anchor.diff:    int playlistIndex = 0;
   
     int nextIndex() {
  -    int index = playedAudios.indexWhere((element) => playlistIndex == element);
  -    if (index + 1 == playedAudios.length) {
  -      return playedAudios.first;
  -    } else {
  -      return playedAudios[index + 1];
  +    int index = indexList.indexWhere((element) => playlistIndex == element);
  +    if (index + 1 == indexList.length) {
  +      return indexList.first;
  +    } else {
  +      return indexList[index + 1];
       }
     }
   
     int previousIndex() {
  -    int index = playedAudios.indexWhere((element) => playlistIndex == element);
  +    int index = indexList.indexWhere((element) => playlistIndex == element);
       if (index == 0) {
  -      return playedAudios.last;
  -    } else {
  -      return playedAudios[index - 1];
  +      return indexList.last;
  +    } else {
  +      return indexList[index - 1];
       }
     }
   
     selectNext() {
  -    int index = playedAudios.indexWhere((element) => playlistIndex == element);
  +    int index = indexList.indexWhere((element) => playlistIndex == element);
       if (hasNext()) {
         index = index + 1;
       }
       playlistIndex = index;
     }
   
  -  List<int> playedAudios = [];
  -
  -  fillPlayedAudios() {
  +  List<int> indexList = [];
  +
  +  sortAudios() {
       for (var i = 0; i < this.playlist.audios.length; i++) {
  -      playedAudios.add(i);
  +      indexList.add(i);
       }
     }
   
     clearPlayeAudio(bool shuffle) {
  -    playedAudios.clear();
  +    indexList.clear();
       if (shuffle) {
         shuffleAudios();
       } else {
  -      fillPlayedAudios();
  +      sortAudios();
       }
     }
   
     shuffleAudios() {
       for (var i = 0; i < this.playlist.audios.length; i++) {
         int index = _shuffleNumbers();
  -      playedAudios.add(index);
  +      indexList.add(index);
       }
     }
   
     int _shuffleNumbers() {
       Random random = Random();
       int index = random.nextInt(playlist.audios.length);
  -    if (playedAudios.contains(index)) {
  +    if (indexList.contains(index)) {
         index = _shuffleNumbers();
       }
       return index;
  
positive.code:   }
  
    bool hasPrev() {
      int index = playedAudios.indexWhere((element) => playlistIndex == element);
      return index > 0;
    }
  
    void selectPrev() {
      playlistIndex--;
      if (playlistIndex < 0) {
        playlistIndex = 0;
      }
  
negative.code: 
    int moveTo(int index) {
      if (index < 0) {
        playlistIndex = indexList.indexWhere((element) => element == 0);
      } else {
        playlistIndex = indexList.indexWhere((element) => element == index);
      }
      return playlistIndex;
    }
  