feat: parallel youtube downloads

lib/controller/ffmpeg_controller.dart

@@ -139,7 +139,12 @@ class NamidaFFMPEG {
     final audioFile = File(audioPath);
     final originalStats = keepOriginalFileStats ? await audioFile.stat() : null;
 
-    final cacheFile = File("${AppDirs.APP_CACHE}/${audioPath.hashCode}.${audioPath.getExtension}");
+    String ext = 'm4a';
+    try {
+      ext = audioPath.getExtension;
+    } catch (_) {}
+
+    final cacheFile = File("${AppDirs.APP_CACHE}/${audioPath.hashCode}.$ext");
     final didSuccess = await _ffmpegExecute('-i "$audioPath" -i "$thumbnailPath" -map 0:a -map 1 -codec copy -disposition:v attached_pic -y "${cacheFile.path}"');
     final canSafelyMoveBack = didSuccess && await cacheFile.exists() && await cacheFile.length() > 0;
     if (canSafelyMoveBack) {

lib/youtube/controller/parallel_downloads_controller.dart

@@ -0,0 +1,48 @@
+import 'dart:async';
+
+import 'package:get/get_rx/get_rx.dart';
+
+import 'package:namida/core/extensions.dart';
+
+class YoutubeParallelDownloadsHandler {
+  static final YoutubeParallelDownloadsHandler inst = YoutubeParallelDownloadsHandler._internal();
+  YoutubeParallelDownloadsHandler._internal();
+
+  /// Use this to wait till a place has been freed.
+  Future<void>? get waitForParallelCompleter => _maxParallelDownloadsCompleter?.future;
+
+  /// Max number of items that can be downloaded simultaniously.
+  int get maxParallelDownloadingItems => _maxParallelDownloadingItems.value;
+
+  void refreshCompleterStatus() => _tryReAssignMaxParallelDownloadsCompleter();
+
+  int _currentDownloadingItemsCount = 0;
+  final _maxParallelDownloadingItems = 1.obs;
+
+  /// used to control the parallel process, stopping the download loop or continuing it.
+  Completer<void>? _maxParallelDownloadsCompleter;
+
+  /// updates the current downloading items number, and triggering completer re-assign
+  /// which either stops ongoing downloads or continues them.
+  set currentDownloadingItemsCount(int value) {
+    _currentDownloadingItemsCount = value;
+    _tryReAssignMaxParallelDownloadsCompleter();
+  }
+
+  void inc() => currentDownloadingItemsCount = _currentDownloadingItemsCount + 1;
+  void dec() => currentDownloadingItemsCount = _currentDownloadingItemsCount - 1;
+
+  void setMaxParalellDownloads(int count) {
+    _maxParallelDownloadingItems.value = count.withMinimum(1);
+    _tryReAssignMaxParallelDownloadsCompleter();
+  }
+
+  void _tryReAssignMaxParallelDownloadsCompleter() {
+    if (_currentDownloadingItemsCount >= _maxParallelDownloadingItems.value) {
+      _maxParallelDownloadsCompleter ??= Completer<void>();
+    } else {
+      _maxParallelDownloadsCompleter?.completeIfWasnt();
+      _maxParallelDownloadsCompleter = null;
+    }
+  }
+}