Ztorrent/internal/torrent/engine.go

package torrent

import (
	"context"
	"crypto/rand"
	"crypto/sha1"
	"encoding/hex"
	"errors"
	"fmt"
	"io"
	"log"
	"net"
	"os"
	"path/filepath"
	"sort"
	"strconv"
	"strings"
	"sync"
	"time"
	"unicode"

	"github.com/veggiedefender/torrent-client/internal/torrentfile"
	"github.com/veggiedefender/torrent-client/internal/tracker"
)

type Engine struct {
	mu sync.RWMutex

	torrent  *torrentfile.TorrentFile
	peers    []PeerStatus
	trackers []TrackerStatus
	peerID   [20]byte

	cancel context.CancelFunc

	lastErr error
	phase   string

	downloadedBytes int64
	completedPieces int
	totalPieces     int
	outputPath      string
	outputRoot      string
	downloadSpeed   float64
	speedAt         time.Time
	speedBytes      int64
}

type Status struct {
	Loaded      bool
	Name        string
	Length      int
	PieceLength int
	PieceCount  int

	CompletedPieces int
	DownloadedBytes int64
	DownloadSpeed   float64
	TotalBytes      int64
	OutputPath      string

	Files    []torrentfile.File
	Announce string

	PeerCount int
	Peers     []PeerStatus
	Trackers  []TrackerStatus

	PeerID    string
	Progress  float64
	Phase     string
	LastError string
}

type PeerStatus struct {
	Address          string
	Port             uint16
	Source           string
	State            string
	Error            string
	DownloadedPieces int
}

type TrackerStatus struct {
	URL       string
	State     string
	PeerCount int
	Error     string
}

const (
	trackerPort            = 6881
	trackerNumWant         = 200
	trackerAnnounceTimeout = 6 * time.Second
	downloadStallTimeout   = 90 * time.Second
	reannounceInterval     = 25 * time.Second
	idleRetryDelay         = 2 * time.Second
)

func NewEngine() *Engine {
	return &Engine{
		peerID: generatePeerID(),
		phase:  "idle",
	}
}

func (e *Engine) LoadTorrent(path, outputRoot string) error {
	e.Stop()
	e.resetStateForNewLoad()
	e.setPhase("loading_metadata")
	outputRoot = normalizeOutputRoot(outputRoot)
	log.Printf("loading torrent metadata from %s", path)
	debugf("download root selected: %s", outputRoot)

	tf, err := torrentfile.Open(path)
	if err != nil {
		log.Printf("failed to parse torrent %s: %v", path, err)
		e.setError(err)
		e.setPhase("failed")
		return err
	}

	downloadCtx, cancel := context.WithCancel(context.Background())
	e.swapCancel(cancel)

	queryCtx, queryCancel := context.WithTimeout(downloadCtx, 30*time.Second)
	defer queryCancel()

	e.setPhase("querying_trackers")
	peers, trackerStatuses := e.queryTrackers(queryCtx, tf, tracker.AnnounceOptions{
		PeerID:  e.peerID,
		Port:    trackerPort,
		NumWant: trackerNumWant,
		Timeout: trackerAnnounceTimeout,
	})

	e.mu.Lock()
	e.torrent = tf
	e.trackers = trackerStatuses
	e.peers = peers
	e.totalPieces = len(tf.PieceHashes)
	e.downloadedBytes = 0
	e.completedPieces = 0
	e.outputRoot = outputRoot
	e.outputPath = buildOutputPath(tf, outputRoot)
	e.downloadSpeed = 0
	e.speedAt = time.Now()
	e.speedBytes = 0
	if len(peers) == 0 {
		warnErr := fmt.Errorf("no peers yet via %d trackers", len(trackerStatuses))
		if trErr := firstTrackerError(trackerStatuses); trErr != nil {
			warnErr = fmt.Errorf("no peers yet: %w", trErr)
		}
		e.lastErr = warnErr
	} else {
		e.lastErr = nil
	}
	e.phase = "ready"
	e.mu.Unlock()
	log.Printf("torrent loaded: name=%s size=%d peers=%d output=%s", tf.Name, tf.Length, len(peers), e.outputPath)

	go e.runDownload(downloadCtx, tf)
	return nil
}

func (e *Engine) Stop() {
	e.mu.Lock()
	cancel := e.cancel
	e.cancel = nil
	if cancel != nil {
		e.phase = "stopped"
	}
	e.mu.Unlock()

	if cancel != nil {
		cancel()
	}
}

func (e *Engine) Progress() float64 {
	e.mu.RLock()
	defer e.mu.RUnlock()
	return e.progressUnsafe()
}

func (e *Engine) Status() Status {
	e.mu.RLock()
	defer e.mu.RUnlock()

	status := Status{
		Loaded:          e.torrent != nil,
		PeerCount:       len(e.peers),
		Peers:           append([]PeerStatus(nil), e.peers...),
		Trackers:        append([]TrackerStatus(nil), e.trackers...),
		PeerID:          string(e.peerID[:]),
		Progress:        e.progressUnsafe(),
		Phase:           e.phase,
		CompletedPieces: e.completedPieces,
		DownloadedBytes: e.downloadedBytes,
		DownloadSpeed:   e.downloadSpeed,
		OutputPath:      e.outputPath,
	}

	if e.torrent != nil {
		status.Name = e.torrent.Name
		status.Length = e.torrent.Length
		status.TotalBytes = int64(e.torrent.Length)
		status.PieceLength = e.torrent.PieceLength
		status.PieceCount = len(e.torrent.PieceHashes)
		status.Files = append(status.Files, e.torrent.Files...)
		status.Announce = e.torrent.Announce
	}
	if e.lastErr != nil {
		status.LastError = e.lastErr.Error()
	}

	return status
}

func (e *Engine) queryTrackers(ctx context.Context, tf *torrentfile.TorrentFile, opts tracker.AnnounceOptions) ([]PeerStatus, []TrackerStatus) {
	trackersToTry := collectTrackersToTry(tf)
	peerMap := make(map[string]PeerStatus)
	statuses := make([]TrackerStatus, 0, len(trackersToTry))

	for _, announce := range trackersToTry {
		log.Printf("tracker announce: %s", announce)
		perTrackerCtx, cancel := context.WithTimeout(ctx, opts.Timeout)
		peers, err := tracker.GetPeersFromURL(perTrackerCtx, announce, tf, opts)
		cancel()

		st := TrackerStatus{URL: announce}
		if err != nil {
			st.State = "error"
			st.Error = err.Error()
			log.Printf("tracker announce failed %s: %v", announce, err)
			statuses = append(statuses, st)
			continue
		}

		if len(peers) == 0 {
			st.State = "empty"
		} else {
			st.State = "ok"
			st.PeerCount = len(peers)
		}
		statuses = append(statuses, st)
		log.Printf("tracker announce result %s: peers=%d state=%s", announce, st.PeerCount, st.State)

		for _, peer := range peers {
			addPeer(peerMap, peer, announce)
		}
	}

	peerList := make([]PeerStatus, 0, len(peerMap))
	for _, peer := range peerMap {
		peerList = append(peerList, peer)
	}
	sort.Slice(peerList, func(i, j int) bool {
		if peerList[i].Address == peerList[j].Address {
			return peerList[i].Port < peerList[j].Port
		}
		return peerList[i].Address < peerList[j].Address
	})

	return peerList, statuses
}

func (e *Engine) runDownload(ctx context.Context, tf *torrentfile.TorrentFile) {
	defer e.clearCancel()

	e.setPhase("preparing_download")
	log.Printf("preparing download to %s", e.outputRoot)

	partPath, partFile, err := createPartFile(tf, e.outputRoot)
	if err != nil {
		log.Printf("failed to create part file: %v", err)
		e.setTerminalError("failed", fmt.Errorf("create temp file: %w", err))
		return
	}
	defer partFile.Close()

	scheduler := newPieceScheduler(len(tf.PieceHashes))
	workerCtx, workerCancel := context.WithCancel(ctx)
	var workersWG sync.WaitGroup

	startedWorkers := make(map[string]struct{})
	workerDoneCh := make(chan string, 512)
	startWorkers := func(peers []PeerStatus) {
		for _, peer := range peers {
			key := peerStatusKey(peer.Address, peer.Port)
			if _, exists := startedWorkers[key]; exists {
				continue
			}
			startedWorkers[key] = struct{}{}
			workersWG.Add(1)
			go func(workerKey string, p PeerStatus) {
				defer workersWG.Done()
				e.runPeerWorker(workerCtx, tf, p, partFile, scheduler)
				select {
				case workerDoneCh <- workerKey:
				default:
				}
			}(key, peer)
		}
	}

	var cleanupOnce sync.Once
	cleanupWorkers := func() {
		cleanupOnce.Do(func() {
			workerCancel()
			scheduler.Stop()
			workersWG.Wait()
		})
	}
	defer cleanupWorkers()

	e.setPhase("downloading")
	log.Printf("download started: pieces=%d peers=%d", len(tf.PieceHashes), len(e.snapshotPeers()))
	lastProgressAt := time.Now()
	lastReannounceAt := time.Now()
	startWorkers(e.snapshotPeers())
	heartbeatTicker := time.NewTicker(idleRetryDelay)
	defer heartbeatTicker.Stop()
	downloadCompleted := false

	for !downloadCompleted {
		select {
		case <-ctx.Done():
			e.setPhase("stopped")
			e.setError(nil)
			return
		case _, ok := <-scheduler.Progress():
			if !ok {
				downloadCompleted = true
				continue
			}
			lastProgressAt = time.Now()
			debugf("piece completed: %d/%d", e.currentCompletedPieces(), len(tf.PieceHashes))
		case <-scheduler.Done():
			downloadCompleted = true
		case workerKey := <-workerDoneCh:
			delete(startedWorkers, workerKey)
			debugf("worker finished for peer %s", workerKey)
		case <-heartbeatTicker.C:
			e.updateSpeed(time.Now())
			if time.Since(lastReannounceAt) >= reannounceInterval {
				e.setPhase("querying_trackers")
				refreshCtx, cancel := context.WithTimeout(ctx, 20*time.Second)
				added := e.refreshPeersFromTrackers(refreshCtx, tf)
				cancel()
				lastReannounceAt = time.Now()
				e.setPhase("downloading")
				debugf("reannounce complete, added peers=%d total=%d", added, len(e.snapshotPeers()))
			}
			startWorkers(e.snapshotPeers())

			if time.Since(lastProgressAt) >= downloadStallTimeout {
				log.Printf("download stalled: no progress for %s", downloadStallTimeout.Round(time.Second))
				e.setTerminalError("stalled", fmt.Errorf("download stalled: no piece progress for %s", downloadStallTimeout.Round(time.Second)))
				return
			}
		}
	}

	cleanupWorkers()

	e.setPhase("writing_files")
	if err := materializeDownloadedFiles(tf, partPath, e.outputRoot); err != nil {
		log.Printf("failed to materialize files: %v", err)
		e.setTerminalError("failed", fmt.Errorf("write output files: %w", err))
		return
	}

	if err := os.Remove(partPath); err != nil && !errors.Is(err, os.ErrNotExist) {
		log.Printf("failed to remove part file %s: %v", partPath, err)
	}

	e.mu.Lock()
	e.downloadedBytes = int64(tf.Length)
	e.completedPieces = len(tf.PieceHashes)
	e.phase = "completed"
	e.lastErr = nil
	e.downloadSpeed = 0
	e.mu.Unlock()
	log.Printf("download completed: %s", e.outputPath)
}

func (e *Engine) runPeerWorker(
	ctx context.Context,
	tf *torrentfile.TorrentFile,
	peer PeerStatus,
	partFile *os.File,
	scheduler *pieceScheduler,
) {
	e.setPeerState(peer.Address, peer.Port, "connecting", "")

	peerAddr := net.JoinHostPort(peer.Address, strconv.Itoa(int(peer.Port)))
	debugf("starting peer worker for %s", peerAddr)
	client, err := newPeerClient(ctx, peerAddr, tf.InfoHash, e.peerID, len(tf.PieceHashes))
	if err != nil {
		if ctx.Err() != nil {
			e.setPeerState(peer.Address, peer.Port, "stopped", "")
		} else {
			log.Printf("peer %s connection failed: %v", peerAddr, err)
			e.setPeerState(peer.Address, peer.Port, "error", err.Error())
		}
		return
	}
	defer client.Close()

	e.setPeerState(peer.Address, peer.Port, "ready", "")
	consecutiveFailures := 0

	for {
		if ctx.Err() != nil {
			e.setPeerState(peer.Address, peer.Port, "stopped", "")
			return
		}

		have, hasInfo := client.PieceAvailability()
		task, ok, err := scheduler.Acquire(ctx, have, hasInfo)
		if err != nil {
			if ctx.Err() != nil {
				e.setPeerState(peer.Address, peer.Port, "stopped", "")
			} else {
				log.Printf("peer %s scheduler acquire failed: %v", peerAddr, err)
				e.setPeerState(peer.Address, peer.Port, "error", err.Error())
			}
			return
		}

		if !ok {
			select {
			case <-ctx.Done():
				e.setPeerState(peer.Address, peer.Port, "stopped", "")
				return
			case <-scheduler.Done():
				e.setPeerState(peer.Address, peer.Port, "done", "")
				return
			case <-time.After(500 * time.Millisecond):
			}
			continue
		}

		e.setPeerState(peer.Address, peer.Port, "requesting", "")
		pieceSize := pieceSizeForIndex(tf, task.Index)
		pieceData, err := client.DownloadPiece(ctx, task.Index, pieceSize)
		if err != nil {
			if _, reportErr := scheduler.Report(ctx, task.Index, false); reportErr != nil && ctx.Err() == nil {
				log.Printf("scheduler report failure for piece %d after peer error: %v", task.Index, reportErr)
			}
			if ctx.Err() != nil {
				e.setPeerState(peer.Address, peer.Port, "stopped", "")
				return
			}

			consecutiveFailures++
			log.Printf("peer %s disconnected: %v", peerAddr, err)
			e.setPeerState(peer.Address, peer.Port, "error", err.Error())
			if shouldDropPeer(err) || consecutiveFailures >= 3 {
				return
			}
			continue
		}

		hash := sha1.Sum(pieceData)
		if hash != tf.PieceHashes[task.Index] {
			if _, reportErr := scheduler.Report(ctx, task.Index, false); reportErr != nil && ctx.Err() == nil {
				log.Printf("scheduler report hash mismatch for piece %d failed: %v", task.Index, reportErr)
			}
			consecutiveFailures++
			log.Printf("peer %s piece %d hash mismatch", peerAddr, task.Index)
			e.setPeerState(peer.Address, peer.Port, "error", fmt.Sprintf("piece %d hash mismatch", task.Index))
			if consecutiveFailures >= 3 {
				return
			}
			continue
		}

		offset := int64(task.Index * tf.PieceLength)
		if _, err := partFile.WriteAt(pieceData, offset); err != nil {
			if _, reportErr := scheduler.Report(ctx, task.Index, false); reportErr != nil && ctx.Err() == nil {
				log.Printf("scheduler report write failure for piece %d failed: %v", task.Index, reportErr)
			}
			log.Printf("io error writing piece %d from peer %s: %v", task.Index, peerAddr, err)
			e.setPeerState(peer.Address, peer.Port, "error", fmt.Sprintf("write piece %d: %v", task.Index, err))
			return
		}

		accepted, reportErr := scheduler.Report(ctx, task.Index, true)
		if reportErr != nil {
			if ctx.Err() != nil {
				e.setPeerState(peer.Address, peer.Port, "stopped", "")
			} else {
				e.setPeerState(peer.Address, peer.Port, "error", reportErr.Error())
			}
			return
		}
		if accepted {
			e.recordPieceComplete(peer.Address, peer.Port, len(pieceData))
			debugf("peer %s committed piece %d", peerAddr, task.Index)
		} else {
			debugf("scheduler rejected piece %d report from %s", task.Index, peerAddr)
		}

		consecutiveFailures = 0
		e.setPeerState(peer.Address, peer.Port, "active", "")
	}
}

func shouldDropPeer(err error) bool {
	if err == nil {
		return false
	}
	if errors.Is(err, context.Canceled) || errors.Is(err, context.DeadlineExceeded) {
		return true
	}
	if isTimeout(err) {
		return false
	}

	msg := strings.ToLower(err.Error())
	switch {
	case strings.Contains(msg, "peer choked"):
		return false
	case strings.Contains(msg, "peer did not unchoke"):
		return false
	case strings.Contains(msg, "hash mismatch"):
		return false
	default:
		return true
	}
}

func (e *Engine) snapshotPeers() []PeerStatus {
	e.mu.RLock()
	defer e.mu.RUnlock()
	return append([]PeerStatus(nil), e.peers...)
}

func (e *Engine) refreshPeersFromTrackers(ctx context.Context, tf *torrentfile.TorrentFile) int {
	peers, trackerStatuses := e.queryTrackers(ctx, tf, tracker.AnnounceOptions{
		PeerID:     e.peerID,
		Port:       trackerPort,
		Downloaded: e.currentDownloadedBytes(),
		NumWant:    trackerNumWant,
		Timeout:    trackerAnnounceTimeout,
	})

	e.mu.Lock()
	defer e.mu.Unlock()

	if len(trackerStatuses) > 0 {
		e.trackers = trackerStatuses
	}
	if len(peers) == 0 {
		waitErr := fmt.Errorf("waiting for peers via %d trackers", len(trackerStatuses))
		if trErr := firstTrackerError(trackerStatuses); trErr != nil {
			waitErr = fmt.Errorf("waiting for peers: %w", trErr)
		}
		e.lastErr = waitErr
		debugf("reannounce returned no peers")
		return 0
	}
	e.lastErr = nil

	seen := make(map[string]struct{}, len(e.peers))
	for _, existingPeer := range e.peers {
		seen[peerStatusKey(existingPeer.Address, existingPeer.Port)] = struct{}{}
	}

	added := 0
	for _, peer := range peers {
		key := peerStatusKey(peer.Address, peer.Port)
		if _, ok := seen[key]; ok {
			continue
		}
		e.peers = append(e.peers, peer)
		seen[key] = struct{}{}
		added++
	}

	if added > 0 {
		sort.Slice(e.peers, func(i, j int) bool {
			if e.peers[i].Address == e.peers[j].Address {
				return e.peers[i].Port < e.peers[j].Port
			}
			return e.peers[i].Address < e.peers[j].Address
		})
	}
	if added > 0 {
		log.Printf("discovered %d new peers (total=%d)", added, len(e.peers))
	}

	return added
}

func (e *Engine) currentDownloadedBytes() int64 {
	e.mu.RLock()
	defer e.mu.RUnlock()
	return e.downloadedBytes
}

func (e *Engine) currentCompletedPieces() int {
	e.mu.RLock()
	defer e.mu.RUnlock()
	return e.completedPieces
}

func (e *Engine) recordPieceComplete(address string, port uint16, pieceSize int) {
	e.mu.Lock()
	defer e.mu.Unlock()
	e.downloadedBytes += int64(pieceSize)
	e.completedPieces++
	e.updateSpeedLocked(time.Now())
	for i := range e.peers {
		if e.peers[i].Address == address && e.peers[i].Port == port {
			e.peers[i].DownloadedPieces++
			break
		}
	}
}

func (e *Engine) setPeerState(address string, port uint16, state, errText string) {
	e.mu.Lock()
	defer e.mu.Unlock()
	for i := range e.peers {
		if e.peers[i].Address == address && e.peers[i].Port == port {
			e.peers[i].State = state
			e.peers[i].Error = errText
			break
		}
	}
}

func (e *Engine) setTerminalError(phase string, err error) {
	e.mu.Lock()
	defer e.mu.Unlock()
	e.phase = phase
	e.lastErr = err
	e.downloadSpeed = 0
}

func (e *Engine) progressUnsafe() float64 {
	if e.torrent != nil && e.torrent.Length > 0 {
		if e.phase == "completed" {
			return 1
		}
		if e.downloadedBytes > 0 {
			progress := float64(e.downloadedBytes) / float64(e.torrent.Length)
			if progress > 1 {
				return 1
			}
			return progress
		}
	}

	switch e.phase {
	case "idle", "failed", "stopped", "stalled", "tracker_errors":
		return 0
	case "loading_metadata":
		return 0.05
	case "querying_trackers":
		return 0.12
	case "ready":
		return 0.2
	case "preparing_download":
		return 0.25
	case "downloading":
		return 0.3
	case "writing_files":
		return 0.95
	default:
		return 0
	}
}

func (e *Engine) resetStateForNewLoad() {
	e.mu.Lock()
	defer e.mu.Unlock()
	e.torrent = nil
	e.peers = nil
	e.trackers = nil
	e.lastErr = nil
	e.phase = "idle"
	e.downloadedBytes = 0
	e.completedPieces = 0
	e.totalPieces = 0
	e.outputPath = ""
	e.outputRoot = ""
	e.downloadSpeed = 0
	e.speedAt = time.Time{}
	e.speedBytes = 0
}

func (e *Engine) updateSpeed(now time.Time) {
	e.mu.Lock()
	defer e.mu.Unlock()
	e.updateSpeedLocked(now)
}

func (e *Engine) updateSpeedLocked(now time.Time) {
	if e.speedAt.IsZero() {
		e.speedAt = now
		e.speedBytes = e.downloadedBytes
		e.downloadSpeed = 0
		return
	}
	elapsed := now.Sub(e.speedAt).Seconds()
	if elapsed < 0.8 {
		return
	}
	delta := e.downloadedBytes - e.speedBytes
	if delta < 0 {
		delta = 0
	}
	e.downloadSpeed = float64(delta) / elapsed
	e.speedAt = now
	e.speedBytes = e.downloadedBytes
}

func (e *Engine) setError(err error) {
	e.mu.Lock()
	defer e.mu.Unlock()
	e.lastErr = err
}

func (e *Engine) setPhase(phase string) {
	e.mu.Lock()
	defer e.mu.Unlock()
	e.phase = phase
}

func (e *Engine) swapCancel(cancel context.CancelFunc) {
	e.mu.Lock()
	defer e.mu.Unlock()
	if e.cancel != nil {
		e.cancel()
	}
	e.cancel = cancel
}

func (e *Engine) clearCancel() {
	e.mu.Lock()
	defer e.mu.Unlock()
	if e.cancel != nil {
		e.cancel()
		e.cancel = nil
	}
}

func generatePeerID() [20]byte {
	const prefix = "-ZT0001-"
	const alphabet = "0123456789abcdefghijklmnopqrstuvwxyz"

	var id [20]byte
	copy(id[:], prefix)

	buf := make([]byte, len(id)-len(prefix))
	if _, err := rand.Read(buf); err != nil {
		now := time.Now().UnixNano()
		for i := range buf {
			buf[i] = byte(now >> (i * 8))
		}
	}

	for i, b := range buf {
		id[len(prefix)+i] = alphabet[int(b)%len(alphabet)]
	}

	return id
}

func addPeer(peerMap map[string]PeerStatus, peer tracker.Peer, source string) {
	key := peerStatusKey(peer.IP.String(), peer.Port)
	if _, exists := peerMap[key]; exists {
		return
	}
	peerMap[key] = PeerStatus{
		Address: peer.IP.String(),
		Port:    peer.Port,
		Source:  source,
		State:   "discovered",
	}
}

func peerStatusKey(address string, port uint16) string {
	return net.JoinHostPort(address, strconv.Itoa(int(port)))
}

func collectTrackersToTry(tf *torrentfile.TorrentFile) []string {
	const fallbackTrackers = `
udp://open.stealth.si:80/announce
udp://tracker.opentrackr.org:1337/announce
udp://tracker.openbittorrent.com:6969/announce
udp://tracker.torrent.eu.org:451/announce
https://tracker.opentrackr.org:443/announce
http://tracker.opentrackr.org:1337/announce
`

	seen := make(map[string]struct{})
	list := make([]string, 0, len(tf.Trackers)+8)

	add := func(tr string) {
		if tr == "" {
			return
		}
		if _, ok := seen[tr]; ok {
			return
		}
		seen[tr] = struct{}{}
		list = append(list, tr)
	}

	for _, tr := range tf.Trackers {
		add(tr)
	}
	for _, tr := range splitLines(fallbackTrackers) {
		add(tr)
	}

	return list
}

func splitLines(s string) []string {
	parts := strings.Split(s, "\n")
	out := make([]string, 0, len(parts))
	for _, part := range parts {
		trimmed := strings.TrimSpace(part)
		if trimmed == "" {
			continue
		}
		out = append(out, trimmed)
	}
	return out
}

func firstTrackerError(statuses []TrackerStatus) error {
	for _, st := range statuses {
		if st.State == "error" && st.Error != "" {
			return errors.New(st.Error)
		}
	}
	return nil
}

func createPartFile(tf *torrentfile.TorrentFile, outputRoot string) (string, *os.File, error) {
	partsDir := filepath.Join(outputRoot, ".ztorrent-parts")
	if err := os.MkdirAll(partsDir, 0o755); err != nil {
		return "", nil, err
	}

	hashPrefix := hex.EncodeToString(tf.InfoHash[:6])
	name := sanitizePathPart(tf.Name) + "-" + hashPrefix + ".part"
	partPath := filepath.Join(partsDir, name)

	f, err := os.OpenFile(partPath, os.O_CREATE|os.O_RDWR|os.O_TRUNC, 0o644)
	if err != nil {
		return "", nil, err
	}
	if err := f.Truncate(int64(tf.Length)); err != nil {
		f.Close()
		return "", nil, err
	}

	return partPath, f, nil
}

func materializeDownloadedFiles(tf *torrentfile.TorrentFile, partPath, outputRoot string) error {
	partFile, err := os.Open(partPath)
	if err != nil {
		return err
	}
	defer partFile.Close()

	if err := os.MkdirAll(outputRoot, 0o755); err != nil {
		return err
	}

	offset := int64(0)
	for _, f := range tf.Files {
		targetPath, err := safeOutputPath(outputRoot, f.Path)
		if err != nil {
			return err
		}
		if err := os.MkdirAll(filepath.Dir(targetPath), 0o755); err != nil {
			return err
		}

		out, err := os.OpenFile(targetPath, os.O_CREATE|os.O_WRONLY|os.O_TRUNC, 0o644)
		if err != nil {
			return err
		}

		_, copyErr := io.CopyN(out, io.NewSectionReader(partFile, offset, int64(f.Length)), int64(f.Length))
		closeErr := out.Close()
		if copyErr != nil {
			return copyErr
		}
		if closeErr != nil {
			return closeErr
		}

		offset += int64(f.Length)
	}

	if offset != int64(tf.Length) {
		return fmt.Errorf("written bytes mismatch: expected %d, wrote %d", tf.Length, offset)
	}

	return nil
}

func safeOutputPath(root, rel string) (string, error) {
	clean := filepath.Clean(filepath.FromSlash(rel))
	if clean == "." || clean == "" {
		return "", errors.New("invalid output file path")
	}
	if filepath.IsAbs(clean) || strings.HasPrefix(clean, "..") {
		return "", fmt.Errorf("unsafe output path %q", rel)
	}
	return filepath.Join(root, clean), nil
}

func buildOutputPath(tf *torrentfile.TorrentFile, outputRoot string) string {
	return filepath.Join(outputRoot, tf.Name)
}

func sanitizePathPart(s string) string {
	s = strings.TrimSpace(s)
	if s == "" {
		return "torrent"
	}

	var b strings.Builder
	for _, r := range s {
		switch {
		case unicode.IsLetter(r), unicode.IsDigit(r), r == '-', r == '_', r == '.':
			b.WriteRune(r)
		default:
			b.WriteByte('_')
		}
	}

	out := strings.Trim(b.String(), "._")
	if out == "" {
		return "torrent"
	}
	return out
}

func normalizeOutputRoot(root string) string {
	root = strings.TrimSpace(root)
	if root == "" {
		cwd, err := os.Getwd()
		if err == nil && cwd != "" {
			return cwd
		}
		return "."
	}
	clean := filepath.Clean(root)
	return clean
}