class Comp {
  static final Comp _singleton = Comp._internal();
  int _pc;
  int _acc;
  Set<int> _loopTest;
  int _swapJmp = -1;
  int _swapNop = -1;

  factory Comp() {
    return _singleton;
  }

  // singleton's constructor
  Comp._internal() {
    reset();
  }

  void reset() {
    _pc = 0;
    _acc = 0;
    _loopTest = {};
  }

  void swapJmp(int index) {
    _swapJmp = index;
    _swapNop = -1;
  }

  void swapNop(int index) {
    _swapNop = index;
    _swapJmp = -1;
  }

  int get pc => _pc;
  int get acc => _acc;

  bool execute(String command) {
    var _newPC = _pc;
    var _newAcc = _acc;

    // print(command);
    var parse = command.split(' ');
    var _val = int.parse(parse[1]);
    var instruction = parse[0];
    if (_swapJmp >= 0 && _swapJmp == _pc) {
      instruction = 'nop';
    } 
    if (_swapNop >= 0 && _swapNop == _pc) {
      instruction = 'jmp';
    } 
    // print('$instruction:$_val  pc=$_pc  swpJmp=$_swapJmp  swpNop=$_swapNop');
    switch (instruction) {
      case 'nop':
        _newPC++;
        break;
      case 'acc':
        _newAcc += _val;
        _newPC++;
        break;
      case 'jmp':
        _newPC += _val;
        break;
      default:
      print('********** ERROR **********');
        break;
    }
    if (_loopTest.contains(_newPC)) {
      return false;
    }
    _pc = _newPC;
    _acc = _newAcc;
    _loopTest.add(_pc);
    return true;
  }

  @override
  String toString() => 'pc:$_pc -- acc:$_acc';
}