// Package scheduler drives the "schedule" trigger kind. It ticks on a
// fixed interval, scans every enabled schedule trigger, and dispatches
// the ones whose next-fire window has elapsed through the same
// FanOutForTrigger path the inbound HTTP webhook uses.
//
// The scheduler is intentionally simple:
//
//   - Tick on `tickInterval` (default 30s).
//   - For every trigger with Kind=="schedule", parse its config to get
//     the interval, compute (LastFiredAt + interval), and if now >=
//     that target, fire.
//   - On fire: build a plugin.InboundEvent{Kind: "schedule"} and call
//     handler.FanOutForTrigger. last_fired_at is persisted BEFORE the
//     dispatch runs so a panicking Match cannot wedge the row into a
//     tight retry loop — a failed deploy waits one full interval
//     before retry, which is the correct trade-off for a periodic
//     refresh trigger.
//   - A never-fired trigger (LastFiredAt == "") fires on the next
//     tick — operator-friendly for testing "did I configure it right?".
//
// Per-trigger errors are logged but do not abort the tick.
package scheduler

import (
	"context"
	"log/slog"
	"sync"
	"time"

	"github.com/alexei/tinyforge/internal/metrics"
	"github.com/alexei/tinyforge/internal/store"
	"github.com/alexei/tinyforge/internal/workload/plugin"
	"github.com/alexei/tinyforge/internal/workload/plugin/trigger/schedule"
)

// Scheduler owns the background tick loop.
type Scheduler struct {
	store        *store.Store
	dispatcher   fanOutFn
	tickInterval time.Duration
	clock        func() time.Time // overridable for tests

	startOnce sync.Once
	stopOnce  sync.Once
	cancel    context.CancelFunc
	wg        sync.WaitGroup
}

// fanOutFn is the internal callback shape — narrower than the public
// FanOutTrigger interface so the wiring in cmd/server/main.go can pass
// a closure directly without standing up a wrapper type.
type fanOutFn func(ctx context.Context, trg store.Trigger, evt plugin.InboundEvent) error

// New constructs a Scheduler bound to `st` that dispatches via `fanOut`.
// `tickInterval` controls how often the loop wakes up to check
// schedules; values <=0 fall back to 30s. Tick intervals longer than 5
// minutes are clamped so a misconfigured value can't silently disable
// schedules.
//
// `fanOut` should call webhook.Handler.FanOutForTrigger and return its
// error (or nil); the per-binding result slice is discarded — the
// scheduler does not need to know per-binding outcomes, only whether
// the dispatch itself failed.
func New(st *store.Store, fanOut fanOutFn, tickInterval time.Duration) *Scheduler {
	clamped := tickInterval
	if clamped <= 0 {
		clamped = 30 * time.Second
	}
	if clamped > 5*time.Minute {
		clamped = 5 * time.Minute
	}
	if clamped != tickInterval && tickInterval != 0 {
		slog.Warn("scheduler: tick interval clamped",
			"requested", tickInterval, "applied", clamped)
	}
	return &Scheduler{
		store:        st,
		dispatcher:   fanOut,
		tickInterval: clamped,
		clock:        func() time.Time { return time.Now().UTC() },
	}
}

// Start launches the loop. Idempotent — repeat calls are no-ops, not
// goroutine leaks. Mirrors the reconciler's lifecycle.
func (s *Scheduler) Start(ctx context.Context) {
	s.startOnce.Do(func() {
		ctx, cancel := context.WithCancel(ctx)
		s.cancel = cancel
		s.wg.Add(1)
		go s.loop(ctx)
	})
}

// Stop cancels the context and waits for the in-flight tick. Idempotent
// via sync.Once — second call returns immediately without panicking on
// a double cancel.
func (s *Scheduler) Stop() {
	s.stopOnce.Do(func() {
		if s.cancel != nil {
			s.cancel()
		}
	})
	s.wg.Wait()
}

func (s *Scheduler) loop(ctx context.Context) {
	defer s.wg.Done()
	// First sweep at boot so a daily schedule does not idle 24h after a
	// restart before it picks up rows whose window already elapsed.
	s.TickOnce(ctx)

	ticker := time.NewTicker(s.tickInterval)
	defer ticker.Stop()
	for {
		select {
		case <-ctx.Done():
			return
		case <-ticker.C:
			s.TickOnce(ctx)
		}
	}
}

// TickOnce runs a single sweep. Exposed for tests and for the boot
// kick. On error per-trigger the loop continues with the next row.
func (s *Scheduler) TickOnce(ctx context.Context) {
	metrics.SchedulerTicksTotal.Inc()
	rows, err := s.store.ListTriggers("schedule")
	if err != nil {
		slog.Warn("scheduler: list triggers", "error", err)
		return
	}
	now := s.clock()
	for _, t := range rows {
		if !s.shouldFire(t, now) {
			continue
		}
		s.fire(ctx, t, now)
	}
}

// shouldFire decides whether to dispatch trg at `now`. Returns true if:
//   - the trigger's interval is parseable, AND
//   - last_fired_at is empty (never fired) OR now >= lastFired + interval.
//
// Unparseable last_fired_at or interval are logged once and treated as
// "do not fire" — the operator needs to fix the config; the scheduler
// must not loop on a broken row.
func (s *Scheduler) shouldFire(t store.Trigger, now time.Time) bool {
	interval, err := schedule.IntervalOfRaw(t.Config)
	if err != nil {
		slog.Warn("scheduler: bad interval", "trigger", t.Name, "error", err)
		return false
	}
	// Defense-in-depth against a hand-inserted row that bypassed
	// Validate (manual SQL, restore, ad-hoc migration). Validate
	// already enforces the floor on the create path; this re-check
	// keeps the loop honest if anything sneaks past it.
	if interval < schedule.MinInterval {
		slog.Warn("scheduler: interval below minimum, ignoring",
			"trigger", t.Name, "interval", interval, "minimum", schedule.MinInterval)
		return false
	}
	if t.LastFiredAt == "" {
		return true
	}
	last, err := time.Parse(time.RFC3339, t.LastFiredAt)
	if err != nil {
		slog.Warn("scheduler: bad last_fired_at", "trigger", t.Name,
			"value", t.LastFiredAt, "error", err)
		// Treat as never-fired so the operator's fix-by-redeploy doesn't
		// require a manual DB poke.
		return true
	}
	if now.Before(last.Add(interval)) {
		return false
	}
	// Catch-up warning: a trigger whose last_fired_at is many intervals
	// old (paused-then-resumed, restored from backup, or just left
	// running while the dispatcher was down) WILL fire on this tick.
	// Log a one-line warning so the operator can recognize the "surprise
	// burst at restart" pattern in audit logs. We still fire — silent
	// no-fire would be worse — but the warning explains why.
	if overdue := now.Sub(last); overdue > catchUpWarnThreshold*interval {
		slog.Warn("scheduler: catch-up fire (very overdue)",
			"trigger", t.Name, "overdue", overdue, "interval", interval)
	}
	return true
}

// catchUpWarnThreshold is the multiplier on `interval` past which a
// fire is logged as "catch-up." 2× means a daily schedule whose last
// fire was more than 48h ago gets a warning at next tick. Chosen so
// the warning fires on "wedged for many intervals" without alerting on
// the every-tick lag a healthy 30s-tick scheduler accumulates against
// a sub-minute interval. Bigger threshold = noisier-quiet trade-off;
// 2× is the smallest value that excludes single-tick lag.
const catchUpWarnThreshold = 2

// fire dispatches one trigger and records the new last_fired_at.
//
// We persist last_fired_at BEFORE calling the dispatcher so a panic
// inside Match cannot wedge the row into a tight loop. Down-side: a
// deploy that fails leaves the scheduler waiting one full interval
// before retry — acceptable because the trigger is a periodic refresh,
// not a critical-path retry mechanism.
func (s *Scheduler) fire(ctx context.Context, t store.Trigger, now time.Time) {
	// Belt-and-suspenders: ListTriggersByKind only returns "schedule"
	// rows, but if a future caller wires fire() differently this guard
	// keeps the scheduler from blindly dispatching a kind it isn't
	// designed for.
	if t.Kind != "schedule" {
		slog.Warn("scheduler: refusing to fire non-schedule kind",
			"trigger", t.Name, "kind", t.Kind)
		return
	}
	ts := now.Format(time.RFC3339)
	if err := s.store.SetTriggerLastFired(t.ID, ts); err != nil {
		slog.Warn("scheduler: persist last_fired_at", "trigger", t.Name, "error", err)
		return
	}
	evt := plugin.InboundEvent{
		Kind:     "schedule",
		Schedule: &plugin.ScheduleEvent{FiredAt: now},
	}
	if err := s.dispatcher(ctx, t, evt); err != nil {
		slog.Warn("scheduler: dispatch", "trigger", t.Name, "error", err)
		return
	}
	metrics.SchedulerDispatchedTotal.Inc()
	slog.Info("scheduler: fired", "trigger", t.Name, "kind", t.Kind, "at", ts)
}