// Package mqttsubscriber subscribes to playlist-changed MQTT notifications.
// It is safe for concurrent use and applies client-side debouncing so that
// a burst of messages within a 3-second window triggers at most one callback.
package mqttsubscriber

import (
	"time"

	mqtt "github.com/eclipse/paho.mqtt.golang"
)

const (
	// debounceDuration is the minimum interval between two callback invocations.
	// Any MQTT message arriving while the timer is still running resets it.
	// 500ms reicht aus um Bursts zu absorbieren, ohne die Latenz merklich zu erhöhen.
	debounceDuration = 500 * time.Millisecond

	// playlistChangedTopicTemplate is the topic the backend publishes to.
	playlistChangedTopic = "signage/screen/%s/playlist-changed"
)

// PlaylistChangedFunc is called when a debounced playlist-changed notification arrives.
type PlaylistChangedFunc func()

// Subscriber listens for playlist-changed notifications on MQTT and calls the
// provided callback at most once per debounceDuration.
type Subscriber struct {
	client   mqtt.Client
	timer    *time.Timer
	onChange PlaylistChangedFunc

	// timerC serializes timer resets through a dedicated goroutine.
	resetC chan struct{}
	stopC  chan struct{}
}

// Topic returns the MQTT topic for a given screenSlug.
func Topic(screenSlug string) string {
	return "signage/screen/" + screenSlug + "/playlist-changed"
}

// New creates a Subscriber that connects to broker and subscribes to the
// playlist-changed topic for screenSlug.  onChange is called (in its own
// goroutine) at most once per debounceDuration.
//
// Returns nil when broker is empty — callers must handle nil.
func New(broker, screenSlug, username, password string, onChange PlaylistChangedFunc) *Subscriber {
	if broker == "" {
		return nil
	}

	s := &Subscriber{
		onChange: onChange,
		resetC:   make(chan struct{}, 16),
		stopC:    make(chan struct{}),
	}

	topic := Topic(screenSlug)

	opts := mqtt.NewClientOptions().
		AddBroker(broker).
		SetClientID("morz-agent-sub-" + screenSlug).
		SetCleanSession(true).
		SetAutoReconnect(true).
		SetConnectRetry(true).
		SetConnectRetryInterval(10 * time.Second).
		SetOnConnectHandler(func(c mqtt.Client) {
			// Re-subscribe after reconnect.
			c.Subscribe(topic, 0, func(_ mqtt.Client, _ mqtt.Message) { //nolint:errcheck
				select {
				case s.resetC <- struct{}{}:
				default: // channel full — debounce timer will fire anyway
				}
			})
		})

	if username != "" {
		opts.SetUsername(username)
		opts.SetPassword(password)
	}

	s.client = mqtt.NewClient(opts)
	s.client.Connect() // non-blocking; paho retries in background

	go s.run()
	return s
}

// run is the debounce loop.  It resets a timer on every incoming signal.
// When the timer fires the onChange callback is called once in a goroutine.
func (s *Subscriber) run() {
	var timer *time.Timer
	for {
		select {
		case <-s.stopC:
			if timer != nil {
				timer.Stop()
			}
			return
		case <-s.resetC:
			if timer != nil {
				timer.Stop()
			}
			timer = time.AfterFunc(debounceDuration, func() {
				go s.onChange()
			})
		}
	}
}

// Close disconnects the MQTT client and stops the debounce loop.
func (s *Subscriber) Close() {
	if s == nil {
		return
	}
	close(s.stopC)
	s.client.Disconnect(250)
}