tix-tracker 0.12.0

//! Recorded-event classification and the idempotency/provenance index (mission
//! `burndown-scope-analytics-01M3C9J1`, WP01 T001–T006).
//!
//! # What this module is
//!
//! The vocabulary every later child consumes, and the one place the mission decides
//! *what* a recorded event means:
//!
//! - [`AnalyticsScope`] — the selected cycle/milestone, its period, and the
//!   optional closed-snapshot binding (`data-model.md` §1).
//! - [`AnalyticsEvent`] — one recorded contribution, with its stable identity, its
//!   class, its optional day, and its provenance (`data-model.md` §2).
//! - [`EventClass`] — the closed four-name classification.
//! - [`EventIndex`] — the contribution set, deduplicated by event id (C-002).
//!
//! # Recorded only, and nothing else
//!
//! Classification reads the History the tree actually holds. It never infers a
//! scope change that was not logged, never treats a missing day as zero, and never
//! compares a status *string*: a completion is decided by the category the
//! transition **recorded** (`transition_accepted`'s `to_category`), read through the
//! shared [#183 vocabulary](crate::workflow::WorkflowCategory) rather than by
//! matching a column name against a hard-coded list (C-001/C-008).
//!
//! # The four classes, and why they are the right partition
//!
//! FR-002's whole claim is that scope growth is distinguishable from completion, so
//! the partition splits the two answers a burndown chart needs on a given day:
//!
//! | class | recorded decision | count-level delta |
//! |---|---|---|
//! | `scope_addition` | `parent_set`, or a `cycle_membership_changed` that **added** a cycle | `Some(1)` for `parent_set`, `None` when only the detail can say |
//! | `completion` | `transition_accepted` whose recorded `to_category` is `completed` | `Some(1)` |
//! | `estimate_change` | `estimate_changed` | `None` (the unit is the caller's mode) |
//! | `unclassified` | every other recorded kind | `None` |
//!
//! Three things follow from that, and each is deliberate:
//!
//! 1. **A move that is not a completion is not a completion.** `transition_accepted`
//!    into `active`/`backlog`/`canceled` is a plain move and lands in
//!    `unclassified` rather than being counted as progress.
//! 2. **A removal has no class.** The vocabulary is closed at four names, and neither
//!    a `parent_cleared` nor a membership change that only *removed* cycles is an
//!    addition; both stay `unclassified` and visible, which is the honest answer for
//!    an event the class table cannot place. They are never read as growth. A
//!    membership change is read by **direction** — the detail names the before and
//!    after lists, and the event is a `scope_addition` only when the after list has a
//!    cycle the before list did not — so `cycles: Default -> none` is a removal and
//!    `cycles: none -> Default` is an addition.
//! 3. **Every other kind keeps its record.** All of #184's time-mutation kinds, the
//!    relationship/triage/health/project/epic/cycle-lifecycle kinds, `archived`,
//!    `restored` and `blocked_move_acknowledged` classify as `unclassified` — never
//!    as completion or scope — so a time entry can never move a burndown line.
//!
//! # What a class cannot see: a write nobody recorded
//!
//! A class is only as wide as the record. `tix edit --estimate` and `tix edit
//! --cycles` append the two kinds above, and `tix edit --parent` already appended its
//! own, so what the **edit seam** writes is visible here. A path that writes a field
//! without appending anything — the bulk patch service reaches `Store::update`
//! directly — leaves no event, so such a change cannot appear in `estimate_change` or
//! `scope_addition` no matter how the series is folded. That is a **coverage limit**
//! a surface reports as an explicit marker, never a licence to read the current
//! frontmatter back into the past: reconstructing unrecorded history is the mission's
//! non-goal and would defeat FR-002's whole distinction.
//!
//! # Idempotency is the index's, not the caller's care
//!
//! [`EventIndex::contribute`] deduplicates by [`AnalyticsEvent::event_id`] and
//! reports what it did ([`Contribution`]). Classifying the same recorded event twice
//! therefore contributes once, which is C-002's "the same recorded event produces
//! the same contribution"; the guard lives in the one function every contribution
//! goes through, so a caller cannot forget it.
//!
//! # Provenance
//!
//! A History event's identity is `<source-ref>#<n>`, where `n` is the entry's
//! **ordinal among the section's parsed events**, not its slot in the raw line list.
//! The section is append-only, so a new line lands at the end: an unparseable line a
//! human **inserts** occupies no ordinal, cannot renumber the events after it, and two
//! reads of one file agree on every id. The one hand edit that still moves an id is a
//! line that was a readable event and was later **mangled into unreadability** — the
//! record genuinely lost that event, so the ordinals after it close up. That is the
//! deliberate trade: the only alternative, hashing the entry's content, would give two
//! identical events appended twice the same id and make C-002's "one recorded event,
//! one contribution" collapse two real events into one. A closed snapshot's identity
//! is the snapshot's own stable `snapshot_id`, which is also its event id — so
//! repeated exports of one closed snapshot contribute once.

use std::collections::{HashMap, HashSet};
use std::fmt;

use crate::error::{Result, TixError};
use crate::model::{
    History, HistoryEntry, HistoryEvent, HistoryEventKind, TransitionDecision, validate_date,
};
use crate::planning::Planner;
use crate::workflow::WorkflowCategory;

/// Which planning entity a scope selects (`data-model.md` §1).
#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
pub enum ScopeKind {
    /// A cycle (hand-written in `[cycles.<name>]` or a generated instance).
    Cycle,
    /// A milestone.
    Milestone,
}

impl ScopeKind {
    /// Every kind, so a surface can enumerate the closed set.
    pub const ALL: [ScopeKind; 2] = [ScopeKind::Cycle, ScopeKind::Milestone];

    /// The canonical spelling used in output and comparisons.
    pub const fn as_str(self) -> &'static str {
        match self {
            ScopeKind::Cycle => "cycle",
            ScopeKind::Milestone => "milestone",
        }
    }

    /// Parse a canonical spelling; `None` for anything outside the vocabulary.
    pub fn parse(raw: &str) -> Option<Self> {
        match raw.trim().to_ascii_lowercase().as_str() {
            "cycle" => Some(ScopeKind::Cycle),
            "milestone" => Some(ScopeKind::Milestone),
            _ => None,
        }
    }
}

impl fmt::Display for ScopeKind {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.write_str(self.as_str())
    }
}

/// The inclusive calendar period a series covers (`data-model.md` §1).
///
/// Both bounds are **caller-supplied** `YYYY-MM-DD` strings — this module reads no
/// clock — and each is optional because a cycle need not declare its dates: an
/// absent bound is "unbounded on that side", which is coverage, not a guess. The
/// bounds come from existing planning data (`[`cycle_scope`]`/`[`milestone_scope`]`),
/// never from an assumption about when work started.
///
/// Comparison is `&str` comparison, which is chronological for that fixed-width
/// shape and needs no date arithmetic.
#[derive(Debug, Clone, PartialEq, Eq, Default)]
pub struct Period {
    /// Inclusive start, or `None` for "unbounded from the beginning".
    pub start: Option<String>,
    /// Inclusive end, or `None` for "still open".
    pub end: Option<String>,
}

impl Period {
    /// A validated period: each present bound must be a canonical calendar date and
    /// `start <= end` must hold.
    ///
    /// A malformed bound is refused here rather than silently dropped, because a
    /// period that quietly lost an end date would turn a closed series into an open
    /// one — the one thing C-006 forbids.
    pub fn new(start: Option<String>, end: Option<String>) -> Result<Self> {
        let period = Self { start, end };
        for (field, value) in [("start", &period.start), ("end", &period.end)] {
            if let Some(date) = value
                && !validate_date(date)
            {
                return Err(TixError::Config(format!(
                    "invalid_period: `{field}` is `{date}`, not a `YYYY-MM-DD` calendar date"
                )));
            }
        }
        if let (Some(start), Some(end)) = (&period.start, &period.end)
            && start > end
        {
            return Err(TixError::Config(format!(
                "invalid_period: start `{start}` is after end `{end}`"
            )));
        }
        Ok(period)
    }

    /// A period with neither bound.
    pub fn unbounded() -> Self {
        Self::default()
    }

    /// Whether the period constrains anything at all.
    pub fn is_bounded(&self) -> bool {
        self.start.is_some() || self.end.is_some()
    }

    /// Whether a canonical `YYYY-MM-DD` date falls inside the period, **both bounds
    /// inclusive**.
    ///
    /// A date that is not a canonical calendar date is *not* contained: the caller
    /// reports the unreadable day as coverage rather than placing it somewhere.
    pub fn contains(&self, date: &str) -> bool {
        if !validate_date(date) {
            return false;
        }
        if let Some(start) = self.start.as_deref()
            && date < start
        {
            return false;
        }
        if let Some(end) = self.end.as_deref()
            && date > end
        {
            return false;
        }
        true
    }
}

/// The selected cycle/milestone and its period context (`data-model.md` §1).
///
/// The `snapshot_id` is the closed-series binding: when it is present the scope
/// refers to a **closed** historical view, and events after the period's end are not
/// contributions to it (C-006 — a later edit never rewrites a closed series).
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct AnalyticsScope {
    /// Which planning entity the `id` names.
    pub kind: ScopeKind,
    /// The stable planning identity (a cycle name, a milestone name).
    pub id: String,
    /// The inclusive period context.
    pub period: Period,
    /// The closed snapshot this series is bound to, when it is bound to one.
    pub snapshot_id: Option<String>,
}

impl AnalyticsScope {
    /// A scope. A blank id is refused: an unnamed scope cannot be reported or
    /// compared, and defaulting it would hide the caller's mistake.
    pub fn new(kind: ScopeKind, id: impl Into<String>, period: Period) -> Result<Self> {
        let id = id.into();
        if id.trim().is_empty() {
            return Err(TixError::Config(format!(
                "invalid_scope: a {} scope needs a name",
                kind.as_str()
            )));
        }
        Ok(Self {
            kind,
            id,
            period,
            snapshot_id: None,
        })
    }

    /// A cycle scope.
    pub fn cycle(id: impl Into<String>, period: Period) -> Result<Self> {
        Self::new(ScopeKind::Cycle, id, period)
    }

    /// A milestone scope.
    pub fn milestone(id: impl Into<String>, period: Period) -> Result<Self> {
        Self::new(ScopeKind::Milestone, id, period)
    }

    /// Bind this scope to a closed snapshot's stable identity (C-006).
    pub fn with_snapshot_id(mut self, snapshot_id: impl Into<String>) -> Self {
        let snapshot_id = snapshot_id.into();
        self.snapshot_id = (!snapshot_id.trim().is_empty()).then_some(snapshot_id);
        self
    }

    /// The inclusive period start, when the planning data declares one.
    pub fn period_start(&self) -> Option<&str> {
        self.period.start.as_deref()
    }

    /// The inclusive period end, when the planning data declares one.
    pub fn period_end(&self) -> Option<&str> {
        self.period.end.as_deref()
    }

    /// Whether this scope is closed: bound to a snapshot or bounded by an end date.
    ///
    /// A closed scope's series stops at its period, which is what keeps a later edit
    /// from extending a historical view.
    pub fn is_closed(&self) -> bool {
        self.snapshot_id.is_some() || self.period.end.is_some()
    }

    /// Whether `date` belongs to this scope's period. An event with no readable day
    /// is handled by [`EventIndex::contribute`], not here.
    pub fn contains(&self, date: &str) -> bool {
        self.period.contains(date)
    }
}

impl fmt::Display for AnalyticsScope {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        let start = self.period_start().unwrap_or("open");
        let end = self.period_end().unwrap_or("open");
        write!(f, "{} `{}` {start}..{end}", self.kind, self.id)?;
        if let Some(snapshot_id) = self.snapshot_id.as_deref() {
            write!(f, " snapshot {snapshot_id}")?;
        }
        Ok(())
    }
}

/// Resolve a cycle scope through the existing planning registry (T001).
///
/// The period is the cycle's **own** declared `start_date`/`due_date`, so it comes
/// from existing planning data rather than from an assumption; a cycle that declares
/// neither is unbounded on that side, which the series reports as coverage.
pub fn cycle_scope(planner: &Planner, name: &str) -> Result<AnalyticsScope> {
    let cycle = planner
        .cycle(name)
        .ok_or_else(|| TixError::NotFound(format!("cycle `{name}`")))?;
    let period = Period::new(cycle.start_date.clone(), cycle.due_date.clone())?;
    AnalyticsScope::cycle(name, period)
}

/// Resolve a milestone scope through the existing planning registry (T001).
///
/// A milestone declares a target date and no start, so the period is bounded on its
/// end only — the same value the projection later forecasts against.
pub fn milestone_scope(planner: &Planner, name: &str) -> Result<AnalyticsScope> {
    let milestone = planner.milestone(name)?;
    let target = milestone.target_date.trim();
    let end = (!target.is_empty()).then(|| target.to_string());
    let period = Period::new(None, end)?;
    AnalyticsScope::milestone(name, period)
}

/// The closed classification vocabulary (`data-model.md` §2).
#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
pub enum EventClass {
    /// The recorded event adds work to the selected scope's planned set.
    ScopeAddition,
    /// The recorded event completes work in the selected scope.
    Completion,
    /// The recorded event changes the planned size of work in the scope.
    EstimateChange,
    /// A recorded event this classification does not place, or one it can only
    /// partly read. Never counted as progress.
    Unclassified,
}

impl EventClass {
    /// Every class, in the order a surface renders them.
    pub const ALL: [EventClass; 4] = [
        EventClass::ScopeAddition,
        EventClass::Completion,
        EventClass::EstimateChange,
        EventClass::Unclassified,
    ];

    /// The canonical spelling (`data-model.md` §2: `scope_addition`, `completion`,
    /// `estimate_change`, `unclassified`).
    pub const fn as_str(self) -> &'static str {
        match self {
            EventClass::ScopeAddition => "scope_addition",
            EventClass::Completion => "completion",
            EventClass::EstimateChange => "estimate_change",
            EventClass::Unclassified => "unclassified",
        }
    }

    /// Parse a canonical spelling; `None` for anything outside the vocabulary.
    pub fn parse(raw: &str) -> Option<Self> {
        match raw.trim().to_ascii_lowercase().as_str() {
            "scope_addition" => Some(EventClass::ScopeAddition),
            "completion" => Some(EventClass::Completion),
            "estimate_change" => Some(EventClass::EstimateChange),
            "unclassified" => Some(EventClass::Unclassified),
            _ => None,
        }
    }

    /// Whether the event carries a class other than [`EventClass::Unclassified`].
    pub const fn is_classified(self) -> bool {
        !matches!(self, EventClass::Unclassified)
    }
}

impl fmt::Display for EventClass {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.write_str(self.as_str())
    }
}

/// Where a recorded contribution came from (`data-model.md` §2).
#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
pub enum EventSource {
    /// An append-only History entry.
    History,
    /// A closed cycle/milestone snapshot's own recorded value.
    Snapshot,
    /// A recorded frontmatter value.
    Frontmatter,
}

impl EventSource {
    /// Every source, in the order a surface renders them.
    pub const ALL: [EventSource; 3] = [
        EventSource::History,
        EventSource::Snapshot,
        EventSource::Frontmatter,
    ];

    /// The canonical spelling used in output.
    pub const fn as_str(self) -> &'static str {
        match self {
            EventSource::History => "history",
            EventSource::Snapshot => "snapshot",
            EventSource::Frontmatter => "frontmatter",
        }
    }

    /// Parse a canonical spelling; `None` for anything outside the vocabulary.
    pub fn parse(raw: &str) -> Option<Self> {
        match raw.trim().to_ascii_lowercase().as_str() {
            "history" => Some(EventSource::History),
            "snapshot" => Some(EventSource::Snapshot),
            "frontmatter" => Some(EventSource::Frontmatter),
            _ => None,
        }
    }
}

impl fmt::Display for EventSource {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.write_str(self.as_str())
    }
}

/// One recorded contribution to a scope's series (`data-model.md` §2).
///
/// Every field is either read from the record or explicitly `None`. `delta` is the
/// **count-level** contribution of the event and is `None` where the event does not
/// carry one at that precision — a repoint, a membership change and an estimate
/// change all name their before/after values in [`Self::detail`], and inventing a
/// number for them would be the arithmetic C-003 forbids. `unit` is reserved for the
/// estimate mode's explicit conversion (WP03) and is `None` here.
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct AnalyticsEvent {
    /// Stable identity, and therefore the idempotency key (C-002).
    pub event_id: String,
    /// The canonical `YYYY-MM-DD` day the event is placed on, or `None` when the
    /// recorded timestamp carried no readable calendar date. `None` is coverage, not
    /// a fabricated day (C-005).
    pub date: Option<String>,
    /// The closed classification.
    pub class: EventClass,
    /// The issue the event belongs to, when the source ref is an issue number.
    pub issue: Option<u32>,
    /// The scope the event was classified against.
    pub scope_id: Option<String>,
    /// The count-level contribution, when the event carries one.
    pub delta: Option<i64>,
    /// The estimate unit, for the estimate mode's conversions (WP03).
    pub unit: Option<String>,
    /// Where the contribution came from.
    pub source: EventSource,
    /// The closed snapshot the contribution came from, when it is snapshot evidence.
    pub snapshot_id: Option<String>,
    /// The recorded detail, verbatim, so a reader can see the before/after values
    /// the classification did not turn into numbers.
    pub detail: String,
}

impl AnalyticsEvent {
    /// Classify one **recorded** History entry (T002–T004).
    ///
    /// `source_ref` names the file the entry lives in (an issue number, a generated
    /// cycle id) and `index` is its ordinal among that section's **parsed events**
    /// (see [`EventIndex::contribute_history`], which is what assigns it); together
    /// they are the event's stable identity, so re-reading the same file classifies
    /// the same event to the same id.
    pub fn from_history(
        source_ref: &str,
        index: usize,
        scope: &AnalyticsScope,
        event: &HistoryEvent,
    ) -> Self {
        let (class, delta) = classify(event);
        Self {
            event_id: format!("{source_ref}#{index}"),
            date: calendar_date(&event.timestamp),
            class,
            issue: source_ref.trim().parse::<u32>().ok(),
            scope_id: Some(scope.id.clone()),
            delta,
            unit: None,
            source: EventSource::History,
            snapshot_id: None,
            detail: event.detail.clone(),
        }
    }

    /// A contribution read from an immutable closed snapshot (T003/T005).
    ///
    /// The caller supplies values it **read from** the snapshot; nothing here
    /// derives a number from a schedule or a configuration, because those are
    /// exactly what a later edit is allowed to change (C-006). The event id is the
    /// snapshot's own stable `snapshot_id`, so the same closed snapshot contributes
    /// once however many times it is read.
    pub fn from_snapshot(
        snapshot_id: &str,
        date: Option<&str>,
        class: EventClass,
        scope_id: Option<&str>,
        delta: Option<i64>,
    ) -> Self {
        Self {
            event_id: format!("snapshot:{}", snapshot_id.trim()),
            date: date
                .map(str::trim)
                .filter(|d| validate_date(d))
                .map(str::to_string),
            class,
            issue: None,
            scope_id: scope_id.map(str::to_string),
            delta,
            unit: None,
            source: EventSource::Snapshot,
            snapshot_id: Some(snapshot_id.trim().to_string()),
            detail: String::new(),
        }
    }

    /// Whether the event is placed on `date`.
    pub fn is_on(&self, date: &str) -> bool {
        self.date.as_deref() == Some(date)
    }
}

/// What [`EventIndex::contribute`] did with one event.
#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
pub enum Contribution {
    /// The event was recorded as a contribution.
    Added,
    /// The event's id was already present: a repeated classification contributes
    /// once (C-002).
    Duplicate,
    /// The event carries a day outside the scope's period, so it is not a
    /// contribution to this series. Counted, never discarded silently.
    OutsidePeriod,
}

impl Contribution {
    /// The canonical spelling used in output.
    pub const fn as_str(self) -> &'static str {
        match self {
            Contribution::Added => "added",
            Contribution::Duplicate => "duplicate",
            Contribution::OutsidePeriod => "outside_period",
        }
    }

    /// Whether this contribution changed the index.
    pub const fn contributed(self) -> bool {
        matches!(self, Contribution::Added)
    }
}

/// The contribution set for one scope: contributions in first-seen order,
/// deduplicated by event id (T005).
///
/// Inflating this type is deliberate — the count-mode fold (WP02) reads it once, so
/// a contribution cannot be counted twice by being walked twice, and the dedupe
/// lives here rather than at each caller.
#[derive(Debug, Clone)]
pub struct EventIndex {
    scope: AnalyticsScope,
    order: Vec<String>,
    events: HashMap<String, AnalyticsEvent>,
    /// Ids already refused for falling outside the scope's period.
    ///
    /// Remembered for the same reason the admitted ids are: one recorded event is one
    /// event, so classifying it again must report a duplicate rather than counting a
    /// second exclusion. Without this, a repeated fold inflated
    /// [`Self::outside_period`] and a caller reading it as a coverage size saw a count
    /// of *classifications*.
    outside_ids: HashSet<String>,
    duplicates: usize,
    outside_period: usize,
}

impl EventIndex {
    /// An empty index for a scope.
    pub fn new(scope: AnalyticsScope) -> Self {
        Self {
            scope,
            order: Vec::new(),
            events: HashMap::new(),
            outside_ids: HashSet::new(),
            duplicates: 0,
            outside_period: 0,
        }
    }

    /// The scope every contribution in this index was classified against.
    pub fn scope(&self) -> &AnalyticsScope {
        &self.scope
    }

    /// Contribute one event, reporting what happened.
    ///
    /// The duplicate check covers ids already **admitted and** ids already refused for
    /// being outside the period, and it comes first; the period check comes second. So
    /// classifying one recorded event twice is inert whichever side of the period it
    /// fell on, and both counters measure recorded events rather than classifications.
    pub fn contribute(&mut self, event: AnalyticsEvent) -> Contribution {
        if self.events.contains_key(&event.event_id) || self.outside_ids.contains(&event.event_id) {
            self.duplicates += 1;
            return Contribution::Duplicate;
        }
        if let Some(date) = event.date.as_deref()
            && !self.scope.contains(date)
        {
            self.outside_ids.insert(event.event_id);
            self.outside_period += 1;
            return Contribution::OutsidePeriod;
        }
        self.order.push(event.event_id.clone());
        self.events.insert(event.event_id.clone(), event);
        Contribution::Added
    }

    /// Contribute every parsed entry of a History section, returning how many were
    /// newly added.
    ///
    /// The ordinal an event is identified by counts **parsed events only**: an
    /// unparseable line a human inserted occupies no ordinal, so it cannot renumber
    /// the events after it and two classifications of one file agree on every id. A
    /// line that was a readable event and was mangled into unreadability does still
    /// close up the ordinals behind it — see the module's provenance note for why that
    /// is the accepted trade rather than a content hash.
    pub fn contribute_history(&mut self, source_ref: &str, history: &History) -> usize {
        let scope = self.scope.clone();
        let mut added = 0;
        let mut index = 0;
        for entry in &history.entries {
            let HistoryEntry::Event(event) = entry else {
                continue;
            };
            let event = AnalyticsEvent::from_history(source_ref, index, &scope, event);
            index += 1;
            if self.contribute(event).contributed() {
                added += 1;
            }
        }
        added
    }

    /// The contributed events, in first-seen order.
    pub fn events(&self) -> impl Iterator<Item = &AnalyticsEvent> + '_ {
        self.order.iter().filter_map(|id| self.events.get(id))
    }

    /// The contributed events of one class, in first-seen order.
    pub fn of_class(&self, class: EventClass) -> impl Iterator<Item = &AnalyticsEvent> + '_ {
        self.events().filter(move |event| event.class == class)
    }

    /// How many events of one class contributed.
    pub fn count_of(&self, class: EventClass) -> usize {
        self.of_class(class).count()
    }

    /// How many contributions the index holds.
    pub fn len(&self) -> usize {
        self.events.len()
    }

    /// Whether the index holds no contribution at all — which is the honest answer
    /// for a project whose files carry no events, not an error.
    pub fn is_empty(&self) -> bool {
        self.events.is_empty()
    }

    /// How many repeated classifications the index rejected (C-002).
    pub fn duplicates(&self) -> usize {
        self.duplicates
    }

    /// How many recorded events fell outside the scope's period and were therefore
    /// not contributions — an **event** count, not a count of classifications, because
    /// an id already refused here is remembered (C-002). Coverage context: a non-zero
    /// count says the period excluded recorded history, never that the history is
    /// absent.
    pub fn outside_period(&self) -> usize {
        self.outside_period
    }
}

/// The recorded day a History timestamp names, when it carries one.
///
/// The timestamp is RFC 3339 (`2026-09-25T16:00:00Z`), so its first ten characters
/// are the calendar date; anything that is not a canonical date — including a
/// truncated or hand-mangled stamp — is `None` rather than a coerced guess.
pub fn calendar_date(timestamp: &str) -> Option<String> {
    let stamp = timestamp.trim();
    let date = stamp.get(..10)?;
    validate_date(date).then(|| date.to_string())
}

/// Classify one recorded event kind.
///
/// See the module documentation for the partition and why each entry is where it is.
fn classify(event: &HistoryEvent) -> (EventClass, Option<i64>) {
    match event.kind {
        // The hierarchy's membership kinds: the issue joins a scope. A *repoint*
        // names both parents in its detail, so it carries no count-level delta —
        // the departing scope's reduction is not something one event can express.
        HistoryEventKind::ParentSet => (EventClass::ScopeAddition, Some(1)),
        HistoryEventKind::ParentChanged => (EventClass::ScopeAddition, None),
        // WP01's own producer (T046/T047): a membership edit routinely adds and
        // removes in one command, so the direction is read from the detail the
        // producer writes. Only a change that **added** a cycle is an addition; a
        // change that only removed one is not growth and stays visible as unclassified.
        HistoryEventKind::CycleMembershipChanged => membership_outcome(&event.detail),
        // WP01's other producer: the planned size changed, not the membership.
        HistoryEventKind::EstimateChanged => (EventClass::EstimateChange, None),
        HistoryEventKind::TransitionAccepted => completion_outcome(event),
        // Everything else stays visible and unclassified — including every one of
        // #184's time-mutation kinds, which must never move a burndown line.
        _ => (EventClass::Unclassified, None),
    }
}

/// Whether a recorded transition completed the issue, read through the shared
/// category vocabulary the decision *recorded*.
fn completion_outcome(event: &HistoryEvent) -> (EventClass, Option<i64>) {
    let Ok(decision) = TransitionDecision::from_event(event) else {
        // A malformed or hand-edited decision is not counted as progress; the raw
        // detail is still carried on the event for a reader.
        return (EventClass::Unclassified, None);
    };
    match WorkflowCategory::from_key(&decision.to_category) {
        Some(category) if category.is_complete() => (EventClass::Completion, Some(1)),
        // A move into any other category is a move. `canceled` is terminal and is
        // *not* complete, and an unknown category is never read as one.
        _ => (EventClass::Unclassified, None),
    }
}

/// Whether a recorded membership change **added** a cycle, read from the detail the
/// producer writes.
///
/// The grammar is the one `tix edit --cycles` appends (`src/commands/edit.rs`):
/// `cycles: <before> -> <after>`, each side a comma-joined list of cycle names or
/// `none`. `--cycles` splits on commas, so a name cannot contain one, which is what
/// makes the list unambiguous. A detail that does not have that shape (a hand edit, a
/// future writer) is not guessed at: it classifies as unclassified and keeps its text
/// on the event, so a reader sees exactly what could not be read.
///
/// Only an **addition** counts. A change that removed cycles — `cycles: Default ->
/// none` — or merely reordered them has nothing to add, so it is not growth; the
/// closed four-name vocabulary has no removal class, and reading one as an addition
/// would be the exact confusion FR-002 exists to prevent.
///
/// The delta is `None` because the count a membership change carries is per *joined*
/// cycle, and which cycle the series is for is the fold's question (WP02); a
/// scope-agnostic `Some(1)` here would credit a cycle the event never named.
fn membership_outcome(detail: &str) -> (EventClass, Option<i64>) {
    let Some((before, after)) = membership_lists(detail) else {
        return (EventClass::Unclassified, None);
    };
    if after.iter().any(|name| !before.contains(name)) {
        (EventClass::ScopeAddition, None)
    } else {
        (EventClass::Unclassified, None)
    }
}

/// The before/after cycle lists a `cycle_membership_changed` detail names.
fn membership_lists(detail: &str) -> Option<(Vec<String>, Vec<String>)> {
    let rest = detail.trim().strip_prefix("cycles:")?;
    let (before, after) = rest.split_once("->")?;
    Some((membership_list(before), membership_list(after)))
}

/// One side of the membership detail: the names, or empty for `none`.
fn membership_list(side: &str) -> Vec<String> {
    let side = side.trim();
    if side.is_empty() || side == "none" {
        return Vec::new();
    }
    side.split(',')
        .map(str::trim)
        .filter(|name| !name.is_empty())
        .map(str::to_string)
        .collect()
}

#[cfg(test)]
mod tests {
    #[allow(clippy::wildcard_imports)]
    use super::*;
    use crate::model::utc_now_rfc3339;

    fn scope() -> AnalyticsScope {
        AnalyticsScope::cycle("Sprint 7", Period::new(None, None).unwrap()).unwrap()
    }

    fn history_event(kind: HistoryEventKind, detail: &str) -> HistoryEvent {
        HistoryEvent::new("2026-09-25T16:00:00Z", "alice", kind, detail)
    }

    #[test]
    fn t005_the_period_bounds_are_validated_and_inclusive() {
        let period = Period::new(Some("2026-09-01".into()), Some("2026-09-30".into())).unwrap();
        assert!(period.contains("2026-09-01"), "start is inclusive");
        assert!(period.contains("2026-09-30"), "end is inclusive");
        assert!(!period.contains("2026-08-31"), "the day before is out");
        assert!(!period.contains("2026-10-01"), "the day after is out");
        // The load-bearing case for the validator: a malformed day that *sorts inside*
        // the bounds (`2026-09-1` is greater than `2026-09-01` and less than
        // `2026-09-30` character by character), so the bounds comparison alone would
        // admit it. Deleting `validate_date` from `contains` makes this assertion fail.
        assert!(
            !period.contains("2026-09-1"),
            "a malformed day inside the bounds is still not placed"
        );
        assert!(
            !period.contains("2026-09-01T00:00:00Z"),
            "a stamp is not a day either"
        );
        assert!(
            !period.contains("not-a-date"),
            "an unreadable day is not placed"
        );
        assert!(Period::unbounded().contains("1999-01-01"), "open is open");

        let backwards = Period::new(Some("2026-09-30".into()), Some("2026-09-01".into()));
        assert!(backwards.is_err(), "a backwards period is refused");
        let malformed = Period::new(Some("2026-9-1".into()), None);
        assert!(malformed.is_err(), "a non-canonical bound is refused");
    }

    #[test]
    fn t006_a_blank_scope_name_is_refused() {
        assert!(AnalyticsScope::cycle("  ", Period::unbounded()).is_err());
    }

    #[test]
    fn t005_a_duplicate_event_contributes_once() {
        let mut index = EventIndex::new(scope());
        let event = history_event(HistoryEventKind::EstimateChanged, "estimate: none -> 3");
        let classified = AnalyticsEvent::from_history("7", 0, index.scope(), &event);
        assert_eq!(index.contribute(classified.clone()), Contribution::Added);
        assert_eq!(
            index.contribute(classified.clone()),
            Contribution::Duplicate
        );
        assert_eq!(index.len(), 1, "the same event contributed once");
        assert_eq!(index.duplicates(), 1);
        assert_eq!(index.of_class(EventClass::EstimateChange).count(), 1);
    }

    #[test]
    fn t001_the_scope_classifies_only_the_event_it_was_given() {
        let index = EventIndex::new(scope());
        let event = history_event(HistoryEventKind::ParentSet, "parent none -> #1");
        let classified = AnalyticsEvent::from_history("7", 3, index.scope(), &event);
        assert_eq!(classified.event_id, "7#3");
        assert_eq!(classified.issue, Some(7));
        assert_eq!(classified.scope_id.as_deref(), Some("Sprint 7"));
        assert_eq!(classified.date.as_deref(), Some("2026-09-25"));
        assert_eq!(classified.class, EventClass::ScopeAddition);
        assert_eq!(classified.delta, Some(1));
        assert_eq!(classified.source, EventSource::History);
    }

    #[test]
    fn t005_an_unreadable_timestamp_is_no_day_rather_than_a_guess() {
        assert_eq!(
            calendar_date("2026-09-25T16:00:00Z").as_deref(),
            Some("2026-09-25")
        );
        assert_eq!(
            calendar_date("2026-13-45T00:00:00Z"),
            None,
            "not a real date"
        );
        assert_eq!(calendar_date("2026-09"), None, "truncated");
        assert_eq!(calendar_date(""), None);
    }

    #[test]
    fn t005_a_snapshot_contribution_is_identified_by_its_snapshot() {
        let mut index = EventIndex::new(scope());
        let event = AnalyticsEvent::from_snapshot(
            "cs-1:000001@2026-09-30T00:00:00Z",
            Some("2026-09-30"),
            EventClass::Completion,
            Some("cs-1:000001"),
            Some(2),
        );
        assert_eq!(index.contribute(event.clone()), Contribution::Added);
        assert_eq!(index.contribute(event), Contribution::Duplicate);
        assert_eq!(index.len(), 1);
        let stored = index.events().next().unwrap();
        assert_eq!(stored.event_id, "snapshot:cs-1:000001@2026-09-30T00:00:00Z");
        assert_eq!(stored.source, EventSource::Snapshot);
        assert_eq!(
            stored.snapshot_id.as_deref(),
            Some("cs-1:000001@2026-09-30T00:00:00Z"),
            "the closed snapshot's identity is retained verbatim"
        );
    }

    #[test]
    fn t005_a_time_mutation_kind_is_never_progress() {
        for kind in [
            HistoryEventKind::TimeEntryCreated,
            HistoryEventKind::TimeEntryUpdated,
            HistoryEventKind::TimeEntryDeleted,
        ] {
            let event = history_event(kind, "entry t-1; created 30m on 2026-09-25 by alice");
            let classified = AnalyticsEvent::from_history("7", 0, &scope(), &event);
            assert_eq!(
                classified.class,
                EventClass::Unclassified,
                "{kind} must not be read as progress"
            );
            assert_eq!(classified.delta, None);
        }
    }

    #[test]
    fn t003_a_parent_cleared_event_is_not_growth() {
        let event = history_event(HistoryEventKind::ParentCleared, "parent #1 -> none");
        let classified = AnalyticsEvent::from_history("7", 0, &scope(), &event);
        assert_eq!(classified.class, EventClass::Unclassified);
        assert_eq!(classified.delta, None, "a removal is never counted as +1");
    }

    #[test]
    fn t003_a_membership_change_is_read_by_direction() {
        for (detail, class) in [
            ("cycles: none -> Default", EventClass::ScopeAddition),
            (
                "cycles: Default -> Default, Sprint 7",
                EventClass::ScopeAddition,
            ),
            ("cycles: Default -> none", EventClass::Unclassified),
            (
                "cycles: Default, Sprint 7 -> Default",
                EventClass::Unclassified,
            ),
            (
                "cycles: Default, Sprint 7 -> Sprint 7, Default",
                EventClass::Unclassified,
            ),
            ("mangled", EventClass::Unclassified),
            ("cycles: Default ->", EventClass::Unclassified),
        ] {
            let event = history_event(HistoryEventKind::CycleMembershipChanged, detail);
            let classified = AnalyticsEvent::from_history("7", 0, &scope(), &event);
            assert_eq!(classified.class, class, "{detail}");
            assert_eq!(classified.delta, None, "{detail}");
            assert_eq!(classified.detail, detail, "the record is kept verbatim");
        }
    }

    #[test]
    fn t005_a_repeated_out_of_period_event_is_a_duplicate_not_a_second_exclusion() {
        let period = Period::new(Some("2026-09-01".into()), Some("2026-09-30".into())).unwrap();
        let bounded = AnalyticsScope::cycle("Sprint 7", period).unwrap();
        let mut index = EventIndex::new(bounded);
        let event = HistoryEvent::new(
            "2026-10-05T00:00:00Z",
            "alice",
            HistoryEventKind::ParentSet,
            "parent none -> #2",
        );
        let classified = AnalyticsEvent::from_history("1", 0, index.scope(), &event);
        assert_eq!(
            index.contribute(classified.clone()),
            Contribution::OutsidePeriod
        );
        assert_eq!(
            index.contribute(classified),
            Contribution::Duplicate,
            "one recorded event, however often it is classified"
        );
        assert_eq!(
            index.outside_period(),
            1,
            "an event count, not a count of classifications"
        );
        assert_eq!(index.duplicates(), 1);
        assert_eq!(index.len(), 0);
    }

    #[test]
    fn t004_a_malformed_transition_record_is_not_a_completion() {
        let event = history_event(HistoryEventKind::TransitionAccepted, "mangled");
        let classified = AnalyticsEvent::from_history("7", 0, &scope(), &event);
        assert_eq!(classified.class, EventClass::Unclassified);
        assert_eq!(classified.detail, "mangled", "the raw record is kept");
    }

    #[test]
    fn t005_the_round_trip_of_a_recorded_file_is_idempotent() {
        let mut history = History::new();
        history.append(HistoryEvent::new(
            utc_now_rfc3339(),
            "alice",
            HistoryEventKind::EstimateChanged,
            "estimate: none -> 3",
        ));
        let mut index = EventIndex::new(scope());
        assert_eq!(index.contribute_history("7", &history), 1);
        assert_eq!(index.contribute_history("7", &history), 0, "already seen");
        assert_eq!(index.len(), 1, "the same file contributes once");
    }
}