// Package mqttclient subscribes to device telemetry topics over MQTT.
package mqttclient

import (
	"fmt"
	"log/slog"
	"time"

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

// Handler receives one message's raw payload and the time it arrived.
type Handler func(payload []byte, receivedAt time.Time)

type Config struct {
	BrokerURL string // e.g. tcp://mosquitto:1883
	ClientID  string
	Topic     string // e.g. devices/+/telemetry
	QoS       byte
}

type Subscriber struct {
	client mqtt.Client
}

// Connect dials the broker and (re-)subscribes to cfg.Topic on every
// successful connection, so a dropped connection resubscribes automatically
// once auto-reconnect succeeds.
func Connect(cfg Config, handler Handler, logger *slog.Logger) (*Subscriber, error) {
	opts := mqtt.NewClientOptions().
		AddBroker(cfg.BrokerURL).
		SetClientID(cfg.ClientID).
		SetAutoReconnect(true).
		SetConnectRetry(true).
		SetOnConnectHandler(func(c mqtt.Client) {
			token := c.Subscribe(cfg.Topic, cfg.QoS, func(_ mqtt.Client, msg mqtt.Message) {
				handler(msg.Payload(), time.Now())
			})
			token.Wait()
			if err := token.Error(); err != nil {
				logger.Error("mqtt subscribe failed", "topic", cfg.Topic, "error", err)
			}
		}).
		SetConnectionLostHandler(func(_ mqtt.Client, err error) {
			logger.Warn("mqtt connection lost", "error", err)
		})

	client := mqtt.NewClient(opts)
	token := client.Connect()
	token.Wait()
	if err := token.Error(); err != nil {
		return nil, fmt.Errorf("connect to mqtt broker %s: %w", cfg.BrokerURL, err)
	}
	return &Subscriber{client: client}, nil
}

// Close disconnects, waiting up to quiesceMS for in-flight work to settle.
func (s *Subscriber) Close() {
	s.client.Disconnect(250)
}