Function permutation_count 

pub fn permutation_count(n: u64, r: u64) -> u128

Number of permutations: P(n, r) = n! / (n - r)!.

Examples

use mathverse_core::integer::permutation_count;

assert_eq!(permutation_count(5, 3), 60);
assert_eq!(permutation_count(5, 0), 1);
assert_eq!(permutation_count(3, 5), 0);