A translation from Ruby to Rust in the book :pp.018-020[「もっとプログラマ脳を鍛える数学パズル」](https://www.shoeisha.co.jp/book/detail/9784798153612)

main.rs

fn main() {
    println!("{n}P{r} = {x}", n = 5, r = 2, x = permutation(5, 2));
    println!("{n}C{r} = {x}", n = 5, r = 2, x = combination(5, 2));
}

pub fn permutation(n: i64, r: i64) -> i64 {
    let mut result = 1;
    for i in (n - r + 1)..=n {
        result *= i;
    }
    return result;
}

pub fn combination(n: i64, r: i64) -> i64 {
    let mut result = 1;
    for i in 1..=r {
        result = result * (n - i + 1) / i;
    }
    return result;
}

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

    #[test]
    fn test_permutation() {
        assert_eq!(permutation(1, 1), 1);
        assert_eq!(permutation(6, 6), 720);
        assert_eq!(permutation(5, 2), 20);
    }

    #[test]
    fn test_combination() {
        assert_eq!(combination(1, 1), 1);
        assert_eq!(combination(6, 6), 1);
        assert_eq!(combination(5, 2), 10);
    }
}