package services

import (
	"archive/tar"
	"archive/zip"
	"bufio"
	"compress/gzip"
	"context"
	"errors"
	"fmt"
	"io"
	"net/http"
	"os"
	"path/filepath"
	"strings"
	"sync"
	"sync/atomic"
	"time"

	"fastfileviewer/backend/internal/models"
	"gorm.io/gorm"
)

type taskEvent struct {
	TaskID   uint
	Status   models.TaskStatus
	Progress int
	LogLine  string
	Error    string
}

type TaskService struct {
	db      *gorm.DB
	fs      *FSService
	queue   chan uint
	workers int

	mu          sync.RWMutex
	subscribers map[uint]map[chan taskEvent]struct{}
}

const (
	minMultiPartSize = 16 * 1024 * 1024
	maxDownloadParts = 8
	partRetryLimit   = 3
)

func NewTaskService(database *gorm.DB, fs *FSService, workers int) *TaskService {
	if workers < 1 {
		workers = 1
	}
	return &TaskService{
		db:          database,
		fs:          fs,
		queue:       make(chan uint, 256),
		workers:     workers,
		subscribers: map[uint]map[chan taskEvent]struct{}{},
	}
}

func (s *TaskService) Start(ctx context.Context) {
	for i := 0; i < s.workers; i++ {
		go s.worker(ctx)
	}
}

func (s *TaskService) CreateDownloadTask(url, targetPath string) (*models.Task, error) {
	task := &models.Task{
		Type:       models.TaskTypeDownload,
		Status:     models.TaskStatusQueued,
		Source:     url,
		TargetPath: targetPath,
		Progress:   0,
	}
	if err := s.db.Create(task).Error; err != nil {
		return nil, err
	}
	s.queue <- task.ID
	return task, nil
}

func (s *TaskService) CreateExtractTask(sourceArchive, targetPath string) (*models.Task, error) {
	task := &models.Task{
		Type:       models.TaskTypeExtract,
		Status:     models.TaskStatusQueued,
		Source:     sourceArchive,
		TargetPath: targetPath,
		Progress:   0,
	}
	if err := s.db.Create(task).Error; err != nil {
		return nil, err
	}
	s.queue <- task.ID
	return task, nil
}

func (s *TaskService) ListTasks(limit int) ([]models.Task, error) {
	if limit <= 0 || limit > 100 {
		limit = 50
	}
	var tasks []models.Task
	err := s.db.Order("id desc").Limit(limit).Find(&tasks).Error
	return tasks, err
}

func (s *TaskService) GetTask(id uint) (*models.Task, error) {
	var task models.Task
	if err := s.db.First(&task, id).Error; err != nil {
		return nil, err
	}
	return &task, nil
}

func (s *TaskService) Subscribe(taskID uint) (chan taskEvent, func()) {
	ch := make(chan taskEvent, 16)
	s.mu.Lock()
	if s.subscribers[taskID] == nil {
		s.subscribers[taskID] = map[chan taskEvent]struct{}{}
	}
	s.subscribers[taskID][ch] = struct{}{}
	s.mu.Unlock()

	cancel := func() {
		s.mu.Lock()
		defer s.mu.Unlock()
		delete(s.subscribers[taskID], ch)
		close(ch)
		if len(s.subscribers[taskID]) == 0 {
			delete(s.subscribers, taskID)
		}
	}
	return ch, cancel
}

func (s *TaskService) worker(ctx context.Context) {
	for {
		select {
		case <-ctx.Done():
			return
		case id := <-s.queue:
			_ = s.runTask(id)
		}
	}
}

func (s *TaskService) runTask(id uint) error {
	task, err := s.GetTask(id)
	if err != nil {
		return err
	}

	s.setStatus(task, models.TaskStatusRunning, 1, "task started", "")
	switch task.Type {
	case models.TaskTypeDownload:
		err = s.handleDownload(task)
	case models.TaskTypeExtract:
		err = s.handleExtract(task)
	default:
		err = errors.New("unsupported task type")
	}

	if err != nil {
		s.setStatus(task, models.TaskStatusFailed, task.Progress, "", err.Error())
		return err
	}
	s.setStatus(task, models.TaskStatusSuccess, 100, "task completed", "")
	return nil
}

func (s *TaskService) handleDownload(task *models.Task) error {
	target, err := s.fs.ResolvePath(task.TargetPath)
	if err != nil {
		return err
	}
	if err := os.MkdirAll(filepath.Dir(target), 0o755); err != nil {
		return err
	}

	s.appendLog(task, "starting download: "+task.Source)
	size, supportsRange, err := probeDownload(task.Source)
	if err == nil && supportsRange && size >= minMultiPartSize {
		parts := int(size / (8 * 1024 * 1024))
		if parts < 2 {
			parts = 2
		}
		if parts > maxDownloadParts {
			parts = maxDownloadParts
		}
		s.appendLog(task, fmt.Sprintf("range download enabled: parts=%d, size=%d bytes", parts, size))
		if err := s.multiPartDownload(task, task.Source, target, size, parts); err == nil {
			s.appendLog(task, "download complete")
			return nil
		} else {
			s.appendLog(task, "multi-part failed, fallback to single stream: "+err.Error())
			_ = os.Remove(target)
		}
	}

	if err := s.singlePartDownload(task, task.Source, target); err != nil {
		return err
	}
	s.appendLog(task, "download complete")
	return nil
}

func probeDownload(url string) (size int64, supportsRange bool, err error) {
	req, err := http.NewRequest(http.MethodHead, url, nil)
	if err != nil {
		return 0, false, err
	}
	resp, err := http.DefaultClient.Do(req) // #nosec G107 user-provided URLs are expected in this app design
	if err != nil {
		return 0, false, err
	}
	defer resp.Body.Close()
	if resp.StatusCode < 200 || resp.StatusCode >= 400 {
		return 0, false, fmt.Errorf("probe failed: %s", resp.Status)
	}
	size = resp.ContentLength
	supportsRange = strings.Contains(strings.ToLower(resp.Header.Get("Accept-Ranges")), "bytes")
	return size, supportsRange, nil
}

func (s *TaskService) singlePartDownload(task *models.Task, url, target string) error {
	resp, err := http.Get(url) // #nosec G107 user-provided URLs are expected in this app design
	if err != nil {
		return err
	}
	defer resp.Body.Close()

	if resp.StatusCode < 200 || resp.StatusCode >= 300 {
		return fmt.Errorf("download failed: %s", resp.Status)
	}

	out, err := os.Create(target)
	if err != nil {
		return err
	}
	defer out.Close()

	total := resp.ContentLength
	var copied int64
	reader := bufio.NewReader(resp.Body)
	buf := make([]byte, 256*1024)
	lastTick := time.Now()
	for {
		n, readErr := reader.Read(buf)
		if n > 0 {
			if _, err := out.Write(buf[:n]); err != nil {
				return err
			}
			copied += int64(n)
			if total > 0 && time.Since(lastTick) > 500*time.Millisecond {
				progress := int((copied * 100) / total)
				if progress > 99 {
					progress = 99
				}
				s.setProgress(task, progress)
				lastTick = time.Now()
			}
		}
		if readErr == io.EOF {
			break
		}
		if readErr != nil {
			return readErr
		}
	}
	return nil
}

func (s *TaskService) multiPartDownload(task *models.Task, url, target string, total int64, parts int) error {
	file, err := os.OpenFile(target, os.O_CREATE|os.O_WRONLY|os.O_TRUNC, 0o644)
	if err != nil {
		return err
	}
	defer file.Close()
	if err := file.Truncate(total); err != nil {
		return err
	}

	var downloaded atomic.Int64
	stopProgress := make(chan struct{})
	go func() {
		ticker := time.NewTicker(500 * time.Millisecond)
		defer ticker.Stop()
		for {
			select {
			case <-stopProgress:
				return
			case <-ticker.C:
				progress := int((downloaded.Load() * 100) / total)
				if progress > 99 {
					progress = 99
				}
				s.setProgress(task, progress)
			}
		}
	}()

	var wg sync.WaitGroup
	errCh := make(chan error, parts)
	chunkSize := total / int64(parts)

	for i := 0; i < parts; i++ {
		start := int64(i) * chunkSize
		end := start + chunkSize - 1
		if i == parts-1 {
			end = total - 1
		}

		wg.Add(1)
		go func(partID int, rangeStart, rangeEnd int64) {
			defer wg.Done()
			var lastErr error
			for attempt := 1; attempt <= partRetryLimit; attempt++ {
				err := downloadRange(url, file, rangeStart, rangeEnd, &downloaded)
				if err == nil {
					return
				}
				lastErr = err
				time.Sleep(time.Duration(attempt) * 400 * time.Millisecond)
			}
			errCh <- fmt.Errorf("part %d failed: %w", partID, lastErr)
		}(i+1, start, end)
	}

	wg.Wait()
	close(stopProgress)
	close(errCh)
	for err := range errCh {
		if err != nil {
			return err
		}
	}
	s.setProgress(task, 99)
	return nil
}

func downloadRange(url string, out *os.File, start, end int64, downloaded *atomic.Int64) error {
	req, err := http.NewRequest(http.MethodGet, url, nil)
	if err != nil {
		return err
	}
	req.Header.Set("Range", fmt.Sprintf("bytes=%d-%d", start, end))

	resp, err := http.DefaultClient.Do(req) // #nosec G107 user-provided URLs are expected in this app design
	if err != nil {
		return err
	}
	defer resp.Body.Close()

	if resp.StatusCode != http.StatusPartialContent {
		return fmt.Errorf("server does not honor range request: %s", resp.Status)
	}

	reader := bufio.NewReader(resp.Body)
	buf := make([]byte, 256*1024)
	offset := start
	expected := end - start + 1
	var written int64

	for {
		n, readErr := reader.Read(buf)
		if n > 0 {
			if _, err := out.WriteAt(buf[:n], offset); err != nil {
				return err
			}
			offset += int64(n)
			written += int64(n)
			downloaded.Add(int64(n))
		}
		if readErr == io.EOF {
			break
		}
		if readErr != nil {
			return readErr
		}
	}
	if written != expected {
		return fmt.Errorf("range incomplete: expected %d got %d", expected, written)
	}
	return nil
}

func (s *TaskService) handleExtract(task *models.Task) error {
	src, err := s.fs.ResolvePath(task.Source)
	if err != nil {
		return err
	}
	target, err := s.fs.ResolvePath(task.TargetPath)
	if err != nil {
		return err
	}
	if err := os.MkdirAll(target, 0o755); err != nil {
		return err
	}

	lower := strings.ToLower(src)
	switch {
	case strings.HasSuffix(lower, ".zip"):
		return s.extractZip(task, src, target)
	case strings.HasSuffix(lower, ".tar.gz"), strings.HasSuffix(lower, ".tgz"):
		return s.extractTarGz(task, src, target)
	default:
		return errors.New("unsupported archive format")
	}
}

func (s *TaskService) extractZip(task *models.Task, src, target string) error {
	r, err := zip.OpenReader(src)
	if err != nil {
		return err
	}
	defer r.Close()

	total := len(r.File)
	if total == 0 {
		return nil
	}

	for i, f := range r.File {
		dest := filepath.Join(target, f.Name)
		if !isSubPath(target, dest) {
			return fmt.Errorf("zip slip detected: %s", f.Name)
		}
		if f.FileInfo().IsDir() {
			if err := os.MkdirAll(dest, 0o755); err != nil {
				return err
			}
		} else {
			if err := os.MkdirAll(filepath.Dir(dest), 0o755); err != nil {
				return err
			}
			in, err := f.Open()
			if err != nil {
				return err
			}
			out, err := os.Create(dest)
			if err != nil {
				in.Close()
				return err
			}
			_, copyErr := io.Copy(out, in)
			in.Close()
			out.Close()
			if copyErr != nil {
				return copyErr
			}
		}
		progress := int((int64(i+1) * 100) / int64(total))
		if progress > 99 {
			progress = 99
		}
		s.setProgress(task, progress)
	}
	s.appendLog(task, "zip extract complete")
	return nil
}

func (s *TaskService) extractTarGz(task *models.Task, src, target string) error {
	f, err := os.Open(src)
	if err != nil {
		return err
	}
	defer f.Close()

	gzr, err := gzip.NewReader(f)
	if err != nil {
		return err
	}
	defer gzr.Close()

	tr := tar.NewReader(gzr)
	for {
		hdr, err := tr.Next()
		if err == io.EOF {
			break
		}
		if err != nil {
			return err
		}
		dest := filepath.Join(target, hdr.Name)
		if !isSubPath(target, dest) {
			return fmt.Errorf("tar path escape detected: %s", hdr.Name)
		}

		switch hdr.Typeflag {
		case tar.TypeDir:
			if err := os.MkdirAll(dest, 0o755); err != nil {
				return err
			}
		case tar.TypeReg:
			if err := os.MkdirAll(filepath.Dir(dest), 0o755); err != nil {
				return err
			}
			out, err := os.Create(dest)
			if err != nil {
				return err
			}
			if _, err := io.Copy(out, tr); err != nil {
				out.Close()
				return err
			}
			out.Close()
		}
	}
	s.setProgress(task, 99)
	s.appendLog(task, "tar.gz extract complete")
	return nil
}

func isSubPath(root, maybeChild string) bool {
	root = filepath.Clean(root)
	maybeChild = filepath.Clean(maybeChild)
	if root == maybeChild {
		return true
	}
	return strings.HasPrefix(maybeChild, root+string(os.PathSeparator))
}

func (s *TaskService) setProgress(task *models.Task, progress int) {
	task.Progress = progress
	s.db.Model(task).Updates(map[string]interface{}{
		"progress": progress,
	})
	s.publish(task.ID, taskEvent{
		TaskID:   task.ID,
		Status:   task.Status,
		Progress: progress,
	})
}

func (s *TaskService) appendLog(task *models.Task, line string) {
	task.Logs = strings.TrimSpace(task.Logs + "\n" + line)
	s.db.Model(task).Update("logs", task.Logs)
	s.publish(task.ID, taskEvent{
		TaskID:   task.ID,
		Status:   task.Status,
		Progress: task.Progress,
		LogLine:  line,
	})
}

func (s *TaskService) setStatus(task *models.Task, status models.TaskStatus, progress int, logLine, errorMsg string) {
	task.Status = status
	task.Progress = progress
	task.Error = errorMsg
	updates := map[string]interface{}{
		"status":   status,
		"progress": progress,
		"error":    errorMsg,
	}
	if logLine != "" {
		task.Logs = strings.TrimSpace(task.Logs + "\n" + logLine)
		updates["logs"] = task.Logs
	}
	s.db.Model(task).Updates(updates)
	s.publish(task.ID, taskEvent{
		TaskID:   task.ID,
		Status:   status,
		Progress: progress,
		LogLine:  logLine,
		Error:    errorMsg,
	})
}

func (s *TaskService) publish(taskID uint, event taskEvent) {
	s.mu.RLock()
	defer s.mu.RUnlock()
	for ch := range s.subscribers[taskID] {
		select {
		case ch <- event:
		default:
		}
	}
}