Function count_permutations 

pub fn count_permutations(
    n_total: u64,
    n_choose: u64,
) -> Result<u64, CombinatoricsError>

Count permutations - the number of ways to pick n_choose items from n_total, where order matters.

Formula

P(n, r) = n! / (n-r)!

Not defined for r > n.