package leaderelection

import (
	"context"
	"encoding/json"
	"fmt"
	"math/rand"
	"os"
	"sync"
	"time"

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

// redisLeaderElection implements LeaderElection using Redis.
type redisLeaderElection struct {
	config              *Config
	redis               redis.UniversalClient
	logger              *zap.Logger
	instanceID          string
	hostname            string
	isLeader            bool
	leaderMutex         sync.RWMutex
	becomeLeaderCbs     []func()
	loseLeadershipCbs   []func()
	callbackMutex       sync.RWMutex
	stopChan            chan struct{}
	stoppedChan         chan struct{}
	leaderStartTime     time.Time
	lastHeartbeat       time.Time
	lastHeartbeatMutex  sync.RWMutex
}

// NewRedisLeaderElection creates a new Redis-based leader election instance.
func NewRedisLeaderElection(
	config *Config,
	redisClient redis.UniversalClient,
	logger *zap.Logger,
) (LeaderElection, error) {
	logger = logger.Named("LeaderElection")

	// Validate configuration
	if err := config.Validate(); err != nil {
		return nil, fmt.Errorf("invalid configuration: %w", err)
	}

	// Generate instance ID if not provided
	instanceID := config.InstanceID
	if instanceID == "" {
		hostname, err := os.Hostname()
		if err != nil {
			hostname = "unknown"
		}
		// Add random suffix to make it unique
		instanceID = fmt.Sprintf("%s-%d", hostname, rand.Intn(100000))
		logger.Info("Generated instance ID", zap.String("instance_id", instanceID))
	}

	// Get hostname if not provided
	hostname := config.Hostname
	if hostname == "" {
		h, err := os.Hostname()
		if err != nil {
			hostname = "unknown"
		} else {
			hostname = h
		}
	}

	return &redisLeaderElection{
		config:            config,
		redis:             redisClient,
		logger:            logger,
		instanceID:        instanceID,
		hostname:          hostname,
		isLeader:          false,
		becomeLeaderCbs:   make([]func(), 0),
		loseLeadershipCbs: make([]func(), 0),
		stopChan:          make(chan struct{}),
		stoppedChan:       make(chan struct{}),
	}, nil
}

// Start begins participating in leader election.
func (le *redisLeaderElection) Start(ctx context.Context) error {
	le.logger.Info("Starting leader election",
		zap.String("instance_id", le.instanceID),
		zap.String("hostname", le.hostname),
		zap.Duration("lock_ttl", le.config.LockTTL),
		zap.Duration("heartbeat_interval", le.config.HeartbeatInterval),
	)

	defer close(le.stoppedChan)

	// Main election loop
	ticker := time.NewTicker(le.config.RetryInterval)
	defer ticker.Stop()

	// Try to become leader immediately on startup
	le.tryBecomeLeader(ctx)

	for {
		select {
		case <-ctx.Done():
			le.logger.Info("Context cancelled, stopping leader election")
			le.releaseLeadership(context.Background())
			return ctx.Err()

		case <-le.stopChan:
			le.logger.Info("Stop signal received, stopping leader election")
			le.releaseLeadership(context.Background())
			return nil

		case <-ticker.C:
			if le.IsLeader() {
				// If we're the leader, send heartbeat
				if err := le.sendHeartbeat(ctx); err != nil {
					le.logger.Error("Failed to send heartbeat, lost leadership",
						zap.Error(err))
					le.setLeaderStatus(false)
					le.executeCallbacks(le.loseLeadershipCbs)
				}
			} else {
				// If we're not the leader, try to become leader
				le.tryBecomeLeader(ctx)
			}
		}
	}
}

// tryBecomeLeader attempts to acquire leadership.
func (le *redisLeaderElection) tryBecomeLeader(ctx context.Context) {
	// Try to set the leader key with NX (only if not exists) and EX (expiry)
	success, err := le.redis.SetNX(ctx, le.config.RedisKeyName, le.instanceID, le.config.LockTTL).Result()
	if err != nil {
		le.logger.Error("Failed to attempt leader election",
			zap.Error(err))
		return
	}

	if success {
		// We became the leader!
		le.logger.Info("🎉 Became the leader!",
			zap.String("instance_id", le.instanceID))

		le.leaderStartTime = time.Now()
		le.setLeaderStatus(true)
		le.updateLeaderInfo(ctx)
		le.executeCallbacks(le.becomeLeaderCbs)
	} else {
		// Someone else is the leader
		if !le.IsLeader() {
			// Only log if we weren't already aware
			currentLeader, _ := le.GetLeaderID()
			le.logger.Debug("Another instance is the leader",
				zap.String("leader_id", currentLeader))
		}
	}
}

// sendHeartbeat renews the leader lock.
func (le *redisLeaderElection) sendHeartbeat(ctx context.Context) error {
	// Verify we still hold the lock
	currentValue, err := le.redis.Get(ctx, le.config.RedisKeyName).Result()
	if err != nil {
		return fmt.Errorf("failed to get current lock value: %w", err)
	}

	if currentValue != le.instanceID {
		return fmt.Errorf("lock held by different instance: %s", currentValue)
	}

	// Renew the lock
	err = le.redis.Expire(ctx, le.config.RedisKeyName, le.config.LockTTL).Err()
	if err != nil {
		return fmt.Errorf("failed to renew lock: %w", err)
	}

	// Update heartbeat time
	le.setLastHeartbeat(time.Now())

	// Update leader info
	le.updateLeaderInfo(ctx)

	le.logger.Debug("Heartbeat sent",
		zap.String("instance_id", le.instanceID))

	return nil
}

// updateLeaderInfo updates the leader information in Redis.
func (le *redisLeaderElection) updateLeaderInfo(ctx context.Context) {
	info := &LeaderInfo{
		InstanceID:    le.instanceID,
		Hostname:      le.hostname,
		StartedAt:     le.leaderStartTime,
		LastHeartbeat: le.getLastHeartbeat(),
	}

	data, err := json.Marshal(info)
	if err != nil {
		le.logger.Error("Failed to marshal leader info", zap.Error(err))
		return
	}

	// Set with same TTL as lock
	err = le.redis.Set(ctx, le.config.RedisInfoKeyName, data, le.config.LockTTL).Err()
	if err != nil {
		le.logger.Error("Failed to update leader info", zap.Error(err))
	}
}

// releaseLeadership voluntarily releases leadership.
func (le *redisLeaderElection) releaseLeadership(ctx context.Context) {
	if !le.IsLeader() {
		return
	}

	le.logger.Info("Releasing leadership voluntarily",
		zap.String("instance_id", le.instanceID))

	// Only delete if we're still the owner
	script := `
		if redis.call("GET", KEYS[1]) == ARGV[1] then
			return redis.call("DEL", KEYS[1])
		else
			return 0
		end
	`

	_, err := le.redis.Eval(ctx, script, []string{le.config.RedisKeyName}, le.instanceID).Result()
	if err != nil {
		le.logger.Error("Failed to release leadership", zap.Error(err))
	}

	// Delete leader info
	le.redis.Del(ctx, le.config.RedisInfoKeyName)

	le.setLeaderStatus(false)
	le.executeCallbacks(le.loseLeadershipCbs)
}

// IsLeader returns true if this instance is the leader.
func (le *redisLeaderElection) IsLeader() bool {
	le.leaderMutex.RLock()
	defer le.leaderMutex.RUnlock()
	return le.isLeader
}

// GetLeaderID returns the ID of the current leader.
func (le *redisLeaderElection) GetLeaderID() (string, error) {
	ctx := context.Background()
	leaderID, err := le.redis.Get(ctx, le.config.RedisKeyName).Result()
	if err == redis.Nil {
		return "", nil
	}
	if err != nil {
		return "", fmt.Errorf("failed to get leader ID: %w", err)
	}
	return leaderID, nil
}

// GetLeaderInfo returns information about the current leader.
func (le *redisLeaderElection) GetLeaderInfo() (*LeaderInfo, error) {
	ctx := context.Background()
	data, err := le.redis.Get(ctx, le.config.RedisInfoKeyName).Result()
	if err == redis.Nil {
		return nil, nil
	}
	if err != nil {
		return nil, fmt.Errorf("failed to get leader info: %w", err)
	}

	var info LeaderInfo
	if err := json.Unmarshal([]byte(data), &info); err != nil {
		return nil, fmt.Errorf("failed to unmarshal leader info: %w", err)
	}

	return &info, nil
}

// OnBecomeLeader registers a callback for when this instance becomes leader.
func (le *redisLeaderElection) OnBecomeLeader(callback func()) {
	le.callbackMutex.Lock()
	defer le.callbackMutex.Unlock()
	le.becomeLeaderCbs = append(le.becomeLeaderCbs, callback)
}

// OnLoseLeadership registers a callback for when this instance loses leadership.
func (le *redisLeaderElection) OnLoseLeadership(callback func()) {
	le.callbackMutex.Lock()
	defer le.callbackMutex.Unlock()
	le.loseLeadershipCbs = append(le.loseLeadershipCbs, callback)
}

// Stop gracefully stops leader election.
func (le *redisLeaderElection) Stop() error {
	le.logger.Info("Stopping leader election")
	close(le.stopChan)

	// Wait for the election loop to finish (with timeout)
	select {
	case <-le.stoppedChan:
		le.logger.Info("Leader election stopped successfully")
		return nil
	case <-time.After(5 * time.Second):
		le.logger.Warn("Timeout waiting for leader election to stop")
		return fmt.Errorf("timeout waiting for leader election to stop")
	}
}

// GetInstanceID returns this instance's unique identifier.
func (le *redisLeaderElection) GetInstanceID() string {
	return le.instanceID
}

// setLeaderStatus updates the leader status (thread-safe).
func (le *redisLeaderElection) setLeaderStatus(isLeader bool) {
	le.leaderMutex.Lock()
	defer le.leaderMutex.Unlock()
	le.isLeader = isLeader
}

// setLastHeartbeat updates the last heartbeat time (thread-safe).
func (le *redisLeaderElection) setLastHeartbeat(t time.Time) {
	le.lastHeartbeatMutex.Lock()
	defer le.lastHeartbeatMutex.Unlock()
	le.lastHeartbeat = t
}

// getLastHeartbeat gets the last heartbeat time (thread-safe).
func (le *redisLeaderElection) getLastHeartbeat() time.Time {
	le.lastHeartbeatMutex.RLock()
	defer le.lastHeartbeatMutex.RUnlock()
	return le.lastHeartbeat
}

// executeCallbacks executes a list of callbacks in separate goroutines.
func (le *redisLeaderElection) executeCallbacks(callbacks []func()) {
	le.callbackMutex.RLock()
	defer le.callbackMutex.RUnlock()

	for _, callback := range callbacks {
		go func(cb func()) {
			defer func() {
				if r := recover(); r != nil {
					le.logger.Error("Panic in leader election callback",
						zap.Any("panic", r))
				}
			}()
			cb()
		}(callback)
	}
}