tix-tracker 0.10.0

//! The time mutation service (mission `per-issue-time-tracking-01M3C9A4`, WP02
//! T007–T012).
//!
//! Three intents over one issue's time entries — **create**, **edit**, **delete** —
//! behind one API that every later surface (CLI, TUI, MCP, portable) calls. The
//! *current projection* is the issue's `time_entries` list; the **History** is
//! append-only, and an applied mutation appends **exactly one** event naming the
//! entry and what changed (C-003, C-004, NFR-002).
//!
//! # Two halves, so a refusal cannot write
//!
//! [`intent`] is the pure half: it validates the whole request against the issue it
//! was handed and returns the one event the mutation *would* append, together with
//! the before/after values. It takes `&Issue` and writes nothing, so a refusal is
//! inert by construction rather than by care. [`apply`] is the writing half: it
//! resolves the issue, runs [`intent`], stages the projection and the event **in
//! memory**, and only then calls the store's existing write seam — once. A refusal
//! returns before that seam is reached, so the tree is byte-identical, and a store
//! failure on the single write leaves nothing behind because nothing else was
//! written (T011). The whole write is wrapped in [`Store::transaction`], the seam
//! every multi-step command uses, so the layout reconcile `Store::update` performs
//! cannot leave a half-moved file either.
//!
//! # One event grammar, one owner
//!
//! The `detail` of a time event is built here and nowhere else, and it is the same
//! grammar for all three kinds:
//!
//! ```text
//! create: entry <id>; created <value>[; reason: <reason>]
//! edit:   entry <id>; updated <value> -> <value>; reason: <reason>
//! delete: entry <id>; deleted <value>; reason: <reason>
//! value:  <minutes>m[ on <date>] by <author>[: <note>]
//! ```
//!
//! so a corrected or deleted entry's **prior value stays readable** in History and
//! no prior event is rewritten, reordered or dropped (C-004). `<id>` is the machine
//! field: [`entry_id_in`] recovers it from any time event, which is what the
//! never-reuse guard asks. `?m`/`by ?` mark a value a malformed entry did not carry
//! — never a silent zero (C-005).
//!
//! Two guarantees make that grammar hold for values this service did not author.
//! The rendering is **total**: a stored note, author or date may carry a line break
//! (a YAML literal block round-trips one), and [`entry_summary`] collapses it to one
//! line, so one applied mutation appends exactly one entry whatever the file holds —
//! refusing instead would make a malformed entry unremovable, which C-005 forbids
//! (see `one_line`). And an id a **create** mints or accepts is a clean token — one
//! line, no `;`, no surrounding whitespace (`check_entry_id`) — so the id a create
//! stores and the id an event records are the same string, which is what makes the
//! never-reuse guard exact (C-008).
//!
//! # What this service does not touch
//!
//! Nothing here changes an issue's status, ownership, estimate, body, relationships
//! or labels (C-008): a mutation assigns `time_entries`, appends one event, and
//! bumps `updated` — the field every write path bumps. An edit names the fields it
//! replaces and leaves the rest, including the entry's preserved unknown metadata,
//! exactly as they were (C-007). The comment this module builds is the
//! `History`-line safety rule; there is no clock read except the crate's one clock,
//! used only when the caller supplied no instant.

use std::fmt;

use crate::error::{Result, TixError};
use crate::model::{HistoryEvent, HistoryEventKind, Issue, utc_now_rfc3339};
use crate::storage::Store;
use crate::time_tracking::entry::{TimeEntry, TimeEntryError};

// ---------------------------------------------------------------------------
// The field patch
// ---------------------------------------------------------------------------

/// One field's requested change: leave it, clear it, or set it.
///
/// A three-state patch rather than `Option<Option<T>>` for the reason
/// [`crate::projects::Patch`] and [`crate::bulk::patch::PatchValue`] are: "the
/// caller did not mention this field" and "the caller asked to empty it" are
/// different requests, and an `Option` cannot tell them apart. Clearing an optional
/// field removes the key; clearing the required `author` produces an entry that
/// validation refuses by name, rather than a silent blank.
#[derive(Debug, Clone, PartialEq, Eq, Default)]
pub enum Patch<T> {
    /// Leave the field as it is.
    #[default]
    Unchanged,
    /// Remove the value.
    Clear,
    /// Replace the value.
    Set(T),
}

impl<T> Patch<T> {
    /// Whether this patch leaves the field alone.
    pub fn is_unchanged(&self) -> bool {
        matches!(self, Patch::Unchanged)
    }
}

// ---------------------------------------------------------------------------
// The intents (T007–T009)
// ---------------------------------------------------------------------------

/// A new entry to create, with the id optional.
///
/// The id is `Option` because `tix time add` has no `--entry` flag (contract
/// `cli-surface.md`): the service mints one through [`new_entry_id`] so the CLI,
/// the TUI and MCP cannot each invent their own shape. A caller that *does* own an
/// id — a portable import, a test — names it with [`NewTimeEntry::with_id`].
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct NewTimeEntry {
    /// The stable id, or `None` for one generated when the intent is built.
    pub id: Option<String>,
    /// The effort, in whole minutes at or above zero.
    pub duration_minutes: i64,
    /// The calendar date the effort belongs to, `YYYY-MM-DD`, when known.
    pub date: Option<String>,
    /// Who did the work.
    pub author: String,
    /// Optional free text.
    pub note: Option<String>,
}

impl NewTimeEntry {
    /// A new entry of `duration_minutes` minutes, attributed to `author`.
    pub fn new(duration_minutes: i64, author: impl Into<String>) -> Self {
        Self {
            id: None,
            duration_minutes,
            date: None,
            author: author.into(),
            note: None,
        }
    }

    /// Create under this stable id instead of a generated one.
    pub fn with_id(mut self, id: impl Into<String>) -> Self {
        self.id = Some(id.into());
        self
    }

    /// Record the calendar date the effort belongs to.
    pub fn on(mut self, date: impl Into<String>) -> Self {
        self.date = Some(date.into());
        self
    }

    /// Attach free text.
    pub fn with_note(mut self, note: impl Into<String>) -> Self {
        self.note = Some(note.into());
        self
    }

    /// The [`TimeEntry`] this intent describes, with an id minted when none was
    /// given.
    fn resolve(self) -> TimeEntry {
        TimeEntry {
            id: self.id.unwrap_or_else(new_entry_id),
            duration_minutes: Some(self.duration_minutes),
            date: self.date,
            author: self.author,
            note: self.note,
            extensions: crate::time_tracking::TimeEntryExtensions::default(),
        }
    }
}

/// The fields of an entry edit: each left alone unless it is named (T008).
///
/// A patch rather than a replacement entry, because `tix time edit … --minutes 30`
/// names one field and must not silently drop the entry's date, note or preserved
/// unknown metadata. The merge is `apply_to`, so the CLI, the TUI and MCP
/// share one definition of "what an edit means" instead of three.
#[derive(Debug, Clone, Default, PartialEq, Eq)]
pub struct TimeEntryEdit {
    /// The replacement duration, in whole minutes.
    pub duration_minutes: Patch<i64>,
    /// The replacement date, or `Clear` for none.
    pub date: Patch<String>,
    /// The replacement author. `Clear` is refused: an entry without attribution is
    /// not auditable.
    pub author: Patch<String>,
    /// The replacement note, or `Clear` for none.
    pub note: Patch<String>,
}

impl TimeEntryEdit {
    /// An edit that names no field yet.
    pub fn new() -> Self {
        Self::default()
    }

    /// Replace the duration.
    pub fn duration(mut self, duration_minutes: i64) -> Self {
        self.duration_minutes = Patch::Set(duration_minutes);
        self
    }

    /// Replace the date.
    pub fn date(mut self, date: impl Into<String>) -> Self {
        self.date = Patch::Set(date.into());
        self
    }

    /// Remove the date.
    pub fn clear_date(mut self) -> Self {
        self.date = Patch::Clear;
        self
    }

    /// Replace the author.
    pub fn author(mut self, author: impl Into<String>) -> Self {
        self.author = Patch::Set(author.into());
        self
    }

    /// Replace the note.
    pub fn note(mut self, note: impl Into<String>) -> Self {
        self.note = Patch::Set(note.into());
        self
    }

    /// Remove the note.
    pub fn clear_note(mut self) -> Self {
        self.note = Patch::Clear;
        self
    }

    /// Whether the edit names no field at all.
    pub fn is_empty(&self) -> bool {
        self.duration_minutes.is_unchanged()
            && self.date.is_unchanged()
            && self.author.is_unchanged()
            && self.note.is_unchanged()
    }

    /// Apply this patch to `before`, preserving the id and the preserved unknown
    /// metadata (C-007).
    ///
    /// The id is the entry's identity and the extensions are another build's data:
    /// neither is a field this edit can name, so both survive by construction.
    fn apply_to(&self, before: &TimeEntry) -> TimeEntry {
        let mut after = before.clone();
        if let Patch::Set(minutes) = self.duration_minutes {
            after.duration_minutes = Some(minutes);
        }
        match &self.date {
            Patch::Unchanged => {}
            Patch::Clear => after.date = None,
            Patch::Set(date) => after.date = Some(date.clone()),
        }
        match &self.author {
            Patch::Unchanged => {}
            Patch::Clear => after.author = String::new(),
            Patch::Set(author) => after.author = author.clone(),
        }
        match &self.note {
            Patch::Unchanged => {}
            Patch::Clear => after.note = None,
            Patch::Set(note) => after.note = Some(note.clone()),
        }
        after
    }
}

/// One mutation intent (data-model §2): the create, edit or delete being asked for.
#[derive(Debug, Clone, PartialEq, Eq)]
pub enum TimeIntent {
    /// Add an entry to the current projection.
    Create(NewTimeEntry),
    /// Correct the entry `entry_id`, naming the fields to replace.
    Edit {
        /// The stable id of the entry to correct.
        entry_id: String,
        /// The fields to replace, leave and clear.
        change: TimeEntryEdit,
    },
    /// Remove `entry_id` from the current projection.
    Delete {
        /// The stable id of the entry to remove.
        entry_id: String,
    },
}

impl TimeIntent {
    /// The id this intent targets, when it names one. A create names one only when
    /// the caller supplied it ([`NewTimeEntry::with_id`]).
    pub fn entry_id(&self) -> Option<&str> {
        match self {
            TimeIntent::Create(new) => new.id.as_deref(),
            TimeIntent::Edit { entry_id, .. } | TimeIntent::Delete { entry_id } => {
                Some(entry_id.as_str())
            }
        }
    }
}

/// Which of the three mutations an intent is.
///
/// A private total mapping onto the model's pre-declared kinds, so the kind and the
/// `detail` verb are chosen once rather than per call site (WP01 pre-declared
/// `TimeEntryCreated`/`Updated`/`Deleted` precisely so this file consumes them).
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
enum TimeAction {
    Create,
    Edit,
    Delete,
}

impl TimeAction {
    fn of(intent: &TimeIntent) -> Self {
        match intent {
            TimeIntent::Create(_) => TimeAction::Create,
            TimeIntent::Edit { .. } => TimeAction::Edit,
            TimeIntent::Delete { .. } => TimeAction::Delete,
        }
    }

    fn kind(self) -> HistoryEventKind {
        match self {
            TimeAction::Create => HistoryEventKind::TimeEntryCreated,
            TimeAction::Edit => HistoryEventKind::TimeEntryUpdated,
            TimeAction::Delete => HistoryEventKind::TimeEntryDeleted,
        }
    }

    /// The verb the `detail` grammar uses.
    fn word(self) -> &'static str {
        match self {
            TimeAction::Create => "created",
            TimeAction::Edit => "updated",
            TimeAction::Delete => "deleted",
        }
    }

    /// The label a refusal names when this action is missing its reason.
    fn label(self) -> &'static str {
        match self {
            TimeAction::Create => "create",
            TimeAction::Edit => "edit",
            TimeAction::Delete => "delete",
        }
    }
}

// ---------------------------------------------------------------------------
// Attribution (T010)
// ---------------------------------------------------------------------------

/// Who is making the mutation, when, and why (data-model §2's `actor`,
/// `timestamp`, `reason`).
///
/// `None` for the actor and the instant is not an omission: it means the crate's
/// own answers, [`crate::storage::actor_identity`] and
/// [`crate::model::utc_now_rfc3339`], so a surface that has no opinion attributes
/// the mutation the same way every other `tix` write does, and a *test* can pin the
/// line without a clock.
#[derive(Debug, Clone, Default, PartialEq, Eq)]
pub struct TimeAttribution {
    /// Who performed the mutation; `None` for the configured git identity.
    pub actor: Option<String>,
    /// The instant recorded, RFC 3339; `None` for now.
    pub timestamp: Option<String>,
    /// Why a correction or a deletion was made. Required for those two actions
    /// (`contracts/cli-surface.md` spells `--reason <text>` non-optional), and
    /// optional for a create.
    pub reason: Option<String>,
}

impl TimeAttribution {
    /// Attribution with no explicit actor, instant or reason.
    pub fn new() -> Self {
        Self::default()
    }

    /// Record `actor` instead of the configured identity.
    pub fn by(mut self, actor: impl Into<String>) -> Self {
        self.actor = Some(actor.into());
        self
    }

    /// Record `timestamp` instead of now.
    pub fn at(mut self, timestamp: impl Into<String>) -> Self {
        self.timestamp = Some(timestamp.into());
        self
    }

    /// Record why the mutation is being made.
    pub fn because(mut self, reason: impl Into<String>) -> Self {
        self.reason = Some(reason.into());
        self
    }

    /// The actor to record, or the crate's own identity.
    fn actor(&self) -> std::result::Result<String, TimeMutationError> {
        let actor = match self.actor.as_deref() {
            Some(actor) => actor.trim().to_string(),
            None => crate::storage::actor_identity(),
        };
        if actor.is_empty() {
            return Err(TimeMutationError::MissingActor);
        }
        check_history_text("actor", &actor, false)?;
        Ok(actor)
    }

    /// The instant to record: the supplied one, shape-checked, or now.
    ///
    /// The shape check reuses the crate's one instant validator
    /// ([`crate::cycle_schedule::snapshot::validate_timestamp`]) rather than
    /// re-implementing it, exactly as [`TimeEntry`] reuses
    /// [`crate::model::validate_date`]: a caller-supplied instant that is not the
    /// crate's `YYYY-MM-DDTHH:MM:SSZ` shape would be stored as evidence that reads
    /// like a timestamp and is not one.
    fn timestamp(&self) -> std::result::Result<String, TimeMutationError> {
        match self.timestamp.as_deref().map(str::trim) {
            Some(timestamp) => {
                let timestamp = timestamp.to_string();
                if !crate::cycle_schedule::snapshot::validate_timestamp(&timestamp) {
                    return Err(TimeMutationError::InvalidTimestamp { timestamp });
                }
                Ok(timestamp)
            }
            None => Ok(utc_now_rfc3339()),
        }
    }

    /// The reason for `action`, which an edit and a delete require.
    fn reason(&self, action: TimeAction) -> std::result::Result<Option<String>, TimeMutationError> {
        let reason = self
            .reason
            .as_deref()
            .map(str::trim)
            .filter(|reason| !reason.is_empty());
        match (reason, action) {
            (Some(reason), _) => {
                check_history_text("reason", reason, false)?;
                Ok(Some(reason.to_string()))
            }
            (None, TimeAction::Create) => Ok(None),
            (None, action) => Err(TimeMutationError::MissingReason {
                action: action.label(),
            }),
        }
    }
}

/// One mutation request: the issue to resolve, the intent, and its attribution.
///
/// [`Self::edit`] and [`Self::delete`] take the reason as an argument rather than an
/// optional setter, because the contract requires one for those two actions: a
/// caller cannot forget it and then wonder why nothing happened. Validation still
/// refuses a blank one, so an empty flag does not become an empty audit line.
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct TimeMutationRequest {
    /// The issue selector, exactly as the caller typed it.
    pub issue: String,
    /// What to do to its current entries.
    pub intent: TimeIntent,
    /// Who, when and why.
    pub attribution: TimeAttribution,
}

impl TimeMutationRequest {
    /// A create request.
    pub fn create(issue: impl Into<String>, entry: NewTimeEntry) -> Self {
        Self {
            issue: issue.into(),
            intent: TimeIntent::Create(entry),
            attribution: TimeAttribution::new(),
        }
    }

    /// An edit request. The reason is required by the contract and is carried here.
    pub fn edit(
        issue: impl Into<String>,
        entry_id: impl Into<String>,
        change: TimeEntryEdit,
        reason: impl Into<String>,
    ) -> Self {
        Self {
            issue: issue.into(),
            intent: TimeIntent::Edit {
                entry_id: entry_id.into(),
                change,
            },
            attribution: TimeAttribution::new().because(reason),
        }
    }

    /// A delete request. The reason is required by the contract and is carried here.
    pub fn delete(
        issue: impl Into<String>,
        entry_id: impl Into<String>,
        reason: impl Into<String>,
    ) -> Self {
        Self {
            issue: issue.into(),
            intent: TimeIntent::Delete {
                entry_id: entry_id.into(),
            },
            attribution: TimeAttribution::new().because(reason),
        }
    }

    /// Attribute the mutation to `actor` instead of the configured identity.
    pub fn by(mut self, actor: impl Into<String>) -> Self {
        self.attribution = self.attribution.by(actor);
        self
    }

    /// Record `timestamp` instead of now.
    pub fn at(mut self, timestamp: impl Into<String>) -> Self {
        self.attribution = self.attribution.at(timestamp);
        self
    }
}

// ---------------------------------------------------------------------------
// Refusals (T010)
// ---------------------------------------------------------------------------

/// Why a mutation was refused, before anything was written.
///
/// Every variant is raised by [`intent`], which cannot write: reaching one of these
/// *is* the guarantee that the issue's bytes and History are untouched. A replacement
/// entry's own field problems are carried as WP01's [`TimeEntryError`] values rather
/// than re-spelled, so the CLI, the TUI, MCP and `tix lint` report one condition
/// under one name (C-006).
#[derive(Debug, Clone, PartialEq, Eq)]
pub enum TimeMutationError {
    /// The issue carries no entry with this id, so there is nothing to edit or
    /// delete.
    UnknownEntry {
        /// The id exactly as the caller wrote it.
        entry_id: String,
    },
    /// A create named an id already in use — in the current projection, or in any
    /// time event this issue's History records.
    ///
    /// The History half is what makes "ids are never reused" true rather than
    /// aspirational (C-008): a deleted entry leaves no frontmatter trace, and
    /// re-adding its id would make the audit trail read as a correction of an entry
    /// that no longer exists.
    DuplicateEntryId {
        /// The id that is already taken.
        entry_id: String,
    },
    /// A create named an id padded with leading or trailing whitespace.
    ///
    /// An id is an identity, and padding makes two *different* strings out of one
    /// entry: [`entry_id_in`] trims what it recovers from an event, so a stored
    /// `e-abc ` and a stored `e-abc` would both answer `Some("e-abc")` — the second
    /// create is let through while the stored ids disagree, and an id that was
    /// deleted comes back under the padded spelling. The crate already refuses a
    /// padded name where the name becomes an identity
    /// ([`crate::projects::duplicate_name_reason`], `crate::config`'s id rule).
    PaddedEntryId {
        /// The id as supplied, padding and all.
        entry_id: String,
    },
    /// An edit named no field, so it asks for nothing.
    EmptyEdit {
        /// The entry the empty edit targeted.
        entry_id: String,
    },
    /// The replacement entry failed validation; the problems are field-aware.
    InvalidEntry {
        /// The entry id being created or corrected.
        entry_id: String,
        /// Every field-level problem, in field order.
        problems: Vec<TimeEntryError>,
    },
    /// The mutation has no actor to attribute it to.
    MissingActor,
    /// The supplied instant is not the crate's RFC 3339 shape.
    InvalidTimestamp {
        /// The off-shape value, as supplied.
        timestamp: String,
    },
    /// An edit or a delete carried no reason, which the contract requires.
    MissingReason {
        /// The action that requires one: `edit` or `delete`.
        action: &'static str,
    },
    /// Text that cannot enter a one-line `## History` entry.
    UnsafeHistoryText {
        /// The field it came from.
        field: &'static str,
        /// What about it cannot be written.
        hazard: &'static str,
    },
}

impl TimeMutationError {
    /// The stable, snake_case name a surface reports this refusal under.
    ///
    /// A replacement entry's problem reports *its own* field-aware kind —
    /// `invalid_duration`, `invalid_date`, `missing_author`, `malformed_entry` from
    /// `data-model.md` §5 — so a bad duration is one condition with one name whether
    /// it was read from a file or supplied to a mutation.
    pub fn kind(&self) -> &'static str {
        match self {
            TimeMutationError::UnknownEntry { .. } => "unknown_entry",
            TimeMutationError::DuplicateEntryId { .. } => "duplicate_entry_id",
            TimeMutationError::PaddedEntryId { .. } => "padded_entry_id",
            TimeMutationError::EmptyEdit { .. } => "empty_edit",
            TimeMutationError::InvalidEntry { problems, .. } => problems
                .first()
                .map_or("malformed_entry", TimeEntryError::kind),
            TimeMutationError::MissingActor => "missing_actor",
            TimeMutationError::InvalidTimestamp { .. } => "invalid_timestamp",
            TimeMutationError::MissingReason { .. } => "missing_reason",
            TimeMutationError::UnsafeHistoryText { .. } => "unsafe_history_text",
        }
    }

    /// The offending input, when this refusal belongs to one.
    pub fn field(&self) -> &'static str {
        match self {
            TimeMutationError::UnknownEntry { .. } | TimeMutationError::EmptyEdit { .. } => "entry",
            TimeMutationError::DuplicateEntryId { .. }
            | TimeMutationError::PaddedEntryId { .. } => "id",
            TimeMutationError::InvalidEntry { problems, .. } => {
                problems.first().map_or("entry", TimeEntryError::field)
            }
            TimeMutationError::MissingActor => "actor",
            TimeMutationError::InvalidTimestamp { .. } => "timestamp",
            TimeMutationError::MissingReason { .. } => "reason",
            TimeMutationError::UnsafeHistoryText { field, .. } => field,
        }
    }

    /// The entry this refusal names, when it names one.
    pub fn entry_id(&self) -> Option<&str> {
        match self {
            TimeMutationError::UnknownEntry { entry_id }
            | TimeMutationError::DuplicateEntryId { entry_id }
            | TimeMutationError::PaddedEntryId { entry_id }
            | TimeMutationError::EmptyEdit { entry_id }
            | TimeMutationError::InvalidEntry { entry_id, .. } => Some(entry_id.as_str()),
            _ => None,
        }
    }

    /// The single-line explanation, without the kind prefix.
    pub fn message(&self) -> String {
        match self {
            TimeMutationError::UnknownEntry { entry_id } => format!(
                "no time entry `{entry_id}` on this issue; the current projection holds only the \
                 entries that were never deleted, and a deleted id is never reused"
            ),
            TimeMutationError::DuplicateEntryId { entry_id } => format!(
                "time entry `{entry_id}` already exists on this issue — in its current entries or \
                 in its History; ids are stable and never reused, so choose a new one"
            ),
            TimeMutationError::PaddedEntryId { entry_id } => format!(
                "time entry id {entry_id:?} has leading or trailing whitespace (the crate's rule \
                 for a name that becomes an identity); the stored id keeps its whitespace while a \
                 History event's id is recovered with it trimmed, so a padded spelling reads as \
                 the id it merely resembles and would let a deleted one back in"
            ),
            TimeMutationError::EmptyEdit { entry_id } => format!(
                "the edit of `{entry_id}` names no field (duration_minutes, date, author, note); \
                 there is nothing to change"
            ),
            TimeMutationError::InvalidEntry { entry_id, problems } => {
                let listed: Vec<String> = problems
                    .iter()
                    .map(|problem| format!("{} ({})", problem, problem.field()))
                    .collect();
                format!("time entry `{entry_id}` is invalid: {}", listed.join("; "))
            }
            TimeMutationError::MissingActor => {
                ("a time mutation records who made it; the actor is blank, and an entry without \
                 attribution is not auditable")
                    .to_string()
            }
            TimeMutationError::InvalidTimestamp { timestamp } => format!(
                "timestamp {timestamp:?} is not the crate's `YYYY-MM-DDTHH:MM:SSZ` instant shape; a \
                 mutation records the instant it happened rather than one that merely looks like \
                 evidence"
            ),
            TimeMutationError::MissingReason { action } => format!(
                "a time {action} requires `--reason`: the reason is what the audit trail keeps \
                 when the current value is replaced or removed"
            ),
            TimeMutationError::UnsafeHistoryText { field, hazard } => format!(
                "the {field} contains {hazard}; a `## History` entry is one line carrying the \
                 entry id, and text that breaks it would misread as an event nobody wrote"
            ),
        }
    }
}

impl fmt::Display for TimeMutationError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "{}: {}", self.kind(), self.message())
    }
}

impl std::error::Error for TimeMutationError {}

/// A refusal becomes user input error, so `?` works at the call site.
///
/// `<kind>: <message>` is already a single-line diagnostic, so it takes the crate's
/// existing `invalid input:` prefix without doubling it — the same boundary
/// [`crate::projects::ProjectError`] crosses.
impl From<TimeMutationError> for TixError {
    fn from(err: TimeMutationError) -> TixError {
        TixError::InvalidInput(err.to_string())
    }
}

/// Refuse an id that cannot be written into, and recovered from, a History detail.
///
/// Asked only where a **new** identity is minted — a create — because that is the one
/// boundary where the id can be held to a shape. (An edit or a delete names an id the
/// file already holds and is matched on its exact bytes, so a hand-edited padded id
/// stays editable and removable rather than being stranded.) Two rules, both about
/// the id being the machine field [`entry_id_in`] recovers:
///
/// * one line and no `;` — [`check_history_text`]'s, since the detail separates the
///   id from the action with `; ` and a break would forge an entry;
/// * no leading or trailing whitespace — the crate's existing rule for a name that
///   becomes an identity (`crate::projects::duplicate_name_reason`, `crate::config`'s
///   id rule). It is not cosmetic here: [`entry_id_in`] trims what it recovers, so a
///   stored `e-abc ` and a stored `e-abc` would both answer `Some("e-abc")` — the
///   second create is let through while the stored ids disagree, and an id that was
///   deleted comes back under the padded spelling.
fn check_entry_id(entry_id: &str) -> std::result::Result<(), TimeMutationError> {
    check_history_text("id", entry_id, true)?;
    if entry_id.trim() != entry_id {
        return Err(TimeMutationError::PaddedEntryId {
            entry_id: entry_id.to_string(),
        });
    }
    Ok(())
}

/// Refuse text that cannot enter a one-line `## History` entry.
///
/// A line break forges a second entry, so no field a **caller supplies** accepts one.
/// Text the **file already carries** is not refused but normalized by [`one_line`] at
/// the point it is rendered, because a stored value must never make a mutation
/// impossible; see that function for why the two directions differ. `;` is refused
/// only where it is the *id*'s separator (`check_semicolon`), because the id is the
/// one field [`entry_id_in`] recovers and an id carrying the separator would make the
/// never-reuse guard answer about the wrong entry.
///
/// The em dash (U+2014) is deliberately **allowed** here, where
/// [`crate::cycle_schedule::lifecycle`]'s `history_text_hazard` refuses it. The two
/// positions differ: a `detail` is the fourth of four fields, and
/// [`HistoryEvent::parse`] splits on its **first three** separators, so an em dash
/// inside a detail costs no parse. Refusing it would make the delete of an entry
/// whose note contains one impossible — and a delete must carry the prior value it
/// removes (C-004). `a_note_containing_an_em_dash_still_writes_one_line` pins both
/// halves of that.
fn check_history_text(
    field: &'static str,
    text: &str,
    check_semicolon: bool,
) -> std::result::Result<(), TimeMutationError> {
    if text.contains(['\n', '\r']) {
        return Err(TimeMutationError::UnsafeHistoryText {
            field,
            hazard: "a line break",
        });
    }
    if check_semicolon && text.contains(';') {
        return Err(TimeMutationError::UnsafeHistoryText {
            field,
            hazard: "the `;` that separates the entry id from its values",
        });
    }
    Ok(())
}

// ---------------------------------------------------------------------------
// The event grammar (one owner)
// ---------------------------------------------------------------------------

/// The prefix every time-mutation detail opens with, so the entry id is the first
/// thing a reader (and [`entry_id_in`]) sees.
const DETAIL_PREFIX: &str = "entry ";

/// The separator between the id and the action.
const DETAIL_FIELD_SEP: &str = "; ";

/// The verb separating the entry id from its values.
const VALUE_ARROW: &str = " -> ";

/// The key the reason is recorded under.
const REASON_KEY: &str = "; reason: ";

/// One value as it is written **inside** a History detail: one line, always.
///
/// This renderer is **total** over whatever a file already holds, and that is the
/// whole point rather than tidiness. A stored `note` (or `author`, or `date`) may
/// legitimately carry an internal line break — a YAML literal block round-trips one
/// into the model — and a `## History` entry is one line whose fields are split
/// positionally. Written through verbatim, the second line of such a value parses as
/// an authentic event nobody wrote, attributed to an actor no surface ever
/// validated: one applied mutation would append two entries, and the log stops being
/// evidence (C-003, C-004, NFR-002).
///
/// The rule differs from [`check_history_text`]'s on purpose, and the difference is
/// load-bearing: a break in a value a **caller supplies** is refused, because a
/// caller can be asked to rephrase, while a break in a value the **file already
/// carries** is collapsed to a single space, because a stored value must never make
/// a mutation impossible — the contract requires a malformed entry to stay readable,
/// removable and repairable (C-005, C-004). Nothing is lost: the current projection
/// keeps the value exactly as it is; only the one-line rendering normalizes it.
fn one_line(value: &str) -> String {
    let mut out = String::with_capacity(value.len());
    for part in value
        .split(['\n', '\r'])
        .map(str::trim)
        .filter(|part| !part.is_empty())
    {
        if !out.is_empty() {
            out.push(' ');
        }
        out.push_str(part);
    }
    out
}

/// One entry's values, as they are written into a History detail.
///
/// Readable first and exact where it must be: minutes at documented precision
/// (`?m` when a malformed entry carries none, never zero), the calendar date when
/// there is one, the author, and the note last so free text cannot be mistaken for
/// a field. This is the one rendering of a prior value the audit trail gets, which is
/// why a delete can keep one (C-004) and a correction can show before and after.
///
/// Every value goes through `one_line`, so the rendering is one line whatever the
/// entry holds — the guarantee is the renderer's, not the caller's, because a
/// delete renders a value no caller supplied.
pub fn entry_summary(entry: &TimeEntry) -> String {
    let mut out = match entry.minutes() {
        Some(minutes) => format!("{minutes}m"),
        None => "?m".to_string(),
    };
    if let Some(date) = entry
        .date
        .as_deref()
        .map(one_line)
        .filter(|d| !d.is_empty())
    {
        out.push_str(&format!(" on {date}"));
    }
    let author = one_line(&entry.author);
    out.push_str(" by ");
    out.push_str(if author.is_empty() { "?" } else { &author });
    if let Some(note) = entry
        .note
        .as_deref()
        .map(one_line)
        .filter(|n| !n.is_empty())
    {
        out.push_str(": ");
        out.push_str(&note);
    }
    out
}

/// The stable entry id a time-mutation detail names, when it names one.
///
/// The inverse half of the grammar above, and the reason the id is written first:
/// the never-reuse guard asks every time event in an issue's History which entry it
/// was about, and this is the one place that answer is read. The recovered id is
/// trimmed, and every id a create accepts is already trimmed (`check_entry_id`), so
/// the trimmed answer and the stored id agree for every id this service can mint —
/// which is exactly what a padded id would break.
pub fn entry_id_in(detail: &str) -> Option<&str> {
    let rest = detail.strip_prefix(DETAIL_PREFIX)?;
    let (entry_id, _) = rest.split_once(DETAIL_FIELD_SEP)?;
    let entry_id = entry_id.trim();
    if entry_id.is_empty() {
        None
    } else {
        Some(entry_id)
    }
}

/// Whether a kind is one of the three this service appends.
fn is_time_entry_kind(kind: HistoryEventKind) -> bool {
    matches!(
        kind,
        HistoryEventKind::TimeEntryCreated
            | HistoryEventKind::TimeEntryUpdated
            | HistoryEventKind::TimeEntryDeleted
    )
}

/// The one `detail` line for an applied mutation.
fn detail_for(
    action: TimeAction,
    entry_id: &str,
    before: Option<&TimeEntry>,
    after: Option<&TimeEntry>,
    reason: Option<&str>,
) -> String {
    let mut out = format!(
        "{DETAIL_PREFIX}{entry_id}{DETAIL_FIELD_SEP}{}",
        action.word()
    );
    match action {
        TimeAction::Create => {
            if let Some(after) = after {
                out.push(' ');
                out.push_str(&entry_summary(after));
            }
        }
        TimeAction::Edit => {
            let before = before.map_or_else(|| "?".to_string(), entry_summary);
            let after = after.map_or_else(|| "?".to_string(), entry_summary);
            out.push(' ');
            out.push_str(&before);
            out.push_str(VALUE_ARROW);
            out.push_str(&after);
        }
        TimeAction::Delete => {
            if let Some(before) = before {
                out.push(' ');
                out.push_str(&entry_summary(before));
            }
        }
    }
    if let Some(reason) = reason {
        out.push_str(REASON_KEY);
        out.push_str(reason);
    }
    out
}

// ---------------------------------------------------------------------------
// The pure half (T007–T010)
// ---------------------------------------------------------------------------

/// One validated intent: the single event it would append, the values it touches,
/// and whether it changes anything.
///
/// `changed == false` is the only case with `event == None`, and it means the request
/// asked for exactly what is already there: no event, no write, no `updated` bump —
/// the same no-op rule [`crate::commands::edit`] applies to `--parent` and the cycle
/// lifecycle applies to a repeated action. "Exactly one event per applied mutation"
/// therefore holds in both directions: a write means one event, an event means a
/// write.
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct TimeIntentOutcome {
    /// The event kind this mutation appends.
    pub kind: HistoryEventKind,
    /// The entry this mutation is about.
    pub entry_id: String,
    /// The prior value, for an edit or a delete.
    pub before: Option<TimeEntry>,
    /// The new value, for a create or an edit.
    pub after: Option<TimeEntry>,
    /// The one event appended when this mutation is applied.
    pub event: Option<HistoryEvent>,
    /// Whether applying this mutation changes the issue at all.
    pub changed: bool,
}

/// Validate a mutation against the issue it targets and build its one event (T007–T010).
///
/// Takes `&Issue` and writes nothing: every refusal is inert by construction. The
/// caller supplies the issue because resolving the selector is a store read, and a
/// refusal must not be a read's side effect — [`apply`] is the pairing that does both.
pub fn intent(
    issue: &Issue,
    request: &TimeMutationRequest,
) -> std::result::Result<TimeIntentOutcome, TimeMutationError> {
    let action = TimeAction::of(&request.intent);
    let actor = request.attribution.actor()?;
    let timestamp = request.attribution.timestamp()?;
    let reason = request.attribution.reason(action)?;
    match &request.intent {
        TimeIntent::Create(new) => {
            let entry = new.clone().resolve();
            if let Some(problems) = validation_problems(&entry) {
                return Err(TimeMutationError::InvalidEntry {
                    entry_id: entry.id.clone(),
                    problems,
                });
            }
            check_entry_id(&entry.id)?;
            check_current_id_is_free(issue, &entry.id)?;
            let id = entry.id.clone();
            let event = HistoryEvent::new(
                timestamp,
                actor,
                action.kind(),
                detail_for(action, &id, None, Some(&entry), reason.as_deref()),
            );
            Ok(TimeIntentOutcome {
                kind: action.kind(),
                entry_id: id,
                before: None,
                after: Some(entry),
                event: Some(event),
                changed: true,
            })
        }
        TimeIntent::Edit { entry_id, change } => {
            check_history_text("id", entry_id, true)?;
            if change.is_empty() {
                return Err(TimeMutationError::EmptyEdit {
                    entry_id: entry_id.clone(),
                });
            }
            let before = current_entry(issue, entry_id)?.clone();
            let after = change.apply_to(&before);
            if let Some(problems) = validation_problems(&after) {
                return Err(TimeMutationError::InvalidEntry {
                    entry_id: entry_id.clone(),
                    problems,
                });
            }
            check_history_text("author", &after.author, false)?;
            if let Some(note) = after.note.as_deref() {
                check_history_text("note", note, false)?;
            }
            if after == before {
                return Ok(TimeIntentOutcome {
                    kind: action.kind(),
                    entry_id: entry_id.clone(),
                    before: Some(before),
                    after: Some(after),
                    event: None,
                    changed: false,
                });
            }
            let event = HistoryEvent::new(
                timestamp,
                actor,
                action.kind(),
                detail_for(
                    action,
                    entry_id,
                    Some(&before),
                    Some(&after),
                    reason.as_deref(),
                ),
            );
            Ok(TimeIntentOutcome {
                kind: action.kind(),
                entry_id: entry_id.clone(),
                before: Some(before),
                after: Some(after),
                event: Some(event),
                changed: true,
            })
        }
        TimeIntent::Delete { entry_id } => {
            check_history_text("id", entry_id, true)?;
            let before = current_entry(issue, entry_id)?.clone();
            let event = HistoryEvent::new(
                timestamp,
                actor,
                action.kind(),
                detail_for(action, entry_id, Some(&before), None, reason.as_deref()),
            );
            Ok(TimeIntentOutcome {
                kind: action.kind(),
                entry_id: entry_id.clone(),
                before: Some(before),
                after: None,
                event: Some(event),
                changed: true,
            })
        }
    }
}

/// The entry's field problems, or `None` when it is valid.
fn validation_problems(entry: &TimeEntry) -> Option<Vec<TimeEntryError>> {
    let problems = entry.problems();
    if problems.is_empty() {
        None
    } else {
        Some(problems)
    }
}

/// The index of the current entry with this id, if there is one.
fn current_index(issue: &Issue, entry_id: &str) -> Option<usize> {
    issue
        .time_entries
        .iter()
        .position(|entry| entry.id == entry_id)
}

/// The current entry with this id, or the refusal naming it (T009).
fn current_entry<'a>(
    issue: &'a Issue,
    entry_id: &str,
) -> std::result::Result<&'a TimeEntry, TimeMutationError> {
    current_index(issue, entry_id)
        .map(|index| &issue.time_entries[index])
        .ok_or_else(|| TimeMutationError::UnknownEntry {
            entry_id: entry_id.to_string(),
        })
}

/// Refuse an id that is already spoken for, now or anywhere in History (C-008).
fn check_current_id_is_free(
    issue: &Issue,
    entry_id: &str,
) -> std::result::Result<(), TimeMutationError> {
    let taken_now = current_index(issue, entry_id).is_some();
    let taken_before = issue.history.events().into_iter().any(|event| {
        is_time_entry_kind(event.kind) && entry_id_in(&event.detail) == Some(entry_id)
    });
    if taken_now || taken_before {
        return Err(TimeMutationError::DuplicateEntryId {
            entry_id: entry_id.to_string(),
        });
    }
    Ok(())
}

// ---------------------------------------------------------------------------
// The writing half (T011)
// ---------------------------------------------------------------------------

/// One applied mutation: the issue as written, and what was appended.
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct AppliedTimeMutation {
    /// The issue after the mutation, with `filename` the path it now lives at.
    pub issue: Issue,
    /// The validated intent that was applied.
    pub outcome: TimeIntentOutcome,
}

impl AppliedTimeMutation {
    /// The one event appended, or `None` when the request changed nothing.
    pub fn event(&self) -> Option<&HistoryEvent> {
        self.outcome.event.as_ref()
    }
}

/// Apply a mutation: validate, stage in memory, and write once (T011).
///
/// The order is the contract. The issue is resolved, [`intent`] validates and builds
/// the one event, the projection and the event are staged on the in-memory issue, and
/// only then does the store's own write seam run — inside a transaction, so the
/// layout reconcile it performs rolls back with the write. A refusal returns before
/// any of that, so the file's bytes and its History are unchanged; a store failure
/// leaves nothing, because the staged issue was never partially written. The
/// mutation bumps `updated` exactly as every other write path does, and touches no
/// other issue field (C-008).
pub fn apply(store: &Store, request: &TimeMutationRequest) -> Result<AppliedTimeMutation> {
    let mut issue = store.resolve_issue(&request.issue)?;
    let outcome = intent(&issue, request).map_err(TixError::from)?;
    if outcome.changed {
        stage(&mut issue, &outcome);
        issue.bump_updated();
        issue.filename = store.transaction(|| store.update(&issue.filename, &issue))?;
    }
    Ok(AppliedTimeMutation { issue, outcome })
}

/// Stage an outcome on the issue: the current projection, then the one event.
///
/// The projection keeps the file's own order: a create appends, an edit replaces the
/// entry at the index it found, and a delete removes it. That is the order the file
/// already had, so an unrelated mutation writes the smallest diff; the *display* order
/// is [`TimeEntry::order_key`], which surfaces sort by (data-model §6).
fn stage(issue: &mut Issue, outcome: &TimeIntentOutcome) {
    match (&outcome.after, current_index(issue, &outcome.entry_id)) {
        (Some(after), Some(index)) => issue.time_entries[index] = after.clone(),
        (Some(after), None) => issue.time_entries.push(after.clone()),
        (None, Some(index)) => {
            issue.time_entries.remove(index);
        }
        // Validation refuses a delete of an unknown entry and a create of a taken
        // id, so this arm is unreachable for every request that reaches `stage`.
        (None, None) => {}
    }
    if let Some(event) = &outcome.event {
        issue.history.append(event.clone());
    }
}

/// A fresh stable entry id.
///
/// `e-` plus the crate's collision-resistant id
/// ([`crate::id::generate_issue_id`], Crockford base32, never all digits), so an
/// entry id is generated without a second random source and can never be read as an
/// issue number. Minted once, at the intent, so the id the projection carries and the
/// id the event names are the same string by construction.
pub fn new_entry_id() -> String {
    format!("e-{}", crate::id::generate_issue_id())
}