tix-tracker 0.12.0

//! Confirmed, idempotent future-cycle generation (WP04, T018–T023).
//!
//! # Only a confirmed action writes, and it writes stable keys
//!
//! [`generate`] is the only writer in the cycle-schedule boundary, and it writes
//! only when the caller says so: [`GenerationRequest::confirmed`] must be set, and
//! an unconfirmed request is refused before anything is read or written. That is
//! C-004's "detection is read-only, every mutation is explicit" expressed as a
//! type rather than a convention — [`crate::cycle_schedule::evaluate`] is handed no
//! store and has no way to reach this function.
//!
//! The unit of work is the **stable key** `(schedule-id, sequence)`, handed out by
//! [`ScheduleIndex::next_key`] and never renumbered or invented. The requested batch
//! is the look-ahead the request carries: the next key, then one step of the
//! schedule's own [`RecurrenceRule`] per further candidate — for a request carrying
//! the schedule's own `look_ahead`, exactly the candidates
//! [`crate::cycle_schedule::evaluate`] reports, so a surface generates what its own
//! warning showed it (C-008). One [`GenerationOutcome`] comes back per requested key:
//! `created`, `no-op`, or `rejected`.
//!
//! # Idempotency is a lookup, and the lookup is durable
//!
//! A key that already exists is a **no-op**: nothing is written, nothing is
//! advanced, and the same bytes stay on disk (C-006/NFR-002). A rejection **ends the
//! batch** instead of skipping ahead, so generation can never create a sequence
//! beyond a gap — the same reason a skipped cycle is *recorded* rather than omitted.
//!
//! Existence is decided against the records that are on disk *now*. The passed
//! [`ScheduleIndex`] supplies the key chain — the caller's view of the next
//! candidate — but an index is a value this very operation invalidates, so a second
//! call under the same snapshot would otherwise re-create a key it had already
//! written. The stored records are the authority, which is what makes a repeated
//! call under the same stable snapshot a no-op rather than a duplicate identity.
//!
//! The chain is anchored the same way. Its **start** is the position the records
//! prove — the authoritative index built inside this call — unless the caller's
//! snapshot is *behind* it, in which case the caller's earlier position is kept: the
//! window that snapshot confirmed has already been generated, so its keys report
//! `no-op` instead of the batch jumping forward past candidates the surface never
//! showed (C-004), and any key of that window the records do not hold is filled. In
//! the other direction, a caller whose snapshot is *ahead* of the records — a file
//! removed since — is pulled back to the records, so generation can never create a
//! sequence beyond a hole. The steps between candidates are always shared-rule steps.
//! Neither direction can name an identity twice or skip a sequence.
//!
//! # The write order is the contract's, and this is the reason
//!
//! A creation is two writes: the **instance record** first (through
//! [`CycleStore::save_record`], the path a generated cycle takes), then the
//! **persisted cursor** (through the existing atomic [`Config::save`] seam — no
//! second seam is added).
//!
//! The order is not arbitrary. A crash between the two writes leaves an instance
//! with a stale cursor, which WP02's index recomputes from the instances and reports
//! as `cursor_drift`, and which a re-run handles as a `no-op` for that key. The
//! reverse order would leave a **skipped sequence** — a cursor that has moved past an
//! instance that does not exist — which nothing can reconstruct, and which C-006
//! forbids above a duplicate identity. Recoverable staleness beats unrecoverable
//! loss.
//!
//! And because this process is *alive* when the cursor write merely returns an
//! error, the two writes are also compensated as a unit: a failed cursor write
//! removes the instances this batch just wrote (and the directories the write
//! created), so a *returned* failure is completely inert too — the tree is
//! byte-identical to what it was before the call. A crash gets the first guarantee;
//! a returned error gets both.
//!
//! # The reference label, the path it becomes, and what is not identity
//!
//! An instance's name is `"<schedule name> <start date> <sequence>"` — `Weekly sprint
//! 2026-01-05 000001`. It is a **mutable reference label** an issue's `cycles` value
//! can hold and the normal cycle machinery resolves; the start date makes it readable
//! and sortable, and the zero-padded sequence makes it unique for good. Identity is
//! never the name: it is the generated id `{schedule-id}:{sequence}` (C-003), so
//! renaming a cycle changes what a human reads and nothing else.
//!
//! A label is written to a file whose stem is `slugify(name)`, and `slugify` caps
//! that stem at 80 characters by cutting the **end** — which is exactly where a
//! label's discriminating part lives. So the schedule-name component is truncated
//! *here*, to the documented [`NAME_COMPONENT_BUDGET`] that leaves the canonical date
//! and the sequence intact, and two instances of one schedule can therefore never
//! slug to one path. Because a path is still not an identity, every write also checks
//! the **path** it is about to take against the store's files: a file already
//! occupied by another identity is a `name_collision` refusal — naming the path and
//! both identities — with zero writes, never a silent overwrite that loses a
//! sequence. Likewise a derived *name* that is already taken (by another generated
//! cycle or by a hand-written one) is refused, through the identity index's own
//! resolution.
//!
//! # No issue is touched, and nothing reads a clock
//!
//! Generation moves no issue, rewrites no `## History`, and invents no second
//! grammar: the audit trail for one generation is the instance record plus the
//! cursor's `last_generated_key`, the evidence WP02's identity doc names. The
//! injected [`RuleContext`] is the caller's as-of date — reported on the
//! [`GenerationReport`] and validated (a named or malformed context timezone is
//! refused, never normalized) — and is deliberately **not** used to derive a date:
//! every date comes from the schedule's own anchor through the shared arithmetic
//! owner, so the same schedule and cursor generate the same instances whenever they
//! run (C-009).
//!
//! # One arithmetic owner
//!
//! Cadence steps are [`RecurrenceRule::next_occurrence`] steps, and a created
//! instance's due date is `start_date + duration` computed by the *same* owner: the
//! duration is expressed as a rule anchored at the instance's own start, whose first
//! occurrence strictly after that anchor is the due date. Month-end re-anchoring,
//! leap days and the interval bound therefore stay in one place — and a duration the
//! shared owner refuses (a single step above its bound) is a field-aware refusal,
//! not arithmetic this module invents.

use std::collections::BTreeMap;
use std::path::{Path, PathBuf};

use time::Date;

use crate::config::Config;
use crate::cycle_schedule::config::{CycleScheduleConfiguration, CycleScheduleId, ScheduleState};
use crate::cycle_schedule::identity::{
    CycleReference, GeneratedCycle, GeneratedCycleId, GeneratedCycleState, GenerationCursor,
    NextCycleKey, ScheduleIndex, cadence_rule,
};
use crate::cycle_store::{CycleRecord, CycleStore};
use crate::error::{Result, render_diagnostics};
use crate::recurrence::rule::{RecurrenceRule, RuleContext, RuleKind};

/// The explicit, confirmed intent to generate a schedule's look-ahead (T018).
///
/// Every field is supplied and none is inferred: the schedule, the caller's injected
/// as-of date, how much look-ahead to apply, who asked, and *that they confirmed*. A
/// surface builds one from a warning it showed the user; nothing constructs a
/// confirmed one on the user's behalf.
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct GenerationRequest {
    /// The schedule to generate for, by stable id.
    pub schedule_id: CycleScheduleId,
    /// The caller's injected as-of date (and timezone policy). Never a clock.
    pub context: RuleContext,
    /// How many candidates to apply, starting at [`ScheduleIndex::next_key`].
    ///
    /// `0` requests nothing, which is the coherent reading WP03's evaluator already
    /// fixes: a schedule with no look-ahead has no candidate question to ask.
    pub look_ahead: u32,
    /// Who confirmed the action — the caller's audit context. Generation persists no
    /// History event, so this is reported, not stored.
    pub actor: String,
    /// Whether the caller confirmed the action. An unconfirmed request is inert.
    pub confirmed: bool,
}

impl GenerationRequest {
    /// A request that is **not** yet confirmed: naming a schedule is not consent, so
    /// a caller has to say so explicitly.
    pub fn new(
        schedule_id: CycleScheduleId,
        context: RuleContext,
        look_ahead: u32,
        actor: impl Into<String>,
    ) -> Self {
        Self {
            schedule_id,
            context,
            look_ahead,
            actor: actor.into(),
            confirmed: false,
        }
    }

    /// Record the confirmation.
    #[must_use]
    pub fn confirm(mut self) -> Self {
        self.confirmed = true;
        self
    }
}

/// Why a generation was refused, spelled in `snake_case` for payloads and tests.
///
/// The vocabulary is deliberately shared with the other cycle-schedule diagnostics
/// where the fact is the same (`missing_cadence`, `invalid_cadence`,
/// `unresolvable_date` are WP03's spellings, and `reference_collision` is WP02's), so
/// one refusal reads the same whichever surface produced it (C-008).
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
pub enum GenerationDiagnosticKind {
    /// The request was not confirmed, so nothing was even read.
    NotConfirmed,
    /// No schedule with that id is defined.
    UnknownSchedule,
    /// The schedule is paused or ended, so generation is not allowed.
    ScheduleNotActive,
    /// The schedule names no cadence family, so no candidate can be derived.
    MissingCadence,
    /// The cadence resolves to a rule the shared validator refuses.
    InvalidCadence,
    /// The schedule's display name, which the reference label is built from, is blank.
    InvalidScheduleName,
    /// The duration is missing or a step the shared arithmetic owner refuses.
    InvalidDuration,
    /// The schedule's generation position is past the last usable sequence, so there
    /// is no candidate to create.
    NoCandidate,
    /// A date generation needs — a candidate's start, or a due date — cannot be
    /// derived.
    UnresolvableDate,
    /// The injected date context carries a timezone this crate refuses to normalize.
    InvalidContext,
    /// The reference label the instance would take is already taken.
    ReferenceCollision,
    /// The file the instance would be written to is already occupied by another
    /// identity — the path, not the name, is what a write takes.
    NameCollision,
    /// The instance record could not be written. Nothing else was attempted.
    SchedulingFailed,
    /// The instance was written but the cursor could not be persisted; the batch was
    /// rolled back.
    CursorNotPersisted,
}

impl GenerationDiagnosticKind {
    /// The contract's spelling, used verbatim in payloads and tests.
    pub const fn as_str(self) -> &'static str {
        match self {
            Self::NotConfirmed => "not_confirmed",
            Self::UnknownSchedule => "unknown_schedule",
            Self::ScheduleNotActive => "schedule_not_active",
            Self::MissingCadence => "missing_cadence",
            Self::InvalidCadence => "invalid_cadence",
            Self::InvalidScheduleName => "invalid_schedule_name",
            Self::InvalidDuration => "invalid_duration",
            Self::NoCandidate => "no_candidate",
            Self::UnresolvableDate => "unresolvable_date",
            Self::InvalidContext => "invalid_context",
            Self::ReferenceCollision => "reference_collision",
            Self::NameCollision => "name_collision",
            Self::SchedulingFailed => "scheduling_failure",
            Self::CursorNotPersisted => "cursor_not_persisted",
        }
    }
}

/// One field-aware reason a generation was refused (T022).
///
/// Rendered through the crate's one diagnostic renderer, so the generator, the
/// evaluator, the CLI and the MCP tools spell the same refusal the same way.
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct GenerationDiagnostic {
    /// What was wrong.
    pub kind: GenerationDiagnosticKind,
    /// The configuration or cycle path the caller must fix, e.g.
    /// `cycle_schedules.cs-weekly.duration.count`.
    pub field: String,
    /// One sentence saying what to do about it.
    pub message: String,
}

impl GenerationDiagnostic {
    /// A finding about `field`.
    fn new(
        kind: GenerationDiagnosticKind,
        field: impl Into<String>,
        message: impl Into<String>,
    ) -> Self {
        Self {
            kind,
            field: field.into(),
            message: message.into(),
        }
    }

    /// Render through the crate's one diagnostic renderer.
    #[must_use]
    pub fn render(&self) -> String {
        render_diagnostics(
            "E-tix-cycle-schedule",
            std::iter::once((self.kind.as_str(), self.message.as_str())),
        )
    }
}

/// The result of one confirmed generation attempt for one requested key
/// (`data-model.md` §7, T022).
///
/// Exactly one of three shapes, and each is a *result* rather than an error: a
/// surface reports one outcome per candidate, so a refusal is data it shows instead
/// of an exception it catches.
#[derive(Debug, Clone, PartialEq, Eq)]
pub enum GenerationOutcome {
    /// The instance was created and the cursor advanced past it.
    Created {
        /// The schedule it belongs to.
        schedule_id: CycleScheduleId,
        /// Its stable key.
        key: GeneratedCycleId,
        /// The instance itself: identity, label, effective dates and state.
        cycle: GeneratedCycle,
    },
    /// The key was already taken. Nothing was written and nothing advanced.
    NoOp {
        /// The schedule.
        schedule_id: CycleScheduleId,
        /// The key that was already taken.
        key: GeneratedCycleId,
        /// The lifecycle state of the instance that holds it.
        state: GeneratedCycleState,
        /// One sentence saying which instance holds it.
        message: String,
    },
    /// The key (or the whole request) was refused. Nothing was written.
    Rejected {
        /// The schedule the request named.
        schedule_id: CycleScheduleId,
        /// The key the refusal concerns, when the refusal is about one candidate.
        key: Option<GeneratedCycleId>,
        /// Every reason, in the order they were found.
        diagnostics: Vec<GenerationDiagnostic>,
    },
}

impl GenerationOutcome {
    /// The stable word a surface prints: `created`, `no-op` or `rejected`.
    #[must_use]
    pub fn as_str(&self) -> &'static str {
        match self {
            Self::Created { .. } => "created",
            Self::NoOp { .. } => "no-op",
            Self::Rejected { .. } => "rejected",
        }
    }

    /// Whether an instance was created.
    #[must_use]
    pub fn is_created(&self) -> bool {
        matches!(self, Self::Created { .. })
    }

    /// Whether the key was already taken.
    #[must_use]
    pub fn is_noop(&self) -> bool {
        matches!(self, Self::NoOp { .. })
    }

    /// Whether the key (or the request) was refused.
    #[must_use]
    pub fn is_rejected(&self) -> bool {
        matches!(self, Self::Rejected { .. })
    }

    /// The schedule the outcome belongs to.
    #[must_use]
    pub fn schedule_id(&self) -> &CycleScheduleId {
        match self {
            Self::Created { schedule_id, .. }
            | Self::NoOp { schedule_id, .. }
            | Self::Rejected { schedule_id, .. } => schedule_id,
        }
    }

    /// The stable key, when the outcome concerns one.
    #[must_use]
    pub fn key(&self) -> Option<&GeneratedCycleId> {
        match self {
            Self::Created { key, .. } | Self::NoOp { key, .. } => Some(key),
            Self::Rejected { key, .. } => key.as_ref(),
        }
    }

    /// The created instance, present on `created` only.
    #[must_use]
    pub fn cycle(&self) -> Option<&GeneratedCycle> {
        match self {
            Self::Created { cycle, .. } => Some(cycle),
            _ => None,
        }
    }

    /// The lifecycle state of the instance the outcome is about, when there is one.
    #[must_use]
    pub fn state(&self) -> Option<GeneratedCycleState> {
        match self {
            Self::Created { cycle, .. } => Some(cycle.state),
            Self::NoOp { state, .. } => Some(*state),
            Self::Rejected { .. } => None,
        }
    }

    /// Every diagnostic, empty for `created` and `no-op`.
    #[must_use]
    pub fn diagnostics(&self) -> &[GenerationDiagnostic] {
        match self {
            Self::Rejected { diagnostics, .. } => diagnostics,
            _ => &[],
        }
    }

    /// One line for a surface: what happened, and to what.
    #[must_use]
    pub fn render(&self) -> String {
        match self {
            Self::Created { key, cycle, .. } => format!(
                "created `{key}` (`{}`, {} → {})",
                cycle.name, cycle.start_date, cycle.due_date
            ),
            Self::NoOp { key, message, .. } => format!("no-op for `{key}`: {message}"),
            Self::Rejected {
                key, diagnostics, ..
            } => {
                let first = diagnostics
                    .first()
                    .map(GenerationDiagnostic::render)
                    .unwrap_or_else(|| {
                        format!(
                            "`{}` rejected",
                            key.as_ref().map_or("request", GeneratedCycleId::as_str)
                        )
                    });
                if diagnostics.len() > 1 {
                    format!("{first} (and {} more)", diagnostics.len() - 1)
                } else {
                    first
                }
            }
        }
    }

    /// A per-key refusal.
    fn refused(
        schedule_id: CycleScheduleId,
        key: Option<GeneratedCycleId>,
        diagnostic: GenerationDiagnostic,
    ) -> Self {
        Self::Rejected {
            schedule_id,
            key,
            diagnostics: vec![diagnostic],
        }
    }
}

/// What one confirmed generation pass did (T022).
///
/// Carries the request's audit context (who confirmed it, and the as-of date they
/// supplied), one [`GenerationOutcome`] per requested key in request order, and the
/// schedule's cursor after the pass — the next-generation context a surface shows
/// without re-deriving it.
#[derive(Debug, Clone, PartialEq)]
pub struct GenerationReport {
    actor: String,
    as_of: Date,
    schedule_id: CycleScheduleId,
    outcomes: Vec<GenerationOutcome>,
    cursor: Option<GenerationCursor>,
}

impl GenerationReport {
    /// Who confirmed the pass.
    #[must_use]
    pub fn actor(&self) -> &str {
        &self.actor
    }

    /// The injected as-of date the caller supplied.
    #[must_use]
    pub fn as_of(&self) -> Date {
        self.as_of
    }

    /// The schedule the pass was for.
    #[must_use]
    pub fn schedule_id(&self) -> &CycleScheduleId {
        &self.schedule_id
    }

    /// One outcome per requested key, in request order.
    #[must_use]
    pub fn outcomes(&self) -> &[GenerationOutcome] {
        &self.outcomes
    }

    /// How many outcomes the pass produced.
    #[must_use]
    pub fn len(&self) -> usize {
        self.outcomes.len()
    }

    /// Whether the pass requested no key at all (`look_ahead = 0`).
    #[must_use]
    pub fn is_empty(&self) -> bool {
        self.outcomes.is_empty()
    }

    /// Whether every outcome is a refusal.
    #[must_use]
    pub fn is_refused(&self) -> bool {
        !self.outcomes.is_empty() && self.outcomes.iter().all(GenerationOutcome::is_rejected)
    }

    /// The schedule's cursor after the pass; absent only when the schedule is unknown.
    #[must_use]
    pub fn cursor(&self) -> Option<&GenerationCursor> {
        self.cursor.as_ref()
    }

    /// The `created` outcomes.
    pub fn created(&self) -> impl Iterator<Item = &GenerationOutcome> {
        self.outcomes.iter().filter(|outcome| outcome.is_created())
    }

    /// The `no-op` outcomes.
    pub fn no_ops(&self) -> impl Iterator<Item = &GenerationOutcome> {
        self.outcomes.iter().filter(|outcome| outcome.is_noop())
    }

    /// The `rejected` outcomes.
    pub fn rejected(&self) -> impl Iterator<Item = &GenerationOutcome> {
        self.outcomes.iter().filter(|outcome| outcome.is_rejected())
    }

    /// One line for a surface.
    #[must_use]
    pub fn render(&self) -> String {
        if self.outcomes.is_empty() {
            return format!(
                "{}: nothing to generate for `{}` (no look-ahead requested)",
                self.actor, self.schedule_id
            );
        }
        format!(
            "{}: {} created, {} no-op, {} rejected for `{}`",
            self.actor,
            self.created().count(),
            self.no_ops().count(),
            self.rejected().count(),
            self.schedule_id
        )
    }

    /// A report whose pass was refused before any key could be derived.
    fn refused(
        as_of: Date,
        actor: &str,
        schedule_id: CycleScheduleId,
        diagnostic: GenerationDiagnostic,
    ) -> Self {
        Self::refused_many(as_of, actor, schedule_id, vec![diagnostic])
    }

    /// A report whose pass was refused with every reason it found.
    fn refused_many(
        as_of: Date,
        actor: &str,
        schedule_id: CycleScheduleId,
        diagnostics: Vec<GenerationDiagnostic>,
    ) -> Self {
        Self {
            actor: actor.to_string(),
            as_of,
            outcomes: vec![GenerationOutcome::Rejected {
                schedule_id: schedule_id.clone(),
                key: None,
                diagnostics,
            }],
            schedule_id,
            cursor: None,
        }
    }
}

/// Generate the look-ahead `request` names, if it is confirmed and still free.
///
/// Returns a report rather than an error for every *decision* the operation makes;
/// only a genuine I/O failure while reading the stores propagates. `config` is the
/// caller's registry and is mutated only after the cursor has been persisted, so a
/// failed pass leaves the in-memory configuration exactly as it was.
///
/// `root` is the project root: it locates the cycle records (`docs/cycles`) and the
/// `tix.toml` the cursor is persisted through. `index` is the caller's already-built
/// read model ([`ScheduleIndex`]) — it supplies the next key, and the durable records
/// decide whether that key is free.
#[allow(clippy::too_many_lines)]
pub fn generate(
    config: &mut Config,
    root: &Path,
    index: &ScheduleIndex,
    request: &GenerationRequest,
) -> Result<GenerationReport> {
    let as_of = request.context.today;
    let schedule_id = request.schedule_id.clone();

    // T018/C-004: consent first. An unconfirmed request touches nothing at all — not
    // the record store, not the configuration, not one byte.
    if !request.confirmed {
        return Ok(GenerationReport::refused(
            as_of,
            &request.actor,
            schedule_id.clone(),
            GenerationDiagnostic::new(
                GenerationDiagnosticKind::NotConfirmed,
                format!("cycle_schedules.{schedule_id}"),
                "generation must be confirmed: detection reports a candidate, a person or a \
                 caller decides, and only then is anything written",
            ),
        ));
    }

    // The injected context is the caller's as-of date. A timezone this crate refuses
    // to resolve is reported rather than normalized (C-005/C-009) — but note the
    // context is *not* used to derive a date: generation is anchored at the
    // schedule's own dates, so the same schedule and cursor produce the same
    // instances whenever they run.
    if let Some(timezone) = request.context.timezone.as_ref()
        && !timezone.is_supported()
    {
        return Ok(GenerationReport::refused(
            as_of,
            &request.actor,
            schedule_id.clone(),
            GenerationDiagnostic::new(
                GenerationDiagnosticKind::InvalidContext,
                format!("cycle_schedules.{schedule_id}.timezone"),
                format!(
                    "the injected date context carries `{timezone}`, which is not a fixed UTC \
                     offset; generation will not normalize an ambiguous zone"
                ),
            ),
        ));
    }

    // T019: every stale or incomplete request is refused before a write, with the
    // field that has to change.
    let Some(record) = index.schedule(&schedule_id) else {
        return Ok(GenerationReport::refused(
            as_of,
            &request.actor,
            schedule_id.clone(),
            GenerationDiagnostic::new(
                GenerationDiagnosticKind::UnknownSchedule,
                format!("cycle_schedules.{schedule_id}"),
                format!("no cycle schedule `{schedule_id}` is defined"),
            ),
        ));
    };
    if record.state != ScheduleState::Active {
        let fix = if record.state.is_terminal() {
            "an ended schedule is terminal; generation is refused for good"
        } else {
            "resume it explicitly before generating"
        };
        return Ok(GenerationReport::refused(
            as_of,
            &request.actor,
            schedule_id.clone(),
            GenerationDiagnostic::new(
                GenerationDiagnosticKind::ScheduleNotActive,
                format!("cycle_schedules.{schedule_id}.state"),
                format!(
                    "schedule `{schedule_id}` is {}; {fix}",
                    record.state.as_str()
                ),
            ),
        ));
    }
    let Some(rule) = cadence_rule(record) else {
        return Ok(GenerationReport::refused(
            as_of,
            &request.actor,
            schedule_id.clone(),
            GenerationDiagnostic::new(
                GenerationDiagnosticKind::MissingCadence,
                format!("cycle_schedules.{schedule_id}.cadence.kind"),
                format!(
                    "schedule `{schedule_id}` names no cadence family, so no candidate can be \
                     derived"
                ),
            ),
        ));
    };
    if let Err(error) = rule.validate() {
        let diagnostics = error
            .diagnostics()
            .iter()
            .map(|diagnostic| {
                GenerationDiagnostic::new(
                    GenerationDiagnosticKind::InvalidCadence,
                    format!(
                        "cycle_schedules.{schedule_id}.{}",
                        cadence_field(diagnostic.field)
                    ),
                    diagnostic.message.clone(),
                )
            })
            .collect();
        return Ok(GenerationReport::refused_many(
            as_of,
            &request.actor,
            schedule_id.clone(),
            diagnostics,
        ));
    }
    if record.name.trim().is_empty() {
        return Ok(GenerationReport::refused(
            as_of,
            &request.actor,
            schedule_id.clone(),
            GenerationDiagnostic::new(
                GenerationDiagnosticKind::InvalidScheduleName,
                format!("cycle_schedules.{schedule_id}.name"),
                format!(
                    "schedule `{schedule_id}` has a blank display name, so no reference label can \
                     be derived for an instance"
                ),
            ),
        ));
    }
    // `None` is the read model's own answer for an ended schedule and for one whose
    // sequence space is used up: there is no candidate, so there is nothing to
    // generate. The *records* are asked, not the caller's snapshot, so the position is
    // the one the project actually holds.
    let store = CycleStore::new(root);
    let config_path = root.join("tix.toml");
    let durable = ScheduleIndex::build(config, &store.list()?);
    // The paths the store already holds, by filename: a write takes a path, and a path
    // is not an identity (two names can slug to one stem).
    let mut files = stored_files(&store)?;
    let Some(durable_next) = durable.next_key(&schedule_id) else {
        return Ok(GenerationReport::refused(
            as_of,
            &request.actor,
            schedule_id.clone(),
            GenerationDiagnostic::new(
                GenerationDiagnosticKind::NoCandidate,
                format!("cycle_schedules.{schedule_id}.cursor"),
                format!(
                    "schedule `{schedule_id}` has no next candidate: its generation position is \
                     past the last sequence the identity's fixed width can hold"
                ),
            ),
        ));
    };
    let cursor_before = durable.cursor(&schedule_id);
    // The directories the instance write will create, so a rolled-back batch can
    // remove them again and leave no trace at all.
    let missing_dirs = missing_ancestors(store.dir());

    // T019: the chain is anchored at the authoritative position. A caller whose
    // snapshot is *ahead* of the records (a file was removed since) is pulled back to
    // the records, so generation can never create a sequence beyond a hole; a caller
    // whose snapshot is *behind* keeps its earlier position, where the keys the records
    // already hold are no-ops and any key they do not is filled — the batch never
    // replaces the confirmed window with a later one the surface did not show (C-004).
    let first = match index.next_key(&schedule_id) {
        Some(caller_next) if caller_next.sequence < durable_next.sequence => caller_next,
        _ => durable_next,
    };

    let keys = candidate_keys(&rule, &first, request.look_ahead);
    let mut outcomes: Vec<GenerationOutcome> = Vec::new();
    let mut created: Vec<usize> = Vec::new();

    for key in keys {
        let id = key.id.clone();
        // T019: the stable key decides, and the *records* decide it. A key that exists
        // is a no-op; a key only a stale snapshot remembers — whose file is gone — is
        // not, which is the same "the snapshot is ahead of the records" case the chain
        // anchor handles.
        if let Some(existing) = durable.cycle(&id) {
            outcomes.push(GenerationOutcome::NoOp {
                schedule_id: schedule_id.clone(),
                key: id,
                state: existing.state,
                message: format!(
                    "`{}` already holds it (`{}`); the stable key is idempotent and nothing was \
                     written",
                    existing.name,
                    existing.state.as_str()
                ),
            });
            continue;
        }
        // T020: a candidate needs a date. Without one there is nothing to name, and
        // the batch stops rather than skipping ahead over a gap.
        let Some(start_date) = key.start_date.clone() else {
            outcomes.push(GenerationOutcome::refused(
                schedule_id.clone(),
                Some(id.clone()),
                GenerationDiagnostic::new(
                    GenerationDiagnosticKind::UnresolvableDate,
                    format!("cycle_schedules.{schedule_id}.cadence"),
                    format!(
                        "candidate `{id}` has no resolvable start date: the schedule's cadence \
                         produced none from the instance it steps from"
                    ),
                ),
            ));
            break;
        };
        // The instance's label must resolve to it and to nothing else (T010).
        let name = reference_name(&record.name, &start_date, key.sequence);
        if !matches!(durable.resolve(&name), CycleReference::Unknown { .. }) {
            outcomes.push(GenerationOutcome::refused(
                schedule_id.clone(),
                Some(id.clone()),
                GenerationDiagnostic::new(
                    GenerationDiagnosticKind::ReferenceCollision,
                    format!("cycles.{name}"),
                    format!(
                        "the reference label `{name}` an issue would hold already answers to \
                         another cycle; rename that cycle or the schedule before generating"
                    ),
                ),
            ));
            break;
        }
        // ...and the *path* it takes must not belong to another identity either. Two
        // different names can slugify to one stem, so the name check above is not
        // enough: without this, the second key of a batch could overwrite the first
        // and the record would survive in neither.
        //
        // `files` is built from what the record store could *read*, and a write is an
        // unconditional replace — so the one occupant the map cannot see (a file with
        // unparseable frontmatter, an interrupted write, a stray non-cycle `.md`) is
        // exactly the one a write would silently destroy. The path itself is therefore
        // asked too, and an unreadable occupant is a refusal, not a free slot.
        let filename = instance_filename(&name);
        let occupant = files.get(&filename);
        let path_taken = occupant.is_some() || store.dir().join(&filename).exists();
        if path_taken {
            let holds_this_key = occupant
                .and_then(|record| GeneratedCycle::from_record(record).ok().flatten())
                .filter(|existing| existing.id == id);
            if let Some(existing) = holds_this_key {
                // The path is already *this* key's record: the key is taken, so this is
                // a no-op like any other existing key — never a rewrite.
                outcomes.push(GenerationOutcome::NoOp {
                    schedule_id: schedule_id.clone(),
                    key: id,
                    state: existing.state,
                    message: format!(
                        "`docs/cycles/{filename}` already holds this key (`{}`); nothing was \
                         written",
                        existing.state.as_str()
                    ),
                });
                continue;
            }
            let other = match occupant {
                Some(record) => match GeneratedCycle::from_record(record).ok().flatten() {
                    Some(existing) => format!("`{}`", existing.id),
                    None => "a hand-written cycle".to_string(),
                },
                None => format!(
                    "a file `docs/cycles/{filename}` this build cannot read as a cycle record"
                ),
            };
            outcomes.push(GenerationOutcome::refused(
                schedule_id.clone(),
                Some(id.clone()),
                GenerationDiagnostic::new(
                    GenerationDiagnosticKind::NameCollision,
                    format!("docs/cycles/{filename}"),
                    format!(
                        "`{id}` and {other} would be written to the same file \
                         `docs/cycles/{filename}`; that can only lose one of them, so \
                         nothing was written — move the file that is already there"
                    ),
                ),
            ));
            break;
        }
        let due_date = match due_date(&schedule_id, record, &start_date) {
            Ok(due_date) => due_date,
            Err(diagnostic) => {
                outcomes.push(GenerationOutcome::refused(
                    schedule_id.clone(),
                    Some(id.clone()),
                    diagnostic,
                ));
                break;
            }
        };
        let cycle = GeneratedCycle {
            id: id.clone(),
            schedule_id: schedule_id.clone(),
            sequence: key.sequence,
            name,
            start_date,
            due_date,
            state: GeneratedCycleState::Active,
        };
        // T021, first half: the instance first, through the one generated-cycle write
        // path. A failure here means nothing else is attempted — the cursor must not
        // move for an instance that does not exist.
        match store.save_record(&cycle.to_record(), None) {
            Ok(()) => {
                created.push(outcomes.len());
                // The path is taken now: a later key of this batch that computed the
                // same filename would be a collision, never a silent overwrite.
                files.insert(filename, cycle.to_record());
                outcomes.push(GenerationOutcome::Created {
                    schedule_id: schedule_id.clone(),
                    key: id,
                    cycle,
                });
            }
            Err(error) => {
                outcomes.push(GenerationOutcome::refused(
                    schedule_id.clone(),
                    Some(id.clone()),
                    GenerationDiagnostic::new(
                        GenerationDiagnosticKind::SchedulingFailed,
                        format!("cycles.{id}"),
                        format!(
                            "the instance record for `{id}` could not be written: {error}; the \
                             cursor was not advanced and nothing else was attempted"
                        ),
                    ),
                ));
                break;
            }
        }
    }

    // T021, second half: the cursor advances only after successful confirmed work,
    // and only to the position the instances now prove.
    let cursor = if created.is_empty() {
        cursor_before
    } else {
        let after = ScheduleIndex::build(config, &store.list()?);
        let cursor = after.cursor(&schedule_id);
        let previous = config
            .cycle_schedules
            .get(schedule_id.as_str())
            .and_then(|record| record.cursor.clone());
        if let Some(record) = config.cycle_schedules.get_mut(schedule_id.as_str()) {
            record.cursor = Some(cursor.clone());
        }
        match config.save(&config_path) {
            Ok(()) => cursor,
            Err(error) => {
                // Alive, so the batch is compensated as a unit: restore the in-memory
                // cursor, remove the instances this pass wrote, and report every one of
                // them as refused. (The instance-first order above is what protects a
                // *crash*; this is what protects a returned error.)
                if let Some(record) = config.cycle_schedules.get_mut(schedule_id.as_str()) {
                    record.cursor = previous;
                }
                let keys: Vec<String> = created
                    .iter()
                    .filter_map(|index| outcomes[*index].key().map(GeneratedCycleId::to_string))
                    .collect();
                let rollback = roll_back(&store, &outcomes, &created, &missing_dirs);
                let message = match rollback {
                    None => format!(
                        "the generation cursor for `{schedule_id}` could not be persisted through \
                         `{}`: {error}; {} was rolled back, so the project is exactly as it was \
                         and a retry starts from the same key",
                        config_path.display(),
                        keys.join(", ")
                    ),
                    Some(failures) => format!(
                        "the generation cursor for `{schedule_id}` could not be persisted through \
                         `{}`: {error}; rolling back {} also failed ({failures}), so those \
                         instances remain and a re-run will report them as a no-op while the \
                         index reports `cursor_drift`",
                        config_path.display(),
                        keys.join(", ")
                    ),
                };
                for index in &created {
                    outcomes[*index] = GenerationOutcome::refused(
                        schedule_id.clone(),
                        None,
                        GenerationDiagnostic::new(
                            GenerationDiagnosticKind::CursorNotPersisted,
                            format!("cycle_schedules.{schedule_id}.cursor"),
                            message.clone(),
                        ),
                    );
                }
                cursor_before
            }
        }
    };

    Ok(GenerationReport {
        actor: request.actor.clone(),
        as_of,
        schedule_id,
        outcomes,
        cursor: Some(cursor),
    })
}

/// `slugify`'s cap on a filename stem, and therefore the longest stem a name can
/// produce.
const FILENAME_STEM_LIMIT: usize = 80;

/// The length of a canonical `YYYY-MM-DD` date, which the shared arithmetic owner
/// always produces.
const CANONICAL_DATE_LEN: usize = 10;

/// How much of a schedule's display name a reference label keeps.
///
/// The label becomes a filename stem through `slugify`, which caps the stem at
/// [`FILENAME_STEM_LIMIT`] characters by cutting the **end** — and the end is the
/// part that tells two instances apart. Truncating the schedule-name component
/// *here* to a budget that leaves the canonical date and the zero-padded sequence
/// intact makes that cut impossible: the whole label slugs to at most
/// `62 + 1 + 10 + 1 + 6 = 80` characters, whatever the schedule is called. (A char of
/// the label contributes at most one byte to the stem, and separator runs collapse,
/// so the stem can never be longer than the label.)
const NAME_COMPONENT_BUDGET: usize =
    FILENAME_STEM_LIMIT - 1 - CANONICAL_DATE_LEN - 1 - GeneratedCycleId::SEQUENCE_WIDTH;

/// The reference label an issue's `cycles` value holds for a generated instance.
///
/// `"<schedule name> <start date> <sequence>"` — `Weekly sprint 2026-01-05 000001`.
/// The date is the human part (canonical, so labels sort chronologically); the
/// zero-padded sequence is the identity's own discriminator, so even two cycles that
/// somehow shared a start date could never share a label. The schedule-name component
/// is truncated to [`NAME_COMPONENT_BUDGET`] characters first, which is what keeps
/// the label's slug inside `slugify`'s cap (see that constant). It is only ever a
/// *label*: the identity is the generated id `{schedule-id}:{sequence}` (C-003).
fn reference_name(schedule_name: &str, start_date: &str, sequence: u32) -> String {
    let component: String = schedule_name
        .trim()
        .chars()
        .take(NAME_COMPONENT_BUDGET)
        .collect();
    let component = component.trim_end();
    format!(
        "{component} {start_date} {sequence:0width$}",
        width = GeneratedCycleId::SEQUENCE_WIDTH
    )
}

/// The file `CycleStore::save_record` will write `name` to.
///
/// Computed exactly the way the store computes it (`slugify(name) + ".md"`), so the
/// collision preflight asks about the path the write will actually take rather than
/// making a second guess at it.
fn instance_filename(name: &str) -> String {
    format!("{}.md", crate::slug::slugify(name))
}

/// Every stored cycle record, keyed by the filename it lives in.
///
/// A [`ScheduleIndex`] is keyed by identity and by name; a write takes a **path**, and
/// neither key is one. Two different names can slugify to one stem, so the path is
/// checked directly — that is the only view from which a clobber is visible.
fn stored_files(store: &CycleStore) -> Result<BTreeMap<String, CycleRecord>> {
    Ok(crate::records::RecordStore::new(store.dir())
        .list::<CycleRecord>()?
        .into_iter()
        .map(|record| (record.filename, record.value))
        .collect())
}

/// The requested candidate keys: the next key, then one shared-rule step each.
///
/// `look_ahead` counts the candidates, matching WP03's evaluator so a surface
/// generates what its own warning listed. A step that the calendar cannot resolve
/// leaves the following key without a date; the key is still returned so the caller
/// can report the refusal at that key, and the batch stops there.
fn candidate_keys(
    rule: &RecurrenceRule,
    first: &NextCycleKey,
    look_ahead: u32,
) -> Vec<NextCycleKey> {
    let mut keys = Vec::new();
    let mut date = first.start_date.clone();
    for sequence in (first.sequence..).take(look_ahead as usize) {
        if sequence >= GeneratedCycleId::SEQUENCE_LIMIT {
            break;
        }
        keys.push(NextCycleKey {
            schedule_id: first.schedule_id.clone(),
            sequence,
            id: GeneratedCycleId::new(&first.schedule_id, sequence),
            start_date: date.clone(),
        });
        let Some(start) = date else {
            break;
        };
        date = next_start(rule, &start);
    }
    keys
}

/// One cadence step after `from`, through the shared arithmetic owner.
fn next_start(rule: &RecurrenceRule, from: &str) -> Option<String> {
    let date = as_date(from)?;
    rule.next_occurrence(&RuleContext::on(date))
        .ok()
        .map(|key| key.as_str().to_string())
}

/// A created instance's due date: `start_date + duration`, through the shared owner.
///
/// The duration is expressed as the rule the shared owner already understands — a
/// step of `count` `unit`s anchored at the instance's own start — and its first
/// occurrence strictly after that anchor *is* the due date. Month-end re-anchoring
/// and leap days are therefore the shared implementation's, not this module's; a step
/// the owner refuses (a single step above its bound) is reported at the field the
/// user wrote.
fn due_date(
    schedule_id: &CycleScheduleId,
    record: &CycleScheduleConfiguration,
    start_date: &str,
) -> std::result::Result<String, GenerationDiagnostic> {
    let field = format!("cycle_schedules.{schedule_id}.duration");
    let Some(unit) = record.duration.unit else {
        return Err(GenerationDiagnostic::new(
            GenerationDiagnosticKind::InvalidDuration,
            format!("{field}.unit"),
            "duration.unit: how long one cycle lasts is not configured; give `days`, `weeks` or \
             `months`",
        ));
    };
    if record.duration.count == 0 {
        return Err(GenerationDiagnostic::new(
            GenerationDiagnosticKind::InvalidDuration,
            format!("{field}.count"),
            "duration.count: a cycle must last at least one unit",
        ));
    }
    let Some(start) = as_date(start_date) else {
        return Err(GenerationDiagnostic::new(
            GenerationDiagnosticKind::UnresolvableDate,
            format!("{field}.count"),
            format!(
                "the candidate's start date `{start_date}` is not a `YYYY-MM-DD` calendar date, \
                 so its duration cannot be added"
            ),
        ));
    };
    let rule = RecurrenceRule {
        kind: RuleKind::Interval,
        interval: Some(record.duration.count),
        anchor: Some(start_date.to_string()),
        timezone: record.timezone.clone(),
        expression: None,
        unit: Some(unit),
        extensions: BTreeMap::new(),
    };
    rule.next_occurrence(&RuleContext::on(start))
        .map(|due| due.as_str().to_string())
        .map_err(|error| {
            let first = error.first();
            GenerationDiagnostic::new(
                GenerationDiagnosticKind::InvalidDuration,
                format!("{field}.{}", duration_field(first.field)),
                first.message.clone(),
            )
        })
}

/// A rule field name, as the schedule configuration spells it.
///
/// The rule's `anchor` is the schedule's `start_date` and its `timezone` is the
/// schedule's own key, which is the same translation WP03's evaluator applies to a
/// cadence finding — repeated here because that helper is private to its module.
fn cadence_field(field: &str) -> String {
    match field {
        "anchor" => "start_date".to_string(),
        "timezone" => "timezone".to_string(),
        other => format!("cadence.{other}"),
    }
}

/// A rule field name, as a duration is spelled in the configuration.
///
/// The duration's rule carries the instance's start date as its anchor, so a finding
/// about that anchor is a finding about the instance's own date; its step is the
/// duration's `count`.
fn duration_field(field: &str) -> String {
    match field {
        "interval" => "count".to_string(),
        "anchor" => "start_date".to_string(),
        "timezone" => "timezone".to_string(),
        other => other.to_string(),
    }
}

/// A canonical `YYYY-MM-DD` date as the shared arithmetic owner's value.
///
/// The rule that a stored date is a real calendar date belongs to
/// [`crate::model::validate_date`]; this is the string→value conversion
/// [`RuleContext`] needs. The other cycle-schedule modules each keep their own copy
/// private for the same reason — the crate enables no `time` parsing feature — so it
/// is repeated here rather than re-exported.
fn as_date(value: &str) -> Option<Date> {
    if !crate::model::validate_date(value) {
        return None;
    }
    let year: i32 = value.get(0..4)?.parse().ok()?;
    let month: u8 = value.get(5..7)?.parse().ok()?;
    let day: u8 = value.get(8..10)?.parse().ok()?;
    Date::from_calendar_date(year, time::Month::try_from(month).ok()?, day).ok()
}

/// The directories below `dir` that do not exist yet, deepest first.
///
/// A write creates its directory chain on demand, so a rolled-back batch has to
/// remove the chain it created as well as the files in it, or it would leave a
/// (now empty) `docs/cycles/` behind that changes what the next `Config::save` sees.
fn missing_ancestors(dir: &Path) -> Vec<PathBuf> {
    let mut missing = Vec::new();
    let mut current = Some(dir.to_path_buf());
    while let Some(path) = current {
        if path.exists() {
            break;
        }
        missing.push(path.clone());
        current = path.parent().map(Path::to_path_buf);
    }
    missing
}

/// Remove the instances this batch wrote, and the directories it created.
///
/// Returns `None` when the compensation was complete, or the names it could not
/// remove — a compensation that itself failed must be reported, not hidden.
fn roll_back(
    store: &CycleStore,
    outcomes: &[GenerationOutcome],
    created: &[usize],
    missing_dirs: &[PathBuf],
) -> Option<String> {
    let mut failures = Vec::new();
    for index in created {
        let Some(cycle) = outcomes[*index].cycle() else {
            continue;
        };
        if let Err(error) = store.delete(&cycle.name) {
            failures.push(format!("`{}`: {error}", cycle.name));
        }
    }
    for dir in missing_dirs {
        let empty = std::fs::read_dir(dir)
            .map(|mut entries| entries.next().is_none())
            .unwrap_or(false);
        if empty {
            let _ = std::fs::remove_dir(dir);
        }
    }
    if failures.is_empty() {
        None
    } else {
        Some(failures.join("; "))
    }
}

#[cfg(test)]
mod tests {
    #[allow(clippy::wildcard_imports)]
    use super::*;
    use crate::cycle_schedule::config::{Cadence, CycleDuration, CycleScheduleExtensions};
    use crate::recurrence::rule::{IntervalUnit, RuleDiagnostic, RuleDiagnosticKind, Timezone};

    fn schedule_id() -> CycleScheduleId {
        CycleScheduleId::new("cs-weekly").unwrap()
    }

    fn schedule(duration: CycleDuration) -> CycleScheduleConfiguration {
        CycleScheduleConfiguration {
            name: "Weekly sprint".to_string(),
            cadence: Cadence::new(RuleKind::Weekly),
            duration,
            start_date: "2026-01-05".to_string(),
            timezone: Some(Timezone::parse("UTC")),
            look_ahead: 2,
            state: ScheduleState::Active,
            cursor: None,
            extensions: CycleScheduleExtensions::default(),
        }
    }

    #[test]
    fn a_request_is_unconfirmed_until_it_says_so() {
        let context =
            RuleContext::on(Date::from_calendar_date(2026, time::Month::January, 5).unwrap());
        let request = GenerationRequest::new(schedule_id(), context.clone(), 2, "alice");
        assert!(!request.confirmed, "naming a schedule is not consent");
        let confirmed = request.clone().confirm();
        assert!(confirmed.confirmed);
        assert_eq!(confirmed.actor, "alice");
        assert_eq!(confirmed.look_ahead, 2);
        assert_eq!(confirmed.context, context);
    }

    #[test]
    fn the_reference_label_keeps_its_discriminating_suffix_through_any_truncation() {
        assert_eq!(
            reference_name("Weekly sprint", "2026-01-05", 1),
            "Weekly sprint 2026-01-05 000001"
        );
        assert!(
            reference_name("Weekly sprint", "2026-01-05", 1)
                < reference_name("Weekly sprint", "2026-01-12", 2),
            "a canonical date keeps the labels in chronological order"
        );

        // The longest possible schedule name cannot push the date or the sequence out
        // of the stem `slugify` derives. That is the property that keeps two instances
        // of one schedule on two paths, whatever the schedule is called.
        let long = "x".repeat(500);
        let first = reference_name(&long, "2026-01-05", 1);
        let second = reference_name(&long, "2026-01-12", 2);
        assert_eq!(crate::slug::slugify(&first).len(), FILENAME_STEM_LIMIT);
        assert_ne!(crate::slug::slugify(&first), crate::slug::slugify(&second));
        assert!(instance_filename(&first).ends_with("-2026-01-05-000001.md"));
        assert!(instance_filename(&second).ends_with("-2026-01-12-000002.md"));
        assert_eq!(
            instance_filename(&first),
            instance_filename(&first),
            "the path a name becomes is deterministic"
        );
        for sequence in [1, 999_999] {
            let name = reference_name(&long, "2026-01-05", sequence);
            assert!(crate::slug::slugify(&name).len() <= FILENAME_STEM_LIMIT);
            assert!(instance_filename(&name).contains(&format!("{sequence:06}")));
        }
    }

    #[test]
    fn every_outcome_has_one_stable_word() {
        let id = GeneratedCycleId::new(&schedule_id(), 1);
        let created = GenerationOutcome::Created {
            schedule_id: schedule_id(),
            key: id.clone(),
            cycle: GeneratedCycle {
                id: id.clone(),
                schedule_id: schedule_id(),
                sequence: 1,
                name: "Weekly sprint 2026-01-05".to_string(),
                start_date: "2026-01-05".to_string(),
                due_date: "2026-01-12".to_string(),
                state: GeneratedCycleState::Active,
            },
        };
        assert_eq!(created.as_str(), "created");
        assert!(created.is_created() && created.cycle().is_some());
        assert_eq!(created.key(), Some(&id));
        assert!(created.render().contains("2026-01-12"));

        let noop = GenerationOutcome::NoOp {
            schedule_id: schedule_id(),
            key: id.clone(),
            state: GeneratedCycleState::Skipped,
            message: "already holds it".to_string(),
        };
        assert_eq!(noop.as_str(), "no-op");
        assert!(noop.is_noop());
        assert_eq!(noop.state(), Some(GeneratedCycleState::Skipped));

        let refused = GenerationOutcome::refused(
            schedule_id(),
            Some(id.clone()),
            GenerationDiagnostic::new(
                GenerationDiagnosticKind::ReferenceCollision,
                "cycles.x",
                "already taken",
            ),
        );
        assert_eq!(refused.as_str(), "rejected");
        assert!(refused.is_rejected());
        assert!(refused.render().contains("reference_collision"));
    }

    #[test]
    fn the_diagnostic_kinds_are_the_closed_vocabulary() {
        let spellings: Vec<&str> = [
            GenerationDiagnosticKind::NotConfirmed,
            GenerationDiagnosticKind::UnknownSchedule,
            GenerationDiagnosticKind::ScheduleNotActive,
            GenerationDiagnosticKind::MissingCadence,
            GenerationDiagnosticKind::InvalidCadence,
            GenerationDiagnosticKind::InvalidScheduleName,
            GenerationDiagnosticKind::InvalidDuration,
            GenerationDiagnosticKind::NoCandidate,
            GenerationDiagnosticKind::UnresolvableDate,
            GenerationDiagnosticKind::InvalidContext,
            GenerationDiagnosticKind::ReferenceCollision,
            GenerationDiagnosticKind::NameCollision,
            GenerationDiagnosticKind::SchedulingFailed,
            GenerationDiagnosticKind::CursorNotPersisted,
        ]
        .iter()
        .map(|kind| kind.as_str())
        .collect();
        assert_eq!(
            spellings,
            vec![
                "not_confirmed",
                "unknown_schedule",
                "schedule_not_active",
                "missing_cadence",
                "invalid_cadence",
                "invalid_schedule_name",
                "invalid_duration",
                "no_candidate",
                "unresolvable_date",
                "invalid_context",
                "reference_collision",
                "name_collision",
                "scheduling_failure",
                "cursor_not_persisted",
            ]
        );
    }

    #[test]
    fn a_due_date_is_the_shared_owners_step_including_month_end_and_leap_days() {
        // 2026-01-31 + 1 month: the shared owner re-anchors on February's last day.
        let monthly = schedule(CycleDuration::new(1, IntervalUnit::Months));
        assert_eq!(
            due_date(&schedule_id(), &monthly, "2026-01-31").unwrap(),
            "2026-02-28"
        );
        // A leap day anchor keeps the day-of-month when the target month has it.
        assert_eq!(
            due_date(&schedule_id(), &monthly, "2024-02-29").unwrap(),
            "2024-03-29"
        );
        // Two weeks is exactly fourteen days.
        let fortnight = schedule(CycleDuration::new(2, IntervalUnit::Weeks));
        assert_eq!(
            due_date(&schedule_id(), &fortnight, "2026-01-05").unwrap(),
            "2026-01-19"
        );
    }

    #[test]
    fn a_duration_the_shared_owner_refuses_is_a_field_aware_refusal_at_the_users_field() {
        // A single step above the owner's bound is refused, and the finding is named
        // for the configuration field a user wrote — never approximated by this module.
        let huge = schedule(CycleDuration::new(1001, IntervalUnit::Weeks));
        let error = due_date(&schedule_id(), &huge, "2026-01-05").unwrap_err();
        assert_eq!(error.kind, GenerationDiagnosticKind::InvalidDuration);
        assert_eq!(error.field, "cycle_schedules.cs-weekly.duration.count");
    }

    #[test]
    fn a_missing_duration_unit_and_a_zero_count_are_named_where_they_are_written() {
        let broken = schedule(CycleDuration {
            count: 1,
            unit: None,
            extensions: CycleScheduleExtensions::default(),
        });
        let error = due_date(&schedule_id(), &broken, "2026-01-05").unwrap_err();
        assert_eq!(error.field, "cycle_schedules.cs-weekly.duration.unit");

        let zero = schedule(CycleDuration {
            count: 0,
            unit: Some(IntervalUnit::Weeks),
            extensions: CycleScheduleExtensions::default(),
        });
        let error = due_date(&schedule_id(), &zero, "2026-01-05").unwrap_err();
        assert_eq!(error.field, "cycle_schedules.cs-weekly.duration.count");
    }

    #[test]
    fn rule_fields_are_translated_to_the_configuration_fields_they_came_from() {
        assert_eq!(cadence_field("anchor"), "start_date");
        assert_eq!(cadence_field("interval"), "cadence.interval");
        assert_eq!(duration_field("interval"), "count");
        assert_eq!(duration_field("unit"), "unit");

        // The translation is what turns a shared-owner finding into a path a user can
        // find in `tix.toml`.
        let diagnostic = RuleDiagnostic {
            kind: RuleDiagnosticKind::InvalidAnchor,
            field: "anchor",
            value: "2026-02-30".to_string(),
            message: "anchor: not a real date".to_string(),
        };
        assert_eq!(
            cadence_field(diagnostic.field),
            "start_date",
            "a cadence anchor is the schedule's start_date"
        );
    }

    #[test]
    fn the_candidate_batch_is_the_next_key_then_shared_rule_steps() {
        let record = schedule(CycleDuration::new(1, IntervalUnit::Weeks));
        let rule = cadence_rule(&record).unwrap();
        let first = NextCycleKey {
            schedule_id: schedule_id(),
            sequence: 1,
            id: GeneratedCycleId::new(&schedule_id(), 1),
            start_date: Some("2026-01-05".to_string()),
        };
        let keys = candidate_keys(&rule, &first, 3);
        let dates: Vec<Option<String>> = keys.iter().map(|key| key.start_date.clone()).collect();
        assert_eq!(
            dates,
            vec![
                Some("2026-01-05".to_string()),
                Some("2026-01-12".to_string()),
                Some("2026-01-19".to_string()),
            ]
        );
        assert_eq!(keys[2].id.as_str(), "cs-weekly:000003");
        assert_eq!(
            candidate_keys(&rule, &first, 0),
            [] as [crate::cycle_schedule::identity::NextCycleKey; 0]
        );
    }

    #[test]
    fn missing_ancestors_stops_at_the_first_directory_that_exists() {
        let missing = super::missing_ancestors(Path::new("/definitely/not/a/real/path/cycles"));
        assert_eq!(
            missing.first().map(PathBuf::as_path),
            Some(Path::new("/definitely/not/a/real/path/cycles"))
        );
        assert!(missing.len() > 1, "every missing ancestor is listed");
        assert!(
            missing.len() < 8,
            "the walk stops at the first existing directory"
        );
    }
}