package scheduler

import (
	"context"
	"fmt"
	"hash/fnv"
	"math/rand"
	"time"

	"gitea.maximumdirect.net/ejr/feedkit/event"
	"gitea.maximumdirect.net/ejr/feedkit/sources"
)

type Job struct {
	Source sources.Source
	Every  time.Duration

	// Jitter is the maximum additional delay added before each poll.
	// Example: if Every=15m and Jitter=30s, each poll will occur at:
	//   tick time + random(0..30s)
	//
	// If Jitter == 0, we compute a default jitter based on Every.
	Jitter time.Duration
}

type Logger func(format string, args ...any)

type Scheduler struct {
	Jobs []Job
	Out  chan<- event.Event
	Logf Logger
}

// Run starts one polling goroutine per job.
// Each job runs on its own interval and emits 0..N events per poll.
func (s *Scheduler) Run(ctx context.Context) error {
	if s.Out == nil {
		return fmt.Errorf("scheduler.Run: Out channel is nil")
	}
	if len(s.Jobs) == 0 {
		return fmt.Errorf("scheduler.Run: no jobs configured")
	}

	for _, job := range s.Jobs {
		job := job // capture loop variable
		go s.runJob(ctx, job)
	}

	<-ctx.Done()
	return ctx.Err()
}

func (s *Scheduler) runJob(ctx context.Context, job Job) {
	if job.Source == nil {
		s.logf("scheduler: job has nil source")
		return
	}
	if job.Every <= 0 {
		s.logf("scheduler: job %s has invalid interval", job.Source.Name())
		return
	}

	// Compute jitter: either configured per job, or a sensible default.
	jitter := effectiveJitter(job.Every, job.Jitter)

	// Each worker gets its own RNG (safe + no lock contention).
	seed := time.Now().UnixNano() ^ int64(hashStringFNV32a(job.Source.Name()))
	rng := rand.New(rand.NewSource(seed))

	// Optional startup jitter: avoids all jobs firing at the exact moment the daemon starts.
	if !sleepJitter(ctx, rng, jitter) {
		return
	}

	// Immediate poll at startup (after startup jitter).
	s.pollOnce(ctx, job)

	t := time.NewTicker(job.Every)
	defer t.Stop()

	for {
		select {
		case <-t.C:
			// Per-tick jitter: spreads calls out within the interval.
			if !sleepJitter(ctx, rng, jitter) {
				return
			}
			s.pollOnce(ctx, job)

		case <-ctx.Done():
			return
		}
	}
}

func (s *Scheduler) pollOnce(ctx context.Context, job Job) {
	events, err := job.Source.Poll(ctx)
	if err != nil {
		s.logf("scheduler: poll failed (%s): %v", job.Source.Name(), err)
		return
	}

	for _, e := range events {
		select {
		case s.Out <- e:
		case <-ctx.Done():
			return
		}
	}
}

func (s *Scheduler) logf(format string, args ...any) {
	if s.Logf == nil {
		return
	}
	s.Logf(format, args...)
}

// effectiveJitter chooses a jitter value.
// - If configuredMax > 0, use it (but clamp).
// - Else default to min(every/10, 30s).
// - Clamp to at most every/2 (so jitter can’t delay more than half the interval).
func effectiveJitter(every time.Duration, configuredMax time.Duration) time.Duration {
	if every <= 0 {
		return 0
	}

	j := configuredMax
	if j <= 0 {
		j = every / 10
		if j > 30*time.Second {
			j = 30 * time.Second
		}
	}

	// Clamp jitter so it doesn’t dominate the schedule.
	maxAllowed := every / 2
	if j > maxAllowed {
		j = maxAllowed
	}
	if j < 0 {
		j = 0
	}
	return j
}

// sleepJitter sleeps for a random duration in [0, max].
// Returns false if the context is cancelled while waiting.
func sleepJitter(ctx context.Context, rng *rand.Rand, max time.Duration) bool {
	if max <= 0 {
		return true
	}

	// Int63n requires a positive argument.
	// We add 1 so max itself is attainable.
	n := rng.Int63n(int64(max) + 1)
	d := time.Duration(n)

	timer := time.NewTimer(d)
	defer timer.Stop()

	select {
	case <-timer.C:
		return true
	case <-ctx.Done():
		return false
	}
}

func hashStringFNV32a(s string) uint32 {
	h := fnv.New32a()
	_, _ = h.Write([]byte(s))
	return h.Sum32()
}