package queue

import (
	"context"
	"encoding/json"
	"fmt"
	"strings"
	"time"

	"github.com/redis/go-redis/v9"
)

const (
	defaultMetricsFlushInterval = 500 * time.Millisecond
	defaultLeaseDuration        = 30 * time.Minute
	defaultMaxRetries           = 3
	defaultBlockTimeout         = 1 * time.Second
)

// TaskQueue manages ML experiment tasks via Redis
type TaskQueue struct {
	client      *redis.Client
	ctx         context.Context
	cancel      context.CancelFunc
	metricsCh   chan metricEvent
	metricsDone chan struct{}
	flushEvery  time.Duration
}

type metricEvent struct {
	JobName string
	Metric  string
	Value   float64
}

// Config holds configuration for TaskQueue
type Config struct {
	RedisAddr            string
	RedisPassword        string
	RedisDB              int
	MetricsFlushInterval time.Duration
}

// NewTaskQueue creates a new task queue instance with Redis backend.
func NewTaskQueue(cfg Config) (*TaskQueue, error) {
	var opts *redis.Options
	var err error

	if len(cfg.RedisAddr) > 8 && cfg.RedisAddr[:8] == "redis://" {
		opts, err = redis.ParseURL(cfg.RedisAddr)
		if err != nil {
			return nil, fmt.Errorf("invalid redis url: %w", err)
		}
	} else {
		opts = &redis.Options{
			Addr:     cfg.RedisAddr,
			Password: cfg.RedisPassword,
			DB:       cfg.RedisDB,
			// Connection pooling optimizations
			PoolSize:     50,              // Increase connection pool size
			MinIdleConns: 10,              // Maintain minimum idle connections
			MaxRetries:   3,               // Reduce retries for faster failure
			DialTimeout:  5 * time.Second, // Faster connection timeout
			ReadTimeout:  3 * time.Second, // Faster read timeout
			WriteTimeout: 3 * time.Second, // Faster write timeout
			PoolTimeout:  4 * time.Second, // Faster pool timeout
		}
	}

	rdb := redis.NewClient(opts)

	ctx, cancel := context.WithCancel(context.Background())
	if err := rdb.Ping(ctx).Err(); err != nil {
		cancel()
		return nil, fmt.Errorf("redis connection failed: %w", err)
	}

	flushEvery := cfg.MetricsFlushInterval
	if flushEvery == 0 {
		flushEvery = defaultMetricsFlushInterval
	}

	tq := &TaskQueue{
		client:      rdb,
		ctx:         ctx,
		cancel:      cancel,
		metricsCh:   make(chan metricEvent, 256),
		metricsDone: make(chan struct{}),
		flushEvery:  flushEvery,
	}

	go tq.runMetricsBuffer()
	go tq.runLeaseReclamation() // Start lease reclamation background job

	return tq, nil
}

// AddTask adds a new task to the queue with default retry settings
func (tq *TaskQueue) AddTask(task *Task) error {
	// Set default retry settings if not specified
	if task.MaxRetries == 0 {
		task.MaxRetries = defaultMaxRetries
	}

	taskData, err := json.Marshal(task)
	if err != nil {
		return fmt.Errorf("failed to marshal task: %w", err)
	}

	pipe := tq.client.Pipeline()

	// Store task data
	pipe.Set(tq.ctx, TaskPrefix+task.ID, taskData, 7*24*time.Hour)

	// Add to priority queue (ZSET)
	// Use priority as score (higher priority = higher score)
	pipe.ZAddArgs(tq.ctx, TaskQueueKey, redis.ZAddArgs{
		Members: []redis.Z{
			{
				Score:  float64(task.Priority),
				Member: task.ID,
			},
		},
	})

	// Initialize status
	pipe.HSet(tq.ctx, TaskStatusPrefix+task.JobName,
		"status", task.Status,
		"task_id", task.ID,
		"updated_at", time.Now().Format(time.RFC3339))

	// Queue depth update piggybacks on the same pipeline round-trip to avoid
	// an extra Redis call per enqueue under high load.
	depthCmd := pipe.ZCard(tq.ctx, TaskQueueKey)

	_, err = pipe.Exec(tq.ctx)
	if err != nil {
		return fmt.Errorf("failed to enqueue task: %w", err)
	}

	// Record metrics
	TasksQueued.Inc()

	// Update queue depth
	UpdateQueueDepth(depthCmd.Val())

	return nil
}

func isBlockingUnsupported(err error) bool {
	if err == nil {
		return false
	}
	msg := strings.ToLower(err.Error())
	return strings.Contains(msg, "unknown command") && strings.Contains(msg, "bzpopmax")
}

func (tq *TaskQueue) pollUntilDeadline(workerID string, leaseDuration, blockTimeout time.Duration) (*Task, error) {
	deadline := time.Now().Add(blockTimeout)
	sleep := 25 * time.Millisecond

	for {
		if time.Now().After(deadline) {
			return nil, nil
		}

		task, err := tq.GetNextTaskWithLease(workerID, leaseDuration)
		if err != nil {
			return nil, err
		}
		if task != nil {
			return task, nil
		}

		select {
		case <-tq.ctx.Done():
			return nil, tq.ctx.Err()
		case <-time.After(sleep):
		}
	}
}

// GetNextTask gets the next task without lease (backward compatible)
func (tq *TaskQueue) GetNextTask() (*Task, error) {
	result, err := tq.client.ZPopMax(tq.ctx, TaskQueueKey, 1).Result()
	if err != nil {
		return nil, err
	}
	if len(result) == 0 {
		return nil, nil
	}

	taskID := result[0].Member.(string)
	return tq.GetTask(taskID)
}

// GetNextTaskWithLease gets the next task and acquires a lease
func (tq *TaskQueue) GetNextTaskWithLease(workerID string, leaseDuration time.Duration) (*Task, error) {
	if leaseDuration == 0 {
		leaseDuration = defaultLeaseDuration
	}

	// Pop highest priority task
	result, err := tq.client.ZPopMax(tq.ctx, TaskQueueKey, 1).Result()
	if err != nil {
		return nil, err
	}
	if len(result) == 0 {
		return nil, nil
	}

	taskID := result[0].Member.(string)
	task, err := tq.GetTask(taskID)
	if err != nil {
		// Re-queue the task if we can't fetch it
		tq.client.ZAdd(tq.ctx, TaskQueueKey, redis.Z{
			Score:  result[0].Score,
			Member: taskID,
		})
		return nil, err
	}

	// Acquire lease
	now := time.Now()
	leaseExpiry := now.Add(leaseDuration)
	task.LeaseExpiry = &leaseExpiry
	task.LeasedBy = workerID

	// Update task with lease
	if err := tq.UpdateTask(task); err != nil {
		// Re-queue if update fails
		tq.client.ZAdd(tq.ctx, TaskQueueKey, redis.Z{
			Score:  result[0].Score,
			Member: taskID,
		})
		return nil, err
	}

	return task, nil
}

// GetNextTaskWithLeaseBlocking blocks up to blockTimeout waiting for a task before acquiring a lease.
func (tq *TaskQueue) GetNextTaskWithLeaseBlocking(
	workerID string,
	leaseDuration, blockTimeout time.Duration,
) (*Task, error) {
	if leaseDuration == 0 {
		leaseDuration = defaultLeaseDuration
	}
	if blockTimeout <= 0 {
		blockTimeout = defaultBlockTimeout
	}

	result, err := tq.client.BZPopMax(tq.ctx, blockTimeout, TaskQueueKey).Result()
	if err == redis.Nil {
		return nil, nil
	}
	if err != nil {
		if isBlockingUnsupported(err) {
			return tq.pollUntilDeadline(workerID, leaseDuration, blockTimeout)
		}
		return nil, err
	}

	taskID, ok := result.Member.(string)
	if !ok {
		return nil, fmt.Errorf("unexpected task id type %T", result.Member)
	}

	task, err := tq.GetTask(taskID)
	if err != nil {
		return nil, err
	}

	now := time.Now()
	leaseExpiry := now.Add(leaseDuration)
	task.LeaseExpiry = &leaseExpiry
	task.LeasedBy = workerID

	if err := tq.UpdateTask(task); err != nil {
		return nil, err
	}

	return task, nil
}

// RenewLease renews the lease on a task (heartbeat)
func (tq *TaskQueue) RenewLease(taskID string, workerID string, leaseDuration time.Duration) error {
	if leaseDuration == 0 {
		leaseDuration = defaultLeaseDuration
	}

	task, err := tq.GetTask(taskID)
	if err != nil {
		return err
	}

	// Verify the worker owns the lease
	if task.LeasedBy != workerID {
		return fmt.Errorf("task leased by different worker: %s", task.LeasedBy)
	}

	// Renew lease
	leaseExpiry := time.Now().Add(leaseDuration)
	task.LeaseExpiry = &leaseExpiry

	// Record renewal metric
	RecordLeaseRenewal(workerID)

	return tq.UpdateTask(task)
}

// ReleaseLease releases the lease on a task
func (tq *TaskQueue) ReleaseLease(taskID string, workerID string) error {
	task, err := tq.GetTask(taskID)
	if err != nil {
		return err
	}

	// Verify the worker owns the lease
	if task.LeasedBy != workerID {
		return fmt.Errorf("task leased by different worker: %s", task.LeasedBy)
	}

	// Clear lease
	task.LeaseExpiry = nil
	task.LeasedBy = ""

	return tq.UpdateTask(task)
}

// RetryTask re-queues a failed task with smart backoff based on error category
func (tq *TaskQueue) RetryTask(task *Task) error {
	if task.RetryCount >= task.MaxRetries {
		// Move to dead letter queue
		RecordDLQAddition("max_retries")
		return tq.MoveToDeadLetterQueue(task, "max retries exceeded")
	}

	// Classify the error if it exists
	errorCategory := ErrorUnknown
	if task.Error != "" {
		errorCategory = ClassifyError(fmt.Errorf("%s", task.Error))
	}

	// Check if error is retryable
	if !IsRetryable(errorCategory) {
		RecordDLQAddition(string(errorCategory))
		return tq.MoveToDeadLetterQueue(task, fmt.Sprintf("non-retryable error: %s", errorCategory))
	}

	task.RetryCount++
	task.Status = "queued"
	task.LastError = task.Error // Preserve last error
	task.Error = ""             // Clear current error

	// Calculate smart backoff based on error category
	backoffSeconds := RetryDelay(errorCategory, task.RetryCount)
	nextRetry := time.Now().Add(time.Duration(backoffSeconds) * time.Second)
	task.NextRetry = &nextRetry

	// Clear lease
	task.LeaseExpiry = nil
	task.LeasedBy = ""

	// Record retry metrics
	RecordTaskRetry(task.JobName, errorCategory)

	// Re-queue with same priority
	return tq.AddTask(task)
}

// MoveToDeadLetterQueue moves a task to the dead letter queue
func (tq *TaskQueue) MoveToDeadLetterQueue(task *Task, reason string) error {
	task.Status = "failed"
	task.Error = fmt.Sprintf("DLQ: %s. Last error: %s", reason, task.LastError)

	taskData, err := json.Marshal(task)
	if err != nil {
		return err
	}

	// Store in dead letter queue with timestamp
	key := "task:dlq:" + task.ID

	// Record metrics
	RecordTaskFailure(task.JobName, ClassifyError(fmt.Errorf("%s", task.LastError)))

	pipe := tq.client.Pipeline()
	pipe.Set(tq.ctx, key, taskData, 30*24*time.Hour)
	pipe.ZRem(tq.ctx, TaskQueueKey, task.ID)
	_, err = pipe.Exec(tq.ctx)
	return err
}

// runLeaseReclamation reclaims expired leases every 1 minute
func (tq *TaskQueue) runLeaseReclamation() {
	ticker := time.NewTicker(1 * time.Minute)
	defer ticker.Stop()

	for {
		select {
		case <-tq.ctx.Done():
			return
		case <-ticker.C:
			if err := tq.reclaimExpiredLeases(); err != nil {
				// Log error but continue
				continue
			}
		}
	}
}

// reclaimExpiredLeases finds and re-queues tasks with expired leases
func (tq *TaskQueue) reclaimExpiredLeases() error {
	// Scan for all task keys
	iter := tq.client.Scan(tq.ctx, 0, TaskPrefix+"*", 100).Iterator()
	now := time.Now()

	for iter.Next(tq.ctx) {
		taskKey := iter.Val()
		taskID := taskKey[len(TaskPrefix):]

		task, err := tq.GetTask(taskID)
		if err != nil {
			continue
		}

		// Check if lease expired and task is still running
		if task.LeaseExpiry != nil && task.LeaseExpiry.Before(now) && task.Status == "running" {
			// Lease expired - retry or fail the task
			task.Error = fmt.Sprintf("worker %s lease expired", task.LeasedBy)

			// Record lease expiration
			RecordLeaseExpiration()

			if task.RetryCount < task.MaxRetries {
				// Retry the task
				if err := tq.RetryTask(task); err != nil {
					continue
				}
			} else {
				// Max retries exceeded - move to DLQ
				if err := tq.MoveToDeadLetterQueue(task, "lease expiry after max retries"); err != nil {
					continue
				}
			}
		}
	}

	return iter.Err()
}

// GetTask retrieves a task by ID
func (tq *TaskQueue) GetTask(taskID string) (*Task, error) {
	data, err := tq.client.Get(tq.ctx, TaskPrefix+taskID).Result()
	if err != nil {
		return nil, err
	}

	var task Task
	if err := json.Unmarshal([]byte(data), &task); err != nil {
		return nil, err
	}

	return &task, nil
}

// GetAllTasks retrieves all tasks from the queue
func (tq *TaskQueue) GetAllTasks() ([]*Task, error) {
	// Get all task keys
	keys, err := tq.client.Keys(tq.ctx, TaskPrefix+"*").Result()
	if err != nil {
		return nil, err
	}

	var tasks []*Task
	for _, key := range keys {
		data, err := tq.client.Get(tq.ctx, key).Result()
		if err != nil {
			continue // Skip tasks that can't be retrieved
		}

		var task Task
		if err := json.Unmarshal([]byte(data), &task); err != nil {
			continue // Skip malformed tasks
		}

		tasks = append(tasks, &task)
	}

	return tasks, nil
}

// GetTaskByName retrieves a task by its job name
func (tq *TaskQueue) GetTaskByName(jobName string) (*Task, error) {
	tasks, err := tq.GetAllTasks()
	if err != nil {
		return nil, err
	}

	for _, task := range tasks {
		if task.JobName == jobName {
			return task, nil
		}
	}

	return nil, fmt.Errorf("task with job name '%s' not found", jobName)
}

// CancelTask marks a task as cancelled
func (tq *TaskQueue) CancelTask(taskID string) error {
	task, err := tq.GetTask(taskID)
	if err != nil {
		return err
	}

	// Update task status to cancelled
	task.Status = "cancelled"
	now := time.Now()
	task.EndedAt = &now

	return tq.UpdateTask(task)
}

// UpdateTask updates a task in Redis
func (tq *TaskQueue) UpdateTask(task *Task) error {
	taskData, err := json.Marshal(task)
	if err != nil {
		return err
	}

	pipe := tq.client.Pipeline()
	pipe.Set(tq.ctx, TaskPrefix+task.ID, taskData, 7*24*time.Hour)
	pipe.HSet(tq.ctx, TaskStatusPrefix+task.JobName,
		"status", task.Status,
		"task_id", task.ID,
		"updated_at", time.Now().Format(time.RFC3339))

	_, err = pipe.Exec(tq.ctx)
	return err
}

// UpdateTaskWithMetrics updates task and records metrics
func (tq *TaskQueue) UpdateTaskWithMetrics(task *Task, action string) error {
	if err := tq.UpdateTask(task); err != nil {
		return err
	}

	metricName := "tasks_" + action
	return tq.RecordMetric(task.JobName, metricName, 1)
}

// RecordMetric records a metric value
func (tq *TaskQueue) RecordMetric(jobName, metric string, value float64) error {
	evt := metricEvent{JobName: jobName, Metric: metric, Value: value}
	select {
	case tq.metricsCh <- evt:
		return nil
	default:
		return tq.writeMetrics(jobName, map[string]float64{metric: value})
	}
}

// Heartbeat records worker heartbeat
func (tq *TaskQueue) Heartbeat(workerID string) error {
	return tq.client.HSet(tq.ctx, WorkerHeartbeat,
		workerID, time.Now().Unix()).Err()
}

// QueueDepth returns the number of pending tasks
func (tq *TaskQueue) QueueDepth() (int64, error) {
	return tq.client.ZCard(tq.ctx, TaskQueueKey).Result()
}

// Close closes the task queue and cleans up resources
func (tq *TaskQueue) Close() error {
	tq.cancel()
	<-tq.metricsDone // Wait for metrics buffer to finish
	return tq.client.Close()
}

// GetRedisClient returns the underlying Redis client for direct access
func (tq *TaskQueue) GetRedisClient() *redis.Client {
	return tq.client
}

// WaitForNextTask waits for next task with timeout
func (tq *TaskQueue) WaitForNextTask(ctx context.Context, timeout time.Duration) (*Task, error) {
	if ctx == nil {
		ctx = tq.ctx
	}
	result, err := tq.client.BZPopMax(ctx, timeout, TaskQueueKey).Result()
	if err == redis.Nil {
		return nil, nil
	}
	if err != nil {
		return nil, err
	}
	member, ok := result.Member.(string)
	if !ok {
		return nil, fmt.Errorf("unexpected task id type %T", result.Member)
	}
	return tq.GetTask(member)
}

// runMetricsBuffer buffers and flushes metrics
func (tq *TaskQueue) runMetricsBuffer() {
	defer close(tq.metricsDone)
	ticker := time.NewTicker(tq.flushEvery)
	defer ticker.Stop()
	pending := make(map[string]map[string]float64)
	flush := func() {
		for job, metrics := range pending {
			if err := tq.writeMetrics(job, metrics); err != nil {
				continue
			}
			delete(pending, job)
		}
	}

	for {
		select {
		case <-tq.ctx.Done():
			flush()
			return
		case evt, ok := <-tq.metricsCh:
			if !ok {
				flush()
				return
			}
			if _, exists := pending[evt.JobName]; !exists {
				pending[evt.JobName] = make(map[string]float64)
			}
			pending[evt.JobName][evt.Metric] = evt.Value
		case <-ticker.C:
			flush()
		}
	}
}

// writeMetrics writes metrics to Redis
func (tq *TaskQueue) writeMetrics(jobName string, metrics map[string]float64) error {
	if len(metrics) == 0 {
		return nil
	}
	key := JobMetricsPrefix + jobName
	args := make([]any, 0, len(metrics)*2+2)
	args = append(args, "timestamp", time.Now().Unix())
	for metric, value := range metrics {
		args = append(args, metric, value)
	}
	return tq.client.HSet(context.Background(), key, args...).Err()
}