use super::fragment::{
    MIN_SUPPORTED_SCORE_FRAGMENT_CONTRACT_VERSION, SCORE_FRAGMENT_CONTRACT_VERSION, ScoreFragment,
    ScoreFragmentSelection, extract_score_fragment,
};
use super::notation::{
    Articulation, Barline, ChordSymbol, Clef, CrossStaff, Dynamic, FiguredBassFigure,
    GuitarTechnique, HairpinKind, KeySignature, Lyric, NoteHead, OttavaKind, StyledText,
    TablatureConfig, TimeSignature, TupletInfo,
};
use super::pitch::Pitch;
use super::score::{
    HarpPedalDiagram, InstrumentDefinition, InstrumentRange, Measure, NotationSpanner,
    NotationSpannerKind, Note, NoteAddr, ObjectStyleOverride, Part, PartGroup,
    PercussionInstrument, RegionalTranspositionTarget, Score, ScoreTemplate, ScoreView, Staff,
    StaffKind, StaffPresentation, ViewStyleOverride, respell_score, respell_score_to_key,
    transpose_staff_region_checked,
};
use super::validate::validate;
use crate::Error;
use serde::{Deserialize, Serialize};
use std::collections::BTreeSet;
use uuid::Uuid;

#[derive(Debug, Clone, Serialize, Deserialize)]
#[serde(tag = "type", rename_all = "snake_case")]
pub enum Command {
    AddNote(AddNoteCmd),
    AddPitch(AddPitchCmd),
    SetDuration(SetDurationCmd),
    DeleteNote(DeleteNoteCmd),
    AddMeasure(AddMeasureCmd),
    DeleteMeasure(DeleteMeasureCmd),
    SetTempo(SetTempoCmd),
    NewScore(NewScoreCmd),
    AddHairpin(AddHairpinCmd),
    ToggleTie(ToggleTieCmd),
    SetDynamic(SetDynamicCmd),
    ToggleArticulation(ToggleArticulationCmd),
    SetKeySignature(SetKeySignatureCmd),
    SetTimeSignature(SetTimeSignatureCmd),
    SetBarline(SetBarlineCmd),
    AddPart(AddPartCmd),
    DeletePart(DeletePartCmd),
    ReorderParts(ReorderPartsCmd),
    SetMetadata(SetMetadataCmd),
    SetRehearsalMark(SetRehearsalMarkCmd),
    SetNavigationMark(SetNavigationMarkCmd),
    SetChordSymbol(SetChordSymbolCmd),
    SetHarmonyRange(SetHarmonyRangeCmd),
    SetFiguredBass(SetFiguredBassCmd),
    SetHarpPedalDiagrams(SetHarpPedalDiagramsCmd),
    SetGrace(SetGraceCmd),
    SetOttava(SetOttavaCmd),
    SetLyric(SetLyricCmd),
    SetMultiRest(SetMultiRestCmd),
    AddPedal(AddPedalCmd),
    SetVolta(SetVoltaCmd),
    SetClef(SetClefCmd),
    SetPartName(SetPartNameCmd),
    SetMidiInstrument(SetMidiInstrumentCmd),
    SetPercussionKit(SetPercussionKitCmd),
    SetInstrumentDefinition(SetInstrumentDefinitionCmd),
    SetMeasureInstrumentChange(SetMeasureInstrumentChangeCmd),
    SetMeasureTablatureChange(SetMeasureTablatureChangeCmd),
    UpsertScoreView(UpsertScoreViewCmd),
    RemoveScoreView(RemoveScoreViewCmd),
    SetTranspose(SetTransposeCmd),
    TransposeStaffRegion(TransposeStaffRegionCmd),
    SetTempoAtMeasure(SetTempoAtMeasureCmd),
    SetTempoRampAtMeasure(SetTempoRampAtMeasureCmd),
    PasteVoice(PasteVoiceCmd),
    PasteRange(PasteRangeCmd),
    PasteScoreFragment(PasteScoreFragmentCmd),
    ExchangeVoices(ExchangeVoicesCmd),
    MoveOrCopyVoiceRange(MoveOrCopyVoiceRangeCmd),
    SplitMeasure(SplitMeasureCmd),
    JoinMeasures(JoinMeasuresCmd),
    ImplodeStaves(ImplodeStavesCmd),
    ExplodeVoices(ExplodeVoicesCmd),
    ExplodeChordPitches(ExplodeChordPitchesCmd),
    ScaleVoiceRange(ScaleVoiceRangeCmd),
    SetSystemBreak(SetSystemBreakCmd),
    SetPageBreak(SetPageBreakCmd),
    SetSectionBreak(SetSectionBreakCmd),
    ToggleSlur(ToggleSlurCmd),
    AddStaff(AddStaffCmd),
    DeleteStaff(DeleteStaffCmd),
    SetTuplet(SetTupletCmd),
    RespellScore(RespellScoreCmd),
    RespellScoreToKey(RespellScoreToKeyCmd),
    SetStem(SetStemCmd),
    SetArpeggio(SetArpeggioCmd),
    SetTechniqueText(SetTechniqueTextCmd),
    SetFingering(SetFingeringCmd),
    SetFingerings(SetFingeringsCmd),
    SetStringNumber(SetStringNumberCmd),
    SetTabPosition(SetTabPositionCmd),
    SetTablatureConfig(SetTablatureConfigCmd),
    SetStaffPresentation(SetStaffPresentationCmd),
    SetNoteHead(SetNoteHeadCmd),
    SetCue(SetCueCmd),
    SetUnpitched(SetUnpitchedCmd),
    SetInstrumentId(SetInstrumentIdCmd),
    SetNotePlacement(SetNotePlacementCmd),
    SetGuitarTechnique(SetGuitarTechniqueCmd),
    SetGuitarBendAlter(SetGuitarBendAlterCmd),
    SetGuitarBendCurve(SetGuitarBendCurveCmd),
    SetExpressionText(SetExpressionTextCmd),
    SetMeasureText(SetMeasureTextCmd),
    SetScoreText(SetScoreTextCmd),
    SetScoreStyleOverrides(SetScoreStyleOverridesCmd),
    SetObjectStyleOverrides(SetObjectStyleOverridesCmd),
    ToggleTrillLine(ToggleTrillLineCmd),
    SetGlissando(SetGlissandoCmd),
    SetCrossStaff(SetCrossStaffCmd),
    SetPartGroup(SetPartGroupCmd),
    AddSpanner(AddSpannerCmd),
    UpdateSpanner(UpdateSpannerCmd),
    RemoveSpanner(RemoveSpannerCmd),
    Batch(BatchCmd),
}

#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct AddNoteCmd {
    pub part_index: usize,
    pub staff_index: usize,
    pub measure_index: usize,
    pub voice: usize,
    pub position: usize,
    pub pitch: Option<Pitch>,
    pub duration: Duration,
    pub dot_count: u8,
    pub is_rest: bool,
    #[serde(default)]
    pub tuplet: Option<TupletInfo>,
}

#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct AddPitchCmd {
    pub part_index: usize,
    pub staff_index: usize,
    pub measure_index: usize,
    pub voice: usize,
    pub note_index: usize,
    pub pitch: Pitch,
}

#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct SetDurationCmd {
    pub part_index: usize,
    pub staff_index: usize,
    pub measure_index: usize,
    pub voice: usize,
    pub note_index: usize,
    pub duration: Duration,
    #[serde(default)]
    pub dot_count: u8,
}

/// Add a typed notation span to the score.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct AddSpannerCmd {
    pub spanner: NotationSpanner,
}

/// Replace every mutable property of an existing typed notation span.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct UpdateSpannerCmd {
    pub spanner: NotationSpanner,
}

/// Remove a typed notation span by its stable identity.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct RemoveSpannerCmd {
    pub id: String,
}

#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct DeleteNoteCmd {
    pub note_id: String,
    pub part_index: usize,
    pub staff_index: usize,
    pub measure_index: usize,
    pub voice: usize,
}

#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct AddMeasureCmd {
    pub after_index: usize,
}

#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct DeleteMeasureCmd {
    pub measure_index: usize,
}

#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct SetTempoCmd {
    pub bpm: u16,
}

#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct NewScoreCmd {
    pub title: String,
    pub composer: String,
    pub tempo_bpm: u16,
    pub time_numerator: u8,
    pub time_denominator: u8,
    pub key_fifths: i8,
    pub measure_count: u32,
    /// When set, creates the score from an ensemble template instead of a blank single-part score.
    #[serde(default)]
    pub template: Option<ScoreTemplate>,
}

#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct AddHairpinCmd {
    pub part_index: usize,
    pub staff_index: usize,
    pub measure_index: usize,
    pub voice: usize,
    pub start_note_idx: usize,
    pub end_note_idx: usize,
    pub kind: HairpinKind,
}

#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct ToggleTieCmd {
    pub part_index: usize,
    pub staff_index: usize,
    pub measure_index: usize,
    pub voice: usize,
    pub note_index: usize,
}

#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct SetDynamicCmd {
    pub part_index: usize,
    pub staff_index: usize,
    pub measure_index: usize,
    pub voice: usize,
    pub note_index: usize,
    pub dynamic: Option<Dynamic>,
}

#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct ToggleArticulationCmd {
    pub part_index: usize,
    pub staff_index: usize,
    pub measure_index: usize,
    pub voice: usize,
    pub note_index: usize,
    pub articulation: Articulation,
}

#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct SetKeySignatureCmd {
    pub fifths: i8,
}

#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct SetTimeSignatureCmd {
    pub numerator: u8,
    pub denominator: u8,
}

#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct SetBarlineCmd {
    pub measure_index: usize,
    /// "left" or "right"
    pub side: String,
    pub barline: Barline,
}

#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct AddPartCmd {
    pub name: String,
    pub short_name: String,
    /// Clef variant names for each staff, e.g. ["Treble"] or ["Treble", "Bass"]
    pub clefs: Vec<String>,
    /// MIDI channel (0–15). Default 0.
    #[serde(default)]
    pub midi_channel: u8,
    /// General MIDI program (0–127). Default 0 = Acoustic Grand Piano.
    #[serde(default)]
    pub midi_program: u8,
}

#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct DeletePartCmd {
    pub part_index: usize,
}

/// Reorder every part by its current zero-based index.
///
/// `order` must be a permutation of every current part. Linked views, typed spanners, and
/// contiguous part groups are remapped so they continue to identify the same musical parts.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct ReorderPartsCmd {
    pub order: Vec<usize>,
}

#[derive(Debug, Clone, Default, Serialize, Deserialize)]
pub struct SetMetadataCmd {
    pub title: Option<String>,
    pub composer: Option<String>,
    pub lyricist: Option<String>,
    pub copyright: Option<String>,
    pub work_number: Option<String>,
    pub movement_title: Option<String>,
}

#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct SetRehearsalMarkCmd {
    pub measure_index: usize,
    pub text: Option<String>,
}

#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct SetNavigationMarkCmd {
    pub measure_index: usize,
    /// Known values: "Segno", "Coda", "Fine", "DaCapo", "DaCapoAlFine",
    /// "DaCapoAlCoda", "DalSegno", "DalSegnoAlFine", "DalSegnoAlCoda", "ToCoda".
    pub mark: Option<String>,
}

#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct SetChordSymbolCmd {
    pub part_index: usize,
    pub staff_index: usize,
    pub measure_index: usize,
    pub voice: usize,
    pub note_index: usize,
    pub chord: Option<ChordSymbol>,
}

/// Set or clear the end note of a chord-symbol continuation range.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct SetHarmonyRangeCmd {
    pub part_index: usize,
    pub staff_index: usize,
    pub measure_index: usize,
    pub voice: usize,
    pub note_index: usize,
    pub end: Option<NoteAddr>,
}

/// Replace the structured figured-bass figures attached to a measure.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct SetFiguredBassCmd {
    pub measure_index: usize,
    pub figures: Vec<FiguredBassFigure>,
}

/// Replace the MusicXML-compatible harp pedal diagrams on one staff-local measure.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct SetHarpPedalDiagramsCmd {
    pub part_index: usize,
    pub staff_index: usize,
    pub measure_index: usize,
    pub diagrams: Vec<HarpPedalDiagram>,
}

#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct SetGraceCmd {
    pub part_index: usize,
    pub staff_index: usize,
    pub measure_index: usize,
    pub voice: usize,
    pub note_index: usize,
    pub is_grace: bool,
    /// true = acciaccatura (slash), false = appoggiatura (no slash).
    pub slash: bool,
}

#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct SetOttavaCmd {
    pub part_index: usize,
    pub staff_index: usize,
    pub measure_index: usize,
    pub voice: usize,
    pub note_index: usize,
    pub ottava_start: Option<OttavaKind>,
    pub ottava_end: bool,
}

#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct SetLyricCmd {
    pub part_index: usize,
    pub staff_index: usize,
    pub measure_index: usize,
    pub voice: usize,
    pub note_index: usize,
    pub lyric: Option<Lyric>,
}

#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct SetMultiRestCmd {
    pub measure_index: usize,
    pub count: Option<u8>,
}

#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct SetVoltaCmd {
    pub measure_index: usize,
    pub volta: Option<super::score::VoltaBracket>,
}

#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct SetClefCmd {
    pub part_index: usize,
    pub staff_index: usize,
    pub clef: Clef,
}

#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct SetPartNameCmd {
    pub part_index: usize,
    pub name: String,
    pub short_name: String,
}

#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct SetMidiInstrumentCmd {
    pub part_index: usize,
    /// MIDI channel (0–15).
    pub midi_channel: u8,
    /// General MIDI program number (0–127).
    pub midi_program: u8,
}

/// Replace a part's editable percussion-kit map.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct SetPercussionKitCmd {
    pub part_index: usize,
    pub instruments: Vec<PercussionInstrument>,
}

/// Set or clear a part's stable instrument semantics.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct SetInstrumentDefinitionCmd {
    pub part_index: usize,
    pub definition: Option<InstrumentDefinition>,
}

/// Set or clear an instrument change beginning at a staff-local measure boundary.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct SetMeasureInstrumentChangeCmd {
    pub part_index: usize,
    pub staff_index: usize,
    pub measure_index: usize,
    pub definition: Option<InstrumentDefinition>,
}

/// Set or clear a tuning/capo change beginning at a staff-local measure boundary.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct SetMeasureTablatureChangeCmd {
    pub part_index: usize,
    pub staff_index: usize,
    pub measure_index: usize,
    pub config: Option<TablatureConfig>,
}

/// Create or replace a named linked-part view by stable ID.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct UpsertScoreViewCmd {
    pub view: ScoreView,
}

/// Remove a named linked-part view by stable ID.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct RemoveScoreViewCmd {
    pub id: String,
}

#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct SetTransposeCmd {
    pub part_index: usize,
    pub staff_index: usize,
    /// Semitones to transpose (negative = down). E.g. -2 for Bb clarinet.
    pub semitones: i8,
}

/// Transpose one contiguous staff range through the validated written/concert boundary.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct TransposeStaffRegionCmd {
    pub part_index: usize,
    pub staff_index: usize,
    /// Inclusive physical measure index.
    pub start_measure: usize,
    /// Exclusive physical measure index.
    pub end_measure: usize,
    pub semitones: i8,
    pub target: RegionalTranspositionTarget,
}

#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct SetTempoAtMeasureCmd {
    pub measure_index: usize,
    /// New BPM at this measure. `None` clears any measure-level override.
    pub bpm: Option<u16>,
}

/// Set or clear the target BPM reached at the end of one physical measure.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct SetTempoRampAtMeasureCmd {
    pub measure_index: usize,
    pub target_bpm: Option<u16>,
}

#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct PasteVoiceCmd {
    pub part_index: usize,
    pub staff_index: usize,
    pub measure_index: usize,
    pub voice_index: usize,
    /// Snapshot of the clipboard at paste time — embedded in the command for undo/redo.
    pub notes: Vec<Note>,
}

#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct SetSystemBreakCmd {
    pub measure_index: usize,
    pub value: bool,
}

#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct SetPageBreakCmd {
    pub measure_index: usize,
    pub value: bool,
}

/// Set or clear a semantic section boundary at a physical measure.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct SetSectionBreakCmd {
    pub measure_index: usize,
    pub value: bool,
}

/// A group of commands applied and undone as a single unit.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct BatchCmd {
    pub commands: Vec<Command>,
    /// Optional i18n key / display label override shown in the undo menu.
    /// E.g. `"ApplyAI"`, `"PasteSelection"`. `None` falls back to `"Batch"`.
    #[serde(default)]
    pub label: Option<String>,
}

#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct AddPedalCmd {
    pub part_index: usize,
    pub staff_index: usize,
    pub measure_index: usize,
    pub voice: usize,
    pub start_note_idx: usize,
    pub end_note_idx: usize,
}

/// Replace a contiguous range of voice measures with stored notes (undo-able).
///
/// `measures` contains one `Vec<Note>` per measure to paste, starting at `target_measure`.
/// The target voice of each measure is replaced entirely.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct PasteRangeCmd {
    pub part_index: usize,
    pub staff_index: usize,
    pub voice_index: usize,
    pub target_measure: usize,
    /// One note list per measure, in order.
    pub measures: Vec<Vec<Note>>,
}

/// Paste a versioned, multi-lane score fragment at a canonical destination.
///
/// The destination is the origin for every relative fragment address. The
/// policy determines whether mapped voice measures are replaced, or only
/// pasted into empty/rest-only lanes. CommandStack retains a full undo snapshot.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct PasteScoreFragmentCmd {
    pub fragment: ScoreFragment,
    pub target: NoteAddr,
    #[serde(default)]
    pub policy: ScoreFragmentPastePolicy,
}

/// Collision policy for a score-fragment paste.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Default, Serialize, Deserialize)]
#[serde(rename_all = "snake_case")]
pub enum ScoreFragmentPastePolicy {
    /// Replace every mapped destination lane, matching the v1/v2 behavior.
    #[default]
    Replace,
    /// Preserve any sounding destination lane; paste only into empty or
    /// rest-only lanes, otherwise reject the complete command atomically.
    Merge,
}

/// Exchange two editable voices across an inclusive measure range.
///
/// Both note vectors and their imported MusicXML source voice numbers move
/// together, preserving sparse-voice cursor semantics for later serialization.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct ExchangeVoicesCmd {
    pub part_index: usize,
    pub staff_index: usize,
    pub start_measure: usize,
    pub end_measure: usize,
    pub first_voice: usize,
    pub second_voice: usize,
}

/// Copy or move an inclusive, whole-measure single-voice range.
///
/// The operation reuses the versioned ScoreFragment mapping contract. `move_source`
/// clears the source lane only after the destination has been validated and populated.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct MoveOrCopyVoiceRangeCmd {
    pub source_start: NoteAddr,
    pub source_end: NoteAddr,
    pub target: NoteAddr,
    #[serde(default)]
    pub move_source: bool,
}

/// Split one physical measure across every part and staff at an exact beat boundary.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct SplitMeasureCmd {
    pub measure_index: usize,
    pub split_at_beats: f64,
}

/// Join one physical measure with its following measure across every part and staff.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct JoinMeasuresCmd {
    pub measure_index: usize,
}

/// Move the primary voice from compatible staves into voices of one staff.
///
/// `source_staves` is ordered: its position becomes the destination voice index.
/// It must contain `target_staff` exactly once and contain two to four staves.
/// Every source secondary voice must be empty; this makes the operation lossless
/// and gives a deterministic conflict policy.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct ImplodeStavesCmd {
    pub part_index: usize,
    pub source_staves: Vec<usize>,
    pub target_staff: usize,
    pub start_measure: usize,
    pub end_measure: usize,
}

/// Move voices from one staff into the primary voices of compatible staves.
///
/// `target_staves[voice]` receives the corresponding source voice. The first
/// target must equal `source_staff`, so the command does not discard voice zero.
/// Non-source targets must contain only rests in their primary voice and no
/// secondary voices; otherwise the command fails before changing the score.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct ExplodeVoicesCmd {
    pub part_index: usize,
    pub source_staff: usize,
    pub target_staves: Vec<usize>,
    pub start_measure: usize,
    pub end_measure: usize,
}

/// Distribute pitches from primary-voice chords to the primary voices of
/// compatible staves.
///
/// `target_staves[0]` must be `source_staff`. The original note identity and
/// all note-attached notation remain with that first pitch; derived pitches get
/// fresh note identities and only pitch-local tablature placement. This avoids
/// duplicating directions, lyrics, or typed span endpoints during export.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct ExplodeChordPitchesCmd {
    pub part_index: usize,
    pub source_staff: usize,
    pub target_staves: Vec<usize>,
    pub start_measure: usize,
    pub end_measure: usize,
}

/// Exact power-of-two duration scaling supported by the portable score model.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize)]
#[serde(rename_all = "snake_case")]
pub enum DurationScale {
    Half,
    Double,
}

/// How duration scaling treats the notation ratio of existing tuplets.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize, Default)]
#[serde(rename_all = "snake_case")]
pub enum TupletScalePolicy {
    /// Keep each `actual-notes:normal-notes` ratio while scaling written duration.
    #[default]
    PreserveRatio,
}

/// Scale every note in one voice across an inclusive measure range.
///
/// Tuplets preserve their `actual-notes:normal-notes` ratio, so their written
/// and performed durations change by the same factor. Underfilled scaled
/// measures receive explicit trailing rests; overflow rejects the complete
/// command before any score or history mutation.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct ScaleVoiceRangeCmd {
    pub part_index: usize,
    pub staff_index: usize,
    pub voice: usize,
    pub start_measure: usize,
    pub end_measure: usize,
    pub scale: DurationScale,
    #[serde(default)]
    pub tuplet_policy: TupletScalePolicy,
}

/// Toggle slur_start on `start` note and slur_end on `end` note (cross-measure aware).
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct ToggleSlurCmd {
    pub start: NoteAddr,
    pub end: NoteAddr,
}

/// Add or replace a part group. `None` removes all groups that overlap the range.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct SetPartGroupCmd {
    /// `Some(group)` to add/replace; `None` removes any group whose `[first_part, last_part]` matches.
    pub group: Option<PartGroup>,
}

/// Toggle a trill line span between two notes (start note gets `trill_line_start`, end gets `trill_line_end`).
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct ToggleTrillLineCmd {
    pub start: NoteAddr,
    pub end: NoteAddr,
}

/// Add a new staff to an existing part with the given clef.
///
/// The new staff is appended with empty measures matching the current measure count.
/// Clef values: `"Treble"` | `"Bass"` | `"Alto"` | `"Tenor"` | `"Percussion"`.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct AddStaffCmd {
    pub part_index: usize,
    pub clef: Clef,
}

/// Remove a staff from a part. Fails if it is the last remaining staff.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct DeleteStaffCmd {
    pub part_index: usize,
    pub staff_index: usize,
}

/// Set (or clear) the tuplet info on an existing note.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct SetTupletCmd {
    pub part_index: usize,
    pub staff_index: usize,
    pub measure_index: usize,
    pub voice_index: usize,
    pub note_index: usize,
    /// `None` removes the tuplet; `Some(TupletInfo)` sets it.
    pub tuplet: Option<TupletInfo>,
}

/// Set or clear the stem direction on a note (override). `None` means auto.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct SetStemCmd {
    pub part_index: usize,
    pub staff_index: usize,
    pub measure_index: usize,
    pub voice_index: usize,
    pub note_index: usize,
    /// `None` = auto, `Some(true)` = stem up, `Some(false)` = stem down.
    pub stem_up: Option<bool>,
}

#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct SetArpeggioCmd {
    pub part_index: usize,
    pub staff_index: usize,
    pub measure_index: usize,
    pub voice_index: usize,
    pub note_index: usize,
    /// `Some(true)` = up, `Some(false)` = down, `None` = clear.
    pub direction: Option<bool>,
}

/// Set (or clear) the technique-text annotation on a note ("pizz.", "arco", "con sord.", etc.).
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct SetTechniqueTextCmd {
    pub part_index: usize,
    pub staff_index: usize,
    pub measure_index: usize,
    pub voice: usize,
    pub note_index: usize,
    /// `None` clears the annotation.
    pub text: Option<String>,
}

#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct SetGlissandoCmd {
    pub part_index: usize,
    pub staff_index: usize,
    pub measure_index: usize,
    pub voice: usize,
    pub note_index: usize,
    pub start: bool,
    pub end: bool,
}

#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct SetCrossStaffCmd {
    pub part_index: usize,
    pub staff_index: usize,
    pub measure_index: usize,
    pub voice: usize,
    pub note_index: usize,
    pub placement: Option<CrossStaff>,
}

/// Set (or clear) the fingering number on a note (0 = open/thumb, 1–5 = fingers).
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct SetFingeringCmd {
    pub part_index: usize,
    pub staff_index: usize,
    pub measure_index: usize,
    pub voice: usize,
    pub note_index: usize,
    /// `None` clears the fingering.
    pub fingering: Option<u8>,
}

/// Set all ordered fingering candidates on a note. The first value mirrors
/// the legacy singular `fingering` field; an empty list clears both fields.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct SetFingeringsCmd {
    pub part_index: usize,
    pub staff_index: usize,
    pub measure_index: usize,
    pub voice: usize,
    pub note_index: usize,
    pub fingerings: Vec<u8>,
}

/// Set (or clear) the string number on a note (1 = highest string).
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct SetStringNumberCmd {
    pub part_index: usize,
    pub staff_index: usize,
    pub measure_index: usize,
    pub voice: usize,
    pub note_index: usize,
    /// `None` clears the string number.
    pub string_number: Option<u8>,
}

/// Set (or clear) the tablature string/fret position on a note.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct SetTabPositionCmd {
    pub part_index: usize,
    pub staff_index: usize,
    pub measure_index: usize,
    pub voice: usize,
    pub note_index: usize,
    /// `None` clears the explicit tablature position.
    pub position: Option<super::notation::TabPosition>,
}

/// Set (or clear) the tablature tuning and capo configuration on a staff.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct SetTablatureConfigCmd {
    pub part_index: usize,
    pub staff_index: usize,
    /// `None` clears tablature mode for the staff.
    pub config: Option<TablatureConfig>,
}

/// Replace the renderer-independent presentation settings for one staff.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct SetStaffPresentationCmd {
    pub part_index: usize,
    pub staff_index: usize,
    pub presentation: StaffPresentation,
}

/// Set (or clear) the guitar playing technique on a note (bend, slide, hammer-on, pull-off).
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct SetGuitarTechniqueCmd {
    pub part_index: usize,
    pub staff_index: usize,
    pub measure_index: usize,
    pub voice: usize,
    pub note_index: usize,
    /// `None` clears the technique.
    pub technique: Option<GuitarTechnique>,
}

/// Set (or clear) the MusicXML guitar bend amount in cents on a note.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct SetGuitarBendAlterCmd {
    pub part_index: usize,
    pub staff_index: usize,
    pub measure_index: usize,
    pub voice: usize,
    pub note_index: usize,
    /// `None` clears the bend amount.
    pub alter_cents: Option<i16>,
}

/// Replace the authored bend/hold/release curve on a note.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct SetGuitarBendCurveCmd {
    pub part_index: usize,
    pub staff_index: usize,
    pub measure_index: usize,
    pub voice: usize,
    pub note_index: usize,
    #[serde(default)]
    pub points: Vec<crate::GuitarBendPoint>,
}

/// Set (or clear) the expression/performance text on a measure ("dolce", "espressivo", etc.).
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct SetExpressionTextCmd {
    pub measure_index: usize,
    /// `None` clears the expression text.
    pub text: Option<String>,
}

/// Insert, replace, or remove one measure-level styled text entry.
///
/// `text_index == texts.len()` with `Some(text)` appends an entry. An existing
/// index with `Some(text)` replaces it; an existing index with `None` removes it.
/// `None` at the end, or an index beyond the end, is rejected.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct SetMeasureTextCmd {
    pub part_index: usize,
    pub staff_index: usize,
    pub measure_index: usize,
    pub text_index: usize,
    pub text: Option<StyledText>,
}

/// Insert, replace, or remove one score-level styled text entry.
///
/// `text_index == texts.len()` with `Some(text)` appends an entry. An existing
/// index with `Some(text)` replaces it; an existing index with `None` removes it.
/// `None` at the end, or an index beyond the end, is rejected.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct SetScoreTextCmd {
    pub text_index: usize,
    pub text: Option<StyledText>,
}

/// Replace the ordered score-wide typed presentation defaults.
///
/// Later entries for the same property take precedence when a view is resolved.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct SetScoreStyleOverridesCmd {
    pub overrides: Vec<ViewStyleOverride>,
}

/// Replace every typed object-attached presentation override.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct SetObjectStyleOverridesCmd {
    pub overrides: Vec<ObjectStyleOverride>,
}

/// Mark or unmark a note as a cue note (cue notes have zero beats).
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct SetCueCmd {
    pub part_index: usize,
    pub staff_index: usize,
    pub measure_index: usize,
    pub voice: usize,
    pub note_index: usize,
    pub is_cue: bool,
}

/// Mark or unmark a note as unpitched while retaining its display placement pitch.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct SetUnpitchedCmd {
    pub part_index: usize,
    pub staff_index: usize,
    pub measure_index: usize,
    pub voice: usize,
    pub note_index: usize,
    pub is_unpitched: bool,
}

/// Set or clear a source instrument identifier attached to a note.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct SetInstrumentIdCmd {
    pub part_index: usize,
    pub staff_index: usize,
    pub measure_index: usize,
    pub voice: usize,
    pub note_index: usize,
    pub instrument_id: Option<String>,
}

/// Set or clear MusicXML-compatible note placement offsets in tenths.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct SetNotePlacementCmd {
    pub part_index: usize,
    pub staff_index: usize,
    pub measure_index: usize,
    pub voice: usize,
    pub note_index: usize,
    #[serde(default)]
    pub offset_x: Option<f64>,
    #[serde(default)]
    pub offset_y: Option<f64>,
    #[serde(default)]
    pub relative_x: Option<f64>,
    #[serde(default)]
    pub relative_y: Option<f64>,
}

/// Set the note head shape on a note.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct SetNoteHeadCmd {
    pub part_index: usize,
    pub staff_index: usize,
    pub measure_index: usize,
    pub voice: usize,
    pub note_index: usize,
    pub note_head: NoteHead,
}

/// Respell all pitches in the score to prefer flats or sharps.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct RespellScoreCmd {
    pub prefer_flat: bool,
}

/// Respell all pitches to match the score's key signature (auto-selects flat vs sharp).
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct RespellScoreToKeyCmd {}

struct UndoEntry {
    command: Command,
    snapshot: Score,
}

pub struct CommandStack {
    history: Vec<UndoEntry>,
    future: Vec<(Command, Score)>,
    max_depth: usize,
}

impl CommandStack {
    pub fn new(max_depth: usize) -> Self {
        Self {
            history: Vec::new(),
            future: Vec::new(),
            max_depth,
        }
    }

    pub fn can_undo(&self) -> bool {
        !self.history.is_empty()
    }

    pub fn can_redo(&self) -> bool {
        !self.future.is_empty()
    }

    pub fn execute(&mut self, cmd: Command, score: &mut Score) -> Result<(), Error> {
        let snapshot = score.clone();
        let mut candidate = snapshot.clone();
        apply_command(&cmd, &mut candidate)?;
        if !validate(&candidate).is_valid() {
            return Err(Error::InvalidScore);
        }
        *score = candidate;
        self.history.push(UndoEntry {
            command: cmd,
            snapshot,
        });
        self.future.clear();
        if self.history.len() > self.max_depth {
            self.history.remove(0);
        }
        Ok(())
    }

    pub fn undo(&mut self, score: &mut Score) -> Result<ChangeHint, Error> {
        let entry = self.history.last().ok_or(Error::NothingToUndo)?;
        if !validate(&entry.snapshot).is_valid() {
            return Err(Error::InvalidScore);
        }
        let entry = self.history.pop().ok_or(Error::NothingToUndo)?;
        let hint = command_hint(&entry.command);
        let post_snapshot = score.clone();
        *score = entry.snapshot;
        self.future.push((entry.command, post_snapshot));
        if self.future.len() > self.max_depth {
            self.future.remove(0);
        }
        Ok(hint)
    }

    pub fn redo(&mut self, score: &mut Score) -> Result<ChangeHint, Error> {
        let (_, post) = self.future.last().ok_or(Error::NothingToRedo)?;
        if !validate(post).is_valid() {
            return Err(Error::InvalidScore);
        }
        let (cmd, post) = self.future.pop().ok_or(Error::NothingToRedo)?;
        let hint = command_hint(&cmd);
        let snapshot = score.clone();
        *score = post;
        self.history.push(UndoEntry {
            command: cmd,
            snapshot,
        });
        Ok(hint)
    }

    /// Return the commands applied so far, in execution order.
    /// Suitable for use with [`ScoreEngine::export_history`].
    pub fn history_commands(&self) -> Vec<Command> {
        self.history.iter().map(|e| e.command.clone()).collect()
    }

    /// Label of the command that would be undone next, for UI display (e.g. "Undo: Add Note").
    pub fn undo_label(&self) -> Option<String> {
        self.history.last().map(|e| command_label(&e.command))
    }

    /// Label of the command that would be redone next, for UI display (e.g. "Redo: Add Note").
    pub fn redo_label(&self) -> Option<String> {
        self.future.last().map(|(cmd, _)| command_label(cmd))
    }

    /// i18n key of the command that would be undone next.
    pub fn undo_key(&self) -> Option<String> {
        self.history.last().map(|e| command_key(&e.command))
    }

    /// i18n key of the command that would be redone next.
    pub fn redo_key(&self) -> Option<String> {
        self.future.last().map(|(cmd, _)| command_key(cmd))
    }

    /// Apply a batch of commands as a single undo entry (rollback-safe).
    pub fn batch_execute(&mut self, cmds: Vec<Command>, score: &mut Score) -> Result<(), Error> {
        if cmds.is_empty() {
            return Ok(());
        }
        let snapshot = score.clone();
        let mut candidate = snapshot.clone();
        for cmd in &cmds {
            apply_command(cmd, &mut candidate)?;
        }
        if !validate(&candidate).is_valid() {
            return Err(Error::InvalidScore);
        }
        *score = candidate;
        self.history.push(UndoEntry {
            command: Command::Batch(BatchCmd {
                commands: cmds,
                label: None,
            }),
            snapshot,
        });
        self.future.clear();
        if self.history.len() > self.max_depth {
            self.history.remove(0);
        }
        Ok(())
    }

    /// Apply a batch with an explicit undo-label, as a single rollback-safe entry.
    ///
    /// The `label` appears as the [`command_key`] for undo/redo UI (e.g. `"ApplyAI"`).
    pub fn batch_execute_labeled(
        &mut self,
        cmds: Vec<Command>,
        label: String,
        score: &mut Score,
    ) -> Result<(), Error> {
        if cmds.is_empty() {
            return Ok(());
        }
        let snapshot = score.clone();
        let mut candidate = snapshot.clone();
        for cmd in &cmds {
            apply_command(cmd, &mut candidate)?;
        }
        if !validate(&candidate).is_valid() {
            return Err(Error::InvalidScore);
        }
        *score = candidate;
        self.history.push(UndoEntry {
            command: Command::Batch(BatchCmd {
                commands: cmds,
                label: Some(label),
            }),
            snapshot,
        });
        self.future.clear();
        if self.history.len() > self.max_depth {
            self.history.remove(0);
        }
        Ok(())
    }
}

/// Return a [`ChangeHint`] describing the scope and dirty flags for a command,
/// without executing it.
pub fn command_hint(cmd: &Command) -> ChangeHint {
    use ChangeScope::*;
    macro_rules! hint {
        ($scope:expr, $layout:expr, $playback:expr) => {
            ChangeHint {
                scope: $scope,
                layout_dirty: $layout,
                playback_dirty: $playback,
            }
        };
    }
    macro_rules! meas {
        ($c:expr) => {
            Measures {
                part: $c.part_index,
                staff: $c.staff_index,
                start: $c.measure_index,
                end: $c.measure_index + 1,
            }
        };
    }
    match cmd {
        // Global — full score affected
        Command::NewScore(_)
        | Command::AddPart(_)
        | Command::DeletePart(_)
        | Command::ReorderParts(_)
        | Command::AddMeasure(_)
        | Command::DeleteMeasure(_) => hint!(Global, true, true),

        Command::SetTempo(_) => hint!(Global, false, true),

        Command::SetMetadata(_) => hint!(Global, false, false),

        Command::SetKeySignature(_) => hint!(Global, true, false),

        Command::SetTimeSignature(_) => hint!(Global, true, true),

        Command::SetBarline(_)
        | Command::SetVolta(_)
        | Command::SetRehearsalMark(_)
        | Command::SetNavigationMark(_)
        | Command::SetExpressionText(_) => hint!(Global, false, false),
        Command::SetMeasureText(c) => hint!(
            Measures {
                part: c.part_index,
                staff: c.staff_index,
                start: c.measure_index,
                end: c.measure_index + 1
            },
            false,
            false
        ),
        Command::SetScoreText(_)
        | Command::SetScoreStyleOverrides(_)
        | Command::SetObjectStyleOverrides(_) => hint!(Global, true, false),

        Command::SetMultiRest(_) => hint!(Global, true, false),

        Command::SetTempoAtMeasure(_) => hint!(Global, false, true),
        Command::SetTempoRampAtMeasure(_) => hint!(Global, false, true),

        // Part scope
        Command::SetPartName(c) => hint!(Part(c.part_index), false, false),

        Command::SetMidiInstrument(c) => hint!(Part(c.part_index), false, true),
        Command::SetPercussionKit(c) => hint!(Part(c.part_index), true, true),

        Command::SetInstrumentDefinition(c) => hint!(Part(c.part_index), true, false),
        Command::SetMeasureInstrumentChange(c) => hint!(meas!(c), true, true),
        Command::SetMeasureTablatureChange(c) => hint!(meas!(c), true, true),

        Command::UpsertScoreView(_) | Command::RemoveScoreView(_) => hint!(Global, true, false),

        Command::SetTranspose(c) => hint!(Part(c.part_index), false, true),
        Command::TransposeStaffRegion(c) => hint!(
            Measures {
                part: c.part_index,
                staff: c.staff_index,
                start: c.start_measure,
                end: c.end_measure
            },
            true,
            true
        ),

        Command::SetClef(c) => hint!(Part(c.part_index), true, false),

        // Measure scope
        Command::AddNote(c) => hint!(meas!(c), false, true),
        Command::AddPitch(c) => hint!(meas!(c), false, true),
        Command::SetDuration(c) => hint!(meas!(c), false, true),
        Command::DeleteNote(c) => hint!(meas!(c), false, true),
        Command::PasteVoice(c) => hint!(meas!(c), false, true),
        Command::PasteRange(c) => hint!(
            Measures {
                part: c.part_index,
                staff: c.staff_index,
                start: c.target_measure,
                end: c.target_measure + c.measures.len()
            },
            false,
            true
        ),
        Command::PasteScoreFragment(_) => hint!(Global, true, true),
        Command::ExchangeVoices(c) => hint!(
            Measures {
                part: c.part_index,
                staff: c.staff_index,
                start: c.start_measure.min(c.end_measure),
                end: c.start_measure.max(c.end_measure) + 1,
            },
            true,
            true
        ),
        Command::MoveOrCopyVoiceRange(_) => hint!(Global, true, true),
        Command::SplitMeasure(_) => hint!(Global, true, true),
        Command::JoinMeasures(_) => hint!(Global, true, true),
        Command::ImplodeStaves(_) | Command::ExplodeVoices(_) => hint!(Global, true, true),
        Command::ExplodeChordPitches(_) => hint!(Global, true, true),
        Command::ScaleVoiceRange(c) => hint!(
            Measures {
                part: c.part_index,
                staff: c.staff_index,
                start: c.start_measure.min(c.end_measure),
                end: c.start_measure.max(c.end_measure) + 1,
            },
            true,
            true
        ),
        Command::AddHairpin(c) => hint!(meas!(c), false, true),
        Command::ToggleTie(c) => hint!(meas!(c), false, true),
        Command::SetDynamic(c) => hint!(meas!(c), false, true),
        Command::ToggleArticulation(c) => hint!(meas!(c), false, true),
        Command::SetGrace(c) => hint!(meas!(c), false, true),
        Command::SetOttava(c) => hint!(meas!(c), false, true),
        Command::SetLyric(c) => hint!(meas!(c), false, true),
        Command::AddPedal(c) => hint!(meas!(c), false, true),
        Command::SetChordSymbol(c) => hint!(meas!(c), false, true),
        Command::SetHarmonyRange(c) => hint!(meas!(c), false, true),
        Command::SetFiguredBass(_) => hint!(Global, false, true),
        Command::SetHarpPedalDiagrams(c) => hint!(meas!(c), false, false),

        Command::SetSystemBreak(_) | Command::SetPageBreak(_) | Command::SetSectionBreak(_) => {
            hint!(Global, true, false)
        }

        Command::ToggleSlur(_) | Command::ToggleTrillLine(_) => hint!(Global, true, false),
        Command::SetGlissando(c) => hint!(meas!(c), true, true),
        Command::SetCrossStaff(c) => hint!(meas!(c), true, true),
        Command::AddSpanner(_) | Command::UpdateSpanner(_) | Command::RemoveSpanner(_) => {
            hint!(Global, true, true)
        }

        Command::SetPartGroup(_) => hint!(Global, false, false),

        Command::AddStaff(_) | Command::DeleteStaff(_) => hint!(Global, true, true),

        Command::SetTuplet(c) => hint!(meas!(c), false, true),

        Command::RespellScore(_) | Command::RespellScoreToKey(_) => hint!(Global, true, true),

        Command::SetStem(c) => hint!(meas!(c), false, false),

        Command::SetArpeggio(c) => hint!(meas!(c), false, false),

        Command::SetTechniqueText(c) => hint!(meas!(c), false, false),
        Command::SetFingering(c) => hint!(meas!(c), false, false),
        Command::SetFingerings(c) => hint!(meas!(c), false, false),
        Command::SetStringNumber(c) => hint!(meas!(c), false, false),
        Command::SetTabPosition(c) => hint!(meas!(c), false, false),
        Command::SetTablatureConfig(c) => hint!(Part(c.part_index), true, true),
        Command::SetStaffPresentation(c) => hint!(Part(c.part_index), true, false),
        Command::SetGuitarTechnique(c) => hint!(meas!(c), false, false),
        Command::SetGuitarBendAlter(c) => hint!(meas!(c), false, false),
        Command::SetGuitarBendCurve(c) => hint!(meas!(c), false, false),
        Command::SetNoteHead(c) => hint!(meas!(c), false, false),
        Command::SetCue(c) => hint!(meas!(c), false, true),
        Command::SetUnpitched(c) => hint!(meas!(c), false, true),
        Command::SetInstrumentId(c) => hint!(meas!(c), false, true),
        Command::SetNotePlacement(c) => hint!(meas!(c), true, false),

        Command::Batch(c) => {
            let Some(first) = c.commands.first() else {
                return hint!(Global, false, false);
            };
            let mut merged = command_hint(first);
            for cmd in c.commands.iter().skip(1) {
                merged = merged.merge(command_hint(cmd));
            }
            merged
        }
    }
}

/// Human-readable label for an undoable command (for menu display).
pub fn command_label(cmd: &Command) -> String {
    match cmd {
        Command::AddNote(_) => "Add Note".to_string(),
        Command::AddPitch(_) => "Add Pitch".to_string(),
        Command::SetDuration(_) => "Set Duration".to_string(),
        Command::DeleteNote(_) => "Delete Note".to_string(),
        Command::AddMeasure(_) => "Add Measure".to_string(),
        Command::DeleteMeasure(_) => "Delete Measure".to_string(),
        Command::SetTempo(_) => "Set Tempo".to_string(),
        Command::NewScore(_) => "New Score".to_string(),
        Command::AddHairpin(_) => "Add Hairpin".to_string(),
        Command::ToggleTie(_) => "Toggle Tie".to_string(),
        Command::SetDynamic(_) => "Set Dynamic".to_string(),
        Command::ToggleArticulation(_) => "Toggle Articulation".to_string(),
        Command::SetKeySignature(_) => "Set Key Signature".to_string(),
        Command::SetTimeSignature(_) => "Set Time Signature".to_string(),
        Command::SetBarline(_) => "Set Barline".to_string(),
        Command::AddPart(_) => "Add Part".to_string(),
        Command::DeletePart(_) => "Delete Part".to_string(),
        Command::ReorderParts(_) => "Reorder Parts".to_string(),
        Command::SetMetadata(_) => "Set Metadata".to_string(),
        Command::SetRehearsalMark(_) => "Set Rehearsal Mark".to_string(),
        Command::SetNavigationMark(_) => "Set Navigation Mark".to_string(),
        Command::SetChordSymbol(_) => "Set Chord Symbol".to_string(),
        Command::SetHarmonyRange(_) => "Set Harmony Range".to_string(),
        Command::SetHarpPedalDiagrams(_) => "Set Harp Pedal Diagrams".to_string(),
        Command::SetFiguredBass(_) => "Set Figured Bass".to_string(),
        Command::SetGrace(_) => "Set Grace Note".to_string(),
        Command::SetOttava(_) => "Set Ottava".to_string(),
        Command::SetLyric(_) => "Set Lyric".to_string(),
        Command::SetMultiRest(_) => "Set Multi-Rest".to_string(),
        Command::AddPedal(_) => "Add Pedal".to_string(),
        Command::SetVolta(_) => "Set Volta".to_string(),
        Command::SetClef(_) => "Set Clef".to_string(),
        Command::SetPartName(_) => "Set Part Name".to_string(),
        Command::SetMidiInstrument(_) => "Set MIDI Instrument".to_string(),
        Command::SetPercussionKit(_) => "Set Percussion Kit".to_string(),
        Command::SetInstrumentDefinition(_) => "Set Instrument Definition".to_string(),
        Command::SetMeasureInstrumentChange(_) => "Set Measure Instrument Change".to_string(),
        Command::SetMeasureTablatureChange(_) => "Set Measure Tablature Change".to_string(),
        Command::UpsertScoreView(_) => "Update Score View".to_string(),
        Command::RemoveScoreView(_) => "Remove Score View".to_string(),
        Command::SetTranspose(_) => "Set Transpose".to_string(),
        Command::TransposeStaffRegion(_) => "Transpose Staff Region".to_string(),
        Command::SetTempoAtMeasure(_) => "Set Tempo".to_string(),
        Command::SetTempoRampAtMeasure(_) => "Set Tempo Ramp".to_string(),
        Command::PasteVoice(_) => "Paste Voice".to_string(),
        Command::PasteRange(_) => "Paste Range".to_string(),
        Command::PasteScoreFragment(_) => "Paste Score Fragment".to_string(),
        Command::ExchangeVoices(_) => "Exchange Voices".to_string(),
        Command::MoveOrCopyVoiceRange(c) => if c.move_source {
            "Move Voice Range"
        } else {
            "Copy Voice Range"
        }
        .to_string(),
        Command::SplitMeasure(_) => "Split Measure".to_string(),
        Command::JoinMeasures(_) => "Join Measures".to_string(),
        Command::ImplodeStaves(_) => "Implode Staves".to_string(),
        Command::ExplodeVoices(_) => "Explode Voices".to_string(),
        Command::ExplodeChordPitches(_) => "Explode Chord Pitches".to_string(),
        Command::ScaleVoiceRange(c) => match c.scale {
            DurationScale::Half => "Halve Voice Durations",
            DurationScale::Double => "Double Voice Durations",
        }
        .to_string(),
        Command::SetSystemBreak(_) => "Set System Break".to_string(),
        Command::SetPageBreak(_) => "Set Page Break".to_string(),
        Command::SetSectionBreak(_) => "Set Section Break".to_string(),
        Command::ToggleSlur(_) => "Toggle Slur".to_string(),
        Command::AddStaff(_) => "Add Staff".to_string(),
        Command::DeleteStaff(_) => "Delete Staff".to_string(),
        Command::SetTuplet(c) => if c.tuplet.is_some() {
            "Set Tuplet"
        } else {
            "Clear Tuplet"
        }
        .to_string(),
        Command::SetInstrumentId(_) => "Set Note Instrument".to_string(),
        Command::SetNotePlacement(_) => "Set Note Placement".to_string(),
        Command::SetUnpitched(c) => if c.is_unpitched {
            "Set Unpitched Note"
        } else {
            "Clear Unpitched Note"
        }
        .to_string(),
        Command::RespellScore(c) => if c.prefer_flat {
            "Respell Score (flat)"
        } else {
            "Respell Score (sharp)"
        }
        .to_string(),
        Command::RespellScoreToKey(_) => "Respell Score to Key".to_string(),
        Command::SetStem(_) => "Set Stem".to_string(),
        Command::SetArpeggio(_) => "Set Arpeggio".to_string(),
        Command::SetTechniqueText(_) => "Set Technique Text".to_string(),
        Command::SetFingering(_) => "Set Fingering".to_string(),
        Command::SetFingerings(_) => "Set Fingering Candidates".to_string(),
        Command::SetStringNumber(_) => "Set String Number".to_string(),
        Command::SetTabPosition(_) => "Set Tablature Position".to_string(),
        Command::SetTablatureConfig(_) => "Set Tablature Configuration".to_string(),
        Command::SetStaffPresentation(_) => "Set Staff Presentation".to_string(),
        Command::SetGuitarTechnique(_) => "Set Guitar Technique".to_string(),
        Command::SetGuitarBendAlter(_) => "Set Guitar Bend Alter".to_string(),
        Command::SetGuitarBendCurve(_) => "Set Guitar Bend Curve".to_string(),
        Command::SetNoteHead(_) => "Set Note Head".to_string(),
        Command::SetCue(c) => if c.is_cue {
            "Set Cue Note"
        } else {
            "Clear Cue Note"
        }
        .to_string(),
        Command::SetExpressionText(_) => "Set Expression Text".to_string(),
        Command::SetMeasureText(c) => match c.text {
            Some(_) => "Set Measure Text",
            None => "Remove Measure Text",
        }
        .to_string(),
        Command::SetScoreText(c) => match c.text {
            Some(_) => "Set Score Text",
            None => "Remove Score Text",
        }
        .to_string(),
        Command::SetScoreStyleOverrides(_) => "Set Score Style Defaults".to_string(),
        Command::SetObjectStyleOverrides(_) => "Set Object Style Overrides".to_string(),
        Command::ToggleTrillLine(_) => "Toggle Trill Line".to_string(),
        Command::SetGlissando(_) => "Set Glissando".to_string(),
        Command::SetCrossStaff(_) => "Set Cross-Staff Placement".to_string(),
        Command::SetPartGroup(_) => "Set Part Group".to_string(),
        Command::AddSpanner(_) => "Add Notation Spanner".to_string(),
        Command::UpdateSpanner(_) => "Update Notation Spanner".to_string(),
        Command::RemoveSpanner(_) => "Remove Notation Spanner".to_string(),
        Command::Batch(c) => c.label.clone().unwrap_or_else(|| {
            c.commands
                .first()
                .map(command_label)
                .unwrap_or_else(|| "Batch".to_string())
        }),
    }
}

/// Stable i18n key for an undoable command — camelCase variant name.
///
/// Use this instead of [`command_label`] when the UI translates labels itself.
pub fn command_key(cmd: &Command) -> String {
    match cmd {
        Command::AddNote(_) => "AddNote".to_string(),
        Command::AddPitch(_) => "AddPitch".to_string(),
        Command::SetDuration(_) => "SetDuration".to_string(),
        Command::DeleteNote(_) => "DeleteNote".to_string(),
        Command::AddMeasure(_) => "AddMeasure".to_string(),
        Command::DeleteMeasure(_) => "DeleteMeasure".to_string(),
        Command::SetTempo(_) => "SetTempo".to_string(),
        Command::NewScore(_) => "NewScore".to_string(),
        Command::AddHairpin(_) => "AddHairpin".to_string(),
        Command::ToggleTie(_) => "ToggleTie".to_string(),
        Command::SetDynamic(_) => "SetDynamic".to_string(),
        Command::ToggleArticulation(_) => "ToggleArticulation".to_string(),
        Command::SetKeySignature(_) => "SetKeySignature".to_string(),
        Command::SetTimeSignature(_) => "SetTimeSignature".to_string(),
        Command::SetBarline(_) => "SetBarline".to_string(),
        Command::AddPart(_) => "AddPart".to_string(),
        Command::DeletePart(_) => "DeletePart".to_string(),
        Command::ReorderParts(_) => "ReorderParts".to_string(),
        Command::SetMetadata(_) => "SetMetadata".to_string(),
        Command::SetRehearsalMark(_) => "SetRehearsalMark".to_string(),
        Command::SetNavigationMark(_) => "SetNavigationMark".to_string(),
        Command::SetChordSymbol(_) => "SetChordSymbol".to_string(),
        Command::SetHarmonyRange(_) => "SetHarmonyRange".to_string(),
        Command::SetFiguredBass(_) => "SetFiguredBass".to_string(),
        Command::SetHarpPedalDiagrams(_) => "SetHarpPedalDiagrams".to_string(),
        Command::SetGrace(_) => "SetGrace".to_string(),
        Command::SetOttava(_) => "SetOttava".to_string(),
        Command::SetLyric(_) => "SetLyric".to_string(),
        Command::SetMultiRest(_) => "SetMultiRest".to_string(),
        Command::AddPedal(_) => "AddPedal".to_string(),
        Command::SetVolta(_) => "SetVolta".to_string(),
        Command::SetClef(_) => "SetClef".to_string(),
        Command::SetPartName(_) => "SetPartName".to_string(),
        Command::SetMidiInstrument(_) => "SetMidiInstrument".to_string(),
        Command::SetPercussionKit(_) => "SetPercussionKit".to_string(),
        Command::SetInstrumentDefinition(_) => "SetInstrumentDefinition".to_string(),
        Command::SetMeasureInstrumentChange(_) => "SetMeasureInstrumentChange".to_string(),
        Command::SetMeasureTablatureChange(_) => "SetMeasureTablatureChange".to_string(),
        Command::UpsertScoreView(_) => "UpsertScoreView".to_string(),
        Command::RemoveScoreView(_) => "RemoveScoreView".to_string(),
        Command::SetTranspose(_) => "SetTranspose".to_string(),
        Command::TransposeStaffRegion(_) => "TransposeStaffRegion".to_string(),
        Command::SetTempoAtMeasure(_) => "SetTempoAtMeasure".to_string(),
        Command::SetTempoRampAtMeasure(_) => "SetTempoRampAtMeasure".to_string(),
        Command::PasteVoice(_) => "PasteVoice".to_string(),
        Command::PasteRange(_) => "PasteRange".to_string(),
        Command::PasteScoreFragment(_) => "PasteScoreFragment".to_string(),
        Command::ExchangeVoices(_) => "ExchangeVoices".to_string(),
        Command::MoveOrCopyVoiceRange(_) => "MoveOrCopyVoiceRange".to_string(),
        Command::SplitMeasure(_) => "SplitMeasure".to_string(),
        Command::JoinMeasures(_) => "JoinMeasures".to_string(),
        Command::ImplodeStaves(_) => "ImplodeStaves".to_string(),
        Command::ExplodeVoices(_) => "ExplodeVoices".to_string(),
        Command::ExplodeChordPitches(_) => "ExplodeChordPitches".to_string(),
        Command::ScaleVoiceRange(_) => "ScaleVoiceRange".to_string(),
        Command::SetSystemBreak(_) => "SetSystemBreak".to_string(),
        Command::SetPageBreak(_) => "SetPageBreak".to_string(),
        Command::SetSectionBreak(_) => "SetSectionBreak".to_string(),
        Command::ToggleSlur(_) => "ToggleSlur".to_string(),
        Command::AddStaff(_) => "AddStaff".to_string(),
        Command::DeleteStaff(_) => "DeleteStaff".to_string(),
        Command::SetTuplet(_) => "SetTuplet".to_string(),
        Command::RespellScore(_) => "RespellScore".to_string(),
        Command::RespellScoreToKey(_) => "RespellScoreToKey".to_string(),
        Command::SetStem(_) => "SetStem".to_string(),
        Command::SetArpeggio(_) => "SetArpeggio".to_string(),
        Command::SetTechniqueText(_) => "SetTechniqueText".to_string(),
        Command::SetFingering(_) => "SetFingering".to_string(),
        Command::SetFingerings(_) => "SetFingerings".to_string(),
        Command::SetStringNumber(_) => "SetStringNumber".to_string(),
        Command::SetTabPosition(_) => "SetTabPosition".to_string(),
        Command::SetTablatureConfig(_) => "SetTablatureConfig".to_string(),
        Command::SetStaffPresentation(_) => "SetStaffPresentation".to_string(),
        Command::SetGuitarTechnique(_) => "SetGuitarTechnique".to_string(),
        Command::SetGuitarBendAlter(_) => "SetGuitarBendAlter".to_string(),
        Command::SetGuitarBendCurve(_) => "SetGuitarBendCurve".to_string(),
        Command::SetNoteHead(_) => "SetNoteHead".to_string(),
        Command::SetCue(_) => "SetCue".to_string(),
        Command::SetUnpitched(_) => "SetUnpitched".to_string(),
        Command::SetInstrumentId(_) => "SetInstrumentId".to_string(),
        Command::SetNotePlacement(_) => "SetNotePlacement".to_string(),
        Command::SetExpressionText(_) => "SetExpressionText".to_string(),
        Command::SetMeasureText(_) => "SetMeasureText".to_string(),
        Command::SetScoreText(_) => "SetScoreText".to_string(),
        Command::SetScoreStyleOverrides(_) => "SetScoreStyleOverrides".to_string(),
        Command::SetObjectStyleOverrides(_) => "SetObjectStyleOverrides".to_string(),
        Command::ToggleTrillLine(_) => "ToggleTrillLine".to_string(),
        Command::SetGlissando(_) => "SetGlissando".to_string(),
        Command::SetCrossStaff(_) => "SetCrossStaff".to_string(),
        Command::SetPartGroup(_) => "SetPartGroup".to_string(),
        Command::AddSpanner(_) => "AddSpanner".to_string(),
        Command::UpdateSpanner(_) => "UpdateSpanner".to_string(),
        Command::RemoveSpanner(_) => "RemoveSpanner".to_string(),
        Command::Batch(c) => c.label.clone().unwrap_or_else(|| "Batch".to_string()),
    }
}

pub fn apply_command(cmd: &Command, score: &mut Score) -> Result<(), Error> {
    match cmd {
        Command::AddNote(c) => apply_add_note(c, score),
        Command::AddPitch(c) => apply_add_pitch(c, score),
        Command::SetDuration(c) => apply_set_duration(c, score),
        Command::DeleteNote(c) => apply_delete_note(c, score),
        Command::AddMeasure(c) => apply_add_measure(c, score),
        Command::DeleteMeasure(c) => apply_delete_measure(c, score),
        Command::SetTempo(c) => {
            score.settings.tempo_bpm = c.bpm;
            Ok(())
        }
        Command::NewScore(c) => {
            let mut s = match c.template {
                Some(kind) => Score::template(kind),
                None => Score::new(
                    &c.title,
                    c.tempo_bpm,
                    c.time_numerator,
                    c.time_denominator,
                    c.key_fifths,
                    c.measure_count,
                ),
            };
            if c.template.is_some() {
                s.metadata.title = c.title.clone();
                s.metadata.composer = c.composer.clone();
                s.settings.tempo_bpm = c.tempo_bpm;
                s.settings.time_signature = TimeSignature {
                    numerator: c.time_numerator,
                    denominator: c.time_denominator,
                };
                s.settings.key_signature = KeySignature {
                    fifths: c.key_fifths,
                    mode: "major".to_string(),
                };
                for part in &mut s.parts {
                    for staff in &mut part.staves {
                        staff.measures.clear();
                        for i in 0..c.measure_count {
                            let mut m = Measure::empty(c.time_numerator, c.time_denominator);
                            m.number = i + 1;
                            staff.measures.push(m);
                        }
                    }
                }
            }
            *score = s;
            Ok(())
        }
        Command::AddHairpin(c) => apply_add_hairpin(c, score),
        Command::ToggleTie(c) => apply_toggle_tie(c, score),
        Command::SetDynamic(c) => apply_set_dynamic(c, score),
        Command::ToggleArticulation(c) => apply_toggle_articulation(c, score),
        Command::SetKeySignature(c) => {
            score.settings.key_signature = KeySignature {
                fifths: c.fifths,
                mode: "major".to_string(),
            };
            Ok(())
        }
        Command::SetTimeSignature(c) => apply_set_time_signature(c, score),
        Command::SetBarline(c) => apply_set_barline(c, score),
        Command::AddPart(c) => apply_add_part(c, score),
        Command::DeletePart(c) => apply_delete_part(c, score),
        Command::ReorderParts(c) => apply_reorder_parts(c, score),
        Command::SetMetadata(c) => apply_set_metadata(c, score),
        Command::SetRehearsalMark(c) => {
            for_each_measure_at(score, c.measure_index, |m| {
                m.rehearsal = c.text.clone();
            });
            Ok(())
        }
        Command::SetNavigationMark(c) => {
            for_each_measure_at(score, c.measure_index, |m| {
                m.navigation = c.mark.clone();
            });
            Ok(())
        }
        Command::SetChordSymbol(c) => {
            get_note_mut(
                score,
                c.part_index,
                c.staff_index,
                c.measure_index,
                c.voice,
                c.note_index,
            )?
            .chord_symbol = c.chord.clone();
            Ok(())
        }
        Command::SetHarmonyRange(c) => {
            if let Some(end) = &c.end
                && !score
                    .parts
                    .get(end.part)
                    .and_then(|part| part.staves.get(end.staff))
                    .and_then(|staff| staff.measures.get(end.measure))
                    .and_then(|measure| measure.voices.get(end.voice))
                    .and_then(|voice| voice.get(end.note))
                    .is_some()
            {
                return Err(Error::InvalidCommand(
                    "harmony range end does not point to an existing note".into(),
                ));
            }
            let note = get_note_mut(
                score,
                c.part_index,
                c.staff_index,
                c.measure_index,
                c.voice,
                c.note_index,
            )?;
            let chord = note.chord_symbol.as_mut().ok_or_else(|| {
                Error::InvalidCommand("cannot set a harmony range without a chord symbol".into())
            })?;
            chord.range_end = c.end.clone();
            Ok(())
        }
        Command::SetFiguredBass(c) => {
            for part in &mut score.parts {
                for staff in &mut part.staves {
                    if let Some(measure) = staff.measures.get_mut(c.measure_index) {
                        measure.figured_bass = c.figures.clone();
                    }
                }
            }
            Ok(())
        }
        Command::SetHarpPedalDiagrams(c) => {
            let measure = score
                .parts
                .get_mut(c.part_index)
                .ok_or(Error::PartNotFound(c.part_index))?
                .staves
                .get_mut(c.staff_index)
                .ok_or(Error::StaffNotFound(c.staff_index))?
                .measures
                .get_mut(c.measure_index)
                .ok_or(Error::MeasureNotFound(c.measure_index))?;
            measure.harp_pedal_diagrams = c.diagrams.clone();
            Ok(())
        }
        Command::SetGrace(c) => {
            let note = get_note_mut(
                score,
                c.part_index,
                c.staff_index,
                c.measure_index,
                c.voice,
                c.note_index,
            )?;
            if note.is_rest {
                return Err(Error::InvalidCommand(
                    "cannot make a rest into a grace note".into(),
                ));
            }
            note.is_grace = c.is_grace;
            note.grace_slash = c.slash;
            Ok(())
        }
        Command::SetOttava(c) => {
            let note = get_note_mut(
                score,
                c.part_index,
                c.staff_index,
                c.measure_index,
                c.voice,
                c.note_index,
            )?;
            note.ottava_start = c.ottava_start;
            note.ottava_end = c.ottava_end;
            Ok(())
        }
        Command::SetLyric(c) => {
            get_note_mut(
                score,
                c.part_index,
                c.staff_index,
                c.measure_index,
                c.voice,
                c.note_index,
            )?
            .lyric = c.lyric.clone();
            Ok(())
        }
        Command::SetMultiRest(c) => {
            for_each_measure_at(score, c.measure_index, |m| {
                m.multi_rest_count = c.count;
            });
            Ok(())
        }
        Command::AddPedal(c) => apply_add_pedal(c, score),
        Command::SetVolta(c) => {
            for_each_measure_at(score, c.measure_index, |m| {
                m.volta = c.volta.clone();
            });
            Ok(())
        }
        Command::SetClef(c) => apply_set_clef(c, score),
        Command::SetPartName(c) => apply_set_part_name(c, score),
        Command::SetMidiInstrument(c) => apply_set_midi_instrument(c, score),
        Command::SetPercussionKit(c) => apply_set_percussion_kit(c, score),
        Command::SetInstrumentDefinition(c) => apply_set_instrument_definition(c, score),
        Command::SetMeasureInstrumentChange(c) => apply_set_measure_instrument_change(c, score),
        Command::SetMeasureTablatureChange(c) => apply_set_measure_tablature_change(c, score),
        Command::UpsertScoreView(c) => apply_upsert_score_view(c, score),
        Command::RemoveScoreView(c) => apply_remove_score_view(c, score),
        Command::SetTranspose(c) => apply_set_transpose(c, score),
        Command::TransposeStaffRegion(c) => apply_transpose_staff_region(c, score),
        Command::SetTempoAtMeasure(c) => {
            for_each_measure_at(score, c.measure_index, |m| {
                m.tempo = c.bpm;
            });
            Ok(())
        }
        Command::SetTempoRampAtMeasure(c) => {
            for_each_measure_at(score, c.measure_index, |m| {
                m.tempo_ramp_to = c.target_bpm;
            });
            Ok(())
        }
        Command::PasteVoice(c) => apply_paste_voice(c, score),
        Command::PasteRange(c) => apply_paste_range(c, score),
        Command::PasteScoreFragment(c) => apply_paste_score_fragment(c, score),
        Command::ExchangeVoices(c) => apply_exchange_voices(c, score),
        Command::MoveOrCopyVoiceRange(c) => apply_move_or_copy_voice_range(c, score),
        Command::SplitMeasure(c) => apply_split_measure(c, score),
        Command::JoinMeasures(c) => apply_join_measures(c, score),
        Command::ImplodeStaves(c) => apply_implode_staves(c, score),
        Command::ExplodeVoices(c) => apply_explode_voices(c, score),
        Command::ExplodeChordPitches(c) => apply_explode_chord_pitches(c, score),
        Command::ScaleVoiceRange(c) => apply_scale_voice_range(c, score),
        Command::SetSystemBreak(c) => {
            for_each_measure_at(score, c.measure_index, |m| {
                m.system_break = c.value;
            });
            Ok(())
        }
        Command::SetPageBreak(c) => {
            for_each_measure_at(score, c.measure_index, |m| {
                m.page_break = c.value;
            });
            Ok(())
        }
        Command::SetSectionBreak(c) => {
            if c.measure_index >= score.measure_count() {
                return Err(Error::MeasureNotFound(c.measure_index));
            }
            for_each_measure_at(score, c.measure_index, |m| {
                m.section_break = c.value;
            });
            Ok(())
        }
        Command::ToggleSlur(c) => apply_toggle_slur(c, score),
        Command::AddStaff(c) => apply_add_staff(c, score),
        Command::DeleteStaff(c) => apply_delete_staff(c, score),
        Command::SetTuplet(c) => {
            get_note_mut(
                score,
                c.part_index,
                c.staff_index,
                c.measure_index,
                c.voice_index,
                c.note_index,
            )?
            .tuplet = c.tuplet.clone();
            Ok(())
        }
        Command::RespellScore(c) => {
            respell_score(score, c.prefer_flat);
            Ok(())
        }
        Command::RespellScoreToKey(_) => {
            respell_score_to_key(score);
            Ok(())
        }
        Command::SetStem(c) => {
            get_note_mut(
                score,
                c.part_index,
                c.staff_index,
                c.measure_index,
                c.voice_index,
                c.note_index,
            )?
            .stem_up = c.stem_up;
            Ok(())
        }
        Command::SetArpeggio(c) => {
            get_note_mut(
                score,
                c.part_index,
                c.staff_index,
                c.measure_index,
                c.voice_index,
                c.note_index,
            )?
            .arpeggiate = c.direction;
            Ok(())
        }
        Command::SetTechniqueText(c) => {
            get_note_mut(
                score,
                c.part_index,
                c.staff_index,
                c.measure_index,
                c.voice,
                c.note_index,
            )?
            .technique_text = c.text.clone();
            Ok(())
        }
        Command::SetGlissando(c) => {
            let note = get_note_mut(
                score,
                c.part_index,
                c.staff_index,
                c.measure_index,
                c.voice,
                c.note_index,
            )?;
            note.glissando_start = c.start;
            note.glissando_end = c.end;
            Ok(())
        }
        Command::SetCrossStaff(c) => {
            let staff_count = score
                .parts
                .get(c.part_index)
                .ok_or(Error::PartNotFound(c.part_index))?
                .staves
                .len();
            let note = get_note_mut(
                score,
                c.part_index,
                c.staff_index,
                c.measure_index,
                c.voice,
                c.note_index,
            )?;
            if let Some(ref placement) = c.placement
                && placement.target_staff == c.staff_index
            {
                return Err(Error::InvalidCommand(
                    "cross-staff target must differ from source staff".into(),
                ));
            }
            if let Some(ref placement) = c.placement
                && placement.target_staff >= staff_count
            {
                return Err(Error::StaffNotFound(placement.target_staff));
            }
            note.cross_staff = c.placement.clone();
            Ok(())
        }
        Command::SetFingering(c) => {
            let note = get_note_mut(
                score,
                c.part_index,
                c.staff_index,
                c.measure_index,
                c.voice,
                c.note_index,
            )?;
            note.fingering = c.fingering;
            note.fingerings = c.fingering.into_iter().collect();
            Ok(())
        }
        Command::SetFingerings(c) => {
            let note = get_note_mut(
                score,
                c.part_index,
                c.staff_index,
                c.measure_index,
                c.voice,
                c.note_index,
            )?;
            note.fingerings = c.fingerings.clone();
            note.fingering = note.fingerings.first().copied();
            Ok(())
        }
        Command::SetStringNumber(c) => {
            get_note_mut(
                score,
                c.part_index,
                c.staff_index,
                c.measure_index,
                c.voice,
                c.note_index,
            )?
            .string_number = c.string_number;
            Ok(())
        }
        Command::SetTabPosition(c) => {
            let note = get_note_mut(
                score,
                c.part_index,
                c.staff_index,
                c.measure_index,
                c.voice,
                c.note_index,
            )?;
            note.string_number = c.position.as_ref().map(|position| position.string);
            note.tab_position = c.position.clone();
            note.tab_positions = c.position.iter().cloned().collect();
            Ok(())
        }
        Command::SetGuitarTechnique(c) => {
            get_note_mut(
                score,
                c.part_index,
                c.staff_index,
                c.measure_index,
                c.voice,
                c.note_index,
            )?
            .guitar_technique = c.technique.clone();
            Ok(())
        }
        Command::SetGuitarBendAlter(c) => {
            get_note_mut(
                score,
                c.part_index,
                c.staff_index,
                c.measure_index,
                c.voice,
                c.note_index,
            )?
            .guitar_bend_alter_cents = c.alter_cents;
            Ok(())
        }
        Command::SetGuitarBendCurve(c) => {
            get_note_mut(
                score,
                c.part_index,
                c.staff_index,
                c.measure_index,
                c.voice,
                c.note_index,
            )?
            .guitar_bend_curve = c.points.clone();
            Ok(())
        }
        Command::SetNoteHead(c) => {
            get_note_mut(
                score,
                c.part_index,
                c.staff_index,
                c.measure_index,
                c.voice,
                c.note_index,
            )?
            .note_head = c.note_head.clone();
            Ok(())
        }
        Command::SetCue(c) => {
            get_note_mut(
                score,
                c.part_index,
                c.staff_index,
                c.measure_index,
                c.voice,
                c.note_index,
            )?
            .is_cue = c.is_cue;
            Ok(())
        }
        Command::SetUnpitched(c) => {
            get_note_mut(
                score,
                c.part_index,
                c.staff_index,
                c.measure_index,
                c.voice,
                c.note_index,
            )?
            .is_unpitched = c.is_unpitched;
            Ok(())
        }
        Command::SetInstrumentId(c) => {
            get_note_mut(
                score,
                c.part_index,
                c.staff_index,
                c.measure_index,
                c.voice,
                c.note_index,
            )?
            .instrument_id = c.instrument_id.clone();
            Ok(())
        }
        Command::SetNotePlacement(c) => {
            let note = get_note_mut(
                score,
                c.part_index,
                c.staff_index,
                c.measure_index,
                c.voice,
                c.note_index,
            )?;
            for value in [c.offset_x, c.offset_y, c.relative_x, c.relative_y] {
                if value.is_some_and(|value| !value.is_finite()) {
                    return Err(Error::InvalidCommand(
                        "note placement offsets must be finite".into(),
                    ));
                }
            }
            note.offset_x = c.offset_x;
            note.offset_y = c.offset_y;
            note.relative_x = c.relative_x;
            note.relative_y = c.relative_y;
            Ok(())
        }
        Command::SetExpressionText(c) => {
            for_each_measure_at(score, c.measure_index, |m| {
                m.expression_text = c.text.clone();
            });
            Ok(())
        }
        Command::SetMeasureText(c) => apply_set_measure_text(c, score),
        Command::SetScoreText(c) => apply_set_score_text(c, score),
        Command::SetScoreStyleOverrides(c) => apply_set_score_style_overrides(c, score),
        Command::SetObjectStyleOverrides(c) => apply_set_object_style_overrides(c, score),
        Command::SetTablatureConfig(c) => apply_set_tablature_config(c, score),
        Command::SetStaffPresentation(c) => apply_set_staff_presentation(c, score),
        Command::ToggleTrillLine(c) => apply_toggle_trill_line(c, score),
        Command::SetPartGroup(c) => {
            if let Some(group) = &c.group {
                score
                    .part_groups
                    .retain(|g| g.first_part != group.first_part || g.last_part != group.last_part);
                score.part_groups.push(group.clone());
            } else {
                // When None, the command carries no range info so we clear all groups.
                score.part_groups.clear();
            }
            Ok(())
        }
        Command::AddSpanner(c) => {
            if score
                .spanners
                .iter()
                .any(|spanner| spanner.id == c.spanner.id)
            {
                return Err(Error::InvalidCommand(format!(
                    "notation spanner id already exists: {}",
                    c.spanner.id
                )));
            }
            score.spanners.push(c.spanner.clone());
            Ok(())
        }
        Command::UpdateSpanner(c) => {
            let index = score
                .spanners
                .iter()
                .position(|spanner| spanner.id == c.spanner.id)
                .ok_or_else(|| {
                    Error::InvalidCommand(format!(
                        "notation spanner id does not exist: {}",
                        c.spanner.id
                    ))
                })?;
            let previous = score.spanners[index].clone();
            score.spanners[index] = c.spanner.clone();
            clear_legacy_spanner_endpoints(score, &previous);
            clear_legacy_spanner_endpoints(score, &c.spanner);
            Ok(())
        }
        Command::RemoveSpanner(c) => {
            let index = score
                .spanners
                .iter()
                .position(|spanner| spanner.id == c.id)
                .ok_or_else(|| {
                    Error::InvalidCommand(format!("notation spanner id does not exist: {}", c.id))
                })?;
            let removed = score.spanners.remove(index);
            clear_legacy_spanner_endpoints(score, &removed);
            Ok(())
        }
        Command::Batch(c) => {
            for cmd in &c.commands {
                apply_command(cmd, score)?;
            }
            Ok(())
        }
    }
}

// ── helpers ──────────────────────────────────────────────────────────────────

fn apply_set_tablature_config(cmd: &SetTablatureConfigCmd, score: &mut Score) -> Result<(), Error> {
    let staff = score
        .parts
        .get_mut(cmd.part_index)
        .ok_or(Error::PartNotFound(cmd.part_index))?
        .staves
        .get_mut(cmd.staff_index)
        .ok_or_else(|| Error::InvalidCommand(format!("staff {} out of range", cmd.staff_index)))?;
    staff.tablature = cmd.config.clone();
    if staff.tablature.is_some() {
        staff.presentation.kind = StaffKind::Tablature;
    } else if staff.presentation.kind == StaffKind::Tablature {
        staff.presentation.kind = StaffKind::Standard;
    }
    if staff.tablature.is_none() {
        for measure in &mut staff.measures {
            measure.tablature_change = None;
        }
    }
    Ok(())
}

fn apply_set_staff_presentation(
    cmd: &SetStaffPresentationCmd,
    score: &mut Score,
) -> Result<(), Error> {
    if !(1..=64).contains(&cmd.presentation.lines) {
        return Err(Error::InvalidCommand(format!(
            "staff line count {} is outside 1..=64",
            cmd.presentation.lines
        )));
    }
    if !cmd.presentation.line_distance.is_finite()
        || !(0.1..=16.0).contains(&cmd.presentation.line_distance)
    {
        return Err(Error::InvalidCommand(
            "staff line distance must be finite and within 0.1..=16.0".into(),
        ));
    }
    let staff = score
        .parts
        .get_mut(cmd.part_index)
        .ok_or(Error::PartNotFound(cmd.part_index))?
        .staves
        .get_mut(cmd.staff_index)
        .ok_or_else(|| Error::InvalidCommand(format!("staff {} out of range", cmd.staff_index)))?;
    if cmd.presentation.kind == StaffKind::Tablature && staff.tablature.is_none() {
        return Err(Error::InvalidCommand(
            "tablature staff presentation requires a tablature configuration".into(),
        ));
    }
    staff.presentation = cmd.presentation.clone();
    Ok(())
}

fn get_note_mut(
    score: &mut Score,
    part_index: usize,
    staff_index: usize,
    measure_index: usize,
    voice: usize,
    note_index: usize,
) -> Result<&mut Note, Error> {
    score
        .parts
        .get_mut(part_index)
        .ok_or(Error::PartNotFound(part_index))?
        .staves
        .get_mut(staff_index)
        .ok_or(Error::StaffNotFound(staff_index))?
        .measures
        .get_mut(measure_index)
        .ok_or(Error::MeasureNotFound(measure_index))?
        .voices
        .get_mut(voice)
        .ok_or(Error::VoiceOutOfRange(voice))?
        .get_mut(note_index)
        .ok_or(Error::NoteNotFound(note_index))
}

fn for_each_measure_at(score: &mut Score, index: usize, mut f: impl FnMut(&mut Measure)) {
    for part in &mut score.parts {
        for staff in &mut part.staves {
            if let Some(m) = staff.measures.get_mut(index) {
                f(m);
            }
        }
    }
}

fn apply_set_measure_text(cmd: &SetMeasureTextCmd, score: &mut Score) -> Result<(), Error> {
    let measure = score
        .parts
        .get_mut(cmd.part_index)
        .ok_or(Error::PartNotFound(cmd.part_index))?
        .staves
        .get_mut(cmd.staff_index)
        .ok_or(Error::StaffNotFound(cmd.staff_index))?
        .measures
        .get_mut(cmd.measure_index)
        .ok_or(Error::MeasureNotFound(cmd.measure_index))?;
    if cmd.text_index > measure.texts.len()
        || (cmd.text.is_none() && cmd.text_index == measure.texts.len())
    {
        return Err(Error::InvalidCommand(format!(
            "styled text index {} out of range for {} entries",
            cmd.text_index,
            measure.texts.len()
        )));
    }
    if let Some(text) = &cmd.text {
        if cmd.text_index == measure.texts.len() {
            measure.texts.push(text.clone());
        } else {
            measure.texts[cmd.text_index] = text.clone();
        }
    } else {
        measure.texts.remove(cmd.text_index);
    }
    Ok(())
}

fn apply_set_score_text(cmd: &SetScoreTextCmd, score: &mut Score) -> Result<(), Error> {
    if cmd.text_index > score.texts.len()
        || (cmd.text.is_none() && cmd.text_index == score.texts.len())
    {
        return Err(Error::InvalidCommand(format!(
            "styled score text index {} out of range for {} entries",
            cmd.text_index,
            score.texts.len()
        )));
    }
    if let Some(text) = &cmd.text {
        if cmd.text_index == score.texts.len() {
            score.texts.push(text.clone());
        } else {
            score.texts[cmd.text_index] = text.clone();
        }
    } else {
        score.texts.remove(cmd.text_index);
    }
    Ok(())
}

fn apply_set_score_style_overrides(
    cmd: &SetScoreStyleOverridesCmd,
    score: &mut Score,
) -> Result<(), Error> {
    if cmd
        .overrides
        .iter()
        .any(|override_| !override_.value.is_finite() || !(0.05..=64.0).contains(&override_.value))
    {
        return Err(Error::InvalidCommand(
            "score typed style override values must be finite and within 0.05..=64".into(),
        ));
    }
    score.style_overrides = cmd.overrides.clone();
    Ok(())
}

fn apply_set_object_style_overrides(
    cmd: &SetObjectStyleOverridesCmd,
    score: &mut Score,
) -> Result<(), Error> {
    let mut candidate = score.clone();
    candidate.object_style_overrides = cmd.overrides.clone();
    if validate(&candidate).errors.iter().any(|error| {
        matches!(
            error,
            super::validate::ValidationError::InvalidObjectStyleOverride { .. }
        )
    }) {
        return Err(Error::InvalidCommand(
            "object style overrides require existing targets, finite values within 0.05..=64, and bounded provenance".into(),
        ));
    }
    score.object_style_overrides = cmd.overrides.clone();
    Ok(())
}

fn apply_add_note(cmd: &AddNoteCmd, score: &mut Score) -> Result<(), Error> {
    let ts_beats = score.settings.time_signature.total_beats();
    let note = if cmd.is_rest {
        let mut n = Note::rest(cmd.duration.clone());
        n.dot_count = cmd.dot_count;
        n.tuplet = cmd.tuplet.clone();
        n
    } else {
        let pitch = cmd
            .pitch
            .clone()
            .ok_or_else(|| Error::InvalidCommand("pitch required for non-rest note".into()))?;
        let mut n = Note::new(pitch, cmd.duration.clone());
        n.dot_count = cmd.dot_count;
        n.tuplet = cmd.tuplet.clone();
        n
    };

    let pos = {
        let voice = score
            .parts
            .get_mut(cmd.part_index)
            .ok_or(Error::PartNotFound(cmd.part_index))?
            .staves
            .get_mut(cmd.staff_index)
            .ok_or(Error::StaffNotFound(cmd.staff_index))?
            .measures
            .get_mut(cmd.measure_index)
            .ok_or(Error::MeasureNotFound(cmd.measure_index))?
            .voices
            .get_mut(cmd.voice)
            .ok_or(Error::VoiceOutOfRange(cmd.voice))?;
        let pos = cmd.position.min(voice.len());
        voice.insert(pos, note);
        trim_voice_to_measure(voice, ts_beats);
        pos
    };
    remap_spanners(score, |address| {
        if same_voice(
            address,
            cmd.part_index,
            cmd.staff_index,
            cmd.measure_index,
            cmd.voice,
        ) && address.note >= pos
        {
            let mut shifted = address.clone();
            shifted.note += 1;
            Some(shifted)
        } else {
            Some(address.clone())
        }
    });
    Ok(())
}

fn apply_add_pitch(cmd: &AddPitchCmd, score: &mut Score) -> Result<(), Error> {
    let voice = score
        .parts
        .get_mut(cmd.part_index)
        .ok_or(Error::PartNotFound(cmd.part_index))?
        .staves
        .get_mut(cmd.staff_index)
        .ok_or(Error::StaffNotFound(cmd.staff_index))?
        .measures
        .get_mut(cmd.measure_index)
        .ok_or(Error::MeasureNotFound(cmd.measure_index))?
        .voices
        .get_mut(cmd.voice)
        .ok_or(Error::VoiceOutOfRange(cmd.voice))?;
    let note = voice
        .get_mut(cmd.note_index)
        .ok_or(Error::NoteNotFound(cmd.note_index))?;
    if note.is_rest {
        return Err(Error::InvalidCommand("cannot add pitch to a rest".into()));
    }
    if !note
        .pitches
        .iter()
        .any(|p| p.step == cmd.pitch.step && p.octave == cmd.pitch.octave)
    {
        note.pitches.push(cmd.pitch.clone());
    }
    Ok(())
}

fn apply_set_duration(cmd: &SetDurationCmd, score: &mut Score) -> Result<(), Error> {
    let ts_beats = score.settings.time_signature.total_beats();
    {
        let voice = score
            .parts
            .get_mut(cmd.part_index)
            .ok_or(Error::PartNotFound(cmd.part_index))?
            .staves
            .get_mut(cmd.staff_index)
            .ok_or(Error::StaffNotFound(cmd.staff_index))?
            .measures
            .get_mut(cmd.measure_index)
            .ok_or(Error::MeasureNotFound(cmd.measure_index))?
            .voices
            .get_mut(cmd.voice)
            .ok_or(Error::VoiceOutOfRange(cmd.voice))?;
        let note = voice
            .get_mut(cmd.note_index)
            .ok_or(Error::NoteNotFound(cmd.note_index))?;
        note.duration = cmd.duration.clone();
        note.dot_count = cmd.dot_count;
        trim_voice_to_measure(voice, ts_beats);
    }
    prune_orphaned_spanners(score);
    Ok(())
}

fn apply_delete_note(cmd: &DeleteNoteCmd, score: &mut Score) -> Result<(), Error> {
    let ts_beats = score.settings.time_signature.total_beats();
    let deleted_position = {
        let voice = score
            .parts
            .get_mut(cmd.part_index)
            .ok_or(Error::PartNotFound(cmd.part_index))?
            .staves
            .get_mut(cmd.staff_index)
            .ok_or(Error::StaffNotFound(cmd.staff_index))?
            .measures
            .get_mut(cmd.measure_index)
            .ok_or(Error::MeasureNotFound(cmd.measure_index))?
            .voices
            .get_mut(cmd.voice)
            .ok_or(Error::VoiceOutOfRange(cmd.voice))?;
        let position = voice.iter().position(|note| note.id == cmd.note_id);
        voice.retain(|note| note.id != cmd.note_id);
        pad_voice_to_measure(voice, ts_beats);
        position
    };
    if let Some(position) = deleted_position {
        remap_spanners(score, |address| {
            if !same_voice(
                address,
                cmd.part_index,
                cmd.staff_index,
                cmd.measure_index,
                cmd.voice,
            ) {
                return Some(address.clone());
            }
            if address.note == position {
                None
            } else if address.note > position {
                let mut shifted = address.clone();
                shifted.note -= 1;
                Some(shifted)
            } else {
                Some(address.clone())
            }
        });
    } else {
        prune_orphaned_spanners(score);
    }
    Ok(())
}

fn apply_add_measure(cmd: &AddMeasureCmd, score: &mut Score) -> Result<(), Error> {
    let ts = score.settings.time_signature.clone();
    let insert_at = cmd.after_index.saturating_add(1);
    for part in &mut score.parts {
        for staff in &mut part.staves {
            let insert_at = insert_at.min(staff.measures.len());
            let mut m = Measure::empty(ts.numerator, ts.denominator);
            m.number = insert_at as u32 + 1;
            staff.measures.insert(insert_at, m);
            for (i, measure) in staff.measures.iter_mut().enumerate() {
                measure.number = i as u32 + 1;
            }
        }
    }
    remap_spanners(score, |address| {
        if address.measure >= insert_at {
            let mut shifted = address.clone();
            shifted.measure += 1;
            Some(shifted)
        } else {
            Some(address.clone())
        }
    });
    Ok(())
}

fn apply_delete_measure(cmd: &DeleteMeasureCmd, score: &mut Score) -> Result<(), Error> {
    for part in &mut score.parts {
        for staff in &mut part.staves {
            if cmd.measure_index < staff.measures.len() {
                staff.measures.remove(cmd.measure_index);
                for (i, m) in staff.measures.iter_mut().enumerate() {
                    m.number = i as u32 + 1;
                }
            }
        }
    }
    remap_spanners(score, |address| {
        if address.measure == cmd.measure_index {
            None
        } else if address.measure > cmd.measure_index {
            let mut shifted = address.clone();
            shifted.measure -= 1;
            Some(shifted)
        } else {
            Some(address.clone())
        }
    });
    Ok(())
}

fn apply_add_hairpin(cmd: &AddHairpinCmd, score: &mut Score) -> Result<(), Error> {
    let voice = score
        .parts
        .get_mut(cmd.part_index)
        .ok_or(Error::PartNotFound(cmd.part_index))?
        .staves
        .get_mut(cmd.staff_index)
        .ok_or(Error::StaffNotFound(cmd.staff_index))?
        .measures
        .get_mut(cmd.measure_index)
        .ok_or(Error::MeasureNotFound(cmd.measure_index))?
        .voices
        .get_mut(cmd.voice)
        .ok_or(Error::VoiceOutOfRange(cmd.voice))?;
    if cmd.start_note_idx >= voice.len() {
        return Err(Error::NoteNotFound(cmd.start_note_idx));
    }
    if cmd.end_note_idx >= voice.len() {
        return Err(Error::NoteNotFound(cmd.end_note_idx));
    }
    if cmd.start_note_idx >= cmd.end_note_idx {
        return Err(Error::InvalidCommand(
            "start_note_idx must be less than end_note_idx".into(),
        ));
    }
    for note in voice
        .iter_mut()
        .take(cmd.end_note_idx + 1)
        .skip(cmd.start_note_idx)
    {
        note.hairpin_start = None;
        note.hairpin_end = false;
    }
    voice[cmd.start_note_idx].hairpin_start = Some(cmd.kind);
    voice[cmd.end_note_idx].hairpin_end = true;
    Ok(())
}

fn apply_add_pedal(cmd: &AddPedalCmd, score: &mut Score) -> Result<(), Error> {
    let voice = score
        .parts
        .get_mut(cmd.part_index)
        .ok_or(Error::PartNotFound(cmd.part_index))?
        .staves
        .get_mut(cmd.staff_index)
        .ok_or(Error::StaffNotFound(cmd.staff_index))?
        .measures
        .get_mut(cmd.measure_index)
        .ok_or(Error::MeasureNotFound(cmd.measure_index))?
        .voices
        .get_mut(cmd.voice)
        .ok_or(Error::VoiceOutOfRange(cmd.voice))?;
    if cmd.start_note_idx >= voice.len() {
        return Err(Error::NoteNotFound(cmd.start_note_idx));
    }
    if cmd.end_note_idx >= voice.len() {
        return Err(Error::NoteNotFound(cmd.end_note_idx));
    }
    if cmd.start_note_idx >= cmd.end_note_idx {
        return Err(Error::InvalidCommand(
            "start_note_idx must be less than end_note_idx".into(),
        ));
    }
    for note in voice
        .iter_mut()
        .take(cmd.end_note_idx + 1)
        .skip(cmd.start_note_idx)
    {
        note.pedal_start = false;
        note.pedal_end = false;
    }
    voice[cmd.start_note_idx].pedal_start = true;
    voice[cmd.end_note_idx].pedal_end = true;
    Ok(())
}

fn apply_toggle_tie(cmd: &ToggleTieCmd, score: &mut Score) -> Result<(), Error> {
    let current_tie_start = {
        let v = score
            .parts
            .get(cmd.part_index)
            .ok_or(Error::PartNotFound(cmd.part_index))?
            .staves
            .get(cmd.staff_index)
            .ok_or(Error::StaffNotFound(cmd.staff_index))?
            .measures
            .get(cmd.measure_index)
            .ok_or(Error::MeasureNotFound(cmd.measure_index))?
            .voices
            .get(cmd.voice)
            .ok_or(Error::VoiceOutOfRange(cmd.voice))?;
        v.get(cmd.note_index)
            .ok_or(Error::NoteNotFound(cmd.note_index))?
            .tie_start
    };
    let voice_len = score.parts[cmd.part_index].staves[cmd.staff_index].measures[cmd.measure_index]
        .voices[cmd.voice]
        .len();
    let total_measures = score.parts[cmd.part_index].staves[cmd.staff_index]
        .measures
        .len();

    let new_tie = !current_tie_start;
    score.parts[cmd.part_index].staves[cmd.staff_index].measures[cmd.measure_index].voices
        [cmd.voice][cmd.note_index]
        .tie_start = new_tie;

    if cmd.note_index + 1 < voice_len {
        score.parts[cmd.part_index].staves[cmd.staff_index].measures[cmd.measure_index].voices
            [cmd.voice][cmd.note_index + 1]
            .tie_end = new_tie;
    } else {
        let next_mi = cmd.measure_index + 1;
        if next_mi < total_measures {
            let next_voice = &mut score.parts[cmd.part_index].staves[cmd.staff_index].measures
                [next_mi]
                .voices[cmd.voice];
            if let Some(n) = next_voice.get_mut(0) {
                n.tie_end = new_tie;
            }
        }
    }
    Ok(())
}

fn apply_set_dynamic(cmd: &SetDynamicCmd, score: &mut Score) -> Result<(), Error> {
    get_note_mut(
        score,
        cmd.part_index,
        cmd.staff_index,
        cmd.measure_index,
        cmd.voice,
        cmd.note_index,
    )?
    .dynamic = cmd.dynamic.clone();
    Ok(())
}

fn apply_toggle_articulation(cmd: &ToggleArticulationCmd, score: &mut Score) -> Result<(), Error> {
    let note = get_note_mut(
        score,
        cmd.part_index,
        cmd.staff_index,
        cmd.measure_index,
        cmd.voice,
        cmd.note_index,
    )?;
    if let Some(pos) = note
        .articulations
        .iter()
        .position(|a| a == &cmd.articulation)
    {
        note.articulations.remove(pos);
    } else {
        note.articulations.push(cmd.articulation.clone());
    }
    Ok(())
}

fn apply_set_time_signature(cmd: &SetTimeSignatureCmd, score: &mut Score) -> Result<(), Error> {
    if cmd.numerator == 0 || cmd.denominator == 0 {
        return Err(Error::InvalidCommand(
            "time signature numerator and denominator must be > 0".into(),
        ));
    }
    if ![1u8, 2, 4, 8, 16, 32].contains(&cmd.denominator) {
        return Err(Error::InvalidCommand(format!(
            "invalid time signature denominator: {}",
            cmd.denominator
        )));
    }
    score.settings.time_signature = TimeSignature {
        numerator: cmd.numerator,
        denominator: cmd.denominator,
    };
    let max_beats = score.settings.time_signature.total_beats();
    for part in &mut score.parts {
        for staff in &mut part.staves {
            for measure in &mut staff.measures {
                for voice in &mut measure.voices {
                    trim_voice_to_measure(voice, max_beats);
                    pad_voice_to_measure(voice, max_beats);
                }
            }
        }
    }
    Ok(())
}

fn apply_set_barline(cmd: &SetBarlineCmd, score: &mut Score) -> Result<(), Error> {
    for part in &mut score.parts {
        for staff in &mut part.staves {
            let measure = staff
                .measures
                .get_mut(cmd.measure_index)
                .ok_or(Error::MeasureNotFound(cmd.measure_index))?;
            match cmd.side.as_str() {
                "left" => measure.barline_left = cmd.barline.clone(),
                "right" => measure.barline_right = cmd.barline.clone(),
                _ => {
                    return Err(Error::InvalidCommand(format!(
                        "invalid barline side: '{}'",
                        cmd.side
                    )));
                }
            }
        }
    }
    Ok(())
}

fn apply_add_part(cmd: &AddPartCmd, score: &mut Score) -> Result<(), Error> {
    if cmd.clefs.is_empty() {
        return Err(Error::InvalidCommand(
            "AddPart requires at least one clef".into(),
        ));
    }
    let measure_count = score.measure_count();
    let ts = score.settings.time_signature.clone();
    let mut part = Part::new(&cmd.name, &cmd.short_name);
    part.midi_channel = cmd.midi_channel.min(15);
    part.midi_program = cmd.midi_program;
    for clef_str in &cmd.clefs {
        let clef = match clef_str.as_str() {
            "Bass" => Clef::Bass,
            "Alto" => Clef::Alto,
            "Tenor" => Clef::Tenor,
            "Percussion" => Clef::Percussion,
            _ => Clef::Treble,
        };
        let mut staff = Staff::new(clef);
        for i in 0..measure_count {
            let mut m = Measure::empty(ts.numerator, ts.denominator);
            m.number = i as u32 + 1;
            staff.measures.push(m);
        }
        part.staves.push(staff);
    }
    score.parts.push(part);
    Ok(())
}

fn apply_set_clef(cmd: &SetClefCmd, score: &mut Score) -> Result<(), Error> {
    score
        .parts
        .get_mut(cmd.part_index)
        .ok_or(Error::PartNotFound(cmd.part_index))?
        .staves
        .get_mut(cmd.staff_index)
        .ok_or(Error::StaffNotFound(cmd.staff_index))?
        .clef = cmd.clef.clone();
    Ok(())
}

fn apply_set_part_name(cmd: &SetPartNameCmd, score: &mut Score) -> Result<(), Error> {
    let part = score
        .parts
        .get_mut(cmd.part_index)
        .ok_or(Error::PartNotFound(cmd.part_index))?;
    part.name = cmd.name.clone();
    part.short_name = cmd.short_name.clone();
    Ok(())
}

fn apply_delete_part(cmd: &DeletePartCmd, score: &mut Score) -> Result<(), Error> {
    if cmd.part_index >= score.parts.len() {
        return Err(Error::PartNotFound(cmd.part_index));
    }
    score.parts.remove(cmd.part_index);
    remap_spanners(score, |address| {
        if address.part == cmd.part_index {
            None
        } else if address.part > cmd.part_index {
            let mut shifted = address.clone();
            shifted.part -= 1;
            Some(shifted)
        } else {
            Some(address.clone())
        }
    });
    Ok(())
}

fn apply_reorder_parts(cmd: &ReorderPartsCmd, score: &mut Score) -> Result<(), Error> {
    let count = score.parts.len();
    if cmd.order.len() != count {
        return Err(Error::InvalidCommand(format!(
            "part order must contain exactly {count} entries"
        )));
    }
    let mut old_to_new = vec![usize::MAX; count];
    for (new_index, &old_index) in cmd.order.iter().enumerate() {
        if old_index >= count {
            return Err(Error::PartNotFound(old_index));
        }
        if std::mem::replace(&mut old_to_new[old_index], new_index) != usize::MAX {
            return Err(Error::InvalidCommand(format!(
                "part order contains duplicate index {old_index}"
            )));
        }
    }
    for view in &score.views {
        for &part in &view.parts {
            if part >= count {
                return Err(Error::PartNotFound(part));
            }
        }
        for override_ in &view.staff_kind_overrides {
            if override_.staff.part >= count {
                return Err(Error::PartNotFound(override_.staff.part));
            }
        }
    }
    for spanner in &score.spanners {
        for address in [&spanner.start, &spanner.end] {
            if address.part >= count {
                return Err(Error::PartNotFound(address.part));
            }
        }
    }
    for group in &score.part_groups {
        if group.first_part > group.last_part || group.last_part >= count {
            return Err(Error::InvalidCommand(
                "part group references an invalid part range".into(),
            ));
        }
        let mut remapped: Vec<_> = (group.first_part..=group.last_part)
            .map(|part| old_to_new[part])
            .collect();
        remapped.sort_unstable();
        if remapped
            .windows(2)
            .any(|pair| pair[1] != pair[0].saturating_add(1))
        {
            return Err(Error::InvalidCommand(
                "part order would split an existing part group".into(),
            ));
        }
    }

    let existing = std::mem::take(&mut score.parts);
    score.parts = cmd
        .order
        .iter()
        .map(|&old_index| existing[old_index].clone())
        .collect();
    for view in &mut score.views {
        for part in &mut view.parts {
            *part = old_to_new[*part];
        }
        for override_ in &mut view.staff_kind_overrides {
            override_.staff.part = old_to_new[override_.staff.part];
        }
    }
    for group in &mut score.part_groups {
        group.first_part = old_to_new[group.first_part];
        group.last_part = old_to_new[group.last_part];
        if group.first_part > group.last_part {
            std::mem::swap(&mut group.first_part, &mut group.last_part);
        }
    }
    remap_spanners(score, |address| {
        let mut remapped = address.clone();
        remapped.part = old_to_new[address.part];
        Some(remapped)
    });
    Ok(())
}

fn apply_set_metadata(cmd: &SetMetadataCmd, score: &mut Score) -> Result<(), Error> {
    if let Some(v) = &cmd.title {
        score.metadata.title = v.clone();
    }
    if let Some(v) = &cmd.composer {
        score.metadata.composer = v.clone();
    }
    if let Some(v) = &cmd.lyricist {
        score.metadata.lyricist = v.clone();
    }
    if let Some(v) = &cmd.copyright {
        score.metadata.copyright = v.clone();
    }
    if let Some(v) = &cmd.work_number {
        score.metadata.work_number = v.clone();
    }
    if let Some(v) = &cmd.movement_title {
        score.metadata.movement_title = v.clone();
    }
    Ok(())
}

fn apply_set_midi_instrument(cmd: &SetMidiInstrumentCmd, score: &mut Score) -> Result<(), Error> {
    let part = score
        .parts
        .get_mut(cmd.part_index)
        .ok_or(Error::PartNotFound(cmd.part_index))?;
    part.midi_channel = cmd.midi_channel.min(15);
    part.midi_program = cmd.midi_program;
    Ok(())
}

fn apply_set_percussion_kit(cmd: &SetPercussionKitCmd, score: &mut Score) -> Result<(), Error> {
    validate_percussion_kit(&cmd.instruments)?;
    let part = score
        .parts
        .get_mut(cmd.part_index)
        .ok_or(Error::PartNotFound(cmd.part_index))?;
    part.percussion_instruments = cmd.instruments.clone();
    Ok(())
}

fn apply_set_instrument_definition(
    cmd: &SetInstrumentDefinitionCmd,
    score: &mut Score,
) -> Result<(), Error> {
    if let Some(definition) = &cmd.definition {
        validate_instrument_definition(definition)?;
    }
    let part = score
        .parts
        .get_mut(cmd.part_index)
        .ok_or(Error::PartNotFound(cmd.part_index))?;
    part.instrument = cmd.definition.clone();
    Ok(())
}

fn apply_set_measure_instrument_change(
    cmd: &SetMeasureInstrumentChangeCmd,
    score: &mut Score,
) -> Result<(), Error> {
    if let Some(definition) = &cmd.definition {
        validate_instrument_definition(definition)?;
    }
    score
        .parts
        .get_mut(cmd.part_index)
        .ok_or(Error::PartNotFound(cmd.part_index))?
        .staves
        .get_mut(cmd.staff_index)
        .ok_or(Error::StaffNotFound(cmd.staff_index))?
        .measures
        .get_mut(cmd.measure_index)
        .ok_or(Error::MeasureNotFound(cmd.measure_index))?
        .instrument_change = cmd.definition.clone();
    Ok(())
}

fn apply_set_measure_tablature_change(
    cmd: &SetMeasureTablatureChangeCmd,
    score: &mut Score,
) -> Result<(), Error> {
    let staff = score
        .parts
        .get_mut(cmd.part_index)
        .ok_or(Error::PartNotFound(cmd.part_index))?
        .staves
        .get_mut(cmd.staff_index)
        .ok_or(Error::StaffNotFound(cmd.staff_index))?;
    let base = staff.tablature.as_ref().ok_or_else(|| {
        Error::InvalidCommand("tablature change requires a staff tablature configuration".into())
    })?;
    if let Some(config) = &cmd.config {
        validate_tablature_config(config)?;
        if config.lines != base.lines {
            return Err(Error::InvalidCommand(format!(
                "tablature change line count {} must match staff line count {}",
                config.lines, base.lines
            )));
        }
    }
    staff
        .measures
        .get_mut(cmd.measure_index)
        .ok_or(Error::MeasureNotFound(cmd.measure_index))?
        .tablature_change = cmd.config.clone();
    Ok(())
}

fn validate_tablature_config(config: &TablatureConfig) -> Result<(), Error> {
    if !(1..=64).contains(&config.lines) {
        return Err(Error::InvalidCommand(format!(
            "tablature line count {} is outside 1..=64",
            config.lines
        )));
    }
    if config.tuning_midi.len() > usize::from(config.lines) {
        return Err(Error::InvalidCommand(format!(
            "tablature has {} tunings for {} lines",
            config.tuning_midi.len(),
            config.lines
        )));
    }
    if let Some(midi) = config
        .tuning_midi
        .iter()
        .copied()
        .find(|midi| !(0..=127).contains(midi))
    {
        return Err(Error::InvalidCommand(format!(
            "tablature tuning MIDI {midi} is outside 0..=127"
        )));
    }
    Ok(())
}

fn validate_percussion_kit(instruments: &[PercussionInstrument]) -> Result<(), Error> {
    let mut ids = std::collections::HashSet::new();
    for instrument in instruments {
        if instrument.id.trim().is_empty() {
            return Err(Error::InvalidCommand(
                "percussion instrument id must not be empty".into(),
            ));
        }
        if !ids.insert(instrument.id.as_str()) {
            return Err(Error::InvalidCommand(format!(
                "percussion instrument id '{}' is duplicated",
                instrument.id
            )));
        }
        if let Some(position) = instrument.staff_position
            && !(-32..=32).contains(&position)
        {
            return Err(Error::InvalidCommand(format!(
                "percussion staff position {position} is outside -32..=32"
            )));
        }
        if let Some(voice) = instrument.preferred_voice
            && !(1..=4).contains(&voice)
        {
            return Err(Error::InvalidCommand(format!(
                "percussion preferred voice {voice} is outside 1..=4"
            )));
        }
        if instrument
            .techniques
            .iter()
            .any(|technique| technique.trim().is_empty() || technique.len() > 128)
        {
            return Err(Error::InvalidCommand(
                "percussion techniques must be non-empty and at most 128 bytes".into(),
            ));
        }
    }
    Ok(())
}

fn validate_instrument_definition(definition: &InstrumentDefinition) -> Result<(), Error> {
    if definition.id.trim().is_empty() {
        return Err(Error::InvalidCommand(
            "instrument definition id must not be empty".into(),
        ));
    }
    if !(1..=64).contains(&definition.staff_count) {
        return Err(Error::InvalidCommand(format!(
            "instrument staff count {} is outside 1..=64",
            definition.staff_count
        )));
    }
    if definition.midi_channel > 15 {
        return Err(Error::InvalidCommand(format!(
            "instrument MIDI channel {} is outside 0..=15",
            definition.midi_channel
        )));
    }
    for (name, range) in [
        ("written", definition.written_range),
        ("sounding", definition.sounding_range),
    ] {
        if let Some(InstrumentRange { lowest, highest }) = range
            && lowest > highest
        {
            return Err(Error::InvalidCommand(format!(
                "instrument {name} range has lowest MIDI note {lowest} above highest {highest}"
            )));
        }
    }
    Ok(())
}

fn apply_upsert_score_view(cmd: &UpsertScoreViewCmd, score: &mut Score) -> Result<(), Error> {
    validate_score_view(&cmd.view, score)?;
    if let Some(index) = score.views.iter().position(|view| view.id == cmd.view.id) {
        score.views[index] = cmd.view.clone();
    } else {
        score.views.push(cmd.view.clone());
    }
    Ok(())
}

fn apply_remove_score_view(cmd: &RemoveScoreViewCmd, score: &mut Score) -> Result<(), Error> {
    let index = score
        .views
        .iter()
        .position(|view| view.id == cmd.id)
        .ok_or_else(|| Error::InvalidCommand(format!("score view '{}' does not exist", cmd.id)))?;
    score.views.remove(index);
    Ok(())
}

fn validate_score_view(view: &ScoreView, score: &Score) -> Result<(), Error> {
    if view.id.trim().is_empty() || view.name.trim().is_empty() {
        return Err(Error::InvalidCommand(
            "score view id and name must not be empty".into(),
        ));
    }
    if view.parts.is_empty() {
        return Err(Error::InvalidCommand(
            "score view must select at least one part".into(),
        ));
    }
    let mut selected = vec![false; score.parts.len()];
    for &part in &view.parts {
        if part >= score.parts.len() {
            return Err(Error::PartNotFound(part));
        }
        if std::mem::replace(&mut selected[part], true) {
            return Err(Error::InvalidCommand(format!(
                "score view '{}' selects part {part} more than once",
                view.id
            )));
        }
    }
    if view.layout.measures_per_row.is_some_and(|value| value == 0) {
        return Err(Error::InvalidCommand(
            "score view measures per row must be greater than zero".into(),
        ));
    }
    if view
        .layout
        .typed_style_overrides
        .iter()
        .any(|override_| !override_.value.is_finite() || !(0.05..=64.0).contains(&override_.value))
    {
        return Err(Error::InvalidCommand(
            "score view typed style override values must be finite and within 0.05..=64".into(),
        ));
    }
    for reference in &view.layout.hidden_staves {
        let Some(part) = score.parts.get(reference.part) else {
            return Err(Error::PartNotFound(reference.part));
        };
        if reference.staff >= part.staves.len() {
            return Err(Error::StaffNotFound(reference.staff));
        }
        if !selected[reference.part] {
            return Err(Error::InvalidCommand(
                "score view cannot hide a staff outside its selected parts".into(),
            ));
        }
    }
    let mut overridden = std::collections::HashSet::new();
    for override_ in &view.staff_kind_overrides {
        let reference = override_.staff;
        let Some(part) = score.parts.get(reference.part) else {
            return Err(Error::PartNotFound(reference.part));
        };
        if reference.staff >= part.staves.len() {
            return Err(Error::StaffNotFound(reference.staff));
        }
        if !selected[reference.part] {
            return Err(Error::InvalidCommand(
                "score view cannot override a staff outside its selected parts".into(),
            ));
        }
        if !overridden.insert((reference.part, reference.staff)) {
            return Err(Error::InvalidCommand(
                "score view may override each staff kind at most once".into(),
            ));
        }
        if override_.kind == StaffKind::Tablature
            && part.staves[reference.staff].tablature.is_none()
        {
            return Err(Error::InvalidCommand(
                "tablature score view requires a tablature configuration".into(),
            ));
        }
    }
    let measure_count = score.measure_count();
    for &break_index in view
        .layout
        .system_breaks
        .iter()
        .chain(view.layout.page_breaks.iter())
    {
        if break_index >= measure_count {
            return Err(Error::InvalidCommand(format!(
                "score view break measure {break_index} is out of range"
            )));
        }
    }
    for (key, value) in &view.layout.style_overrides {
        if key.trim().is_empty() || value.len() > 4096 {
            return Err(Error::InvalidCommand(
                "score view style overrides need a non-empty key and a value of at most 4096 bytes"
                    .into(),
            ));
        }
    }
    Ok(())
}

fn apply_set_transpose(cmd: &SetTransposeCmd, score: &mut Score) -> Result<(), Error> {
    score
        .parts
        .get_mut(cmd.part_index)
        .ok_or(Error::PartNotFound(cmd.part_index))?
        .staves
        .get_mut(cmd.staff_index)
        .ok_or(Error::StaffNotFound(cmd.staff_index))?
        .transpose_semitones = cmd.semitones;
    Ok(())
}

fn apply_transpose_staff_region(
    cmd: &TransposeStaffRegionCmd,
    score: &mut Score,
) -> Result<(), Error> {
    *score = transpose_staff_region_checked(
        score,
        cmd.part_index,
        cmd.staff_index,
        cmd.start_measure,
        cmd.end_measure,
        cmd.semitones,
        cmd.target,
    )?;
    Ok(())
}

fn apply_paste_voice(cmd: &PasteVoiceCmd, score: &mut Score) -> Result<(), Error> {
    let endpoint_ids = capture_replaced_endpoint_ids(score, |address| {
        same_voice(
            address,
            cmd.part_index,
            cmd.staff_index,
            cmd.measure_index,
            cmd.voice_index,
        )
    });
    {
        let voice = score
            .parts
            .get_mut(cmd.part_index)
            .ok_or(Error::PartNotFound(cmd.part_index))?
            .staves
            .get_mut(cmd.staff_index)
            .ok_or(Error::StaffNotFound(cmd.staff_index))?
            .measures
            .get_mut(cmd.measure_index)
            .ok_or(Error::MeasureNotFound(cmd.measure_index))?
            .voices
            .get_mut(cmd.voice_index)
            .ok_or(Error::VoiceOutOfRange(cmd.voice_index))?;
        *voice = cmd.notes.clone();
    }
    remap_replaced_spanner_endpoints(score, &endpoint_ids);
    Ok(())
}

fn apply_paste_range(cmd: &PasteRangeCmd, score: &mut Score) -> Result<(), Error> {
    if cmd.voice_index >= 4 {
        return Err(Error::VoiceOutOfRange(cmd.voice_index));
    }
    let endpoint_ids = capture_replaced_endpoint_ids(score, |address| {
        address.part == cmd.part_index
            && address.staff == cmd.staff_index
            && address.voice == cmd.voice_index
            && address.measure >= cmd.target_measure
            && address.measure < cmd.target_measure.saturating_add(cmd.measures.len())
    });
    let part = score
        .parts
        .get_mut(cmd.part_index)
        .ok_or(Error::PartNotFound(cmd.part_index))?;
    let staff = part
        .staves
        .get_mut(cmd.staff_index)
        .ok_or(Error::StaffNotFound(cmd.staff_index))?;
    for (offset, notes) in cmd.measures.iter().enumerate() {
        let mi = cmd.target_measure + offset;
        let measure = staff
            .measures
            .get_mut(mi)
            .ok_or(Error::MeasureNotFound(mi))?;
        measure.voices[cmd.voice_index] = notes.clone();
    }
    remap_replaced_spanner_endpoints(score, &endpoint_ids);
    Ok(())
}

fn apply_exchange_voices(cmd: &ExchangeVoicesCmd, score: &mut Score) -> Result<(), Error> {
    if cmd.first_voice >= 4 {
        return Err(Error::VoiceOutOfRange(cmd.first_voice));
    }
    if cmd.second_voice >= 4 {
        return Err(Error::VoiceOutOfRange(cmd.second_voice));
    }
    if cmd.first_voice == cmd.second_voice {
        return Err(Error::InvalidCommand(
            "exchange voices requires two distinct voices".into(),
        ));
    }
    let start = cmd.start_measure.min(cmd.end_measure);
    let end = cmd.start_measure.max(cmd.end_measure);
    let staff = score
        .parts
        .get_mut(cmd.part_index)
        .ok_or(Error::PartNotFound(cmd.part_index))?
        .staves
        .get_mut(cmd.staff_index)
        .ok_or(Error::StaffNotFound(cmd.staff_index))?;
    if end >= staff.measures.len() {
        return Err(Error::MeasureNotFound(end));
    }
    for measure in &mut staff.measures[start..=end] {
        measure.voices.swap(cmd.first_voice, cmd.second_voice);
        measure
            .source_voice_numbers
            .swap(cmd.first_voice, cmd.second_voice);
    }
    remap_spanners(score, |address| {
        if address.part != cmd.part_index
            || address.staff != cmd.staff_index
            || address.measure < start
            || address.measure > end
        {
            return Some(address.clone());
        }
        let mut remapped = address.clone();
        if address.voice == cmd.first_voice {
            remapped.voice = cmd.second_voice;
        } else if address.voice == cmd.second_voice {
            remapped.voice = cmd.first_voice;
        }
        Some(remapped)
    });
    Ok(())
}

fn apply_move_or_copy_voice_range(
    cmd: &MoveOrCopyVoiceRangeCmd,
    score: &mut Score,
) -> Result<(), Error> {
    if cmd.source_start.part != cmd.source_end.part
        || cmd.source_start.staff != cmd.source_end.staff
        || cmd.source_start.voice != cmd.source_end.voice
    {
        return Err(Error::InvalidCommand(
            "move or copy source endpoints must share part, staff, and voice".into(),
        ));
    }
    if cmd.source_start.voice >= 4 {
        return Err(Error::VoiceOutOfRange(cmd.source_start.voice));
    }
    let source_from = cmd.source_start.measure.min(cmd.source_end.measure);
    let source_to = cmd.source_start.measure.max(cmd.source_end.measure);
    let source_count = source_to - source_from + 1;
    let target_end = cmd
        .target
        .measure
        .checked_add(source_count - 1)
        .ok_or_else(|| Error::InvalidCommand("voice range target overflows".into()))?;
    if cmd.move_source
        && cmd.target.part == cmd.source_start.part
        && cmd.target.staff == cmd.source_start.staff
        && cmd.target.voice == cmd.source_start.voice
        && cmd.target.measure <= source_to
        && target_end >= source_from
    {
        return Err(Error::InvalidCommand(
            "moving a voice range onto itself is not supported".into(),
        ));
    }

    let fragment = extract_score_fragment(
        score,
        &[ScoreFragmentSelection {
            start: cmd.source_start.clone(),
            end: cmd.source_end.clone(),
        }],
    )?;
    apply_paste_score_fragment(
        &PasteScoreFragmentCmd {
            fragment,
            target: cmd.target.clone(),
            policy: ScoreFragmentPastePolicy::Replace,
        },
        score,
    )?;
    if !cmd.move_source {
        return Ok(());
    }

    let settings_time_signature = score.settings.time_signature.clone();
    let staff = score
        .parts
        .get_mut(cmd.source_start.part)
        .ok_or(Error::PartNotFound(cmd.source_start.part))?
        .staves
        .get_mut(cmd.source_start.staff)
        .ok_or(Error::StaffNotFound(cmd.source_start.staff))?;
    if source_to >= staff.measures.len() {
        return Err(Error::MeasureNotFound(source_to));
    }
    for measure in &mut staff.measures[source_from..=source_to] {
        let signature = measure
            .time_sig
            .as_ref()
            .unwrap_or(&settings_time_signature);
        let mut replacement = Vec::new();
        pad_voice_to_measure(&mut replacement, signature.total_beats());
        measure.voices[cmd.source_start.voice] = replacement;
        measure.source_voice_numbers[cmd.source_start.voice] = None;
    }
    prune_orphaned_spanners(score);
    Ok(())
}

fn apply_split_measure(cmd: &SplitMeasureCmd, score: &mut Score) -> Result<(), Error> {
    if !cmd.split_at_beats.is_finite() || cmd.split_at_beats <= 0.0 {
        return Err(Error::InvalidCommand("split point must be positive".into()));
    }
    let mut boundaries = Vec::new();
    for (part_index, part) in score.parts.iter().enumerate() {
        for (staff_index, staff) in part.staves.iter().enumerate() {
            let measure = staff
                .measures
                .get(cmd.measure_index)
                .ok_or(Error::MeasureNotFound(cmd.measure_index))?;
            let expected = measure
                .time_sig
                .as_ref()
                .unwrap_or(&score.settings.time_signature)
                .total_beats();
            if cmd.split_at_beats >= expected - 1e-9 {
                return Err(Error::InvalidCommand(
                    "split point must be inside measure".into(),
                ));
            }
            let mut indices = [0usize; 4];
            for (voice_index, voice) in measure.voices.iter().enumerate() {
                let mut beats = 0.0;
                let mut found = false;
                for (index, note) in voice.iter().enumerate() {
                    beats += note.beats();
                    if (beats - cmd.split_at_beats).abs() < 1e-9 {
                        indices[voice_index] = index + 1;
                        found = true;
                        break;
                    }
                    if beats > cmd.split_at_beats {
                        break;
                    }
                }
                if !voice.is_empty() && !found {
                    return Err(Error::InvalidCommand(
                        "split point must align with every voice note boundary".into(),
                    ));
                }
            }
            boundaries.push((part_index, staff_index, indices));
        }
    }
    for (part_index, staff_index, indices) in &boundaries {
        let measure =
            &mut score.parts[*part_index].staves[*staff_index].measures[cmd.measure_index];
        let mut right = measure.clone();
        for (voice_index, split_index) in indices.iter().enumerate() {
            right.voices[voice_index] = measure.voices[voice_index].split_off(*split_index);
        }
        measure.barline_right = Barline::Normal;
        right.barline_left = Barline::Normal;
        score.parts[*part_index].staves[*staff_index]
            .measures
            .insert(cmd.measure_index + 1, right);
        for (index, measure) in score.parts[*part_index].staves[*staff_index]
            .measures
            .iter_mut()
            .enumerate()
        {
            measure.number = index as u32 + 1;
        }
    }
    remap_spanners(score, |address| {
        if address.measure < cmd.measure_index {
            return Some(address.clone());
        }
        if address.measure > cmd.measure_index {
            let mut shifted = address.clone();
            shifted.measure += 1;
            return Some(shifted);
        }
        let split = boundaries
            .iter()
            .find(|(part, staff, _)| *part == address.part && *staff == address.staff)?
            .2[address.voice];
        if address.note >= split {
            Some(NoteAddr {
                measure: cmd.measure_index + 1,
                note: address.note - split,
                ..address.clone()
            })
        } else {
            Some(address.clone())
        }
    });
    Ok(())
}

fn apply_join_measures(cmd: &JoinMeasuresCmd, score: &mut Score) -> Result<(), Error> {
    let mut offsets = Vec::new();
    for (part_index, part) in score.parts.iter().enumerate() {
        for (staff_index, staff) in part.staves.iter().enumerate() {
            let left = staff
                .measures
                .get(cmd.measure_index)
                .ok_or(Error::MeasureNotFound(cmd.measure_index))?;
            let _right = staff
                .measures
                .get(cmd.measure_index + 1)
                .ok_or(Error::MeasureNotFound(cmd.measure_index + 1))?;
            let mut counts = [0usize; 4];
            for (voice, notes) in left.voices.iter().enumerate() {
                counts[voice] = notes.len();
            }
            offsets.push((part_index, staff_index, counts));
        }
    }
    for (part, staff, _) in &offsets {
        let measures = &mut score.parts[*part].staves[*staff].measures;
        let right = measures.remove(cmd.measure_index + 1);
        let left = &mut measures[cmd.measure_index];
        for voice in 0..4 {
            left.voices[voice].extend(right.voices[voice].clone());
        }
        left.barline_right = right.barline_right;
        for (index, measure) in measures.iter_mut().enumerate() {
            measure.number = index as u32 + 1;
        }
    }
    remap_spanners(score, |address| {
        if address.measure < cmd.measure_index + 1 {
            return Some(address.clone());
        }
        if address.measure > cmd.measure_index + 1 {
            let mut shifted = address.clone();
            shifted.measure -= 1;
            return Some(shifted);
        }
        let offset = offsets
            .iter()
            .find(|(part, staff, _)| *part == address.part && *staff == address.staff)?
            .2[address.voice];
        Some(NoteAddr {
            measure: cmd.measure_index,
            note: address.note + offset,
            ..address.clone()
        })
    });
    Ok(())
}

fn effective_staff_measure_beats(
    staff: &Staff,
    default_time_signature: &TimeSignature,
    measure_index: usize,
) -> Result<f64, Error> {
    let mut time_signature = default_time_signature.clone();
    for measure in staff.measures.iter().take(measure_index.saturating_add(1)) {
        if let Some(signature) = &measure.time_sig {
            time_signature = signature.clone();
        }
    }
    if staff.measures.get(measure_index).is_none() {
        return Err(Error::MeasureNotFound(measure_index));
    }
    Ok(time_signature.total_beats())
}

fn normalized_structural_measure_range(start: usize, end: usize) -> (usize, usize) {
    (start.min(end), start.max(end))
}

fn apply_implode_staves(cmd: &ImplodeStavesCmd, score: &mut Score) -> Result<(), Error> {
    if !(2..=4).contains(&cmd.source_staves.len()) {
        return Err(Error::InvalidCommand(
            "implode requires two to four source staves".into(),
        ));
    }
    let mut unique_staves = BTreeSet::new();
    for &staff_index in &cmd.source_staves {
        if !unique_staves.insert(staff_index) {
            return Err(Error::InvalidCommand(
                "implode source staves must be distinct".into(),
            ));
        }
    }
    if !unique_staves.contains(&cmd.target_staff) {
        return Err(Error::InvalidCommand(
            "implode target staff must be included in source staves".into(),
        ));
    }
    let (start, end) = normalized_structural_measure_range(cmd.start_measure, cmd.end_measure);
    let part = score
        .parts
        .get(cmd.part_index)
        .ok_or(Error::PartNotFound(cmd.part_index))?;
    for &staff_index in &cmd.source_staves {
        if part.staves.get(staff_index).is_none() {
            return Err(Error::StaffNotFound(staff_index));
        }
    }

    // Validate all ranges before mutation.  This is intentionally stricter than
    // layout validation: a source secondary voice or a cross-staff placement
    // would otherwise need a policy that risks silently changing semantics.
    for measure_index in start..=end {
        let mut expected_beats: Option<f64> = None;
        for &staff_index in &cmd.source_staves {
            let staff = &part.staves[staff_index];
            let measure = staff
                .measures
                .get(measure_index)
                .ok_or(Error::MeasureNotFound(measure_index))?;
            let beats = effective_staff_measure_beats(
                staff,
                &score.settings.time_signature,
                measure_index,
            )?;
            if let Some(expected) = expected_beats
                && (expected - beats).abs() > 1e-9
            {
                return Err(Error::InvalidCommand(
                    "implode source staves must have matching measure durations".into(),
                ));
            }
            expected_beats = Some(beats);
            if measure.voices[1..].iter().any(|voice| !voice.is_empty()) {
                return Err(Error::InvalidCommand(
                    "implode requires empty secondary source voices".into(),
                ));
            }
            if measure.voices[0]
                .iter()
                .any(|note| note.cross_staff.is_some())
            {
                return Err(Error::InvalidCommand(
                    "implode does not support cross-staff source notes".into(),
                ));
            }
            let total: f64 = measure.voices[0].iter().map(Note::beats).sum();
            if total > beats + 1e-9 {
                return Err(Error::InvalidCommand(
                    "implode source voice exceeds its measure duration".into(),
                ));
            }
        }
    }

    for measure_index in start..=end {
        let transferred: Vec<(Vec<Note>, Option<u32>)> = cmd
            .source_staves
            .iter()
            .map(|&staff_index| {
                let measure =
                    &score.parts[cmd.part_index].staves[staff_index].measures[measure_index];
                (measure.voices[0].clone(), measure.source_voice_numbers[0])
            })
            .collect();
        {
            let target =
                &mut score.parts[cmd.part_index].staves[cmd.target_staff].measures[measure_index];
            target.voices = [Vec::new(), Vec::new(), Vec::new(), Vec::new()];
            target.source_voice_numbers = [None; 4];
            for (voice_index, (notes, source_voice_number)) in transferred.into_iter().enumerate() {
                target.voices[voice_index] = notes;
                target.source_voice_numbers[voice_index] = source_voice_number;
            }
        }
        for &staff_index in &cmd.source_staves {
            if staff_index == cmd.target_staff {
                continue;
            }
            let beats = effective_staff_measure_beats(
                &score.parts[cmd.part_index].staves[staff_index],
                &score.settings.time_signature,
                measure_index,
            )?;
            let mut rest_voice = Vec::new();
            pad_voice_to_measure(&mut rest_voice, beats);
            let measure =
                &mut score.parts[cmd.part_index].staves[staff_index].measures[measure_index];
            measure.voices = [rest_voice, Vec::new(), Vec::new(), Vec::new()];
            measure.source_voice_numbers = [None; 4];
        }
    }
    remap_spanners(score, |address| {
        if address.part != cmd.part_index || address.measure < start || address.measure > end {
            return Some(address.clone());
        }
        let voice = cmd
            .source_staves
            .iter()
            .position(|&staff| staff == address.staff);
        if address.voice != 0 {
            return Some(address.clone());
        }
        voice
            .map(|voice| NoteAddr {
                staff: cmd.target_staff,
                voice,
                ..address.clone()
            })
            .or_else(|| Some(address.clone()))
    });
    Ok(())
}

fn apply_explode_voices(cmd: &ExplodeVoicesCmd, score: &mut Score) -> Result<(), Error> {
    if !(2..=4).contains(&cmd.target_staves.len()) {
        return Err(Error::InvalidCommand(
            "explode requires two to four target staves".into(),
        ));
    }
    if cmd.target_staves.first() != Some(&cmd.source_staff) {
        return Err(Error::InvalidCommand(
            "explode target staves must begin with the source staff".into(),
        ));
    }
    let mut unique_staves = BTreeSet::new();
    for &staff_index in &cmd.target_staves {
        if !unique_staves.insert(staff_index) {
            return Err(Error::InvalidCommand(
                "explode target staves must be distinct".into(),
            ));
        }
    }
    let (start, end) = normalized_structural_measure_range(cmd.start_measure, cmd.end_measure);
    let part = score
        .parts
        .get(cmd.part_index)
        .ok_or(Error::PartNotFound(cmd.part_index))?;
    let source = part
        .staves
        .get(cmd.source_staff)
        .ok_or(Error::StaffNotFound(cmd.source_staff))?;
    for &staff_index in &cmd.target_staves {
        if part.staves.get(staff_index).is_none() {
            return Err(Error::StaffNotFound(staff_index));
        }
    }

    for measure_index in start..=end {
        let expected =
            effective_staff_measure_beats(source, &score.settings.time_signature, measure_index)?;
        let source_measure = source
            .measures
            .get(measure_index)
            .ok_or(Error::MeasureNotFound(measure_index))?;
        for voice_index in cmd.target_staves.len()..4 {
            if !source_measure.voices[voice_index].is_empty() {
                return Err(Error::InvalidCommand(
                    "explode would discard a source voice without a target staff".into(),
                ));
            }
        }
        for voice_index in 0..cmd.target_staves.len() {
            if source_measure.voices[voice_index]
                .iter()
                .any(|note| note.cross_staff.is_some())
            {
                return Err(Error::InvalidCommand(
                    "explode does not support cross-staff source notes".into(),
                ));
            }
            let total: f64 = source_measure.voices[voice_index]
                .iter()
                .map(Note::beats)
                .sum();
            if total > expected + 1e-9 {
                return Err(Error::InvalidCommand(
                    "explode source voice exceeds its measure duration".into(),
                ));
            }
        }
        for &staff_index in cmd.target_staves.iter().skip(1) {
            let target = &part.staves[staff_index];
            let target_beats = effective_staff_measure_beats(
                target,
                &score.settings.time_signature,
                measure_index,
            )?;
            if (target_beats - expected).abs() > 1e-9 {
                return Err(Error::InvalidCommand(
                    "explode target staves must have matching measure durations".into(),
                ));
            }
            let measure = target
                .measures
                .get(measure_index)
                .ok_or(Error::MeasureNotFound(measure_index))?;
            if measure.voices[0].iter().any(|note| !note.is_rest)
                || measure.voices[1..].iter().any(|voice| !voice.is_empty())
                || measure.source_voice_numbers.iter().any(Option::is_some)
            {
                return Err(Error::InvalidCommand(
                    "explode destination staff must contain only an unnumbered rest voice".into(),
                ));
            }
        }
    }

    for measure_index in start..=end {
        let transferred: Vec<(Vec<Note>, Option<u32>)> = (0..cmd.target_staves.len())
            .map(|voice_index| {
                let measure =
                    &score.parts[cmd.part_index].staves[cmd.source_staff].measures[measure_index];
                (
                    measure.voices[voice_index].clone(),
                    measure.source_voice_numbers[voice_index],
                )
            })
            .collect();
        for (voice_index, &staff_index) in cmd.target_staves.iter().enumerate() {
            let (notes, source_voice_number) = &transferred[voice_index];
            let target =
                &mut score.parts[cmd.part_index].staves[staff_index].measures[measure_index];
            target.voices = [notes.clone(), Vec::new(), Vec::new(), Vec::new()];
            target.source_voice_numbers = [*source_voice_number, None, None, None];
        }
    }
    remap_spanners(score, |address| {
        if address.part != cmd.part_index
            || address.staff != cmd.source_staff
            || address.measure < start
            || address.measure > end
            || address.voice >= cmd.target_staves.len()
        {
            return Some(address.clone());
        }
        Some(NoteAddr {
            staff: cmd.target_staves[address.voice],
            voice: 0,
            ..address.clone()
        })
    });
    Ok(())
}

fn clear_derived_chord_note_notation(note: &mut Note) {
    note.articulations.clear();
    note.dynamic = None;
    note.stem_up = None;
    note.hairpin_start = None;
    note.hairpin_end = false;
    note.chord_symbol = None;
    note.ottava_start = None;
    note.ottava_end = false;
    note.lyric = None;
    note.pedal_start = false;
    note.pedal_end = false;
    note.slur_start = false;
    note.slur_end = false;
    note.arpeggiate = None;
    note.technique_text = None;
    note.glissando_start = false;
    note.glissando_end = false;
    note.cross_staff = None;
    note.fingering = None;
    note.fingerings.clear();
    note.string_number = None;
    note.trill_line_start = false;
    note.trill_line_end = false;
    note.guitar_technique = None;
    note.guitar_bend_alter_cents = None;
    note.guitar_bend_curve.clear();
}

fn exploded_chord_note(source: &Note, pitch_index: usize) -> Note {
    if pitch_index == 0 {
        let mut retained = source.clone();
        if !retained.is_rest {
            retained.pitches = vec![source.pitches[0].clone()];
            retained.tab_positions = source.tab_positions.first().cloned().into_iter().collect();
            retained.tab_position = retained.tab_positions.first().cloned();
        }
        return retained;
    }
    let mut derived = source.clone();
    derived.id = Uuid::new_v4().to_string();
    clear_derived_chord_note_notation(&mut derived);
    if derived.is_rest || pitch_index >= source.pitches.len() {
        derived.is_rest = true;
        derived.is_unpitched = false;
        derived.pitches.clear();
        derived.tab_position = None;
        derived.tab_positions.clear();
    } else {
        derived.pitches = vec![source.pitches[pitch_index].clone()];
        derived.tab_positions = source
            .tab_positions
            .get(pitch_index)
            .cloned()
            .into_iter()
            .collect();
        derived.tab_position = derived.tab_positions.first().cloned();
    }
    derived
}

fn apply_explode_chord_pitches(
    cmd: &ExplodeChordPitchesCmd,
    score: &mut Score,
) -> Result<(), Error> {
    if !(2..=4).contains(&cmd.target_staves.len()) {
        return Err(Error::InvalidCommand(
            "chord explode requires two to four target staves".into(),
        ));
    }
    if cmd.target_staves.first() != Some(&cmd.source_staff) {
        return Err(Error::InvalidCommand(
            "chord explode target staves must begin with the source staff".into(),
        ));
    }
    let mut unique_staves = BTreeSet::new();
    for &staff_index in &cmd.target_staves {
        if !unique_staves.insert(staff_index) {
            return Err(Error::InvalidCommand(
                "chord explode target staves must be distinct".into(),
            ));
        }
    }
    let (start, end) = normalized_structural_measure_range(cmd.start_measure, cmd.end_measure);
    let part = score
        .parts
        .get(cmd.part_index)
        .ok_or(Error::PartNotFound(cmd.part_index))?;
    let source = part
        .staves
        .get(cmd.source_staff)
        .ok_or(Error::StaffNotFound(cmd.source_staff))?;
    for &staff_index in &cmd.target_staves {
        if part.staves.get(staff_index).is_none() {
            return Err(Error::StaffNotFound(staff_index));
        }
    }
    for measure_index in start..=end {
        let expected =
            effective_staff_measure_beats(source, &score.settings.time_signature, measure_index)?;
        let source_measure = source
            .measures
            .get(measure_index)
            .ok_or(Error::MeasureNotFound(measure_index))?;
        for note in &source_measure.voices[0] {
            if note.tuplet.is_some() {
                return Err(Error::InvalidCommand(
                    "chord explode does not support tuplets".into(),
                ));
            }
            if note.cross_staff.is_some() {
                return Err(Error::InvalidCommand(
                    "chord explode does not support cross-staff source notes".into(),
                ));
            }
            if note.is_unpitched || note.is_grace || note.is_cue {
                return Err(Error::InvalidCommand(
                    "chord explode does not support unpitched, grace, or cue notes".into(),
                ));
            }
            if !note.is_rest
                && (note.pitches.is_empty() || note.pitches.len() > cmd.target_staves.len())
            {
                return Err(Error::InvalidCommand(
                    "chord explode pitch count must fit target staves".into(),
                ));
            }
        }
        let total: f64 = source_measure.voices[0].iter().map(Note::beats).sum();
        if total > expected + 1e-9 {
            return Err(Error::InvalidCommand(
                "chord explode source voice exceeds its measure duration".into(),
            ));
        }
        for &staff_index in cmd.target_staves.iter().skip(1) {
            let target = &part.staves[staff_index];
            let target_beats = effective_staff_measure_beats(
                target,
                &score.settings.time_signature,
                measure_index,
            )?;
            if (target_beats - expected).abs() > 1e-9 {
                return Err(Error::InvalidCommand(
                    "chord explode target staves must have matching measure durations".into(),
                ));
            }
            let measure = target
                .measures
                .get(measure_index)
                .ok_or(Error::MeasureNotFound(measure_index))?;
            if measure.voices[0].iter().any(|note| !note.is_rest)
                || measure.voices[1..].iter().any(|voice| !voice.is_empty())
                || measure.source_voice_numbers.iter().any(Option::is_some)
            {
                return Err(Error::InvalidCommand(
                    "chord explode destination staff must contain only an unnumbered rest voice"
                        .into(),
                ));
            }
        }
    }
    for measure_index in start..=end {
        let source_measure =
            &score.parts[cmd.part_index].staves[cmd.source_staff].measures[measure_index];
        let source_voice_number = source_measure.source_voice_numbers[0];
        let mut exploded = vec![Vec::new(); cmd.target_staves.len()];
        for note in &source_measure.voices[0] {
            for (pitch_index, voice) in exploded.iter_mut().enumerate() {
                voice.push(exploded_chord_note(note, pitch_index));
            }
        }
        for (index, &staff_index) in cmd.target_staves.iter().enumerate() {
            let target =
                &mut score.parts[cmd.part_index].staves[staff_index].measures[measure_index];
            target.voices = [exploded[index].clone(), Vec::new(), Vec::new(), Vec::new()];
            target.source_voice_numbers = [source_voice_number, None, None, None];
        }
    }
    Ok(())
}

fn scaled_duration(duration: &Duration, scale: DurationScale) -> Option<Duration> {
    match (duration, scale) {
        (Duration::Whole, DurationScale::Double) | (Duration::SixtyFourth, DurationScale::Half) => {
            None
        }
        (Duration::Whole, DurationScale::Half) | (Duration::Half, DurationScale::Double) => {
            Some(Duration::Half)
        }
        (Duration::Half, DurationScale::Half) | (Duration::Quarter, DurationScale::Double) => {
            Some(Duration::Quarter)
        }
        (Duration::Quarter, DurationScale::Half) | (Duration::Eighth, DurationScale::Double) => {
            Some(Duration::Eighth)
        }
        (Duration::Eighth, DurationScale::Half) | (Duration::Sixteenth, DurationScale::Double) => {
            Some(Duration::Sixteenth)
        }
        (Duration::Sixteenth, DurationScale::Half)
        | (Duration::ThirtySecond, DurationScale::Double) => Some(Duration::ThirtySecond),
        (Duration::ThirtySecond, DurationScale::Half)
        | (Duration::SixtyFourth, DurationScale::Double) => Some(Duration::SixtyFourth),
    }
}

fn uniform_tuplet_ratio(voice: &[Note]) -> Result<Option<TupletInfo>, Error> {
    let mut ratio: Option<TupletInfo> = None;
    for note in voice {
        let Some(tuplet) = &note.tuplet else {
            continue;
        };
        if tuplet.actual_notes == 0 || tuplet.normal_notes == 0 {
            return Err(Error::InvalidCommand(
                "duration scaling requires a non-zero tuplet ratio".into(),
            ));
        }
        if let Some(existing) = &ratio {
            if existing != tuplet {
                return Err(Error::InvalidCommand(
                    "duration scaling requires one shared tuplet ratio per voice".into(),
                ));
            }
        } else {
            ratio = Some(tuplet.clone());
        }
    }
    Ok(ratio)
}

fn pad_voice_to_measure_with_tuplet_ratio(
    voice: &mut Vec<Note>,
    max_beats: f64,
    ratio: &TupletInfo,
) -> Result<(), Error> {
    let mut used: f64 = voice.iter().map(Note::beats).sum();
    let ratio_scale = f64::from(ratio.normal_notes) / f64::from(ratio.actual_notes);
    while max_beats - used > 1e-9 {
        let remaining = max_beats - used;
        let duration = [
            Duration::Whole,
            Duration::Half,
            Duration::Quarter,
            Duration::Eighth,
            Duration::Sixteenth,
            Duration::ThirtySecond,
            Duration::SixtyFourth,
        ]
        .into_iter()
        .find(|duration| duration.beats(0) * ratio_scale <= remaining + 1e-9)
        .ok_or_else(|| {
            Error::InvalidCommand(
                "duration scaling cannot represent the remaining tuplet duration".into(),
            )
        })?;
        let mut rest = Note::rest(duration);
        rest.tuplet = Some(ratio.clone());
        used += rest.beats();
        voice.push(rest);
    }
    Ok(())
}

fn apply_scale_voice_range(cmd: &ScaleVoiceRangeCmd, score: &mut Score) -> Result<(), Error> {
    if cmd.voice >= 4 {
        return Err(Error::VoiceOutOfRange(cmd.voice));
    }
    let (start, end) = normalized_structural_measure_range(cmd.start_measure, cmd.end_measure);
    let staff = score
        .parts
        .get(cmd.part_index)
        .ok_or(Error::PartNotFound(cmd.part_index))?
        .staves
        .get(cmd.staff_index)
        .ok_or(Error::StaffNotFound(cmd.staff_index))?;
    for measure_index in start..=end {
        let measure = staff
            .measures
            .get(measure_index)
            .ok_or(Error::MeasureNotFound(measure_index))?;
        let expected =
            effective_staff_measure_beats(staff, &score.settings.time_signature, measure_index)?;
        let voice = &measure.voices[cmd.voice];
        match cmd.tuplet_policy {
            TupletScalePolicy::PreserveRatio => {
                uniform_tuplet_ratio(voice)?;
            }
        }
        let mut scaled_beats = 0.0;
        for note in voice {
            if scaled_duration(&note.duration, cmd.scale).is_none() {
                return Err(Error::InvalidCommand(
                    "duration scaling exceeds the portable duration range".into(),
                ));
            }
            if !note.is_grace && !note.is_cue {
                scaled_beats += match cmd.scale {
                    DurationScale::Half => note.beats() / 2.0,
                    DurationScale::Double => note.beats() * 2.0,
                };
            }
        }
        if scaled_beats > expected + 1e-9 {
            return Err(Error::InvalidCommand(format!(
                "duration scaling overflows measure {measure_index}: {scaled_beats} beats exceeds {expected}"
            )));
        }
    }

    for measure_index in start..=end {
        let expected = effective_staff_measure_beats(
            &score.parts[cmd.part_index].staves[cmd.staff_index],
            &score.settings.time_signature,
            measure_index,
        )?;
        let voice = &mut score.parts[cmd.part_index].staves[cmd.staff_index].measures
            [measure_index]
            .voices[cmd.voice];
        if voice.is_empty() {
            continue;
        }
        let tuplet_ratio = match cmd.tuplet_policy {
            TupletScalePolicy::PreserveRatio => uniform_tuplet_ratio(voice)?,
        };
        for note in voice.iter_mut() {
            note.duration = scaled_duration(&note.duration, cmd.scale).ok_or_else(|| {
                Error::InvalidCommand("duration scaling exceeds the portable duration range".into())
            })?;
        }
        if let Some(ratio) = tuplet_ratio {
            pad_voice_to_measure_with_tuplet_ratio(voice, expected, &ratio)?;
        } else {
            pad_voice_to_measure(voice, expected);
        }
    }
    Ok(())
}

fn apply_paste_score_fragment(cmd: &PasteScoreFragmentCmd, score: &mut Score) -> Result<(), Error> {
    if !(MIN_SUPPORTED_SCORE_FRAGMENT_CONTRACT_VERSION..=SCORE_FRAGMENT_CONTRACT_VERSION)
        .contains(&cmd.fragment.contract_version)
    {
        return Err(Error::InvalidCommand(format!(
            "unsupported score fragment contract version {}",
            cmd.fragment.contract_version
        )));
    }
    if cmd.fragment.voices.is_empty() {
        return Err(Error::InvalidCommand(
            "cannot paste an empty score fragment".into(),
        ));
    }

    // Resolve every target before mutating the score. CommandStack then also
    // validates the complete candidate before adding history, preserving
    // score/history atomicity for invalid destinations.
    let mut lanes = BTreeSet::new();
    let mut replaced = BTreeSet::new();
    for lane in &cmd.fragment.voices {
        let part_index = cmd
            .target
            .part
            .checked_add(lane.relative_part)
            .ok_or_else(|| Error::InvalidCommand("fragment part target overflows".into()))?;
        let staff_index = cmd
            .target
            .staff
            .checked_add(lane.relative_staff)
            .ok_or_else(|| Error::InvalidCommand("fragment staff target overflows".into()))?;
        let voice_index = cmd
            .target
            .voice
            .checked_add(lane.relative_voice)
            .ok_or_else(|| Error::InvalidCommand("fragment voice target overflows".into()))?;
        if voice_index >= 4 {
            return Err(Error::VoiceOutOfRange(voice_index));
        }
        let staff = score
            .parts
            .get(part_index)
            .ok_or(Error::PartNotFound(part_index))?
            .staves
            .get(staff_index)
            .ok_or(Error::StaffNotFound(staff_index))?;
        if !lanes.insert((part_index, staff_index, voice_index)) {
            return Err(Error::InvalidCommand(
                "fragment contains duplicate destination voice lanes".into(),
            ));
        }
        for measure in &lane.measures {
            let measure_index = cmd
                .target
                .measure
                .checked_add(measure.relative_measure)
                .ok_or_else(|| Error::InvalidCommand("fragment measure target overflows".into()))?;
            if measure_index >= staff.measures.len() {
                return Err(Error::MeasureNotFound(measure_index));
            }
            if !measure.cross_staff_targets.is_empty()
                && measure.cross_staff_targets.len() != measure.notes.len()
            {
                return Err(Error::InvalidCommand(
                    "fragment cross-staff targets do not match note count".into(),
                ));
            }
            if cmd.policy == ScoreFragmentPastePolicy::Merge
                && !measure.notes.is_empty()
                && staff.measures[measure_index].voices[voice_index]
                    .iter()
                    .any(|note| !note.is_rest)
            {
                return Err(Error::InvalidCommand(
                    "fragment merge would overwrite a sounding destination lane".into(),
                ));
            }
            if cmd.policy == ScoreFragmentPastePolicy::Merge
                && measure.attributes.present
                && measure.attributes
                    != super::fragment::ScoreFragmentMeasureAttributes::from_measure(
                        &staff.measures[measure_index],
                    )
            {
                return Err(Error::InvalidCommand(
                    "fragment merge would overwrite destination measure attributes".into(),
                ));
            }
            for cross_staff in measure.cross_staff_targets.iter().flatten() {
                let target_staff = staff_index as i64 + cross_staff.staff_offset;
                if target_staff < 0
                    || score.parts[part_index]
                        .staves
                        .get(target_staff as usize)
                        .is_none()
                {
                    return Err(Error::InvalidCommand(
                        "fragment cross-staff target is outside destination part".into(),
                    ));
                }
                if cross_staff.target_voice.is_some_and(|voice| voice >= 4) {
                    return Err(Error::InvalidCommand(
                        "fragment cross-staff target voice is outside editable range".into(),
                    ));
                }
            }
            replaced.insert((part_index, staff_index, measure_index, voice_index));
        }
    }

    // Replacing a lane invalidates its former typed-span endpoints. Remove
    // those spans explicitly rather than allowing legacy endpoint flags to
    // leave dangling canonical addresses.
    score.spanners.retain(|spanner| {
        !replaced.contains(&(
            spanner.start.part,
            spanner.start.staff,
            spanner.start.measure,
            spanner.start.voice,
        )) && !replaced.contains(&(
            spanner.end.part,
            spanner.end.staff,
            spanner.end.measure,
            spanner.end.voice,
        ))
    });
    for lane in &cmd.fragment.voices {
        let part_index = cmd.target.part + lane.relative_part;
        let staff_index = cmd.target.staff + lane.relative_staff;
        let voice_index = cmd.target.voice + lane.relative_voice;
        let staff = &mut score.parts[part_index].staves[staff_index];
        for measure in &lane.measures {
            let measure_index = cmd.target.measure + measure.relative_measure;
            let target = &mut staff.measures[measure_index];
            if cmd.policy == ScoreFragmentPastePolicy::Merge && measure.notes.is_empty() {
                continue;
            }
            let mut notes = measure.notes.clone();
            for note in &mut notes {
                note.id = Uuid::new_v4().to_string();
            }
            if !measure.cross_staff_targets.is_empty() {
                for (note, cross_staff) in notes.iter_mut().zip(&measure.cross_staff_targets) {
                    note.cross_staff = cross_staff.as_ref().map(|cross_staff| CrossStaff {
                        target_staff: (staff_index as i64 + cross_staff.staff_offset) as usize,
                        target_voice: cross_staff.target_voice,
                    });
                }
            }
            target.voices[voice_index] = notes;
            target.source_voice_numbers[voice_index] = measure.source_voice_number;
            if cmd.policy == ScoreFragmentPastePolicy::Replace {
                measure.attributes.apply_to_measure(target);
            }
        }
    }

    let mut used_spanner_ids = score
        .spanners
        .iter()
        .map(|spanner| spanner.id.clone())
        .collect::<BTreeSet<_>>();
    for source in &cmd.fragment.spanners {
        let mut copied = source.clone();
        copied.start = fragment_destination_address(&cmd.target, &source.start)?;
        copied.end = fragment_destination_address(&cmd.target, &source.end)?;
        if note_at(score, &copied.start).is_none() {
            return Err(Error::NoteNotFound(copied.start.note));
        }
        if note_at(score, &copied.end).is_none() {
            return Err(Error::NoteNotFound(copied.end.note));
        }
        copied.id = unique_fragment_spanner_id(&used_spanner_ids, &source.id);
        used_spanner_ids.insert(copied.id.clone());
        score.spanners.push(copied);
    }
    Ok(())
}

fn fragment_destination_address(target: &NoteAddr, relative: &NoteAddr) -> Result<NoteAddr, Error> {
    Ok(NoteAddr {
        part: target
            .part
            .checked_add(relative.part)
            .ok_or_else(|| Error::InvalidCommand("fragment part target overflows".into()))?,
        staff: target
            .staff
            .checked_add(relative.staff)
            .ok_or_else(|| Error::InvalidCommand("fragment staff target overflows".into()))?,
        measure: target
            .measure
            .checked_add(relative.measure)
            .ok_or_else(|| Error::InvalidCommand("fragment measure target overflows".into()))?,
        voice: target
            .voice
            .checked_add(relative.voice)
            .ok_or_else(|| Error::InvalidCommand("fragment voice target overflows".into()))?,
        note: relative.note,
    })
}

fn unique_fragment_spanner_id(used: &BTreeSet<String>, source_id: &str) -> String {
    let base = format!("{source_id}-copy");
    if !used.contains(&base) {
        return base;
    }
    let mut suffix = 2usize;
    loop {
        let candidate = format!("{base}-{suffix}");
        if !used.contains(&candidate) {
            return candidate;
        }
        suffix += 1;
    }
}

fn apply_toggle_slur(cmd: &ToggleSlurCmd, score: &mut Score) -> Result<(), Error> {
    let new_start = !{
        score
            .parts
            .get(cmd.start.part)
            .ok_or(Error::PartNotFound(cmd.start.part))?
            .staves
            .get(cmd.start.staff)
            .ok_or(Error::StaffNotFound(cmd.start.staff))?
            .measures
            .get(cmd.start.measure)
            .ok_or(Error::MeasureNotFound(cmd.start.measure))?
            .voices
            .get(cmd.start.voice)
            .ok_or(Error::VoiceOutOfRange(cmd.start.voice))?
            .get(cmd.start.note)
            .ok_or(Error::NoteNotFound(cmd.start.note))?
            .slur_start
    };
    let new_end = !{
        score
            .parts
            .get(cmd.end.part)
            .ok_or(Error::PartNotFound(cmd.end.part))?
            .staves
            .get(cmd.end.staff)
            .ok_or(Error::StaffNotFound(cmd.end.staff))?
            .measures
            .get(cmd.end.measure)
            .ok_or(Error::MeasureNotFound(cmd.end.measure))?
            .voices
            .get(cmd.end.voice)
            .ok_or(Error::VoiceOutOfRange(cmd.end.voice))?
            .get(cmd.end.note)
            .ok_or(Error::NoteNotFound(cmd.end.note))?
            .slur_end
    };
    score.parts[cmd.start.part].staves[cmd.start.staff].measures[cmd.start.measure].voices
        [cmd.start.voice][cmd.start.note]
        .slur_start = new_start;
    score.parts[cmd.end.part].staves[cmd.end.staff].measures[cmd.end.measure].voices
        [cmd.end.voice][cmd.end.note]
        .slur_end = new_end;
    Ok(())
}

fn apply_toggle_trill_line(cmd: &ToggleTrillLineCmd, score: &mut Score) -> Result<(), Error> {
    let new_start = !{
        score
            .parts
            .get(cmd.start.part)
            .ok_or(Error::PartNotFound(cmd.start.part))?
            .staves
            .get(cmd.start.staff)
            .ok_or(Error::StaffNotFound(cmd.start.staff))?
            .measures
            .get(cmd.start.measure)
            .ok_or(Error::MeasureNotFound(cmd.start.measure))?
            .voices
            .get(cmd.start.voice)
            .ok_or(Error::VoiceOutOfRange(cmd.start.voice))?
            .get(cmd.start.note)
            .ok_or(Error::NoteNotFound(cmd.start.note))?
            .trill_line_start
    };
    let new_end = !{
        score
            .parts
            .get(cmd.end.part)
            .ok_or(Error::PartNotFound(cmd.end.part))?
            .staves
            .get(cmd.end.staff)
            .ok_or(Error::StaffNotFound(cmd.end.staff))?
            .measures
            .get(cmd.end.measure)
            .ok_or(Error::MeasureNotFound(cmd.end.measure))?
            .voices
            .get(cmd.end.voice)
            .ok_or(Error::VoiceOutOfRange(cmd.end.voice))?
            .get(cmd.end.note)
            .ok_or(Error::NoteNotFound(cmd.end.note))?
            .trill_line_end
    };
    score.parts[cmd.start.part].staves[cmd.start.staff].measures[cmd.start.measure].voices
        [cmd.start.voice][cmd.start.note]
        .trill_line_start = new_start;
    score.parts[cmd.end.part].staves[cmd.end.staff].measures[cmd.end.measure].voices
        [cmd.end.voice][cmd.end.note]
        .trill_line_end = new_end;
    Ok(())
}

fn apply_add_staff(cmd: &AddStaffCmd, score: &mut Score) -> Result<(), Error> {
    let ts = score.settings.time_signature.clone();
    let measure_count = score
        .parts
        .get(cmd.part_index)
        .ok_or(Error::PartNotFound(cmd.part_index))?
        .staves
        .first()
        .map_or(0, |s| s.measures.len());
    let mut staff = Staff::new(cmd.clef.clone());
    for i in 0..measure_count {
        let mut m = Measure::empty(ts.numerator, ts.denominator);
        m.number = i as u32 + 1;
        staff.measures.push(m);
    }
    score.parts[cmd.part_index].staves.push(staff);
    Ok(())
}

fn apply_delete_staff(cmd: &DeleteStaffCmd, score: &mut Score) -> Result<(), Error> {
    let part = score
        .parts
        .get_mut(cmd.part_index)
        .ok_or(Error::PartNotFound(cmd.part_index))?;
    if part.staves.len() <= 1 {
        return Err(Error::CannotDeleteLastStaff);
    }
    if cmd.staff_index >= part.staves.len() {
        return Err(Error::StaffNotFound(cmd.staff_index));
    }
    part.staves.remove(cmd.staff_index);
    remap_spanners(score, |address| {
        if address.part != cmd.part_index {
            Some(address.clone())
        } else if address.staff == cmd.staff_index {
            None
        } else if address.staff > cmd.staff_index {
            let mut shifted = address.clone();
            shifted.staff -= 1;
            Some(shifted)
        } else {
            Some(address.clone())
        }
    });
    Ok(())
}

/// Returns true when an address refers to the given canonical voice.
fn same_voice(address: &NoteAddr, part: usize, staff: usize, measure: usize, voice: usize) -> bool {
    address.part == part
        && address.staff == staff
        && address.measure == measure
        && address.voice == voice
}

fn note_at<'a>(score: &'a Score, address: &NoteAddr) -> Option<&'a Note> {
    score
        .parts
        .get(address.part)
        .and_then(|part| part.staves.get(address.staff))
        .and_then(|staff| staff.measures.get(address.measure))
        .and_then(|measure| measure.voices.get(address.voice))
        .and_then(|voice| voice.get(address.note))
}

fn note_at_mut<'a>(score: &'a mut Score, address: &NoteAddr) -> Option<&'a mut Note> {
    score
        .parts
        .get_mut(address.part)
        .and_then(|part| part.staves.get_mut(address.staff))
        .and_then(|staff| staff.measures.get_mut(address.measure))
        .and_then(|measure| measure.voices.get_mut(address.voice))
        .and_then(|voice| voice.get_mut(address.note))
}

/// Typed spanners are authoritative once edited through the command engine.  Parsers retain
/// legacy endpoint flags for backwards-compatible JSON, but a typed remove/update must not leave
/// those flags able to resurrect stale MusicXML notation during serialization.
fn clear_legacy_spanner_endpoints(score: &mut Score, spanner: &NotationSpanner) {
    let clear_start = |note: &mut Note| match spanner.kind {
        NotationSpannerKind::Slur => note.slur_start = false,
        NotationSpannerKind::Glissando => note.glissando_start = false,
        NotationSpannerKind::TrillLine => note.trill_line_start = false,
        NotationSpannerKind::Pedal => note.pedal_start = false,
        NotationSpannerKind::Ottava => note.ottava_start = None,
    };
    let clear_end = |note: &mut Note| match spanner.kind {
        NotationSpannerKind::Slur => note.slur_end = false,
        NotationSpannerKind::Glissando => note.glissando_end = false,
        NotationSpannerKind::TrillLine => note.trill_line_end = false,
        NotationSpannerKind::Pedal => note.pedal_end = false,
        NotationSpannerKind::Ottava => note.ottava_end = false,
    };
    if let Some(note) = note_at_mut(score, &spanner.start) {
        clear_start(note);
    }
    if let Some(note) = note_at_mut(score, &spanner.end) {
        clear_end(note);
    }
}

/// Remap typed notation endpoints after a structural edit.
///
/// An endpoint that cannot be mapped, or no longer names an existing note, removes its complete
/// span. This deliberately favors an observable removed span over silently retargeting it.
fn remap_spanners(score: &mut Score, mut remap: impl FnMut(&NoteAddr) -> Option<NoteAddr>) {
    let spanners = std::mem::take(&mut score.spanners);
    score.spanners = spanners
        .into_iter()
        .filter_map(|mut spanner| {
            spanner.start = remap(&spanner.start)?;
            spanner.end = remap(&spanner.end)?;
            (note_at(score, &spanner.start).is_some() && note_at(score, &spanner.end).is_some())
                .then_some(spanner)
        })
        .collect();
}

fn prune_orphaned_spanners(score: &mut Score) {
    remap_spanners(score, |address| Some(address.clone()));
}

/// Capture the note identities behind endpoints that will be replaced wholesale.
fn capture_replaced_endpoint_ids(
    score: &Score,
    mut replaced: impl FnMut(&NoteAddr) -> bool,
) -> Vec<(NoteAddr, String)> {
    score
        .spanners
        .iter()
        .flat_map(|spanner| [&spanner.start, &spanner.end])
        .filter(|address| replaced(address))
        .filter_map(|address| {
            note_at(score, address).map(|note| (address.clone(), note.id.clone()))
        })
        .collect()
}

/// Re-resolve endpoints in a replaced voice/range by their original note IDs.
/// Duplicate IDs are treated as ambiguous and remove the affected span instead of guessing.
fn remap_replaced_spanner_endpoints(score: &mut Score, endpoint_ids: &[(NoteAddr, String)]) {
    let mapped: Vec<(NoteAddr, Option<NoteAddr>)> = endpoint_ids
        .iter()
        .map(|(address, note_id)| {
            let matches: Vec<usize> = score
                .parts
                .get(address.part)
                .and_then(|part| part.staves.get(address.staff))
                .and_then(|staff| staff.measures.get(address.measure))
                .and_then(|measure| measure.voices.get(address.voice))
                .map(|voice| {
                    voice
                        .iter()
                        .enumerate()
                        .filter_map(|(index, note)| (note.id == *note_id).then_some(index))
                        .collect()
                })
                .unwrap_or_default();
            let replacement = (matches.len() == 1).then(|| NoteAddr {
                note: matches[0],
                ..address.clone()
            });
            (address.clone(), replacement)
        })
        .collect();
    remap_spanners(score, |address| {
        mapped
            .iter()
            .find(|(old, _)| old == address)
            .map(|(_, replacement)| replacement.clone())
            .unwrap_or_else(|| Some(address.clone()))
    });
}

fn trim_voice_to_measure(voice: &mut Vec<Note>, max_beats: f64) {
    let mut total = 0.0f64;
    let mut cutoff = voice.len();
    for (i, n) in voice.iter().enumerate() {
        total += n.beats();
        if total > max_beats + 1e-9 {
            cutoff = i;
            break;
        }
    }
    voice.truncate(cutoff);
    pad_voice_to_measure(voice, max_beats);
}

fn pad_voice_to_measure(voice: &mut Vec<Note>, max_beats: f64) {
    let mut used: f64 = voice.iter().map(|n| n.beats()).sum();
    while max_beats - used > 1e-9 {
        let remaining = max_beats - used;
        let rest = Note::rest(Duration::whole_filling_beats(remaining));
        used += rest.beats();
        voice.push(rest);
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::ScoreEngine;
    use crate::model::pitch::Step;

    fn default_engine_score() -> Score {
        let mut s = Score::default();
        for part in &mut s.parts {
            for staff in &mut part.staves {
                for (i, m) in staff.measures.iter_mut().enumerate() {
                    m.number = i as u32 + 1;
                }
            }
        }
        s
    }

    #[test]
    fn legacy_scale_voice_range_command_defaults_to_preserve_tuplet_ratio() {
        let command: Command = serde_json::from_str(
            r#"{"type":"scale_voice_range","part_index":0,"staff_index":0,"voice":0,"start_measure":0,"end_measure":0,"scale":"half"}"#,
        )
        .expect("legacy scale command deserializes");
        let Command::ScaleVoiceRange(command) = command else {
            panic!("expected scale command");
        };
        assert_eq!(command.tuplet_policy, TupletScalePolicy::PreserveRatio);
    }

    fn score_with_typed_spanner() -> Score {
        use crate::model::score::{NotationSpanner, NotationSpannerKind};

        let mut score = Score::new("Spanner edits", 120, 4, 4, 0, 1);
        score.parts[0].staves[0].measures[0].voices[0] = (0..4)
            .map(|offset| Note::new(Pitch::new(Step::C, 4 + offset), Duration::Quarter))
            .collect();
        score.spanners.push(NotationSpanner {
            id: "typed-glissando".to_string(),
            kind: NotationSpannerKind::Glissando,
            start: NoteAddr {
                part: 0,
                staff: 0,
                measure: 0,
                voice: 0,
                note: 1,
            },
            end: NoteAddr {
                part: 0,
                staff: 0,
                measure: 0,
                voice: 0,
                note: 2,
            },
            number: Some(1),
            line_type: None,
            text: None,
            placement: None,
            ottava_size: None,
            ottava_type: None,
        });
        score
    }

    fn insert_note_at(position: usize) -> Command {
        Command::AddNote(AddNoteCmd {
            part_index: 0,
            staff_index: 0,
            measure_index: 0,
            voice: 0,
            position,
            pitch: Some(Pitch::new(Step::D, 5)),
            duration: Duration::Quarter,
            dot_count: 0,
            is_rest: false,
            tuplet: None,
        })
    }

    #[test]
    fn add_note_inserts_into_voice() {
        let mut score = default_engine_score();
        let cmd = Command::AddNote(AddNoteCmd {
            part_index: 0,
            staff_index: 0,
            measure_index: 0,
            voice: 0,
            position: 0,
            pitch: Some(Pitch::new(Step::C, 4)),
            duration: Duration::Quarter,
            dot_count: 0,
            is_rest: false,
            tuplet: None,
        });
        apply_command(&cmd, &mut score).unwrap();
        let first = &score.parts[0].staves[0].measures[0].voices[0][0];
        assert!(!first.is_rest);
        assert_eq!(first.pitches[0].step, Step::C);
    }

    #[test]
    fn typed_spanners_follow_note_insertions_at_every_relative_position() {
        for (position, expected_start, expected_end) in [
            (0, 2, 3), // before the span
            (1, 2, 3), // at its start endpoint
            (2, 1, 3), // inside the span
            (4, 1, 2), // at the end of the voice
            (3, 1, 2), // after the span
        ] {
            let mut score = score_with_typed_spanner();
            apply_command(&insert_note_at(position), &mut score).unwrap();
            let span = score.spanners.first().expect("span remains attached");
            assert_eq!(span.start.note, expected_start, "position {position}");
            assert_eq!(span.end.note, expected_end, "position {position}");
        }
    }

    #[test]
    fn deleting_a_typed_spanner_endpoint_removes_the_whole_span_and_undo_redo_is_atomic() {
        let mut score = score_with_typed_spanner();
        let endpoint_id = score.parts[0].staves[0].measures[0].voices[0][1].id.clone();
        let mut stack = CommandStack::new(8);

        stack
            .execute(
                Command::DeleteNote(DeleteNoteCmd {
                    note_id: endpoint_id,
                    part_index: 0,
                    staff_index: 0,
                    measure_index: 0,
                    voice: 0,
                }),
                &mut score,
            )
            .unwrap();
        assert!(score.spanners.is_empty());

        stack.undo(&mut score).unwrap();
        assert_eq!(score.spanners[0].start.note, 1);
        assert_eq!(score.spanners[0].end.note, 2);

        stack.redo(&mut score).unwrap();
        assert!(score.spanners.is_empty());
    }

    #[test]
    fn batched_structural_edits_remap_typed_spanners_and_restore_them_on_undo() {
        let mut score = score_with_typed_spanner();
        let mut stack = CommandStack::new(8);
        stack
            .batch_execute(
                vec![
                    insert_note_at(0),
                    Command::SetTempo(SetTempoCmd { bpm: 144 }),
                ],
                &mut score,
            )
            .unwrap();
        assert_eq!(score.spanners[0].start.note, 2);
        assert_eq!(score.spanners[0].end.note, 3);

        stack.undo(&mut score).unwrap();
        assert_eq!(score.spanners[0].start.note, 1);
        assert_eq!(score.spanners[0].end.note, 2);

        stack.redo(&mut score).unwrap();
        assert_eq!(score.spanners[0].start.note, 2);
        assert_eq!(score.spanners[0].end.note, 3);
    }

    #[test]
    fn typed_spanners_remap_only_the_edited_endpoint_across_staff_and_voice() {
        use crate::model::score::{NotationSpanner, NotationSpannerKind, ScoreTemplate};

        let mut score = Score::template(ScoreTemplate::Piano);
        for staff in &mut score.parts[0].staves {
            let notes: Vec<Note> = (0..4)
                .map(|offset| Note::new(Pitch::new(Step::C, 4 + offset), Duration::Quarter))
                .collect();
            staff.measures[0].voices[0] = notes.clone();
            staff.measures[0].voices[1] = notes;
        }
        score.spanners.push(NotationSpanner {
            id: "cross-staff".to_string(),
            kind: NotationSpannerKind::Slur,
            start: NoteAddr {
                part: 0,
                staff: 0,
                measure: 0,
                voice: 0,
                note: 1,
            },
            end: NoteAddr {
                part: 0,
                staff: 1,
                measure: 0,
                voice: 1,
                note: 2,
            },
            number: Some(2),
            line_type: None,
            text: None,
            placement: None,
            ottava_size: None,
            ottava_type: None,
        });

        let mut stack = CommandStack::new(8);
        stack.execute(insert_note_at(0), &mut score).unwrap();
        let span = &score.spanners[0];
        assert_eq!(span.start.note, 2);
        assert_eq!(span.end.staff, 1);
        assert_eq!(span.end.voice, 1);
        assert_eq!(span.end.note, 2);
    }

    #[test]
    fn set_fingerings_keeps_first_legacy_value_in_sync() {
        let mut score = default_engine_score();
        apply_command(
            &Command::AddNote(AddNoteCmd {
                part_index: 0,
                staff_index: 0,
                measure_index: 0,
                voice: 0,
                position: 0,
                pitch: Some(Pitch::new(Step::C, 4)),
                duration: Duration::Quarter,
                dot_count: 0,
                is_rest: false,
                tuplet: None,
            }),
            &mut score,
        )
        .unwrap();
        apply_command(
            &Command::SetFingerings(SetFingeringsCmd {
                part_index: 0,
                staff_index: 0,
                measure_index: 0,
                voice: 0,
                note_index: 0,
                fingerings: vec![1, 3, 4],
            }),
            &mut score,
        )
        .unwrap();
        let note = &score.parts[0].staves[0].measures[0].voices[0][0];
        assert_eq!(note.fingerings, vec![1, 3, 4]);
        assert_eq!(note.fingering, Some(1));
    }

    #[test]
    fn set_figured_bass_replaces_measure_figures() {
        let mut score = default_engine_score();
        let figures = vec![FiguredBassFigure {
            number: "6".to_string(),
            alter: Some("-1".to_string()),
            prefix: Some("+".to_string()),
            suffix: None,
            extender: false,
        }];
        apply_command(
            &Command::SetFiguredBass(SetFiguredBassCmd {
                measure_index: 0,
                figures: figures.clone(),
            }),
            &mut score,
        )
        .unwrap();
        assert_eq!(score.parts[0].staves[0].measures[0].figured_bass, figures);
    }

    #[test]
    fn set_measure_text_supports_append_replace_remove_and_undo_redo() {
        let mut engine = crate::ScoreEngine::new();
        let address = SetMeasureTextCmd {
            part_index: 0,
            staff_index: 0,
            measure_index: 0,
            text_index: 0,
            text: Some(StyledText {
                style: crate::TextStyle::Technique,
                text: "dolce".to_string(),
                placement: None,
                offset_x: None,
                offset_y: None,
                relative_x: None,
                relative_y: None,
            }),
        };
        engine.apply(Command::SetMeasureText(address)).unwrap();
        assert_eq!(
            engine.score.parts[0].staves[0].measures[0].texts[0],
            StyledText {
                style: crate::TextStyle::Technique,
                text: "dolce".to_string(),
                placement: None,
                offset_x: None,
                offset_y: None,
                relative_x: None,
                relative_y: None,
            }
        );

        engine
            .apply(Command::SetMeasureText(SetMeasureTextCmd {
                text_index: 0,
                text: Some(StyledText {
                    style: crate::TextStyle::RehearsalMark,
                    text: "A".to_string(),
                    placement: None,
                    offset_x: None,
                    offset_y: None,
                    relative_x: None,
                    relative_y: None,
                }),
                ..SetMeasureTextCmd {
                    part_index: 0,
                    staff_index: 0,
                    measure_index: 0,
                    text_index: 0,
                    text: None,
                }
            }))
            .unwrap();
        assert_eq!(
            engine.score.parts[0].staves[0].measures[0].texts[0].style,
            crate::TextStyle::RehearsalMark
        );

        engine
            .apply(Command::SetMeasureText(SetMeasureTextCmd {
                text_index: 0,
                text: None,
                ..SetMeasureTextCmd {
                    part_index: 0,
                    staff_index: 0,
                    measure_index: 0,
                    text_index: 0,
                    text: None,
                }
            }))
            .unwrap();
        assert!(engine.score.parts[0].staves[0].measures[0].texts.is_empty());
        engine.undo().unwrap();
        assert_eq!(engine.score.parts[0].staves[0].measures[0].texts.len(), 1);
        engine.redo().unwrap();
        assert!(engine.score.parts[0].staves[0].measures[0].texts.is_empty());
    }

    #[test]
    fn set_measure_text_rejects_invalid_index_atomically_and_round_trips_json() {
        let mut score = default_engine_score();
        let before = score.clone();
        let command = Command::SetMeasureText(SetMeasureTextCmd {
            part_index: 0,
            staff_index: 0,
            measure_index: 0,
            text_index: 2,
            text: Some(StyledText {
                style: crate::TextStyle::Expression,
                text: "espressivo".to_string(),
                placement: None,
                offset_x: None,
                offset_y: None,
                relative_x: None,
                relative_y: None,
            }),
        });
        let json = serde_json::to_string(&command).unwrap();
        let restored: Command = serde_json::from_str(&json).unwrap();
        assert_eq!(
            serde_json::to_value(&restored).unwrap(),
            serde_json::to_value(&command).unwrap()
        );
        assert!(apply_command(&restored, &mut score).is_err());
        assert_eq!(
            serde_json::to_value(&score).unwrap(),
            serde_json::to_value(&before).unwrap()
        );
    }

    #[test]
    fn set_score_text_supports_append_replace_remove_and_undo_redo() {
        let mut engine = crate::ScoreEngine::new();
        let text = StyledText {
            style: crate::TextStyle::Expression,
            text: "Title".to_string(),
            placement: None,
            offset_x: None,
            offset_y: None,
            relative_x: None,
            relative_y: None,
        };
        engine
            .apply(Command::SetScoreText(SetScoreTextCmd {
                text_index: 0,
                text: Some(text.clone()),
            }))
            .unwrap();
        assert_eq!(engine.score.texts, vec![text.clone()]);

        let mut replacement = text.clone();
        replacement.text = "Subtitle".to_string();
        engine
            .apply(Command::SetScoreText(SetScoreTextCmd {
                text_index: 0,
                text: Some(replacement.clone()),
            }))
            .unwrap();
        assert_eq!(engine.score.texts, vec![replacement]);

        engine
            .apply(Command::SetScoreText(SetScoreTextCmd {
                text_index: 0,
                text: None,
            }))
            .unwrap();
        assert!(engine.score.texts.is_empty());
        engine.undo().unwrap();
        assert_eq!(engine.score.texts.len(), 1);
        engine.redo().unwrap();
        assert!(engine.score.texts.is_empty());
    }

    #[test]
    fn set_score_style_overrides_is_undoable_and_rejects_invalid_values() {
        let mut engine = crate::ScoreEngine::new();
        let overrides = vec![ViewStyleOverride {
            property: super::super::score::ViewStyleProperty::SystemGap,
            value: 2.5,
        }];
        engine
            .apply(Command::SetScoreStyleOverrides(SetScoreStyleOverridesCmd {
                overrides: overrides.clone(),
            }))
            .unwrap();
        assert_eq!(engine.score.style_overrides, overrides);
        engine.undo().unwrap();
        assert!(engine.score.style_overrides.is_empty());
        engine.redo().unwrap();
        assert_eq!(engine.score.style_overrides, overrides);

        let before = engine.score.clone();
        assert!(
            engine
                .apply(Command::SetScoreStyleOverrides(SetScoreStyleOverridesCmd {
                    overrides: vec![ViewStyleOverride {
                        property: super::super::score::ViewStyleProperty::TextScale,
                        value: f32::NAN,
                    }],
                }))
                .is_err()
        );
        assert_eq!(
            serde_json::to_value(&engine.score).unwrap(),
            serde_json::to_value(&before).unwrap()
        );
    }

    #[test]
    fn set_object_style_overrides_is_undoable_and_requires_existing_target() {
        let mut engine = crate::ScoreEngine::new();
        engine.score.texts.push(StyledText {
            style: crate::TextStyle::Expression,
            text: "Allegro".into(),
            placement: None,
            offset_x: None,
            offset_y: None,
            relative_x: None,
            relative_y: None,
        });
        let overrides = vec![ObjectStyleOverride {
            target: super::super::score::ObjectStyleTarget::ScoreText { text_index: 0 },
            property: super::super::score::ViewStyleProperty::TextScale,
            value: 1.2,
            provenance: None,
        }];
        engine
            .apply(Command::SetObjectStyleOverrides(
                SetObjectStyleOverridesCmd {
                    overrides: overrides.clone(),
                },
            ))
            .unwrap();
        assert_eq!(engine.score.object_style_overrides, overrides);
        engine.undo().unwrap();
        assert!(engine.score.object_style_overrides.is_empty());

        assert!(
            engine
                .apply(Command::SetObjectStyleOverrides(
                    SetObjectStyleOverridesCmd {
                        overrides: vec![ObjectStyleOverride {
                            target: super::super::score::ObjectStyleTarget::ScoreText {
                                text_index: 1,
                            },
                            property: super::super::score::ViewStyleProperty::TextScale,
                            value: 1.2,
                            provenance: None,
                        }],
                    }
                ))
                .is_err()
        );
    }

    #[test]
    fn set_harmony_range_is_undoable_and_json_compatible() {
        let mut score = default_engine_score();
        score.parts[0].staves[0].measures[0].voices[0] =
            vec![Note::new(Pitch::new(Step::C, 4), Duration::Whole)];
        score.parts[0].staves[0].measures[0].voices[0][0].chord_symbol = Some(ChordSymbol {
            root: "C".to_owned(),
            kind: "major".to_owned(),
            bass: None,
            placement: None,
            extender: true,
            harmonic_degree: None,
            harmony_function: None,
            harmony_type: None,
            chord_ref: None,
            range_end: None,
            degrees: Vec::new(),
        });
        let mut engine = ScoreEngine::new();
        engine.replace_score(score);
        let command = Command::SetHarmonyRange(SetHarmonyRangeCmd {
            part_index: 0,
            staff_index: 0,
            measure_index: 0,
            voice: 0,
            note_index: 0,
            end: Some(NoteAddr {
                part: 0,
                staff: 0,
                measure: 0,
                voice: 0,
                note: 0,
            }),
        });
        let json = serde_json::to_string(&command).unwrap();
        let restored: Command = serde_json::from_str(&json).unwrap();
        engine.apply(restored).unwrap();
        assert!(
            engine.score.parts[0].staves[0].measures[0].voices[0][0]
                .chord_symbol
                .as_ref()
                .and_then(|chord| chord.range_end.as_ref())
                .is_some()
        );
        engine.undo().unwrap();
        assert!(
            engine.score.parts[0].staves[0].measures[0].voices[0][0]
                .chord_symbol
                .as_ref()
                .is_some_and(|chord| chord.range_end.is_none())
        );
    }

    #[test]
    fn set_harp_pedal_diagrams_is_undoable_and_json_compatible() {
        let mut engine = ScoreEngine::new();
        let mut diagram = HarpPedalDiagram::default();
        diagram.positions[0] = super::super::score::HarpPedalPosition::Flat;
        diagram.positions[6] = super::super::score::HarpPedalPosition::Sharp;
        let command = Command::SetHarpPedalDiagrams(SetHarpPedalDiagramsCmd {
            part_index: 0,
            staff_index: 0,
            measure_index: 0,
            diagrams: vec![diagram.clone()],
        });
        let restored: Command =
            serde_json::from_str(&serde_json::to_string(&command).unwrap()).unwrap();
        engine.apply(restored).unwrap();
        assert_eq!(
            engine.score.parts[0].staves[0].measures[0].harp_pedal_diagrams,
            vec![diagram]
        );
        engine.undo().unwrap();
        assert!(
            engine.score.parts[0].staves[0].measures[0]
                .harp_pedal_diagrams
                .is_empty()
        );
    }

    #[test]
    fn set_note_placement_is_undoable_and_rejects_non_finite_values() {
        let mut score = default_engine_score();
        score.parts[0].staves[0].measures[0].voices[0] =
            vec![Note::new(Pitch::new(Step::C, 4), Duration::Quarter)];
        let mut engine = ScoreEngine::new();
        engine.replace_score(score);
        let command = Command::SetNotePlacement(SetNotePlacementCmd {
            part_index: 0,
            staff_index: 0,
            measure_index: 0,
            voice: 0,
            note_index: 0,
            offset_x: Some(12.5),
            offset_y: Some(-3.0),
            relative_x: Some(1.25),
            relative_y: Some(-0.5),
        });
        engine.apply(command).unwrap();
        let note = &engine.score.parts[0].staves[0].measures[0].voices[0][0];
        assert_eq!(note.offset_x, Some(12.5));
        assert_eq!(note.relative_y, Some(-0.5));
        engine.undo().unwrap();
        assert_eq!(
            engine.score.parts[0].staves[0].measures[0].voices[0][0].offset_x,
            None
        );
        let invalid = Command::SetNotePlacement(SetNotePlacementCmd {
            part_index: 0,
            staff_index: 0,
            measure_index: 0,
            voice: 0,
            note_index: 0,
            offset_x: Some(f64::NAN),
            offset_y: None,
            relative_x: None,
            relative_y: None,
        });
        assert!(engine.apply(invalid).is_err());
        assert_eq!(
            engine.score.parts[0].staves[0].measures[0].voices[0][0].offset_x,
            None
        );
    }

    #[test]
    fn set_guitar_bend_alter_updates_note_and_can_clear() {
        let mut score = default_engine_score();
        apply_command(
            &Command::AddNote(AddNoteCmd {
                part_index: 0,
                staff_index: 0,
                measure_index: 0,
                voice: 0,
                position: 0,
                pitch: Some(Pitch::new(Step::G, 4)),
                duration: Duration::Quarter,
                dot_count: 0,
                is_rest: false,
                tuplet: None,
            }),
            &mut score,
        )
        .unwrap();
        let location = SetGuitarBendAlterCmd {
            part_index: 0,
            staff_index: 0,
            measure_index: 0,
            voice: 0,
            note_index: 0,
            alter_cents: Some(200),
        };
        apply_command(&Command::SetGuitarBendAlter(location.clone()), &mut score).unwrap();
        assert_eq!(
            score.parts[0].staves[0].measures[0].voices[0][0].guitar_bend_alter_cents,
            Some(200)
        );
        let mut cleared = location;
        cleared.alter_cents = None;
        apply_command(&Command::SetGuitarBendAlter(cleared), &mut score).unwrap();
        assert_eq!(
            score.parts[0].staves[0].measures[0].voices[0][0].guitar_bend_alter_cents,
            None
        );
    }

    #[test]
    fn set_guitar_bend_curve_is_undoable_and_json_compatible() {
        let mut score = Score::new("T", 120, 4, 4, 0, 1);
        let command = Command::SetGuitarBendCurve(SetGuitarBendCurveCmd {
            part_index: 0,
            staff_index: 0,
            measure_index: 0,
            voice: 0,
            note_index: 0,
            points: vec![
                crate::GuitarBendPoint {
                    position_per_mille: 0,
                    alter_cents: 0,
                },
                crate::GuitarBendPoint {
                    position_per_mille: 500,
                    alter_cents: 200,
                },
                crate::GuitarBendPoint {
                    position_per_mille: 1000,
                    alter_cents: 0,
                },
            ],
        });
        let decoded: Command =
            serde_json::from_str(&serde_json::to_string(&command).unwrap()).unwrap();
        let mut stack = CommandStack::new(16);
        stack.execute(decoded, &mut score).unwrap();
        assert_eq!(
            score.parts[0].staves[0].measures[0].voices[0][0]
                .guitar_bend_curve
                .len(),
            3
        );
        stack.undo(&mut score).unwrap();
        assert!(
            score.parts[0].staves[0].measures[0].voices[0][0]
                .guitar_bend_curve
                .is_empty()
        );
    }

    #[test]
    fn set_duration_updates_note_and_preserves_measure_capacity() {
        let mut score = default_engine_score();
        apply_command(
            &Command::AddNote(AddNoteCmd {
                part_index: 0,
                staff_index: 0,
                measure_index: 0,
                voice: 0,
                position: 0,
                pitch: Some(Pitch::new(Step::C, 4)),
                duration: Duration::Quarter,
                dot_count: 0,
                is_rest: false,
                tuplet: None,
            }),
            &mut score,
        )
        .unwrap();
        apply_command(
            &Command::SetDuration(SetDurationCmd {
                part_index: 0,
                staff_index: 0,
                measure_index: 0,
                voice: 0,
                note_index: 0,
                duration: Duration::Half,
                dot_count: 1,
            }),
            &mut score,
        )
        .unwrap();
        let voice = &score.parts[0].staves[0].measures[0].voices[0];
        assert_eq!(voice[0].duration, Duration::Half);
        assert_eq!(voice[0].dot_count, 1);
        assert!((voice.iter().map(|note| note.beats()).sum::<f64>() - 4.0).abs() < 1e-9);
    }

    #[test]
    fn set_tempo_updates_score() {
        let mut score = default_engine_score();
        let cmd = Command::SetTempo(SetTempoCmd { bpm: 160 });
        apply_command(&cmd, &mut score).unwrap();
        assert_eq!(score.settings.tempo_bpm, 160);
    }

    #[test]
    fn add_measure_increases_count() {
        let mut score = default_engine_score();
        let before = score.measure_count();
        apply_command(
            &Command::AddMeasure(AddMeasureCmd { after_index: 0 }),
            &mut score,
        )
        .unwrap();
        assert_eq!(score.measure_count(), before + 1);
    }

    #[test]
    fn delete_measure_decreases_count() {
        let mut score = default_engine_score();
        let before = score.measure_count();
        apply_command(
            &Command::DeleteMeasure(DeleteMeasureCmd { measure_index: 0 }),
            &mut score,
        )
        .unwrap();
        assert_eq!(score.measure_count(), before - 1);
    }

    #[test]
    fn undo_restores_score() {
        let mut stack = CommandStack::new(50);
        let mut score = default_engine_score();
        let before = score.settings.tempo_bpm;
        stack
            .execute(Command::SetTempo(SetTempoCmd { bpm: 200 }), &mut score)
            .unwrap();
        assert_eq!(score.settings.tempo_bpm, 200);
        stack.undo(&mut score).unwrap();
        assert_eq!(score.settings.tempo_bpm, before);
    }

    #[test]
    fn redo_reapplies_command() {
        let mut stack = CommandStack::new(50);
        let mut score = default_engine_score();
        stack
            .execute(Command::SetTempo(SetTempoCmd { bpm: 200 }), &mut score)
            .unwrap();
        stack.undo(&mut score).unwrap();
        stack.redo(&mut score).unwrap();
        assert_eq!(score.settings.tempo_bpm, 200);
    }

    #[test]
    fn undo_nothing_returns_error() {
        let mut stack = CommandStack::new(50);
        let mut score = default_engine_score();
        assert!(stack.undo(&mut score).is_err());
    }

    #[test]
    fn add_part_appends_part() {
        let mut score = default_engine_score();
        let before = score.parts.len();
        apply_command(
            &Command::AddPart(AddPartCmd {
                name: "Violin".into(),
                short_name: "Vln.".into(),
                clefs: vec!["Treble".into()],
                midi_channel: 0,
                midi_program: 0,
            }),
            &mut score,
        )
        .unwrap();
        assert_eq!(score.parts.len(), before + 1);
    }

    #[test]
    fn delete_part_removes_part() {
        let mut score = default_engine_score();
        apply_command(
            &Command::AddPart(AddPartCmd {
                name: "Violin".into(),
                short_name: "V.".into(),
                clefs: vec!["Treble".into()],
                midi_channel: 0,
                midi_program: 0,
            }),
            &mut score,
        )
        .unwrap();
        let before = score.parts.len();
        apply_command(
            &Command::DeletePart(DeletePartCmd { part_index: 0 }),
            &mut score,
        )
        .unwrap();
        assert_eq!(score.parts.len(), before - 1);
    }

    #[test]
    fn delete_part_out_of_range_returns_err() {
        let mut score = default_engine_score();
        assert!(
            apply_command(
                &Command::DeletePart(DeletePartCmd { part_index: 99 }),
                &mut score
            )
            .is_err()
        );
    }

    #[test]
    fn delete_part_undo_restores_part() {
        let mut stack = CommandStack::new(50);
        let mut score = default_engine_score();
        apply_command(
            &Command::AddPart(AddPartCmd {
                name: "Violin".into(),
                short_name: "V.".into(),
                clefs: vec!["Treble".into()],
                midi_channel: 0,
                midi_program: 0,
            }),
            &mut score,
        )
        .unwrap();
        let before = score.parts.len();
        stack
            .execute(
                Command::DeletePart(DeletePartCmd { part_index: 0 }),
                &mut score,
            )
            .unwrap();
        assert_eq!(score.parts.len(), before - 1);
        stack.undo(&mut score).unwrap();
        assert_eq!(score.parts.len(), before);
    }

    #[test]
    fn reorder_parts_remaps_linked_views_and_preserves_undo() {
        let mut engine = crate::ScoreEngine::new();
        engine
            .apply(Command::AddPart(AddPartCmd {
                name: "Flute".into(),
                short_name: "Fl.".into(),
                clefs: vec!["Treble".into()],
                midi_channel: 1,
                midi_program: 73,
            }))
            .unwrap();
        engine
            .score
            .views
            .push(ScoreView::linked_part("flute", "Flute", 1));
        engine.score.part_groups.push(PartGroup {
            first_part: 0,
            last_part: 1,
            symbol: super::super::score::PartGroupSymbol::Bracket,
            barlines_connect: false,
        });

        engine
            .apply(Command::ReorderParts(ReorderPartsCmd { order: vec![1, 0] }))
            .unwrap();
        assert_eq!(engine.score.parts[0].name, "Flute");
        assert_eq!(engine.score.views[0].parts, vec![0]);
        engine.undo().unwrap();
        assert_eq!(engine.score.parts[0].name, "Piano");
        assert_eq!(engine.score.views[0].parts, vec![1]);
    }

    #[test]
    fn reorder_parts_rejects_splitting_a_part_group_without_mutation() {
        let mut score = Score::template(ScoreTemplate::StringQuartet);
        score.part_groups.push(PartGroup {
            first_part: 0,
            last_part: 1,
            symbol: super::super::score::PartGroupSymbol::Bracket,
            barlines_connect: false,
        });
        let before = serde_json::to_value(&score).unwrap();

        assert!(
            apply_command(
                &Command::ReorderParts(ReorderPartsCmd {
                    order: vec![0, 2, 1, 3],
                }),
                &mut score,
            )
            .is_err()
        );
        assert_eq!(serde_json::to_value(&score).unwrap(), before);
    }

    #[test]
    fn set_metadata_updates_title() {
        let mut score = default_engine_score();
        apply_command(
            &Command::SetMetadata(SetMetadataCmd {
                title: Some("New Title".into()),
                ..Default::default()
            }),
            &mut score,
        )
        .unwrap();
        assert_eq!(score.metadata.title, "New Title");
    }

    #[test]
    fn set_metadata_none_fields_skipped() {
        let mut score = default_engine_score();
        let original_composer = score.metadata.composer.clone();
        apply_command(
            &Command::SetMetadata(SetMetadataCmd {
                title: Some("X".into()),
                ..Default::default()
            }),
            &mut score,
        )
        .unwrap();
        assert_eq!(score.metadata.composer, original_composer);
    }

    #[test]
    fn set_volta_sets_bracket() {
        use crate::model::score::VoltaBracket;
        let mut score = default_engine_score();
        let volta = VoltaBracket {
            number: 1,
            kind: "begin_end".into(),
        };
        apply_command(
            &Command::SetVolta(SetVoltaCmd {
                measure_index: 0,
                volta: Some(volta.clone()),
            }),
            &mut score,
        )
        .unwrap();
        assert!(score.parts[0].staves[0].measures[0].volta.is_some());
    }

    #[test]
    fn set_volta_none_clears_bracket() {
        use crate::model::score::VoltaBracket;
        let mut score = default_engine_score();
        score.parts[0].staves[0].measures[0].volta = Some(VoltaBracket {
            number: 1,
            kind: "begin_end".into(),
        });
        apply_command(
            &Command::SetVolta(SetVoltaCmd {
                measure_index: 0,
                volta: None,
            }),
            &mut score,
        )
        .unwrap();
        assert!(score.parts[0].staves[0].measures[0].volta.is_none());
    }

    #[test]
    fn set_volta_undo_restores_old() {
        use crate::model::score::VoltaBracket;
        let mut stack = CommandStack::new(50);
        let mut score = default_engine_score();
        stack
            .execute(
                Command::SetVolta(SetVoltaCmd {
                    measure_index: 0,
                    volta: Some(VoltaBracket {
                        number: 1,
                        kind: "begin_end".into(),
                    }),
                }),
                &mut score,
            )
            .unwrap();
        stack.undo(&mut score).unwrap();
        assert!(score.parts[0].staves[0].measures[0].volta.is_none());
    }

    #[test]
    fn set_clef_updates_staff_clef() {
        use crate::model::notation::Clef;
        let mut score = default_engine_score();
        apply_command(
            &Command::SetClef(SetClefCmd {
                part_index: 0,
                staff_index: 0,
                clef: Clef::Bass,
            }),
            &mut score,
        )
        .unwrap();
        assert_eq!(score.parts[0].staves[0].clef, Clef::Bass);
    }

    #[test]
    fn set_clef_out_of_range_returns_err() {
        use crate::model::notation::Clef;
        let mut score = default_engine_score();
        assert!(
            apply_command(
                &Command::SetClef(SetClefCmd {
                    part_index: 99,
                    staff_index: 0,
                    clef: Clef::Bass,
                }),
                &mut score
            )
            .is_err()
        );
    }

    #[test]
    fn set_part_name_updates_name() {
        let mut score = default_engine_score();
        apply_command(
            &Command::SetPartName(SetPartNameCmd {
                part_index: 0,
                name: "Violin".into(),
                short_name: "Vln.".into(),
            }),
            &mut score,
        )
        .unwrap();
        assert_eq!(score.parts[0].name, "Violin");
        assert_eq!(score.parts[0].short_name, "Vln.");
    }

    #[test]
    fn set_part_name_undo_restores_old() {
        let mut stack = CommandStack::new(50);
        let mut score = default_engine_score();
        let original = score.parts[0].name.clone();
        stack
            .execute(
                Command::SetPartName(SetPartNameCmd {
                    part_index: 0,
                    name: "Flute".into(),
                    short_name: "Fl.".into(),
                }),
                &mut score,
            )
            .unwrap();
        stack.undo(&mut score).unwrap();
        assert_eq!(score.parts[0].name, original);
    }

    #[test]
    fn set_metadata_undo_restores_old_title() {
        let mut stack = CommandStack::new(50);
        let mut score = default_engine_score();
        let original = score.metadata.title.clone();
        stack
            .execute(
                Command::SetMetadata(SetMetadataCmd {
                    title: Some("Changed".into()),
                    ..Default::default()
                }),
                &mut score,
            )
            .unwrap();
        assert_ne!(score.metadata.title, original);
        stack.undo(&mut score).unwrap();
        assert_eq!(score.metadata.title, original);
    }

    #[test]
    fn set_midi_instrument_updates_channel_and_program() {
        let mut score = default_engine_score();
        apply_command(
            &Command::SetMidiInstrument(SetMidiInstrumentCmd {
                part_index: 0,
                midi_channel: 2,
                midi_program: 40,
            }),
            &mut score,
        )
        .unwrap();
        assert_eq!(score.parts[0].midi_channel, 2);
        assert_eq!(score.parts[0].midi_program, 40);
    }

    #[test]
    fn set_midi_instrument_clamps_channel_to_15() {
        let mut score = default_engine_score();
        apply_command(
            &Command::SetMidiInstrument(SetMidiInstrumentCmd {
                part_index: 0,
                midi_channel: 20,
                midi_program: 0,
            }),
            &mut score,
        )
        .unwrap();
        assert_eq!(score.parts[0].midi_channel, 15);
    }

    #[test]
    fn set_midi_instrument_undo_restores_old() {
        let mut stack = CommandStack::new(50);
        let mut score = default_engine_score();
        score.parts[0].midi_channel = 3;
        score.parts[0].midi_program = 10;
        stack
            .execute(
                Command::SetMidiInstrument(SetMidiInstrumentCmd {
                    part_index: 0,
                    midi_channel: 9,
                    midi_program: 114,
                }),
                &mut score,
            )
            .unwrap();
        stack.undo(&mut score).unwrap();
        assert_eq!(score.parts[0].midi_channel, 3);
        assert_eq!(score.parts[0].midi_program, 10);
    }

    #[test]
    fn set_transpose_updates_staff() {
        let mut score = default_engine_score();
        apply_command(
            &Command::SetTranspose(SetTransposeCmd {
                part_index: 0,
                staff_index: 0,
                semitones: -2,
            }),
            &mut score,
        )
        .unwrap();
        assert_eq!(score.parts[0].staves[0].transpose_semitones, -2);
    }

    #[test]
    fn set_transpose_out_of_range_returns_err() {
        let mut score = default_engine_score();
        assert!(
            apply_command(
                &Command::SetTranspose(SetTransposeCmd {
                    part_index: 99,
                    staff_index: 0,
                    semitones: -2,
                }),
                &mut score
            )
            .is_err()
        );
    }

    #[test]
    fn transpose_staff_region_is_undoable_and_json_compatible() {
        let mut stack = CommandStack::new(50);
        let mut score = Score::new("Region", 120, 4, 4, 0, 2);
        for measure in &mut score.parts[0].staves[0].measures {
            measure.voices[0] = vec![Note::new(Pitch::new(Step::C, 4), Duration::Quarter)];
        }
        let command = Command::TransposeStaffRegion(TransposeStaffRegionCmd {
            part_index: 0,
            staff_index: 0,
            start_measure: 1,
            end_measure: 2,
            semitones: 2,
            target: RegionalTranspositionTarget::Written,
        });
        let json = serde_json::to_string(&command).expect("command serializes");
        let decoded: Command = serde_json::from_str(&json).expect("command deserializes");
        assert_eq!(command_key(&decoded), "TransposeStaffRegion");
        stack.execute(decoded, &mut score).expect("region applies");
        assert_eq!(
            score.parts[0].staves[0].measures[0].voices[0][0].pitches[0].to_midi(),
            60
        );
        assert_eq!(
            score.parts[0].staves[0].measures[1].voices[0][0].pitches[0].to_midi(),
            62
        );
        stack.undo(&mut score).expect("region undo applies");
        assert_eq!(
            score.parts[0].staves[0].measures[1].voices[0][0].pitches[0].to_midi(),
            60
        );
    }

    #[test]
    fn set_tempo_at_measure_sets_tempo() {
        let mut score = default_engine_score();
        apply_command(
            &Command::SetTempoAtMeasure(SetTempoAtMeasureCmd {
                measure_index: 0,
                bpm: Some(80),
            }),
            &mut score,
        )
        .unwrap();
        assert_eq!(score.parts[0].staves[0].measures[0].tempo, Some(80));
    }

    #[test]
    fn set_tempo_at_measure_none_clears_tempo() {
        let mut score = default_engine_score();
        score.parts[0].staves[0].measures[0].tempo = Some(120);
        apply_command(
            &Command::SetTempoAtMeasure(SetTempoAtMeasureCmd {
                measure_index: 0,
                bpm: None,
            }),
            &mut score,
        )
        .unwrap();
        assert!(score.parts[0].staves[0].measures[0].tempo.is_none());
    }

    #[test]
    fn set_tempo_ramp_at_measure_applies() {
        let mut score = default_engine_score();
        apply_command(
            &Command::SetTempoRampAtMeasure(SetTempoRampAtMeasureCmd {
                measure_index: 0,
                target_bpm: Some(72),
            }),
            &mut score,
        )
        .expect("tempo ramp applies");
        assert_eq!(score.parts[0].staves[0].measures[0].tempo_ramp_to, Some(72));
    }

    // ── batch_execute ─────────────────────────────────────────────────────────

    #[test]
    fn batch_execute_two_commands_single_undo() {
        let mut stack = CommandStack::new(50);
        let mut score = default_engine_score();
        let original_bpm = score.settings.tempo_bpm;
        stack
            .batch_execute(
                vec![
                    Command::SetTempo(SetTempoCmd { bpm: 160 }),
                    Command::SetTempo(SetTempoCmd { bpm: 180 }),
                ],
                &mut score,
            )
            .unwrap();
        assert_eq!(score.settings.tempo_bpm, 180);
        stack.undo(&mut score).unwrap();
        assert_eq!(score.settings.tempo_bpm, original_bpm);
    }

    #[test]
    fn batch_execute_partial_failure_rollback() {
        let mut stack = CommandStack::new(50);
        let mut score = default_engine_score();
        let original_bpm = score.settings.tempo_bpm;
        let result = stack.batch_execute(
            vec![
                Command::SetTempo(SetTempoCmd { bpm: 160 }),
                Command::DeleteNote(DeleteNoteCmd {
                    note_id: "nonexistent".into(),
                    part_index: 99,
                    staff_index: 0,
                    measure_index: 0,
                    voice: 0,
                }),
            ],
            &mut score,
        );
        assert!(result.is_err());
        assert_eq!(score.settings.tempo_bpm, original_bpm);
    }

    #[test]
    fn execute_rejects_invalid_candidate_without_mutating_or_recording_history() {
        let mut stack = CommandStack::new(50);
        let mut score = default_engine_score();
        score.parts[0].staves[0].tablature = Some(crate::TablatureConfig {
            lines: 6,
            tuning_midi: vec![40, 45, 50, 55, 59, 64],
            capo: 0,
        });
        stack
            .execute(
                Command::AddNote(AddNoteCmd {
                    part_index: 0,
                    staff_index: 0,
                    measure_index: 0,
                    voice: 0,
                    position: 0,
                    pitch: Some(Pitch::new(Step::C, 4)),
                    duration: Duration::Quarter,
                    dot_count: 0,
                    is_rest: false,
                    tuplet: None,
                }),
                &mut score,
            )
            .expect("valid pitched note should be accepted");
        let original = score.clone();
        let result = stack.execute(
            Command::SetTabPosition(SetTabPositionCmd {
                part_index: 0,
                staff_index: 0,
                measure_index: 0,
                voice: 0,
                note_index: 0,
                position: Some(crate::TabPosition { string: 7, fret: 0 }),
            }),
            &mut score,
        );
        assert!(matches!(result, Err(Error::InvalidScore)));
        let note = &score.parts[0].staves[0].measures[0].voices[0][0];
        let original_note = &original.parts[0].staves[0].measures[0].voices[0][0];
        assert_eq!(note.pitches, original_note.pitches);
        assert_eq!(note.duration, original_note.duration);
        assert_eq!(note.tab_position, original_note.tab_position);
        assert_eq!(note.tab_positions, original_note.tab_positions);
        assert!(stack.can_undo());
    }

    #[test]
    fn set_tablature_config_is_undoable_and_json_compatible() {
        let mut stack = CommandStack::new(50);
        let mut score = default_engine_score();
        let config = crate::TablatureConfig {
            lines: 6,
            tuning_midi: vec![40, 45, 50, 55, 59, 64],
            capo: 2,
        };
        let command = Command::SetTablatureConfig(SetTablatureConfigCmd {
            part_index: 0,
            staff_index: 0,
            config: Some(config.clone()),
        });
        let json = serde_json::to_string(&command).expect("command should serialize");
        let decoded: Command = serde_json::from_str(&json).expect("command should deserialize");
        assert_eq!(command_key(&decoded), "SetTablatureConfig");
        stack
            .execute(decoded, &mut score)
            .expect("config should apply");
        assert_eq!(score.parts[0].staves[0].tablature, Some(config));
        assert_eq!(
            score.parts[0].staves[0].presentation.kind,
            StaffKind::Tablature
        );
        stack.undo(&mut score).expect("config undo should apply");
        assert!(score.parts[0].staves[0].tablature.is_none());
        assert_eq!(
            score.parts[0].staves[0].presentation.kind,
            StaffKind::Standard
        );
        stack.redo(&mut score).expect("config redo should apply");
        assert_eq!(
            score.parts[0].staves[0]
                .tablature
                .as_ref()
                .map(|tab| tab.capo),
            Some(2)
        );
    }

    #[test]
    fn set_staff_presentation_is_undoable_and_requires_tablature_config() {
        let mut stack = CommandStack::new(50);
        let mut score = default_engine_score();
        let tab = StaffPresentation {
            kind: StaffKind::Tablature,
            lines: 6,
            line_distance: 1.25,
            small: true,
            cutaway: false,
            visible: true,
            notehead_scheme: super::super::score::StaffNoteheadScheme::Standard,
            tablature_rhythm_display: super::super::score::TablatureRhythmDisplay::FretOnly,
            tablature_fret_mark_style: super::super::score::TablatureFretMarkStyle::Arabic,
        };
        let rejected = Command::SetStaffPresentation(SetStaffPresentationCmd {
            part_index: 0,
            staff_index: 0,
            presentation: tab.clone(),
        });
        let before = score.clone();
        assert!(stack.execute(rejected, &mut score).is_err());
        assert_eq!(
            score.parts[0].staves[0].presentation,
            before.parts[0].staves[0].presentation
        );
        assert!(!stack.can_undo());

        stack
            .execute(
                Command::SetTablatureConfig(SetTablatureConfigCmd {
                    part_index: 0,
                    staff_index: 0,
                    config: Some(crate::TablatureConfig {
                        lines: 6,
                        tuning_midi: vec![40, 45, 50, 55, 59, 64],
                        capo: 0,
                    }),
                }),
                &mut score,
            )
            .expect("tablature configuration should apply");
        stack
            .execute(
                Command::SetStaffPresentation(SetStaffPresentationCmd {
                    part_index: 0,
                    staff_index: 0,
                    presentation: tab.clone(),
                }),
                &mut score,
            )
            .expect("staff presentation should apply");
        assert_eq!(stack.undo_key().as_deref(), Some("SetStaffPresentation"));
        assert_eq!(score.parts[0].staves[0].presentation, tab);
        stack
            .undo(&mut score)
            .expect("presentation undo should apply");
        assert_eq!(
            score.parts[0].staves[0].presentation.kind,
            StaffKind::Tablature
        );
        stack
            .redo(&mut score)
            .expect("presentation redo should apply");
        assert_eq!(
            score.parts[0].staves[0].presentation.kind,
            StaffKind::Tablature
        );
    }

    #[test]
    fn set_instrument_definition_is_undoable_and_rejects_invalid_ranges() {
        let mut stack = CommandStack::new(50);
        let mut score = default_engine_score();
        let mut invalid = InstrumentDefinition::new("violin", "Violin");
        invalid.written_range = Some(InstrumentRange {
            lowest: 100,
            highest: 55,
        });
        let before = score.clone();
        assert!(
            stack
                .execute(
                    Command::SetInstrumentDefinition(SetInstrumentDefinitionCmd {
                        part_index: 0,
                        definition: Some(invalid),
                    }),
                    &mut score,
                )
                .is_err()
        );
        assert_eq!(score.parts[0].instrument, before.parts[0].instrument);

        let mut definition = InstrumentDefinition::new("violin", "Violin");
        definition.short_name = "Vln.".to_string();
        definition.family = Some("strings".to_string());
        definition.written_range = Some(InstrumentRange {
            lowest: 55,
            highest: 103,
        });
        definition.sounding_range = definition.written_range;
        definition.default_clefs = vec![Clef::Treble];
        definition.midi_program = 40;
        let command = Command::SetInstrumentDefinition(SetInstrumentDefinitionCmd {
            part_index: 0,
            definition: Some(definition.clone()),
        });
        let json = serde_json::to_string(&command).expect("command should serialize");
        let decoded: Command = serde_json::from_str(&json).expect("command should deserialize");
        assert_eq!(command_key(&decoded), "SetInstrumentDefinition");
        stack
            .execute(decoded, &mut score)
            .expect("definition applies");
        assert_eq!(score.parts[0].instrument, Some(definition));
        stack.undo(&mut score).expect("definition undo applies");
        assert!(score.parts[0].instrument.is_none());
        stack.redo(&mut score).expect("definition redo applies");
        assert_eq!(
            score.parts[0]
                .instrument
                .as_ref()
                .map(|value| value.id.as_str()),
            Some("violin")
        );
    }

    #[test]
    fn set_percussion_kit_is_undoable_and_rejects_invalid_entries() {
        let mut stack = CommandStack::new(50);
        let mut score = default_engine_score();
        let invalid = Command::SetPercussionKit(SetPercussionKitCmd {
            part_index: 0,
            instruments: vec![PercussionInstrument {
                id: "snare".to_string(),
                name: None,
                midi_unpitched: Some(38),
                staff_position: Some(33),
                notehead: None,
                preferred_voice: None,
                techniques: Vec::new(),
            }],
        });
        assert!(stack.execute(invalid, &mut score).is_err());
        assert!(score.parts[0].percussion_instruments.is_empty());

        let kit = vec![PercussionInstrument {
            id: "snare".to_string(),
            name: Some("Acoustic Snare".to_string()),
            midi_unpitched: Some(38),
            staff_position: Some(0),
            notehead: Some(NoteHead::Cross),
            preferred_voice: Some(1),
            techniques: vec!["rim-shot".to_string()],
        }];
        let command = Command::SetPercussionKit(SetPercussionKitCmd {
            part_index: 0,
            instruments: kit.clone(),
        });
        let json = serde_json::to_string(&command).expect("command serializes");
        let decoded: Command = serde_json::from_str(&json).expect("command deserializes");
        assert_eq!(command_key(&decoded), "SetPercussionKit");
        stack.execute(decoded, &mut score).expect("kit applies");
        assert_eq!(score.parts[0].percussion_instruments, kit);
        stack.undo(&mut score).expect("kit undo applies");
        assert!(score.parts[0].percussion_instruments.is_empty());
        stack.redo(&mut score).expect("kit redo applies");
        assert_eq!(score.parts[0].percussion_instruments, kit);
    }

    #[test]
    fn set_measure_instrument_change_is_undoable_and_json_compatible() {
        let mut stack = CommandStack::new(50);
        let mut score = default_engine_score();
        let mut definition = InstrumentDefinition::new("clarinet-bb", "B-flat Clarinet");
        definition.transpose_semitones = -2;
        definition.midi_program = 71;
        let command = Command::SetMeasureInstrumentChange(SetMeasureInstrumentChangeCmd {
            part_index: 0,
            staff_index: 0,
            measure_index: 2,
            definition: Some(definition.clone()),
        });
        let json = serde_json::to_string(&command).expect("command should serialize");
        let decoded: Command = serde_json::from_str(&json).expect("command should deserialize");
        assert_eq!(command_key(&decoded), "SetMeasureInstrumentChange");
        stack.execute(decoded, &mut score).expect("change applies");
        assert_eq!(
            score.parts[0].staves[0].measures[2].instrument_change,
            Some(definition)
        );
        stack.undo(&mut score).expect("undo applies");
        assert!(
            score.parts[0].staves[0].measures[2]
                .instrument_change
                .is_none()
        );
    }

    #[test]
    fn set_measure_tablature_change_is_undoable_and_uses_base_line_count() {
        let mut stack = CommandStack::new(50);
        let mut score = default_engine_score();
        let base = crate::TablatureConfig {
            lines: 6,
            tuning_midi: vec![40, 45, 50, 55, 59, 64],
            capo: 0,
        };
        score.parts[0].staves[0].tablature = Some(base.clone());
        let mut changed = base;
        changed.tuning_midi[0] = 38;
        changed.capo = 2;
        let command = Command::SetMeasureTablatureChange(SetMeasureTablatureChangeCmd {
            part_index: 0,
            staff_index: 0,
            measure_index: 2,
            config: Some(changed.clone()),
        });
        let json = serde_json::to_string(&command).expect("command serializes");
        let decoded: Command = serde_json::from_str(&json).expect("command deserializes");
        assert_eq!(command_key(&decoded), "SetMeasureTablatureChange");
        stack.execute(decoded, &mut score).expect("change applies");
        assert_eq!(
            score.parts[0].staves[0].tablature_at(1),
            score.parts[0].staves[0].tablature
        );
        assert_eq!(score.parts[0].staves[0].tablature_at(2), Some(changed));
        stack.undo(&mut score).expect("change undo applies");
        assert!(
            score.parts[0].staves[0].measures[2]
                .tablature_change
                .is_none()
        );

        let invalid = Command::SetMeasureTablatureChange(SetMeasureTablatureChangeCmd {
            part_index: 0,
            staff_index: 0,
            measure_index: 1,
            config: Some(crate::TablatureConfig {
                lines: 7,
                tuning_midi: vec![40, 45, 50, 55, 59, 64, 69],
                capo: 0,
            }),
        });
        assert!(stack.execute(invalid, &mut score).is_err());
    }

    #[test]
    fn batch_execute_empty_is_noop() {
        let mut stack = CommandStack::new(50);
        let mut score = default_engine_score();
        stack.batch_execute(vec![], &mut score).unwrap();
        assert!(!stack.can_undo());
    }

    // ── BatchCmd.label ────────────────────────────────────────────────────────

    #[test]
    fn batch_label_used_as_command_key() {
        let cmd = Command::Batch(BatchCmd {
            commands: vec![],
            label: Some("ApplyAI".to_string()),
        });
        assert_eq!(command_key(&cmd), "ApplyAI");
    }

    #[test]
    fn batch_no_label_key_is_batch() {
        let cmd = Command::Batch(BatchCmd {
            commands: vec![],
            label: None,
        });
        assert_eq!(command_key(&cmd), "Batch");
    }

    #[test]
    fn batch_label_survives_json_roundtrip() {
        let cmd = Command::Batch(BatchCmd {
            commands: vec![Command::SetTempo(SetTempoCmd { bpm: 120 })],
            label: Some("PasteSelection".to_string()),
        });
        let json = serde_json::to_string(&cmd).unwrap();
        let cmd2: Command = serde_json::from_str(&json).unwrap();
        assert_eq!(command_key(&cmd2), "PasteSelection");
    }

    #[test]
    fn batch_label_in_undo_key() {
        let mut stack = CommandStack::new(50);
        let mut score = default_engine_score();
        let cmd = Command::Batch(BatchCmd {
            commands: vec![Command::SetTempo(SetTempoCmd { bpm: 140 })],
            label: Some("ApplyAI".to_string()),
        });
        stack.execute(cmd, &mut score).unwrap();
        assert_eq!(stack.undo_key(), Some("ApplyAI".to_string()));
    }

    #[test]
    fn undo_returns_change_hint() {
        use crate::model::change_hint::ChangeScope;
        let mut stack = CommandStack::new(50);
        let mut score = default_engine_score();
        stack
            .execute(Command::SetTempo(SetTempoCmd { bpm: 200 }), &mut score)
            .unwrap();
        let hint = stack.undo(&mut score).unwrap();
        assert_eq!(hint.scope, ChangeScope::Global);
        assert!(hint.playback_dirty);
    }

    #[test]
    fn redo_returns_change_hint() {
        use crate::model::change_hint::ChangeScope;
        let mut stack = CommandStack::new(50);
        let mut score = default_engine_score();
        stack
            .execute(Command::SetTempo(SetTempoCmd { bpm: 200 }), &mut score)
            .unwrap();
        stack.undo(&mut score).unwrap();
        let hint = stack.redo(&mut score).unwrap();
        assert_eq!(hint.scope, ChangeScope::Global);
        assert!(hint.playback_dirty);
    }

    // ── Feature A: ToggleSlur ─────────────────────────────────────────────

    #[test]
    fn toggle_slur_sets_start_and_end() {
        let mut score = default_engine_score();
        let cmd = Command::AddNote(AddNoteCmd {
            part_index: 0,
            staff_index: 0,
            measure_index: 0,
            voice: 0,
            position: 0,
            pitch: Some(Pitch::new(Step::C, 4)),
            duration: Duration::Quarter,
            dot_count: 0,
            is_rest: false,
            tuplet: None,
        });
        apply_command(&cmd, &mut score).unwrap();
        apply_command(
            &Command::AddNote(AddNoteCmd {
                part_index: 0,
                staff_index: 0,
                measure_index: 0,
                voice: 0,
                position: 1,
                pitch: Some(Pitch::new(Step::D, 4)),
                duration: Duration::Quarter,
                dot_count: 0,
                is_rest: false,
                tuplet: None,
            }),
            &mut score,
        )
        .unwrap();
        let start = NoteAddr {
            part: 0,
            staff: 0,
            measure: 0,
            voice: 0,
            note: 0,
        };
        let end = NoteAddr {
            part: 0,
            staff: 0,
            measure: 0,
            voice: 0,
            note: 1,
        };
        apply_command(
            &Command::ToggleSlur(ToggleSlurCmd {
                start: start.clone(),
                end: end.clone(),
            }),
            &mut score,
        )
        .unwrap();
        assert!(score.parts[0].staves[0].measures[0].voices[0][0].slur_start);
        assert!(score.parts[0].staves[0].measures[0].voices[0][1].slur_end);
        // toggle off
        apply_command(
            &Command::ToggleSlur(ToggleSlurCmd { start, end }),
            &mut score,
        )
        .unwrap();
        assert!(!score.parts[0].staves[0].measures[0].voices[0][0].slur_start);
        assert!(!score.parts[0].staves[0].measures[0].voices[0][1].slur_end);
    }

    // ── Feature B: AddStaff / DeleteStaff ────────────────────────────────

    #[test]
    fn add_staff_appends_staff_with_correct_measure_count() {
        let mut score = default_engine_score();
        let before = score.parts[0].staves.len();
        let measure_count = score.parts[0].staves[0].measures.len();
        apply_command(
            &Command::AddStaff(AddStaffCmd {
                part_index: 0,
                clef: Clef::Bass,
            }),
            &mut score,
        )
        .unwrap();
        assert_eq!(score.parts[0].staves.len(), before + 1);
        let new_staff = score.parts[0].staves.last().unwrap();
        assert_eq!(new_staff.measures.len(), measure_count);
    }

    #[test]
    fn add_staff_out_of_range_returns_err() {
        let mut score = default_engine_score();
        let result = apply_command(
            &Command::AddStaff(AddStaffCmd {
                part_index: 99,
                clef: Clef::Treble,
            }),
            &mut score,
        );
        assert!(result.is_err());
    }

    #[test]
    fn delete_staff_removes_extra_staff() {
        let mut score = default_engine_score();
        apply_command(
            &Command::AddStaff(AddStaffCmd {
                part_index: 0,
                clef: Clef::Bass,
            }),
            &mut score,
        )
        .unwrap();
        assert_eq!(score.parts[0].staves.len(), 2);
        apply_command(
            &Command::DeleteStaff(DeleteStaffCmd {
                part_index: 0,
                staff_index: 1,
            }),
            &mut score,
        )
        .unwrap();
        assert_eq!(score.parts[0].staves.len(), 1);
    }

    #[test]
    fn delete_last_staff_returns_err() {
        let mut score = default_engine_score();
        assert_eq!(score.parts[0].staves.len(), 1);
        let result = apply_command(
            &Command::DeleteStaff(DeleteStaffCmd {
                part_index: 0,
                staff_index: 0,
            }),
            &mut score,
        );
        assert!(result.is_err());
    }

    // ── Feature D: SetTuplet ──────────────────────────────────────────────

    #[test]
    fn set_tuplet_assigns_and_clears() {
        use crate::model::notation::TupletInfo;
        let mut score = default_engine_score();
        apply_command(
            &Command::AddNote(AddNoteCmd {
                part_index: 0,
                staff_index: 0,
                measure_index: 0,
                voice: 0,
                position: 0,
                pitch: Some(Pitch::new(Step::C, 4)),
                duration: Duration::Quarter,
                dot_count: 0,
                is_rest: false,
                tuplet: None,
            }),
            &mut score,
        )
        .unwrap();
        let ti = TupletInfo {
            actual_notes: 3,
            normal_notes: 2,
        };
        apply_command(
            &Command::SetTuplet(SetTupletCmd {
                part_index: 0,
                staff_index: 0,
                measure_index: 0,
                voice_index: 0,
                note_index: 0,
                tuplet: Some(ti.clone()),
            }),
            &mut score,
        )
        .unwrap();
        assert_eq!(
            score.parts[0].staves[0].measures[0].voices[0][0].tuplet,
            Some(ti)
        );
        apply_command(
            &Command::SetTuplet(SetTupletCmd {
                part_index: 0,
                staff_index: 0,
                measure_index: 0,
                voice_index: 0,
                note_index: 0,
                tuplet: None,
            }),
            &mut score,
        )
        .unwrap();
        assert!(
            score.parts[0].staves[0].measures[0].voices[0][0]
                .tuplet
                .is_none()
        );
    }

    // ── Feature E: RespellScore ───────────────────────────────────────────

    #[test]
    fn respell_score_cmd_changes_all_pitches() {
        use crate::model::pitch::Step;
        let mut score = default_engine_score();
        apply_command(
            &Command::AddNote(AddNoteCmd {
                part_index: 0,
                staff_index: 0,
                measure_index: 0,
                voice: 0,
                position: 0,
                pitch: Some(Pitch::with_alter(Step::C, 4, 1)), // C#4
                duration: Duration::Quarter,
                dot_count: 0,
                is_rest: false,
                tuplet: None,
            }),
            &mut score,
        )
        .unwrap();
        apply_command(
            &Command::RespellScore(RespellScoreCmd { prefer_flat: true }),
            &mut score,
        )
        .unwrap();
        let pitch = &score.parts[0].staves[0].measures[0].voices[0][0].pitches[0];
        assert_eq!(pitch.step, Step::D);
        assert_eq!(pitch.alter, -1); // Db4
    }

    #[test]
    fn typed_spanner_commands_are_atomic_and_undoable() {
        use crate::model::score::{NotationSpanner, NotationSpannerKind};

        let mut score = default_engine_score();
        let address = NoteAddr {
            part: 0,
            staff: 0,
            measure: 0,
            voice: 0,
            note: 0,
        };
        let spanner = NotationSpanner {
            id: "slur-1".to_string(),
            kind: NotationSpannerKind::Slur,
            start: address.clone(),
            end: address,
            number: Some(1),
            line_type: Some("dashed".to_string()),
            text: None,
            placement: Some("above".to_string()),
            ottava_size: None,
            ottava_type: None,
        };
        let mut stack = CommandStack::new(8);
        stack
            .execute(Command::AddSpanner(AddSpannerCmd { spanner }), &mut score)
            .unwrap();
        assert_eq!(score.spanners.len(), 1);
        assert_eq!(stack.undo_key().as_deref(), Some("AddSpanner"));

        let before_duplicate = score.clone();
        let duplicate = score.spanners[0].clone();
        assert!(
            stack
                .execute(
                    Command::AddSpanner(AddSpannerCmd { spanner: duplicate }),
                    &mut score,
                )
                .is_err()
        );
        assert_eq!(score.spanners, before_duplicate.spanners);

        let mut updated = score.spanners[0].clone();
        updated.number = Some(2);
        stack
            .execute(
                Command::UpdateSpanner(UpdateSpannerCmd { spanner: updated }),
                &mut score,
            )
            .unwrap();
        assert_eq!(score.spanners[0].number, Some(2));
        stack.undo(&mut score).unwrap();
        assert_eq!(score.spanners[0].number, Some(1));
        stack.redo(&mut score).unwrap();
        assert_eq!(score.spanners[0].number, Some(2));

        stack
            .execute(
                Command::RemoveSpanner(RemoveSpannerCmd {
                    id: "slur-1".to_string(),
                }),
                &mut score,
            )
            .unwrap();
        assert!(score.spanners.is_empty());
        stack.undo(&mut score).unwrap();
        assert_eq!(score.spanners.len(), 1);
    }

    #[test]
    fn string_quartet_linked_views_roundtrip_and_undo_redo_as_one_contract() {
        let mut score = Score::template(ScoreTemplate::StringQuartet);
        let mut stack = CommandStack::new(16);
        for (part, id, name) in [
            (0, "violin-1", "Violin I"),
            (1, "violin-2", "Violin II"),
            (2, "viola", "Viola"),
            (3, "cello", "Cello"),
        ] {
            stack
                .execute(
                    Command::UpsertScoreView(UpsertScoreViewCmd {
                        view: ScoreView::linked_part(id, name, part),
                    }),
                    &mut score,
                )
                .expect("linked part view applies");
        }
        assert_eq!(score.parts.len(), 4);
        assert_eq!(score.views.len(), 4);
        for (part, view) in score.views.iter().enumerate() {
            assert_eq!(view.parts, vec![part]);
            assert_eq!(score.resolve_view(&view.id).unwrap().parts.len(), 1);
        }

        let restored: Score =
            serde_json::from_str(&serde_json::to_string(&score).expect("quartet score serializes"))
                .expect("quartet score deserializes");
        assert_eq!(restored.views, score.views);

        for _ in 0..4 {
            stack.undo(&mut score).expect("view undo applies");
        }
        assert!(score.views.is_empty());
        for _ in 0..4 {
            stack.redo(&mut score).expect("view redo applies");
        }
        assert_eq!(score.views, restored.views);
    }

    #[test]
    fn section_break_is_atomic_undoable_and_synced_across_staves() {
        let mut score = Score::template(ScoreTemplate::Piano);
        let original = score.clone();
        let mut stack = CommandStack::new(4);
        stack
            .execute(
                Command::SetSectionBreak(SetSectionBreakCmd {
                    measure_index: 2,
                    value: true,
                }),
                &mut score,
            )
            .expect("section break applies");
        assert!(
            score
                .parts
                .iter()
                .flat_map(|part| part.staves.iter())
                .all(|staff| staff.measures[2].section_break)
        );
        stack.undo(&mut score).expect("section break undoes");
        assert_eq!(
            serde_json::to_value(&score).unwrap(),
            serde_json::to_value(&original).unwrap()
        );

        assert!(
            stack
                .execute(
                    Command::SetSectionBreak(SetSectionBreakCmd {
                        measure_index: 99,
                        value: true,
                    }),
                    &mut score,
                )
                .is_err()
        );
        assert_eq!(
            serde_json::to_value(&score).unwrap(),
            serde_json::to_value(&original).unwrap()
        );
    }
}