// Package session owns the daemon's in-memory state of truth and persists a
// snapshot on every change. Transitions go through the pure statemachine. It
// also owns per-session evidence stats: it accumulates active-window time while
// Active, logs raw focus events, and seals each session into the audit chain.
package session

import (
	"context"
	"errors"
	"log"
	"path/filepath"
	"sort"
	"strings"
	"sync"
	"time"

	"antidrift/internal/ai"
	"antidrift/internal/domain"
	"antidrift/internal/evidence"
	"antidrift/internal/statemachine"
	"antidrift/internal/store"

	"github.com/google/uuid"
)

const (
	unavailableTitle = "(evidence unavailable)"
	sessionRetention = 30 * 24 * time.Hour
)

const coachTimeout = 60 * time.Second

const (
	coachIdle    = "idle"
	coachPending = "pending"
	coachReady   = "ready"
	coachError   = "error"
)

const (
	driftDebounce = 10 * time.Second
	driftTimeout  = 30 * time.Second
	nudgeDebounce = 5 * time.Minute
	nudgeTimeout  = 30 * time.Second
)

const recentTitlesMax = 10

const (
	driftIdle     = "idle"
	driftPending  = "pending"
	driftOnTask   = "ontask"
	driftDrifting = "drifting"
)

var ErrNotPlanning = errors.New("session: coaching is only available while planning")

var ErrNotActive = errors.New("session: only available while a commitment is active")

// bucketKey identifies a time bucket; Title is the scrubbed title.
type bucketKey struct{ Class, Title string }

// EvidenceStats is the in-memory accounting for the current session only.
type EvidenceStats struct {
	SessionID   string
	StartedUnix int64
	Buckets     map[bucketKey]time.Duration
	SwitchCount int
	Current     evidence.WindowSnapshot
	lastFocusAt time.Time
	lastKey     bucketKey
	hasLast     bool
}

// Controller holds runtime state and the active commitment behind a mutex.
type Controller struct {
	mu             sync.Mutex
	runtimeState   domain.RuntimeState
	commitment     *domain.Commitment
	deadline       time.Time
	snapshotPath   string
	auditPath      string
	sessionsDir    string
	clock          func() time.Time
	onChange       func()
	latestWindow   evidence.WindowSnapshot
	stats          *EvidenceStats
	outcomePending string
	coach          ai.Coach
	coachStatus    string
	coachProposal  *ai.Proposal
	coachErr       string
	coachGen       int

	allowedClasses []string // durable: the active session's allowed window classes
	judge          ai.DriftJudge
	driftStatus    string
	driftReason    string
	driftGen       int
	nudgeEpoch     int // identifies the current on-task stretch; nudge staleness guard
	lastJudgedAt   time.Time
	judgedClasses  map[string]ai.Verdict

	nudge        ai.Nudger
	recentTitles []string // in-memory ring of recent distinct titles this session
	nudgeMessage string   // current soft advisory ("" = none)
	lastNudgedAt time.Time
}

// CommitmentView is the UI projection of the active commitment.
type CommitmentView struct {
	NextAction       string `json:"next_action"`
	SuccessCondition string `json:"success_condition"`
	TimeboxSecs      int64  `json:"timebox_secs"`
	DeadlineUnixSecs int64  `json:"deadline_unix_secs"`
}

// ProposalView / CoachView project the ephemeral planning-coach state.
type ProposalView struct {
	NextAction       string `json:"next_action"`
	SuccessCondition string `json:"success_condition"`
	TimeboxSecs      int64  `json:"timebox_secs"`

	AllowedWindowClasses []string `json:"allowed_window_classes,omitempty"`
}

// DriftView is the active-only drift projection. Nudge is a separate axis from
// Status: it is populated precisely when Status is "ontask" (semantic drift
// inside an allowed app), so it cannot be folded into the status enum.
type DriftView struct {
	Status string `json:"status"`
	Reason string `json:"reason,omitempty"`
	Nudge  string `json:"nudge,omitempty"`
}

type CoachView struct {
	Status   string        `json:"status"`
	Proposal *ProposalView `json:"proposal,omitempty"`
	Error    string        `json:"error,omitempty"`
}

// WindowView / BucketView / EvidenceView are the evidence projection.
type WindowView struct {
	Class string `json:"class"`
	Title string `json:"title"`
}

type BucketView struct {
	Class   string `json:"class"`
	Title   string `json:"title"`
	Seconds int64  `json:"seconds"`
}

type EvidenceView struct {
	Available   bool         `json:"available"`
	Reason      string       `json:"reason"`
	Current     WindowView   `json:"current"`
	SwitchCount int          `json:"switch_count"`
	Buckets     []BucketView `json:"buckets"`
}

// State is the broadcastable view of the controller.
type State struct {
	RuntimeState domain.RuntimeState `json:"runtime_state"`
	Commitment   *CommitmentView     `json:"commitment"`
	Evidence     *EvidenceView       `json:"evidence"`
	Coach        *CoachView          `json:"coach,omitempty"`
	Drift        *DriftView          `json:"drift,omitempty"`
}

// New loads any persisted snapshot, prunes stale session logs, and rebuilds
// in-memory stats from the raw log if a live session was interrupted.
func New(snapshotPath string) (*Controller, error) {
	s, err := store.Load(snapshotPath)
	if err != nil {
		return nil, err
	}
	dir := filepath.Dir(snapshotPath)
	c := &Controller{
		runtimeState:   s.RuntimeState,
		commitment:     s.Commitment,
		snapshotPath:   snapshotPath,
		auditPath:      filepath.Join(dir, "audit.jsonl"),
		sessionsDir:    filepath.Join(dir, "sessions"),
		clock:          time.Now,
		outcomePending: s.OutcomePending,
	}
	if s.DeadlineUnixSecs > 0 {
		c.deadline = time.Unix(s.DeadlineUnixSecs, 0)
	}
	if c.runtimeState == "" {
		c.runtimeState = domain.RuntimeLocked
	}
	_ = store.PruneOlderThan(c.sessionsDir, sessionRetention, c.clock())
	if c.runtimeState == domain.RuntimeActive && s.SessionID != "" {
		c.allowedClasses = s.AllowedWindowClasses
		// Drift state is not persisted: recompute fresh after restart to avoid
		// stale interrupts. This also initializes judgedClasses (the pipeline
		// writes to it) so a restored Active session never panics on a nil map.
		c.resetDriftLocked()
		c.replayStats(s.SessionID)
	}
	return c, nil
}

// SetClock overrides the time source (tests only). Call before starting a
// session.
func (c *Controller) SetClock(f func() time.Time) {
	c.mu.Lock()
	c.clock = f
	c.mu.Unlock()
}

// SetOnChange registers a callback fired after an evidence-driven state change
// (focus updates). It is invoked with the mutex released.
func (c *Controller) SetOnChange(f func()) {
	c.mu.Lock()
	c.onChange = f
	c.mu.Unlock()
}

func (c *Controller) notify() {
	c.mu.Lock()
	f := c.onChange
	c.mu.Unlock()
	if f != nil {
		f()
	}
}

// State returns the current broadcastable state. Safe for concurrent use.
func (c *Controller) State() State {
	c.mu.Lock()
	defer c.mu.Unlock()
	return c.stateLocked()
}

// Deadline returns the active commitment deadline, or the zero time.
func (c *Controller) Deadline() time.Time {
	c.mu.Lock()
	defer c.mu.Unlock()
	return c.deadline
}

func (c *Controller) stateLocked() State {
	st := State{RuntimeState: c.runtimeState}
	if c.commitment != nil {
		view := &CommitmentView{
			NextAction:       c.commitment.NextAction,
			SuccessCondition: c.commitment.SuccessCondition,
			TimeboxSecs:      c.commitment.TimeboxSecs,
		}
		if !c.deadline.IsZero() {
			view.DeadlineUnixSecs = c.deadline.Unix()
		}
		st.Commitment = view
	}
	if c.stats != nil {
		st.Evidence = &EvidenceView{
			Available:   c.stats.Current.Health.Available,
			Reason:      c.stats.Current.Health.Reason,
			Current:     WindowView{Class: c.stats.Current.Class, Title: c.stats.Current.Title},
			SwitchCount: c.stats.SwitchCount,
			Buckets:     bucketViews(c.stats.Buckets),
		}
	}
	if c.runtimeState == domain.RuntimePlanning {
		status := c.coachStatus
		if status == "" {
			status = coachIdle
		}
		cv := &CoachView{Status: status, Error: c.coachErr}
		if c.coachProposal != nil {
			cv.Proposal = &ProposalView{
				NextAction:           c.coachProposal.NextAction,
				SuccessCondition:     c.coachProposal.SuccessCondition,
				TimeboxSecs:          c.coachProposal.TimeboxSecs,
				AllowedWindowClasses: c.coachProposal.AllowedWindowClasses,
			}
		}
		st.Coach = cv
	}
	if c.runtimeState == domain.RuntimeActive {
		status := c.driftStatus
		if status == "" {
			status = driftIdle
		}
		st.Drift = &DriftView{Status: status, Reason: c.driftReason, Nudge: c.nudgeMessage}
	}
	return st
}

func bucketViews(buckets map[bucketKey]time.Duration) []BucketView {
	out := make([]BucketView, 0, len(buckets))
	for k, d := range buckets {
		out = append(out, BucketView{Class: k.Class, Title: k.Title, Seconds: int64(d.Seconds())})
	}
	sort.Slice(out, func(i, j int) bool { return out[i].Seconds > out[j].Seconds })
	return out
}

func (c *Controller) persistLocked() error {
	snap := store.Snapshot{
		RuntimeState:   c.runtimeState,
		Commitment:     c.commitment,
		OutcomePending: c.outcomePending,
	}
	if !c.deadline.IsZero() {
		snap.DeadlineUnixSecs = c.deadline.Unix()
	}
	if c.stats != nil {
		snap.SessionID = c.stats.SessionID
	}
	snap.AllowedWindowClasses = c.allowedClasses
	return store.Save(c.snapshotPath, snap)
}

// EnterPlanning moves Locked -> Planning.
func (c *Controller) EnterPlanning() error {
	c.mu.Lock()
	defer c.mu.Unlock()
	next, err := statemachine.TransitionRuntime(c.runtimeState, statemachine.EnterPlanning)
	if err != nil {
		return err
	}
	c.runtimeState = next
	c.resetCoachLocked()
	return c.persistLocked()
}

// SetCoach injects the AI coach. Mirrors SetOnChange. A nil coach makes
// RequestCoach degrade gracefully.
func (c *Controller) SetCoach(coach ai.Coach) {
	c.mu.Lock()
	c.coach = coach
	c.mu.Unlock()
}

// resetCoachLocked returns coach state to idle and invalidates any in-flight
// request. Caller holds mu.
func (c *Controller) resetCoachLocked() {
	c.coachStatus = coachIdle
	c.coachProposal = nil
	c.coachErr = ""
	c.coachGen++
}

// AllowedClassesForTest exposes the session allowed classes for tests.
func (c *Controller) AllowedClassesForTest() []string {
	c.mu.Lock()
	defer c.mu.Unlock()
	return append([]string(nil), c.allowedClasses...)
}

// SetDriftJudge injects the AI drift judge. A nil judge keeps local matching
// working; unmatched windows simply stay idle.
func (c *Controller) SetDriftJudge(j ai.DriftJudge) {
	c.mu.Lock()
	c.judge = j
	c.mu.Unlock()
}

// SetNudge injects the AI semantic nudge judge. A nil nudger disables nudging;
// local matching and the drift judge are unaffected.
func (c *Controller) SetNudge(n ai.Nudger) {
	c.mu.Lock()
	c.nudge = n
	c.mu.Unlock()
}

// commitmentLineLocked renders the active commitment as a single line for AI
// prompts, or "" if there is none. Caller holds mu.
func (c *Controller) commitmentLineLocked() string {
	if c.commitment == nil {
		return ""
	}
	return c.commitment.NextAction + " — " + c.commitment.SuccessCondition
}

// recentTitlesForTest returns a copy of the recent-titles ring (test-only).
func (c *Controller) recentTitlesForTest() []string {
	c.mu.Lock()
	defer c.mu.Unlock()
	return append([]string(nil), c.recentTitles...)
}

// resetDriftLocked clears all drift machinery for a fresh session. Caller holds mu.
func (c *Controller) resetDriftLocked() {
	c.driftStatus = driftIdle
	c.driftReason = ""
	c.driftGen++
	c.nudgeEpoch++
	c.lastJudgedAt = time.Time{}
	c.judgedClasses = map[string]ai.Verdict{}
	c.recentTitles = nil
	c.nudgeMessage = ""
	c.lastNudgedAt = time.Time{}
}

// OnTask appends the current window class to the session allowed-context, clears
// drift, drops any cached verdict for that class, and persists. The class now
// matches locally and is never re-judged.
func (c *Controller) OnTask() error {
	c.mu.Lock()
	defer c.mu.Unlock()
	if c.runtimeState != domain.RuntimeActive {
		return ErrNotActive
	}
	var class string
	if c.stats != nil {
		class = c.stats.Current.Class
	}
	if strings.TrimSpace(class) != "" {
		ac := domain.AllowedContext{WindowClasses: c.allowedClasses}
		if !evidence.MatchesAllowed(ac, class, "") {
			c.allowedClasses = append(c.allowedClasses, strings.TrimSpace(class))
		}
		delete(c.judgedClasses, class)
	}
	c.driftStatus = driftOnTask
	c.driftReason = ""
	return c.persistLocked()
}

// Refocus clears the current drift verdict without changing allowed-context, and
// drops the cached verdict for the current class so it may be judged again later.
func (c *Controller) Refocus() error {
	c.mu.Lock()
	defer c.mu.Unlock()
	if c.runtimeState != domain.RuntimeActive {
		return ErrNotActive
	}
	if c.stats != nil {
		delete(c.judgedClasses, c.stats.Current.Class)
	}
	c.driftStatus = driftIdle
	c.driftReason = ""
	return c.persistLocked()
}

// RequestCoach starts an async coach call for intent. Returns ErrNotPlanning if
// not in planning; otherwise never a hard error (failures surface as coach
// state). The proposal is ephemeral and never persisted.
func (c *Controller) RequestCoach(intent string) error {
	c.mu.Lock()
	if c.runtimeState != domain.RuntimePlanning {
		c.mu.Unlock()
		return ErrNotPlanning
	}
	if c.coach == nil {
		c.coachStatus = coachError
		c.coachErr = "coach unavailable"
		c.coachProposal = nil
		c.mu.Unlock()
		c.notify()
		return nil
	}
	c.coachGen++
	gen := c.coachGen
	c.coachStatus = coachPending
	c.coachErr = ""
	c.coachProposal = nil
	coach := c.coach
	c.mu.Unlock()
	c.notify()

	go func() {
		ctx, cancel := context.WithTimeout(context.Background(), coachTimeout)
		defer cancel()
		prop, err := coach.Coach(ctx, intent)

		c.mu.Lock()
		if gen != c.coachGen || c.runtimeState != domain.RuntimePlanning {
			c.mu.Unlock()
			return // stale or left planning: discard
		}
		if err != nil {
			c.coachStatus = coachError
			c.coachErr = coachErrorMessage(err)
			c.coachProposal = nil
		} else {
			c.coachStatus = coachReady
			c.coachProposal = &prop
			c.coachErr = ""
		}
		c.mu.Unlock()
		c.notify()
	}()
	return nil
}

func coachErrorMessage(err error) string {
	switch {
	case errors.Is(err, ai.ErrEmptyResponse), errors.Is(err, ai.ErrNoJSON), errors.Is(err, ai.ErrInvalidProposal):
		return "coach returned an unusable response"
	case errors.Is(err, context.DeadlineExceeded):
		return "coach timed out"
	default:
		return "coach unavailable"
	}
}

// StartManualCommitment validates input, activates a new commitment, mints a
// session, seeds evidence stats from the latest window, and moves Planning ->
// Active.
func (c *Controller) StartManualCommitment(nextAction, successCondition string, timebox time.Duration, allowedClasses []string) error {
	c.mu.Lock()
	defer c.mu.Unlock()
	commitment, err := domain.NewManual(nextAction, successCondition, timebox)
	if err != nil {
		return err
	}
	commitment.State, err = statemachine.TransitionCommitment(commitment.State, statemachine.CommitmentActivate)
	if err != nil {
		return err
	}
	next, err := statemachine.TransitionRuntime(c.runtimeState, statemachine.ActivateAccepted)
	if err != nil {
		return err
	}
	now := c.clock()
	c.runtimeState = next
	c.commitment = &commitment
	c.deadline = now.Add(timebox)
	c.outcomePending = ""
	c.resetCoachLocked()
	c.allowedClasses = append([]string(nil), allowedClasses...)
	c.resetDriftLocked()

	sessionID := "session-" + uuid.Must(uuid.NewV7()).String()
	c.stats = &EvidenceStats{
		SessionID:   sessionID,
		StartedUnix: now.Unix(),
		Buckets:     map[bucketKey]time.Duration{},
	}
	seed := c.latestWindow
	_ = store.AppendFocus(c.sessionsDir, sessionID, focusEvent(now, seed))
	c.applyEvent(now, seed)
	return c.persistLocked()
}

// Complete moves Active -> Review with a "completed" outcome.
func (c *Controller) Complete() error { return c.enterReview("completed") }

// Expire moves Active -> Review with an "expired" outcome (timebox elapsed).
func (c *Controller) Expire() error { return c.enterReview("expired") }

func (c *Controller) enterReview(outcome string) error {
	c.mu.Lock()
	defer c.mu.Unlock()
	next, err := statemachine.TransitionRuntime(c.runtimeState, statemachine.CompleteForReview)
	if err != nil {
		return err
	}
	if c.commitment != nil {
		completed, err := statemachine.TransitionCommitment(c.commitment.State, statemachine.Complete)
		if err != nil {
			return err
		}
		c.commitment.State = completed
	}
	// Flush the final open segment, then freeze accounting.
	if c.stats != nil && c.stats.hasLast {
		c.stats.Buckets[c.stats.lastKey] += c.clock().Sub(c.stats.lastFocusAt)
		c.stats.hasLast = false
	}
	c.runtimeState = next
	c.outcomePending = outcome
	return c.persistLocked()
}

// End moves Review -> Locked, writes the session summary to the audit chain,
// and clears the commitment and stats.
func (c *Controller) End() error {
	c.mu.Lock()
	defer c.mu.Unlock()
	next, err := statemachine.TransitionRuntime(c.runtimeState, statemachine.EndWorkPeriod)
	if err != nil {
		return err
	}
	if c.stats != nil {
		if err := store.AppendSession(c.auditPath, c.buildSummaryLocked()); err != nil {
			// State integrity over audit completeness: the transition still
			// completes. Surfaced for the operator; no auto-retry in M1.
			log.Printf("session: audit append failed: %v", err)
		}
	}
	c.runtimeState = next
	c.commitment = nil
	c.deadline = time.Time{}
	c.stats = nil
	c.outcomePending = ""
	c.allowedClasses = nil
	c.resetDriftLocked()
	return c.persistLocked()
}

func (c *Controller) buildSummaryLocked() store.SessionSummary {
	buckets := make([]store.BucketTotal, 0, len(c.stats.Buckets))
	for k, d := range c.stats.Buckets {
		buckets = append(buckets, store.BucketTotal{Class: k.Class, Title: k.Title, Seconds: int64(d.Seconds())})
	}
	sort.Slice(buckets, func(i, j int) bool { return buckets[i].Seconds > buckets[j].Seconds })
	outcome := c.outcomePending
	if outcome == "" {
		outcome = "completed"
	}
	var na, sc string
	if c.commitment != nil {
		na, sc = c.commitment.NextAction, c.commitment.SuccessCondition
	}
	return store.SessionSummary{
		SessionID:   c.stats.SessionID,
		NextAction:  na,
		SuccessCond: sc,
		Outcome:     outcome,
		StartedUnix: c.stats.StartedUnix,
		EndedUnix:   c.clock().Unix(),
		SwitchCount: c.stats.SwitchCount,
		Buckets:     buckets,
	}
}

// RecordWindow ingests one sensor observation. It accumulates time only while
// Active, always tracks the latest window for display, and fires onChange after
// releasing the mutex.
func (c *Controller) RecordWindow(snap evidence.WindowSnapshot) {
	c.mu.Lock()
	c.latestWindow = snap
	if c.runtimeState != domain.RuntimeActive || c.stats == nil {
		c.mu.Unlock()
		c.notify()
		return
	}
	now := c.clock()
	_ = store.AppendFocus(c.sessionsDir, c.stats.SessionID, focusEvent(now, snap))
	c.applyEvent(now, snap)
	c.recordTitleLocked(snap.Title)
	launch := c.evaluateDriftLocked(now, snap)
	c.mu.Unlock()
	if launch != nil {
		go launch()
	}
	c.notify()
}

// evaluateDriftLocked runs the local-first drift pipeline for one observation
// and updates synchronous drift state. When an async judgment is warranted it
// returns the judging closure; the caller runs it in a goroutine after
// releasing the mutex (the M2 RequestCoach discipline). Caller holds mu and has
// already verified the runtime is Active.
func (c *Controller) evaluateDriftLocked(now time.Time, snap evidence.WindowSnapshot) func() {
	class, title := snap.Class, snap.Title

	// 1. Local match: authoritative on-task, no LLM. This is also the ONLY path
	// where the semantic nudge runs — the drift judge stays silent here, so the
	// two are mutually exclusive per observation.
	ac := domain.AllowedContext{WindowClasses: c.allowedClasses}
	if evidence.MatchesAllowed(ac, class, title) {
		c.driftStatus = driftOnTask
		c.driftReason = ""
		return c.maybeNudgeLocked(now)
	}

	// Past this point the window is not a local on-task match: the on-task
	// stretch (if any) has ended. Void any soft nudge tied to it and advance the
	// epoch so an in-flight nudge result is discarded rather than surfacing stale
	// when the user returns.
	c.nudgeMessage = ""
	c.nudgeEpoch++

	// 2. Per-class cache.
	if v, ok := c.judgedClasses[class]; ok {
		c.applyVerdictLocked(v)
		return nil
	}

	// 3. No judge wired: never block; leave idle.
	if c.judge == nil {
		c.driftStatus = driftIdle
		c.driftReason = ""
		return nil
	}

	// 4. Debounce: at most one judgment per driftDebounce window.
	if !c.lastJudgedAt.IsZero() && now.Sub(c.lastJudgedAt) < driftDebounce {
		return nil
	}

	prevStatus, prevReason := c.driftStatus, c.driftReason
	c.driftGen++
	gen := c.driftGen
	c.lastJudgedAt = now
	c.driftStatus = driftPending
	c.driftReason = ""
	judge := c.judge
	commitment := c.commitmentLineLocked()

	return func() {
		ctx, cancel := context.WithTimeout(context.Background(), driftTimeout)
		defer cancel()
		v, err := judge.JudgeDrift(ctx, commitment, class, title)

		c.mu.Lock()
		if gen != c.driftGen || c.runtimeState != domain.RuntimeActive {
			c.mu.Unlock()
			return // stale: a newer judgment started or the session ended
		}
		if err != nil {
			// Degrade: never fabricate drift. Revert the optimistic pending.
			if c.driftStatus == driftPending {
				c.driftStatus = prevStatus
				c.driftReason = prevReason
			}
			c.mu.Unlock()
			log.Printf("session: drift judge failed: %v", err)
			c.notify()
			return
		}
		c.judgedClasses[class] = v
		if c.stats != nil && c.stats.Current.Class == class {
			c.applyVerdictLocked(v)
		}
		c.mu.Unlock()
		c.notify()
	}
}

// maybeNudgeLocked decides whether to launch a semantic nudge on the on-task
// local-match path. It returns the nudging closure when eligible (judge wired,
// at least two titles of history, debounce elapsed), else nil. The closure
// follows the drift-judge discipline: it runs in a goroutine after the caller
// releases the mutex, re-acquires the lock, guards on nudgeEpoch (the current
// on-task-stretch id), so a nudge whose stretch has since ended — a drift
// episode or a session change — is discarded rather than surfacing stale, never
// fabricates a concern on error, and notifies only after unlocking. Caller holds
// mu and has already verified the runtime is Active.
func (c *Controller) maybeNudgeLocked(now time.Time) func() {
	if c.nudge == nil || len(c.recentTitles) < 2 {
		return nil
	}
	if !c.lastNudgedAt.IsZero() && now.Sub(c.lastNudgedAt) < nudgeDebounce {
		return nil
	}
	c.lastNudgedAt = now
	epoch := c.nudgeEpoch
	nudge := c.nudge
	commitment := c.commitmentLineLocked()
	titles := append([]string(nil), c.recentTitles...)

	return func() {
		ctx, cancel := context.WithTimeout(context.Background(), nudgeTimeout)
		defer cancel()
		msg, err := nudge.Nudge(ctx, commitment, titles)

		c.mu.Lock()
		if epoch != c.nudgeEpoch || c.runtimeState != domain.RuntimeActive {
			c.mu.Unlock()
			return // stale: the on-task stretch ended (drift episode) or the session changed
		}
		if err != nil {
			c.mu.Unlock()
			log.Printf("session: nudge failed: %v", err)
			return // never fabricate a concern; leave any prior message intact
		}
		c.nudgeMessage = msg // "" clears a prior advisory once back on trajectory
		c.mu.Unlock()
		c.notify()
	}
}

// recordTitleLocked appends a non-empty title to the recent ring, skipping a
// consecutive duplicate, and caps the ring at recentTitlesMax. Caller holds mu.
func (c *Controller) recordTitleLocked(title string) {
	t := strings.TrimSpace(title)
	if t == "" {
		return
	}
	if n := len(c.recentTitles); n > 0 && c.recentTitles[n-1] == t {
		return
	}
	c.recentTitles = append(c.recentTitles, t)
	if len(c.recentTitles) > recentTitlesMax {
		c.recentTitles = c.recentTitles[len(c.recentTitles)-recentTitlesMax:]
	}
}

// applyVerdictLocked maps a verdict onto drift state. Caller holds mu.
func (c *Controller) applyVerdictLocked(v ai.Verdict) {
	if v.OnTask {
		c.driftStatus = driftOnTask
		c.driftReason = ""
		return
	}
	c.driftStatus = driftDrifting
	c.driftReason = v.Reason
}

// applyEvent advances stats by one observation: it credits the prior segment to
// the prior bucket, counts a context switch on key change, and records the new
// current window. Used by both live tracking and crash replay. Caller holds mu.
func (c *Controller) applyEvent(now time.Time, snap evidence.WindowSnapshot) {
	if c.stats.hasLast {
		c.stats.Buckets[c.stats.lastKey] += now.Sub(c.stats.lastFocusAt)
	}
	newKey := keyFor(snap)
	if c.stats.hasLast && newKey != c.stats.lastKey {
		c.stats.SwitchCount++
	}
	c.stats.lastKey = newKey
	c.stats.lastFocusAt = now
	c.stats.hasLast = true
	c.stats.Current = snap
}

// replayStats rebuilds in-memory stats from the raw session log after a crash.
func (c *Controller) replayStats(sessionID string) {
	events, err := store.ReplaySession(c.sessionsDir, sessionID)
	if err != nil || len(events) == 0 {
		c.stats = &EvidenceStats{
			SessionID:   sessionID,
			StartedUnix: c.clock().Unix(),
			Buckets:     map[bucketKey]time.Duration{},
		}
		return
	}
	c.stats = &EvidenceStats{
		SessionID:   sessionID,
		StartedUnix: events[0].AtUnixMillis / 1000,
		Buckets:     map[bucketKey]time.Duration{},
	}
	for _, e := range events {
		c.applyEvent(time.UnixMilli(e.AtUnixMillis), snapFromEvent(e))
	}
}

func keyFor(snap evidence.WindowSnapshot) bucketKey {
	if !snap.Health.Available {
		return bucketKey{Class: "", Title: unavailableTitle}
	}
	return bucketKey{Class: snap.Class, Title: evidence.ScrubTitle(snap.Title)}
}

func focusEvent(now time.Time, snap evidence.WindowSnapshot) store.FocusEvent {
	return store.FocusEvent{
		AtUnixMillis: now.UnixMilli(),
		Class:        snap.Class,
		Title:        snap.Title,
		Available:    snap.Health.Available,
		Reason:       snap.Health.Reason,
	}
}

func snapFromEvent(e store.FocusEvent) evidence.WindowSnapshot {
	return evidence.WindowSnapshot{
		Title:  e.Title,
		Class:  e.Class,
		Health: evidence.EvidenceHealth{Available: e.Available, Reason: e.Reason},
	}
}