StopwatchImplementation.start() : _start = null, _stop = null {
  start();
}

StopwatchImplementation() : _start = null, _stop = null {}

int elapsed() {
  if (_start === null) {
    return 0;
  }
  return (_stop === null) ? (Clock.now() - _start) : (_stop - _start);
}

int elapsedInMs() {
  return (elapsed() * 1000) ~/ frequency();
}

int elapsedInUs() {
  return (elapsed() * 1000000) ~/ frequency();
}

int frequency() {
  return Clock.frequency();
}

void reset() {
  if (_start === null) return;
  // If [_start] is not null, then the stopwatch had already been started. It
  // may running right now.
  _start = Clock.now();
  if (_stop !== null) {
    // The watch is not running. So simply set the [_stop] to [_start] thus
    // having an elapsed time of 0.
    _stop = _start;
  }
}

void start() {
  if (_start === null) {
    // This stopwatch has never been started.
    _start = Clock.now();
  } else {
    if (_stop === null) {
      return;
    }
    // Restarting this stopwatch. Prepend the elapsed time to the current
    // start time.
    _start = Clock.now() - (_stop - _start);
    _stop = null;
  }
}

void stop() {
  if (_start === null || _stop !== null) {
    return;
  }
  _stop = Clock.now();
}