minstrel/flutter_client/lib/cache/prefetcher.dart

import 'package:audio_service/audio_service.dart';
import 'package:flutter_riverpod/flutter_riverpod.dart';

import '../player/player_provider.dart';
import 'audio_cache_manager.dart';
import 'cache_settings_provider.dart';
import 'db.dart';

/// Listens to the player's currently-playing track. When it changes,
/// computes the next-N tracks ahead in the queue and pins them via
/// AudioCacheManager (source: autoPrefetch). After pinning, runs an
/// eviction pass against the operator-set cap.
///
/// The window N comes from cacheSettingsProvider.prefetchWindow
/// (default 5, configurable in Settings).
class Prefetcher {
  Prefetcher(this._ref) {
    // The mediaItem stream changes whenever the active track changes
    // (skipNext/skipPrev or natural progression). Settings changes (e.g.
    // operator bumps prefetch window) also trigger a reconcile.
    _ref.listen<AsyncValue<MediaItem?>>(mediaItemProvider, (_, _) => _reconcile());
    _ref.listen<AsyncValue<CacheSettings>>(cacheSettingsProvider, (_, _) => _reconcile());
  }

  final Ref _ref;

  Future<void> _reconcile() async {
    final settings = _ref.read(cacheSettingsProvider).valueOrNull;
    if (settings == null) return;

    final queue = _ref.read(queueProvider).valueOrNull ?? const <MediaItem>[];
    final current = _ref.read(mediaItemProvider).valueOrNull;
    if (queue.isEmpty || current == null) return;

    final currentIdx = queue.indexWhere((m) => m.id == current.id);
    if (currentIdx < 0) return;

    final endIdx =
        (currentIdx + settings.prefetchWindow).clamp(0, queue.length - 1);

    final mgr = _ref.read(audioCacheManagerProvider);

    for (var i = currentIdx; i <= endIdx; i++) {
      final trackId = queue[i].id;
      if (await mgr.isCached(trackId)) continue;
      // Fire-and-forget; downloads happen in the background. Errors
      // inside pin() are caught + cleaned up internally.
      // ignore: unawaited_futures
      mgr.pin(trackId, source: CacheSource.autoPrefetch);
    }

    // Eviction pass after pinning new files.
    if (settings.capBytes > 0) {
      await mgr.evict(targetBytes: settings.capBytes);
    }
  }
}

/// Read this provider once at app start to activate the prefetcher.
/// Constructor wires the listeners.
final prefetcherProvider = Provider<Prefetcher>((ref) {
  return Prefetcher(ref);
});