pub fn gcd(mut n: u64, mut m: u64) -> u64 {
    assert!(n != 0 && m != 0);

    while m != 0 {
        if m < n {
            let t = m;
            m = n;
            n = t;
        }
        m = m % n;
    }

    n
}

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

    #[test]
    fn test_gcd() {
        assert_eq!(gcd(14, 15), 1);

        assert_eq!(
            gcd(
                2 * 3 * 5 * 11 * 17,
                3 * 7 * 11 * 13 * 19
            ),
            3 * 11
        )
    }
}