toni-sync/internal/syncer/playlist.go

package syncer

import (
	"bufio"
	"fmt"
	"os"
	"path/filepath"
	"strings"
)

var playlistExtensions = map[string]bool{
	".m3u":  true,
	".m3u8": true,
}

// findPlaylistFile looks for a single .m3u/.m3u8 file directly inside folder.
// Returns "" if none is present. Returns an error if more than one is found,
// since the order would then be ambiguous.
func findPlaylistFile(folder string, entries []os.DirEntry) (string, error) {
	var found []string
	for _, e := range entries {
		if e.IsDir() {
			continue
		}
		if playlistExtensions[strings.ToLower(filepath.Ext(e.Name()))] {
			found = append(found, e.Name())
		}
	}
	switch len(found) {
	case 0:
		return "", nil
	case 1:
		return filepath.Join(folder, found[0]), nil
	default:
		return "", fmt.Errorf("multiple playlist files found in %s (%s) - keep only one to define the chapter order", folder, strings.Join(found, ", "))
	}
}

// parseM3U reads an m3u/m3u8 playlist and returns the referenced entry paths
// in file order. Blank lines and lines starting with "#" (comments/EXTM3U
// directives) are ignored. Entries are returned exactly as written in the
// file (relative or absolute); resolution against the playlist's folder
// happens in the caller.
func parseM3U(path string) ([]string, error) {
	f, err := os.Open(path)
	if err != nil {
		return nil, fmt.Errorf("reading playlist %s: %w", path, err)
	}
	defer f.Close()

	var entries []string
	scanner := bufio.NewScanner(f)
	for scanner.Scan() {
		line := strings.TrimSpace(scanner.Text())
		if line == "" || strings.HasPrefix(line, "#") {
			continue
		}
		entries = append(entries, line)
	}
	if err := scanner.Err(); err != nil {
		return nil, fmt.Errorf("reading playlist %s: %w", path, err)
	}
	return entries, nil
}

// orderTracksByPlaylist reorders tracks according to the entry order in the
// given m3u file. Tracks not referenced by the playlist are appended
// afterwards in their existing (alphabetical) order, so nothing is silently
// dropped from the sync. Playlist entries that don't match any local track
// are ignored.
func orderTracksByPlaylist(folder, playlistPath string, tracks []Track) ([]Track, error) {
	entries, err := parseM3U(playlistPath)
	if err != nil {
		return nil, err
	}

	byPath := make(map[string]Track, len(tracks))
	for _, t := range tracks {
		byPath[t.Path] = t
	}

	used := make(map[string]bool, len(entries))
	ordered := make([]Track, 0, len(tracks))
	for _, entry := range entries {
		resolved := entry
		if !filepath.IsAbs(resolved) {
			resolved = filepath.Join(folder, resolved)
		}
		resolved = filepath.Clean(resolved)
		if t, ok := byPath[resolved]; ok {
			ordered = append(ordered, t)
			used[resolved] = true
		}
	}

	// Append any local tracks not referenced by the playlist, preserving
	// their existing order, so they aren't silently excluded from sync.
	for _, t := range tracks {
		if !used[t.Path] {
			ordered = append(ordered, t)
		}
	}
	return ordered, nil
}