// Package healthcheck periodically flips devices to offline when they stop
// sending telemetry/acks, per the platform's Device Shadow health-check
// design (a goroutine + graceful shutdown, deliberately simple).
package healthcheck

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

// DeviceStore is the subset of shadow.Store the checker needs, kept as an
// interface so tests don't require a real Redis.
type DeviceStore interface {
	KnownDevices(ctx context.Context) ([]string, error)
	MarkOfflineIfStale(ctx context.Context, deviceID string, timeout time.Duration) (bool, error)
}

// OnOfflineFunc is called once per device that just transitioned to offline.
type OnOfflineFunc func(deviceID string)

type Checker struct {
	store     DeviceStore
	timeout   time.Duration
	interval  time.Duration
	onOffline OnOfflineFunc
	logger    *slog.Logger

	stop chan struct{}
	done chan struct{}
}

func New(store DeviceStore, timeout, interval time.Duration, onOffline OnOfflineFunc, logger *slog.Logger) *Checker {
	return &Checker{
		store:     store,
		timeout:   timeout,
		interval:  interval,
		onOffline: onOffline,
		logger:    logger,
	}
}

// Start begins the periodic scan loop. Call once.
func (c *Checker) Start() {
	c.stop = make(chan struct{})
	c.done = make(chan struct{})
	go c.loop()
}

// Stop signals the loop to exit and waits for it to finish.
func (c *Checker) Stop() {
	close(c.stop)
	<-c.done
}

func (c *Checker) loop() {
	defer close(c.done)
	ticker := time.NewTicker(c.interval)
	defer ticker.Stop()
	for {
		select {
		case <-ticker.C:
			c.checkAll()
		case <-c.stop:
			return
		}
	}
}

func (c *Checker) checkAll() {
	ctx, cancel := context.WithTimeout(context.Background(), c.interval)
	defer cancel()

	ids, err := c.store.KnownDevices(ctx)
	if err != nil {
		c.logger.Error("healthcheck: list known devices failed", "error", err)
		return
	}

	for _, id := range ids {
		changed, err := c.store.MarkOfflineIfStale(ctx, id, c.timeout)
		if err != nil {
			c.logger.Error("healthcheck: check device failed", "device_id", id, "error", err)
			continue
		}
		if changed {
			c.logger.Info("device marked offline", "device_id", id)
			if c.onOffline != nil {
				c.onOffline(id)
			}
		}
	}
}