package queue

import (
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"os"
	"path/filepath"
	"sort"
	"strings"
	"time"

	"github.com/jfraeys/fetch_ml/internal/domain"
)

type FilesystemQueue struct {
	root   string
	ctx    context.Context
	cancel context.CancelFunc
}

type filesystemQueueIndex struct {
	Version   int                        `json:"version"`
	UpdatedAt string                     `json:"updated_at"`
	Tasks     []filesystemQueueIndexTask `json:"tasks"`
}

type filesystemQueueIndexTask struct {
	ID        string `json:"id"`
	Priority  int64  `json:"priority"`
	CreatedAt string `json:"created_at"`
}

func NewFilesystemQueue(root string) (*FilesystemQueue, error) {
	root = strings.TrimSpace(root)
	if root == "" {
		return nil, fmt.Errorf("filesystem queue root is required")
	}
	root = filepath.Clean(root)
	if err := os.MkdirAll(filepath.Join(root, "pending", "entries"), 0750); err != nil {
		return nil, err
	}
	for _, d := range []string{"running", "finished", "failed"} {
		if err := os.MkdirAll(filepath.Join(root, d), 0750); err != nil {
			return nil, err
		}
	}

	ctx, cancel := context.WithCancel(context.Background())
	q := &FilesystemQueue{root: root, ctx: ctx, cancel: cancel}
	_ = q.rebuildIndex()
	return q, nil
}

func (q *FilesystemQueue) Close() error {
	q.cancel()
	return nil
}

func (q *FilesystemQueue) AddTask(task *Task) error {
	if task == nil {
		return fmt.Errorf("task is nil")
	}
	if strings.TrimSpace(task.ID) == "" {
		return fmt.Errorf("task id is required")
	}
	if strings.TrimSpace(task.JobName) == "" {
		return fmt.Errorf("job name is required")
	}
	if task.MaxRetries == 0 {
		task.MaxRetries = defaultMaxRetries
	}
	if task.CreatedAt.IsZero() {
		task.CreatedAt = time.Now().UTC()
	}
	if strings.TrimSpace(task.Status) == "" {
		task.Status = "queued"
	}
	if task.Status != "queued" {
		// For filesystem backend we only enqueue queued tasks.
		// Other status updates should go through UpdateTask.
		task.Status = "queued"
	}

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

	path := q.pendingEntryPath(task.ID)
	if err := writeFileAtomic(path, payload, 0640); err != nil {
		return err
	}
	TasksQueued.Inc()
	if depth, derr := q.QueueDepth(); derr == nil {
		UpdateQueueDepth(depth)
	}
	_ = q.rebuildIndex()
	return nil
}

func (q *FilesystemQueue) GetNextTask() (*Task, error) {
	return q.claimNext("", 0, false)
}

func (q *FilesystemQueue) PeekNextTask() (*Task, error) {
	return q.claimNext("", 0, true)
}

func (q *FilesystemQueue) GetNextTaskWithLease(workerID string, leaseDuration time.Duration) (*Task, error) {
	return q.claimNext(workerID, leaseDuration, false)
}

func (q *FilesystemQueue) GetNextTaskWithLeaseBlocking(
	workerID string,
	leaseDuration, blockTimeout time.Duration,
) (*Task, error) {
	if blockTimeout <= 0 {
		blockTimeout = defaultBlockTimeout
	}
	deadline := time.Now().Add(blockTimeout)
	for {
		t, err := q.claimNext(workerID, leaseDuration, false)
		if err != nil {
			return nil, err
		}
		if t != nil {
			return t, nil
		}
		if time.Now().After(deadline) {
			return nil, nil
		}
		select {
		case <-q.ctx.Done():
			return nil, q.ctx.Err()
		case <-time.After(50 * time.Millisecond):
		}
	}
}

func (q *FilesystemQueue) RenewLease(taskID string, workerID string, leaseDuration time.Duration) error {
	// Single-worker friendly best-effort: update task lease fields if present.
	t, err := q.GetTask(taskID)
	if err != nil {
		return err
	}
	if t.LeasedBy != "" && workerID != "" && t.LeasedBy != workerID {
		return fmt.Errorf("task leased by different worker: %s", t.LeasedBy)
	}
	if leaseDuration == 0 {
		leaseDuration = defaultLeaseDuration
	}
	exp := time.Now().UTC().Add(leaseDuration)
	t.LeaseExpiry = &exp
	if workerID != "" {
		t.LeasedBy = workerID
	}
	RecordLeaseRenewal(workerID)
	return q.UpdateTask(t)
}

func (q *FilesystemQueue) ReleaseLease(taskID string, workerID string) error {
	t, err := q.GetTask(taskID)
	if err != nil {
		return err
	}
	if t.LeasedBy != "" && workerID != "" && t.LeasedBy != workerID {
		return fmt.Errorf("task leased by different worker: %s", t.LeasedBy)
	}
	t.LeaseExpiry = nil
	t.LeasedBy = ""
	return q.UpdateTask(t)
}

func (q *FilesystemQueue) RetryTask(task *Task) error {
	if task.RetryCount >= task.MaxRetries {
		RecordDLQAddition("max_retries")
		return q.MoveToDeadLetterQueue(task, "max retries exceeded")
	}

	failureClass := domain.FailureUnknown
	if task.Error != "" {
		failureClass = domain.ClassifyFailure(0, nil, task.Error)
	}
	if !domain.ShouldAutoRetry(failureClass, task.RetryCount) {
		RecordDLQAddition(string(failureClass))
		return q.MoveToDeadLetterQueue(task, fmt.Sprintf("non-retryable error: %s", failureClass))
	}

	task.RetryCount++
	task.Status = "queued"
	task.LastError = task.Error
	task.Error = ""

	backoffSeconds := domain.RetryDelayForClass(failureClass, task.RetryCount)
	nextRetry := time.Now().UTC().Add(time.Duration(backoffSeconds) * time.Second)
	task.NextRetry = &nextRetry
	task.LeaseExpiry = nil
	task.LeasedBy = ""

	RecordTaskRetry(task.JobName, failureClass)
	return q.AddTask(task)
}

func (q *FilesystemQueue) MoveToDeadLetterQueue(task *Task, reason string) error {
	if task == nil {
		return fmt.Errorf("task is nil")
	}
	task.Status = "failed"
	task.Error = fmt.Sprintf("DLQ: %s. Last error: %s", reason, task.LastError)
	failureClass := domain.ClassifyFailure(0, nil, task.LastError)
	RecordTaskFailure(task.JobName, failureClass)
	return q.UpdateTask(task)
}

func (q *FilesystemQueue) GetTask(taskID string) (*Task, error) {
	path, err := q.findTaskPath(taskID)
	if err != nil {
		return nil, err
	}
	data, err := os.ReadFile(path)
	if err != nil {
		return nil, err
	}
	var t Task
	if err := json.Unmarshal(data, &t); err != nil {
		return nil, err
	}
	return &t, nil
}

func (q *FilesystemQueue) GetAllTasks() ([]*Task, error) {
	paths := make([]string, 0, 32)
	for _, p := range []string{
		filepath.Join(q.root, "pending", "entries"),
		filepath.Join(q.root, "running"),
		filepath.Join(q.root, "finished"),
		filepath.Join(q.root, "failed"),
	} {
		entries, err := os.ReadDir(p)
		if err != nil {
			continue
		}
		for _, e := range entries {
			if e.IsDir() {
				continue
			}
			if !strings.HasSuffix(e.Name(), ".json") {
				continue
			}
			paths = append(paths, filepath.Join(p, e.Name()))
		}
	}

	out := make([]*Task, 0, len(paths))
	for _, path := range paths {
		data, err := os.ReadFile(path)
		if err != nil {
			continue
		}
		var t Task
		if err := json.Unmarshal(data, &t); err != nil {
			continue
		}
		out = append(out, &t)
	}
	return out, nil
}

func (q *FilesystemQueue) GetTaskByName(jobName string) (*Task, error) {
	jobName = strings.TrimSpace(jobName)
	if jobName == "" {
		return nil, fmt.Errorf("job name is required")
	}
	tasks, err := q.GetAllTasks()
	if err != nil {
		return nil, err
	}
	var best *Task
	for _, t := range tasks {
		if t == nil || t.JobName != jobName {
			continue
		}
		if best == nil || t.CreatedAt.After(best.CreatedAt) {
			best = t
		}
	}
	if best == nil {
		return nil, os.ErrNotExist
	}
	return best, nil
}

func (q *FilesystemQueue) CancelTask(taskID string) error {
	t, err := q.GetTask(taskID)
	if err != nil {
		return err
	}
	t.Status = "cancelled"
	now := time.Now().UTC()
	t.EndedAt = &now
	return q.UpdateTask(t)
}

func (q *FilesystemQueue) UpdateTask(task *Task) error {
	if task == nil {
		return fmt.Errorf("task is nil")
	}
	if strings.TrimSpace(task.ID) == "" {
		return fmt.Errorf("task id is required")
	}
	if strings.TrimSpace(task.Status) == "" {
		return fmt.Errorf("task status is required")
	}

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

	dst := q.pathForStatus(task.Status, task.ID)
	if dst == "" {
		// For statuses we don't map yet, keep it in running.
		dst = filepath.Join(q.root, "running", task.ID+".json")
	}
	if err := os.MkdirAll(filepath.Dir(dst), 0750); err != nil {
		return err
	}

	// Best-effort: remove any other copies before writing.
	_ = q.removeTaskFromAllDirs(task.ID)
	if err := writeFileAtomic(dst, payload, 0640); err != nil {
		return err
	}
	if depth, derr := q.QueueDepth(); derr == nil {
		UpdateQueueDepth(depth)
	}
	_ = q.rebuildIndex()
	return nil
}

func (q *FilesystemQueue) UpdateTaskWithMetrics(task *Task, _ string) error {
	return q.UpdateTask(task)
}

func (q *FilesystemQueue) RecordMetric(_, _ string, _ float64) error {
	return nil
}

func (q *FilesystemQueue) Heartbeat(_ string) error {
	return nil
}

func (q *FilesystemQueue) QueueDepth() (int64, error) {
	entries, err := os.ReadDir(filepath.Join(q.root, "pending", "entries"))
	if err != nil {
		return 0, err
	}
	var n int64
	for _, e := range entries {
		if e.IsDir() {
			continue
		}
		if strings.HasSuffix(e.Name(), ".json") {
			n++
		}
	}
	return n, nil
}

func (q *FilesystemQueue) SetWorkerPrewarmState(_ PrewarmState) error { return nil }
func (q *FilesystemQueue) ClearWorkerPrewarmState(_ string) error     { return nil }
func (q *FilesystemQueue) GetWorkerPrewarmState(_ string) (*PrewarmState, error) {
	return nil, nil
}
func (q *FilesystemQueue) GetAllWorkerPrewarmStates() ([]PrewarmState, error) {
	return nil, nil
}
func (q *FilesystemQueue) SignalPrewarmGC() error { return nil }
func (q *FilesystemQueue) PrewarmGCRequestValue() (string, error) {
	return "", nil
}

func (q *FilesystemQueue) claimNext(workerID string, leaseDuration time.Duration, peek bool) (*Task, error) {
	pendingDir := filepath.Join(q.root, "pending", "entries")
	entries, err := os.ReadDir(pendingDir)
	if err != nil {
		return nil, err
	}

	candidates := make([]*Task, 0, len(entries))
	paths := make(map[string]string, len(entries))
	for _, e := range entries {
		if e.IsDir() {
			continue
		}
		if !strings.HasSuffix(e.Name(), ".json") {
			continue
		}
		path := filepath.Join(pendingDir, e.Name())
		data, err := os.ReadFile(path)
		if err != nil {
			continue
		}
		var t Task
		if err := json.Unmarshal(data, &t); err != nil {
			continue
		}
		if t.NextRetry != nil && time.Now().UTC().Before(t.NextRetry.UTC()) {
			continue
		}
		candidates = append(candidates, &t)
		paths[t.ID] = path
	}

	if len(candidates) == 0 {
		return nil, nil
	}

	sort.Slice(candidates, func(i, j int) bool {
		if candidates[i].Priority != candidates[j].Priority {
			return candidates[i].Priority > candidates[j].Priority
		}
		return candidates[i].CreatedAt.Before(candidates[j].CreatedAt)
	})

	chosen := candidates[0]
	if peek {
		return chosen, nil
	}

	src := paths[chosen.ID]
	if src == "" {
		return nil, nil
	}
	dst := filepath.Join(q.root, "running", chosen.ID+".json")
	if err := os.Rename(src, dst); err != nil {
		// Another process might have claimed it.
		if errors.Is(err, os.ErrNotExist) {
			return nil, nil
		}
		return nil, err
	}

	// Refresh from the moved file to avoid race on content.
	data, err := os.ReadFile(dst)
	if err != nil {
		return nil, err
	}
	var t Task
	if err := json.Unmarshal(data, &t); err != nil {
		return nil, err
	}

	now := time.Now().UTC()
	if leaseDuration == 0 {
		leaseDuration = defaultLeaseDuration
	}
	exp := now.Add(leaseDuration)
	t.LeaseExpiry = &exp
	if strings.TrimSpace(workerID) != "" {
		t.LeasedBy = workerID
	}
	// Note: status transitions are handled by worker UpdateTask calls.

	payload, err := json.Marshal(&t)
	if err == nil {
		_ = writeFileAtomic(dst, payload, 0640)
	}
	if depth, derr := q.QueueDepth(); derr == nil {
		UpdateQueueDepth(depth)
	}
	_ = q.rebuildIndex()
	return &t, nil
}

func (q *FilesystemQueue) pendingEntryPath(taskID string) string {
	return filepath.Join(q.root, "pending", "entries", taskID+".json")
}

func (q *FilesystemQueue) pathForStatus(status, taskID string) string {
	switch status {
	case "queued":
		return q.pendingEntryPath(taskID)
	case "running":
		return filepath.Join(q.root, "running", taskID+".json")
	case "completed", "finished":
		return filepath.Join(q.root, "finished", taskID+".json")
	case "failed", "cancelled":
		return filepath.Join(q.root, "failed", taskID+".json")
	default:
		return ""
	}
}

func (q *FilesystemQueue) findTaskPath(taskID string) (string, error) {
	paths := []string{
		q.pendingEntryPath(taskID),
		filepath.Join(q.root, "running", taskID+".json"),
		filepath.Join(q.root, "finished", taskID+".json"),
		filepath.Join(q.root, "failed", taskID+".json"),
	}
	for _, p := range paths {
		if _, err := os.Stat(p); err == nil {
			return p, nil
		}
	}
	return "", os.ErrNotExist
}

func (q *FilesystemQueue) removeTaskFromAllDirs(taskID string) error {
	paths := []string{
		q.pendingEntryPath(taskID),
		filepath.Join(q.root, "running", taskID+".json"),
		filepath.Join(q.root, "finished", taskID+".json"),
		filepath.Join(q.root, "failed", taskID+".json"),
	}
	var outErr error
	for _, p := range paths {
		if err := os.Remove(p); err != nil && !errors.Is(err, os.ErrNotExist) {
			outErr = err
		}
	}
	return outErr
}

func (q *FilesystemQueue) rebuildIndex() error {
	pendingDir := filepath.Join(q.root, "pending", "entries")
	entries, err := os.ReadDir(pendingDir)
	if err != nil {
		return err
	}
	idx := filesystemQueueIndex{Version: 1, UpdatedAt: time.Now().UTC().Format(time.RFC3339)}
	for _, e := range entries {
		if e.IsDir() || !strings.HasSuffix(e.Name(), ".json") {
			continue
		}
		data, err := os.ReadFile(filepath.Join(pendingDir, e.Name()))
		if err != nil {
			continue
		}
		var t Task
		if err := json.Unmarshal(data, &t); err != nil {
			continue
		}
		idx.Tasks = append(idx.Tasks, filesystemQueueIndexTask{ID: t.ID, Priority: t.Priority, CreatedAt: t.CreatedAt.UTC().Format(time.RFC3339Nano)})
	}

	sort.Slice(idx.Tasks, func(i, j int) bool {
		if idx.Tasks[i].Priority != idx.Tasks[j].Priority {
			return idx.Tasks[i].Priority > idx.Tasks[j].Priority
		}
		return idx.Tasks[i].CreatedAt < idx.Tasks[j].CreatedAt
	})

	payload, err := json.MarshalIndent(&idx, "", "  ")
	if err != nil {
		return err
	}
	path := filepath.Join(q.root, "pending", ".queue.json")
	return writeFileAtomic(path, payload, 0640)
}

func writeFileAtomic(path string, data []byte, perm os.FileMode) error {
	dir := filepath.Dir(path)
	if err := os.MkdirAll(dir, 0750); err != nil {
		return err
	}
	tmp := path + ".tmp"
	if err := os.WriteFile(tmp, data, perm); err != nil {
		return err
	}
	return os.Rename(tmp, path)
}