Function random_idx_where 

fn random_idx_where<R, T, P>(
    rng: &mut R,
    slice: &mut [T],
    predicate: P,
) -> Option<usize>
where
    R: Rng,
    P: Fn(&T) -> bool,

Helper: find a random item elt in slice such that predicate(elt) is true. Return the index of that item.

Can arbitrarily reorder slice. This allows us to visit the indices in uniform-at-random order, without having to do any O(N) operations or allocations.