package services

import (
	"context"
	"sync"

	"veza-backend-api/internal/metrics"

	"github.com/google/uuid"
	"github.com/redis/go-redis/v9"
	"go.uber.org/zap"
)

type ChatPubSubService struct {
	redisClient         *redis.Client
	logger              *zap.Logger
	inMemorySubscribers map[string][]chan []byte
	mu                  sync.RWMutex
}

func NewChatPubSubService(redisClient *redis.Client, logger *zap.Logger) *ChatPubSubService {
	if logger == nil {
		logger = zap.NewNop()
	}
	if redisClient == nil {
		// In multi-pod deployments the in-memory fallback silently breaks:
		// messages published on pod A are never seen by subscribers on pod B.
		// Emit a loud startup error so the misconfiguration is noticed.
		logger.Error("Redis unavailable, falling back to in-memory PubSub — cross-instance messages will be lost. Set REDIS_URL and restart for multi-pod correctness")
	}
	return &ChatPubSubService{
		redisClient:         redisClient,
		logger:              logger,
		inMemorySubscribers: make(map[string][]chan []byte),
	}
}

func (s *ChatPubSubService) roomChannel(roomID uuid.UUID) string {
	return "chat:room:" + roomID.String()
}

func (s *ChatPubSubService) Publish(ctx context.Context, roomID uuid.UUID, message []byte) error {
	channel := s.roomChannel(roomID)

	if s.redisClient != nil {
		if err := s.redisClient.Publish(ctx, channel, message).Err(); err != nil {
			metrics.RecordCacheMiss("chat_pubsub")
			// ERROR, not Warn: the in-memory fallback only reaches subscribers
			// on this pod — a multi-pod chat becomes partitioned until Redis
			// recovers. Operators should page on this log line.
			s.logger.Error("Redis publish failed, in-memory fallback will not reach other pods",
				zap.String("channel", channel),
				zap.Error(err),
			)
			s.publishInMemory(channel, message)
			return nil
		}
		metrics.RecordCacheHit("chat_pubsub")
		return nil
	}

	metrics.RecordCacheMiss("chat_pubsub")
	s.publishInMemory(channel, message)
	return nil
}

func (s *ChatPubSubService) Subscribe(ctx context.Context, roomID uuid.UUID) (<-chan []byte, func(), error) {
	channel := s.roomChannel(roomID)

	if s.redisClient != nil {
		pubsub := s.redisClient.Subscribe(ctx, channel)
		ch := make(chan []byte, 256)

		go func() {
			defer close(ch)
			for {
				msg, err := pubsub.ReceiveMessage(ctx)
				if err != nil {
					return
				}
				select {
				case ch <- []byte(msg.Payload):
				default:
					s.logger.Warn("PubSub channel full, dropping message", zap.String("channel", channel))
				}
			}
		}()

		cancel := func() {
			_ = pubsub.Close()
		}
		return ch, cancel, nil
	}

	return s.subscribeInMemory(channel)
}

func (s *ChatPubSubService) PublishPresence(ctx context.Context, event []byte) error {
	channel := "chat:presence"

	if s.redisClient != nil {
		if err := s.redisClient.Publish(ctx, channel, event).Err(); err != nil {
			metrics.RecordCacheMiss("chat_pubsub")
			return err
		}
		metrics.RecordCacheHit("chat_pubsub")
		return nil
	}

	metrics.RecordCacheMiss("chat_pubsub")
	s.publishInMemory(channel, event)
	return nil
}

func (s *ChatPubSubService) SubscribePresence(ctx context.Context) (<-chan []byte, func(), error) {
	channel := "chat:presence"

	if s.redisClient != nil {
		pubsub := s.redisClient.Subscribe(ctx, channel)
		ch := make(chan []byte, 256)

		go func() {
			defer close(ch)
			for {
				msg, err := pubsub.ReceiveMessage(ctx)
				if err != nil {
					return
				}
				select {
				case ch <- []byte(msg.Payload):
				default:
				}
			}
		}()

		cancel := func() {
			_ = pubsub.Close()
		}
		return ch, cancel, nil
	}

	return s.subscribeInMemory(channel)
}

func (s *ChatPubSubService) publishInMemory(channel string, message []byte) {
	s.mu.RLock()
	defer s.mu.RUnlock()

	subscribers, ok := s.inMemorySubscribers[channel]
	if !ok {
		return
	}

	for _, ch := range subscribers {
		select {
		case ch <- message:
		default:
		}
	}
}

func (s *ChatPubSubService) subscribeInMemory(channel string) (<-chan []byte, func(), error) {
	ch := make(chan []byte, 256)

	s.mu.Lock()
	s.inMemorySubscribers[channel] = append(s.inMemorySubscribers[channel], ch)
	s.mu.Unlock()

	cancel := func() {
		s.mu.Lock()
		defer s.mu.Unlock()

		subs := s.inMemorySubscribers[channel]
		for i, sub := range subs {
			if sub == ch {
				s.inMemorySubscribers[channel] = append(subs[:i], subs[i+1:]...)
				break
			}
		}
		close(ch)
	}

	return ch, cancel, nil
}