Function sample_multiple_from_known_length 

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

Samples requested elements uniformly at random without replacement from an iterator whose length is known at runtime. Requires len >= requested.

The caller must ensure that (len, Some(len)) == iter.size_hint(), i.e. the iterator reports its exact length via size_hint. We do not require ExactSizeIterator because that is a compile-time guarantee, whereas our requirement is a runtime condition.

The implementation selects random indices and uses Iterator::nth. For iterators with O(1) nth (e.g., randomly indexable structures), this is very efficient. Selected values are cloned.

This strategy is particularly effective for small requested (≤ 5), since it avoids iterating over the entire set and is typically faster than reservoir sampling.