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

    while b != 0 {
        if b < a {
            let temp = b;
            b = a;
            a = temp;
        }
        b = b % a;
    }

    a
}

#[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
        )
    }
}