// Package lane — sampler.go: periodic occupancy sampler (v7).
//
// Why a dedicated sampler goroutine: /skills/admin/queues shows current
// state but operators need a timeline ("ollama lane was saturated for 4
// hours yesterday afternoon"). Sampling at fixed intervals is the
// simplest way to capture that without instrumenting every Submit/
// complete path. Sampling is best-effort observability — if the
// goroutine dies, charts show a gap; nothing else breaks.
//
// Why in pkg/lane (vs pkg/logic/skills/lane_sampler.go): the sampler
// reads from the lane registry which lives here. The persistence layer
// (skill_lane_samples table) lives in skills, so the sampler takes a
// narrow LaneSampleSink interface — production wires
// `skills.Storage.RecordLaneSample`; tests substitute a fake.
package lane

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

// LaneSampleSink is the persistence surface the sampler writes to.
// Production wires skills.Storage; tests substitute a recording fake.
//
// Why a narrow interface (vs importing skills.Storage): pkg/lane is a
// generic primitive that must NOT import the application's skills
// package — that would create an import cycle. Using a small typed
// interface keeps lane decoupled.
type LaneSampleSink interface {
	RecordLaneSample(ctx context.Context, lane string, running, queued int, sampledAt time.Time) error
}

// LaneSampleSinkFunc adapts a closure to LaneSampleSink. Useful in
// production wiring (mort.go) where the underlying storage method has
// a different shape.
type LaneSampleSinkFunc func(ctx context.Context, lane string, running, queued int, sampledAt time.Time) error

// RecordLaneSample satisfies LaneSampleSink.
func (f LaneSampleSinkFunc) RecordLaneSample(ctx context.Context, lane string, running, queued int, sampledAt time.Time) error {
	if f == nil {
		return nil
	}
	return f(ctx, lane, running, queued, sampledAt)
}

// LaneSamplePurger is the periodic-sweeper surface. Production wires
// skills.Storage.PurgeLaneSamples.
type LaneSamplePurger interface {
	PurgeLaneSamples(ctx context.Context, olderThan time.Time) (int64, error)
}

// LaneSamplePurgerFunc adapts a closure.
type LaneSamplePurgerFunc func(ctx context.Context, olderThan time.Time) (int64, error)

// PurgeLaneSamples satisfies LaneSamplePurger.
func (f LaneSamplePurgerFunc) PurgeLaneSamples(ctx context.Context, olderThan time.Time) (int64, error) {
	if f == nil {
		return 0, nil
	}
	return f(ctx, olderThan)
}

// Sampler periodically reads stats from every lane in the registry and
// writes one sample row per lane via the configured Sink. Optionally
// runs a daily retention sweep that purges samples older than
// RetentionDays via Purger.
//
// Test: sampler_test.go drives Sample() synchronously with a fake
// clock + recording sink.
type Sampler struct {
	registry *Registry
	sink     LaneSampleSink
	purger   LaneSamplePurger

	interval       time.Duration
	retention      time.Duration
	purgeInterval  time.Duration
	clock          func() time.Time

	// run-time state
	mu       sync.Mutex
	running  bool
	stopCh   chan struct{}
	doneCh   chan struct{}
}

// NewSampler constructs the sampler.
//
// interval — sample cadence (typically 30s in production).
// retention — purge cutoff (typically 7d).
// clock=nil → time.Now.
func NewSampler(registry *Registry, sink LaneSampleSink, purger LaneSamplePurger,
	interval, retention time.Duration, clock func() time.Time) *Sampler {
	if interval <= 0 {
		interval = 30 * time.Second
	}
	if retention <= 0 {
		retention = 7 * 24 * time.Hour
	}
	if clock == nil {
		clock = time.Now
	}
	return &Sampler{
		registry:      registry,
		sink:          sink,
		purger:        purger,
		interval:      interval,
		retention:     retention,
		purgeInterval: 24 * time.Hour,
		clock:         clock,
	}
}

// Start launches the sampler goroutine. Cancelling ctx stops it.
// Idempotent — calling Start twice without an intervening Stop is a
// no-op for the second call.
func (s *Sampler) Start(ctx context.Context) {
	s.mu.Lock()
	if s.running {
		s.mu.Unlock()
		return
	}
	s.running = true
	s.stopCh = make(chan struct{})
	s.doneCh = make(chan struct{})
	s.mu.Unlock()

	go s.loop(ctx)
}

// Stop signals the sampler to exit and waits for the goroutine to
// finish. Idempotent.
func (s *Sampler) Stop() {
	s.mu.Lock()
	if !s.running {
		s.mu.Unlock()
		return
	}
	close(s.stopCh)
	doneCh := s.doneCh
	s.running = false
	s.mu.Unlock()
	<-doneCh
}

// Sample runs one sampling pass synchronously. Test entry point —
// production callers use Start.
func (s *Sampler) Sample(ctx context.Context) {
	if s.registry == nil || s.sink == nil {
		return
	}
	now := s.clock()
	for _, l := range s.registry.List() {
		st := l.Stats()
		if err := s.sink.RecordLaneSample(ctx, st.Name, st.Running, st.Queued, now); err != nil {
			// Best-effort observability — log and continue, never block.
			slog.Warn("lane sampler: record failed", "lane", st.Name, "error", err)
		}
	}
}

// PurgeOnce runs one retention sweep synchronously. Test entry point.
func (s *Sampler) PurgeOnce(ctx context.Context) {
	if s.purger == nil {
		return
	}
	cutoff := s.clock().Add(-s.retention)
	if _, err := s.purger.PurgeLaneSamples(ctx, cutoff); err != nil {
		slog.Warn("lane sampler: purge failed", "error", err)
	}
}

// loop is the sampler's main goroutine. Calls Sample at the interval
// cadence and PurgeOnce daily. Exits on ctx.Done OR Stop.
func (s *Sampler) loop(ctx context.Context) {
	defer func() {
		s.mu.Lock()
		if s.doneCh != nil {
			close(s.doneCh)
			s.doneCh = nil
		}
		s.mu.Unlock()
	}()
	sampleTicker := time.NewTicker(s.interval)
	defer sampleTicker.Stop()
	purgeTicker := time.NewTicker(s.purgeInterval)
	defer purgeTicker.Stop()

	for {
		select {
		case <-ctx.Done():
			return
		case <-s.stopCh:
			return
		case <-sampleTicker.C:
			s.Sample(ctx)
		case <-purgeTicker.C:
			s.PurgeOnce(ctx)
		}
	}
}