Function sample_array 

pub fn sample_array<R, const N: usize>(
    rng: &mut R,
    len: usize,
) -> Option<[usize; N]>
where
    R: Rng + ?Sized,

Randomly sample exactly N distinct indices from 0..len, and return them in random order (fully shuffled).

This is implemented via Floyd’s algorithm. Time complexity is O(N^2) and memory complexity is O(N).

Returns None if (and only if) N > len.