class CollatzConjecture {
  int steps(int start) {
    if (start < 1)
      throw new ArgumentError('Only positive integers are allowed');
    if (start == 1) return 0;
    if (start.isEven) {
      return 1 + this.steps(start ~/= 2);
    } else {
      return 1 + this.steps(start * 3 + 1);
    }
  }
}