mobile-music/lib/library/offline_library.dart

import '../subsonic/models.dart';

/// Reconstructs [Album]s and [Artist]s from a flat list of downloaded [Song]s.
/// The offline library only ever has tracks (that's all that's cached on disk),
/// so album/artist entities are synthesized on demand. Pure — no I/O.

/// Compare two strings case-insensitively, treating null as empty (sorts first).
int _byString(String? a, String? b) =>
    (a ?? '').toLowerCase().compareTo((b ?? '').toLowerCase());

/// Compare where a null [a]/[b] always sorts *last*. Takes bare [Comparable] so
/// `int` (`Comparable<num>`) works for disc/track ordering.
int _nullsLast(Comparable? a, Comparable? b) {
  if (a == null && b == null) return 0;
  if (a == null) return 1;
  if (b == null) return -1;
  return a.compareTo(b);
}

/// True for a present, non-blank id/name.
bool _has(String? v) => v != null && v.trim().isNotEmpty;

/// First non-blank value in [values], or null if none.
String? _firstNonNull(Iterable<String?> values) {
  for (final v in values) {
    if (_has(v)) return v;
  }
  return null;
}

/// First non-null value in [values], or null if none. For nullable ints.
int? _firstNonNullInt(Iterable<int?> values) {
  for (final v in values) {
    if (v != null) return v;
  }
  return null;
}

/// Stable album key: prefer [Song.albumId], else fall back to the album NAME.
/// The synthesized [Album.id] mirrors this exactly (see [albumsFromSongs]), so
/// Phase 2 providers can look an album up by the same key it was built under.
String? _albumKey(Song s) => _has(s.albumId) ? s.albumId : s.album;

/// Stable artist key: prefer [Song.artistId], else fall back to the artist NAME.
String? _artistKey(Song s) => _has(s.artistId) ? s.artistId : s.artist;

/// Synthesize [Album]s from downloaded [Song]s, grouped by [_albumKey].
///
/// Songs with neither an albumId nor an album name are skipped — with no album
/// identity they can't form a meaningful album. The synthesized [Album.id] is
/// the albumId when present, else the album *name* itself (the same string used
/// as the grouping key), so lookups by id stay stable across rebuilds.
///
/// Returned albums are sorted by name (case-insensitive ascending) for a stable
/// default order.
List<Album> albumsFromSongs(List<Song> songs) {
  // Preserve first-seen insertion order within groups; output order is imposed
  // by the final sort, so the map's own ordering only needs to be deterministic.
  final groups = <String, List<Song>>{};
  for (final s in songs) {
    final key = _albumKey(s);
    if (key == null) continue; // no album identity → skip
    groups.putIfAbsent(key, () => []).add(s);
  }

  final out = <Album>[];
  groups.forEach((key, group) {
    // id: albumId if any song carries one, else the name-derived key.
    final albumId = _firstNonNull(group.map((s) => s.albumId));
    final id = albumId ?? key;

    final sorted = [...group]
      ..sort((a, b) {
        final d = _nullsLast(a.discNumber, b.discNumber);
        if (d != 0) return d;
        final t = _nullsLast(a.track, b.track);
        return t != 0 ? t : _byString(a.title, b.title);
      });

    out.add(
      Album(
        id: id,
        name: _firstNonNull(group.map((s) => s.album)),
        artist: _firstNonNull(group.map((s) => s.artist)),
        artistId: _firstNonNull(group.map((s) => s.artistId)),
        coverArt: _firstNonNull(group.map((s) => s.coverArt)),
        year: _firstNonNullInt(group.map((s) => s.year)),
        genre: _firstNonNull(group.map((s) => s.genre)),
        songCount: group.length,
        songs: sorted,
      ),
    );
  });

  out.sort((a, b) => _byString(a.name, b.name));
  return out;
}

/// Synthesize [Artist]s from downloaded [Song]s, grouped by [_artistKey].
///
/// Songs with neither an artistId nor an artist name are skipped. Each artist's
/// [Artist.albums] is built by running [albumsFromSongs] over that artist's own
/// songs, and its [Artist.id] follows the same id/name fallback as albums.
///
/// Returned artists are sorted by name (case-insensitive ascending).
List<Artist> artistsFromSongs(List<Song> songs) {
  final groups = <String, List<Song>>{};
  for (final s in songs) {
    final key = _artistKey(s);
    if (key == null) continue; // no artist identity → skip
    groups.putIfAbsent(key, () => []).add(s);
  }

  final out = <Artist>[];
  groups.forEach((key, group) {
    // id: artistId if any song carries one, else the name-derived key.
    final artistId = _firstNonNull(group.map((s) => s.artistId));
    final id = artistId ?? key;

    final albums = albumsFromSongs(group);

    out.add(
      Artist(
        id: id,
        name: _firstNonNull(group.map((s) => s.artist)),
        coverArt: _firstNonNull(albums.map((a) => a.coverArt)),
        albums: albums,
        albumCount: albums.length,
      ),
    );
  });

  out.sort((a, b) => _byString(a.name, b.name));
  return out;
}

/// The synthesized album whose id == [id], or null. Keyed lookup for the album
/// detail provider.
Album? albumFromSongs(List<Song> songs, String id) {
  for (final a in albumsFromSongs(songs)) {
    if (a.id == id) return a;
  }
  return null;
}

/// The synthesized artist whose id == [id], or null. Keyed lookup for the artist
/// detail provider.
Artist? artistFromSongs(List<Song> songs, String id) {
  for (final a in artistsFromSongs(songs)) {
    if (a.id == id) return a;
  }
  return null;
}