Function sample_multiple_l_reservoir 

pub fn sample_multiple_l_reservoir<I, R, T>(
    rng: &mut R,
    iter: I,
    requested: usize,
) -> Vec<T>
where
    R: Rng,
    I: IntoIterator<Item = T>,

Sample multiple random elements uniformly without replacement from a container of unknown length. If more samples are requested than are in the set, the function returns as many items as it can.

The implementation uses Iterator::nth. Randomly indexable structures will have a O(1) nth implementation and will be very efficient. The values are cloned.

This function implements “Algorithm L” from KIM-HUNG LI Reservoir-Sampling Algorithms of Time Complexity O(n(1 + log(N/n))) https://dl.acm.org/doi/pdf/10.1145/198429.198435

This algorithm is significantly faster than the reservoir algorithm in rand and is on par with the “known length” algorithm for large requested values.