package usage

import (
	"context"
	"math/rand"
	"sync"
	"sync/atomic"
	"time"

	"golang.org/x/xerrors"

	"cdr.dev/slog/v3"
	"github.com/coder/coder/v2/coderd/database"
	"github.com/coder/coder/v2/coderd/database/dbauthz"
	"github.com/coder/coder/v2/coderd/pproflabel"
	agplusage "github.com/coder/coder/v2/coderd/usage"
	"github.com/coder/coder/v2/coderd/usage/usagetypes"
	"github.com/coder/quartz"
)

// epoch is a fixed reference point for aligning interval boundaries.
// All replicas use this same epoch so their buckets are identical.
var epoch = time.Date(2023, 1, 1, 0, 0, 0, 0, time.UTC)

const (
	cronDateFormat = "2006-01-02_15:04:05"
)

// HeartbeatFunc generates a heartbeat event and its stable ID.
// It is called periodically by the cron. Returning an error skips
// the insert for that tick and logs a warning.
type HeartbeatFunc func(ctx context.Context) (event usagetypes.HeartbeatEvent, err error)

// CronJob defines a periodic heartbeat job.
type CronJob struct {
	// Name is a human-readable label used in logs.
	Name string
	// Interval is the base duration between ticks.
	Interval time.Duration
	// EventType must match the events generated by the Fn.
	EventType usagetypes.UsageEventType
	// Jitter is the maximum random delay added after the boundary.
	// The actual offset is uniformly distributed in [0, Jitter).
	// This staggers replicas so one is likely to complete the work
	// before others attempt it, allowing them to skip via the
	// existence check (heartbeat inserts are idempotent).
	Jitter time.Duration
	// Fn produces the heartbeat event.
	Fn HeartbeatFunc
}

// Cron runs registered CronJobs on the dbInserter's clock. Stopping
// the context passed to Start cancels all jobs. Daemon restarts
// naturally restart the timers since Start() creates them fresh —
// there is no state to persist or recover.
type Cron struct {
	clock quartz.Clock
	log   slog.Logger
	db    database.Store
	ins   agplusage.Inserter
	jobs  []CronJob

	// cancel cancels the context on all running jobs. If the ctx passed into `Start`
	// is canceled, the jobs will also stop.
	cancel context.CancelFunc

	// wg ensures all job goroutines have exited before Close returns.
	wg sync.WaitGroup

	// startOnce ensures Start is idempotent.
	startOnce sync.Once
	started   atomic.Bool
}

// NewCron creates a Cron that periodically generates and inserts
// heartbeat events. The clock controls all timers so that tests can
// advance time deterministically via quartz.Mock.
func NewCron(clock quartz.Clock, log slog.Logger, db database.Store, ins agplusage.Inserter) *Cron {
	return &Cron{
		clock: clock,
		log:   log,
		db:    db,
		ins:   ins,
	}
}

// Register adds a job. It must be called before Start; calling it
// after Start returns an error.
func (c *Cron) Register(job CronJob) error {
	if !job.EventType.IsHeartbeat() {
		return xerrors.New("event type must be a heartbeat type")
	}
	if c.started.Load() {
		return xerrors.New("cannot register a job after Start has been called")
	}
	c.jobs = append(c.jobs, job)
	return nil
}

// Start launches a goroutine per job. Subsequent calls are no-ops.
// On daemon restart a new Cron should be created.
func (c *Cron) Start(ctx context.Context) {
	c.startOnce.Do(func() {
		c.started.Store(true)
		ctx, c.cancel = context.WithCancel(ctx)
		for _, job := range c.jobs {
			c.wg.Add(1)
			pproflabel.Go(ctx, pproflabel.Service(pproflabel.ServiceUsageEventCron, "job", job.Name), func(ctx context.Context) {
				c.run(ctx, job)
			})
		}
	})
}

// Close cancels all jobs and waits for goroutines to exit.
func (c *Cron) Close() error {
	if c.cancel != nil {
		c.cancel()
	}
	c.wg.Wait()
	return nil
}

func (c *Cron) run(ctx context.Context, job CronJob) {
	//nolint:gocritic // We are a publisher in this function
	ctx = dbauthz.AsUsagePublisher(ctx)
	defer c.wg.Done()
	for {
		boundary, delay := nextTick(c.clock.Now(), job.Interval, job.Jitter)

		// Use a quartz timer so the wait honors ctx cancellation and
		// tests can advance time deterministically.
		timer := c.clock.NewTimer(delay, job.Name)

		select {
		case <-ctx.Done():
			if !timer.Stop() {
				// Drain the channel if the timer already fired.
				<-timer.C
			}
			return
		case <-timer.C:
		}

		// Use the boundary (not wall-clock "now") for the stable ID
		// so all replicas targeting the same boundary produce the
		// same key.
		stableID := string(job.EventType) + ":" + boundary.UTC().Format(cronDateFormat)

		// Skip if this bucket was already recorded — avoids running
		// the potentially expensive heartbeat function for a
		// duplicate.
		exists, err := c.db.UsageEventExistsByID(ctx, stableID)
		if err != nil {
			c.log.Warn(ctx, "cron heartbeat existence check failed",
				slog.F("job", job.Name),
				slog.Error(err),
			)
			continue
		}
		if exists {
			c.log.Debug(ctx, "cron heartbeat already recorded, skipping",
				slog.F("job", job.Name),
				slog.F("id", stableID),
			)
			continue
		}

		event, err := job.Fn(ctx)
		if err != nil {
			c.log.Error(ctx, "cron heartbeat func failed",
				slog.F("job", job.Name),
				slog.Error(err),
			)
			continue
		}

		if event.EventType() != job.EventType {
			c.log.Error(ctx, "cron heartbeat func returned wrong event type",
				slog.F("job", job.Name),
				slog.F("expected", job.EventType),
				slog.F("actual", event.EventType()),
			)
			continue
		}

		if err := c.ins.InsertHeartbeatUsageEvent(ctx, c.db, stableID, event); err != nil {
			c.log.Warn(ctx, "cron heartbeat insert failed",
				slog.F("job", job.Name),
				slog.Error(err),
			)
		}
	}
}

// nextTick computes the delay until the next epoch-aligned boundary
// for the given interval, plus a random jitter in [0, jitter). It
// returns the target boundary and the total delay from now.
func nextTick(now time.Time, interval, jitter time.Duration) (boundary time.Time, delay time.Duration) {
	boundary = nextBoundary(now, interval)
	delay = boundary.Sub(now)
	if jitter > 0 {
		//nolint:gosec // Jitter does not need cryptographic randomness.
		delay += time.Duration(rand.Int63n(int64(jitter)))
	}
	return boundary, delay
}

// nextBoundary returns the first multiple of interval (relative to
// epoch) that is strictly after t.
func nextBoundary(t time.Time, interval time.Duration) time.Time {
	since := t.Sub(epoch)
	n := since / interval
	return epoch.Add((n + 1) * interval)
}