pub fn collatz(mut n: u64) -> u64 {
    assert_ne!(n, 0);
    let c = std::iter::repeat(())
        .map(|_| {
            let x = n;
            n = if n % 2 == 0 {n / 2} else {3 * n + 1};
            x
        })
        .take_while(|&n| n != 1)
        .count();
    c as u64
}

#[cfg(test)]
mod tests {
    use super::collatz;

    #[test]
    fn it_works() {
        let expected = [0, 1, 7, 2, 5, 8, 16, 3, 19];
        let actual = (1 ..= expected.len() as u64).map(collatz).collect::<Vec<_>>();
        assert_eq!(&*actual, expected);
    }
}