package server

import (
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"os"
	"path/filepath"
	"sync"
	"sync/atomic"
	"syscall"
	"time"
)

var (
	osWriteFile = os.WriteFile
	osRename    = os.Rename
)

type VideoRequest struct {
	URL          string `json:"url"`
	OutDir       string `json:"out_dir"`
	MetadataOnly bool   `json:"metadata_only"`
}

type queueItem struct {
	ID      string       `json:"id"`
	Request VideoRequest `json:"request"`
}

// Queue provides a simple FIFO with optional on-disk persistence.
type Queue struct {
	mu         sync.Mutex
	items      []queueItem
	persist    string
	notifyChan chan struct{}
}

func NewQueue(persistPath string) (*Queue, error) {
	q := &Queue{
		persist:    persistPath,
		notifyChan: make(chan struct{}, 1),
	}

	if persistPath != "" {
		if err := q.load(); err != nil {
			return nil, err
		}
	}

	return q, nil
}

func (q *Queue) Enqueue(req VideoRequest) (string, error) {
	q.mu.Lock()
	defer q.mu.Unlock()

	id := generateID()
	item := queueItem{ID: id, Request: req}
	q.items = append(q.items, item)

	if err := q.persistLocked(); err != nil {
		q.items = q.items[:len(q.items)-1]
		return "", err
	}

	q.signal()
	return id, nil
}

func (q *Queue) Next(ctx context.Context) (queueItem, error) {
	for {
		q.mu.Lock()
		if len(q.items) > 0 {
			item := q.items[0]
			q.mu.Unlock()
			return item, nil
		}
		q.mu.Unlock()

		select {
		case <-ctx.Done():
			return queueItem{}, ctx.Err()
		case <-q.notifyChan:
		}
	}
}

func (q *Queue) MarkDone(id string) error {
	q.mu.Lock()
	defer q.mu.Unlock()

	if len(q.items) == 0 {
		return errors.New("queue is empty")
	}

	if q.items[0].ID != id {
		// Find the item anywhere in the queue to avoid leaks if processing order is disrupted.
		index := -1
		for i, item := range q.items {
			if item.ID == id {
				index = i
				break
			}
		}
		if index == -1 {
			return errors.New("queue item not found")
		}
		q.items = append(q.items[:index], q.items[index+1:]...)
	} else {
		q.items = q.items[1:]
	}

	return q.persistLocked()
}

func (q *Queue) signal() {
	select {
	case q.notifyChan <- struct{}{}:
	default:
	}
}

func (q *Queue) Snapshot() []queueItem {
	q.mu.Lock()
	defer q.mu.Unlock()

	items := make([]queueItem, len(q.items))
	copy(items, q.items)
	return items
}

func (q *Queue) load() error {
	data, err := os.ReadFile(q.persist)
	if err != nil {
		if os.IsNotExist(err) {
			return nil
		}
		return err
	}

	var stored []queueItem
	if len(data) == 0 {
		q.items = nil
		return nil
	}

	if err := json.Unmarshal(data, &stored); err != nil {
		return err
	}

	q.items = append(q.items, stored...)
	return nil
}

func (q *Queue) persistLocked() error {
	if q.persist == "" {
		return nil
	}

	if err := os.MkdirAll(filepath.Dir(q.persist), 0o755); err != nil {
		return err
	}

	if len(q.items) == 0 {
		if err := os.Remove(q.persist); err != nil && !os.IsNotExist(err) {
			return err
		}
		return nil
	}

	data, err := json.MarshalIndent(q.items, "", "  ")
	if err != nil {
		return err
	}

	return writeFileAtomic(q.persist, data)
}

var idCounter uint64

func generateID() string {
	count := atomic.AddUint64(&idCounter, 1)
	return fmt.Sprintf("%d-%d", time.Now().UTC().UnixNano(), count)
}

func writeFileAtomic(path string, data []byte) error {
	tmp := path + ".tmp"
	if err := osWriteFile(tmp, data, 0o644); err != nil {
		return err
	}

	if err := osRename(tmp, path); err != nil {
		if errors.Is(err, syscall.EBUSY) {
			if writeErr := osWriteFile(path, data, 0o644); writeErr != nil {
				_ = os.Remove(tmp)
				return writeErr
			}
			return os.Remove(tmp)
		}
		_ = os.Remove(tmp)
		return err
	}

	return nil
}