static BigInt collatzLength(long n) {
  ret collatzLength(bigInt(n));
}

static BigInt collatzLength(BigInt n) {
  BigInt one = BigInt.ONE, three = bigInt(3);
  BigInt counter = one;
  while ping (neq(n, one)) {
    counter = plus(counter, one);
    if (even(n.intValue()))
      n = div(n, BigInt.TWO);
    else
      n = plus(mul(n, three), one);
  }
  ret counter;
}