use alloc::collections::BinaryHeap;

use core::cmp::Ord;

pub(crate) fn k_smallest<T: Ord, I: Iterator<Item = T>>(mut iter: I, k: usize) -> BinaryHeap<T> {

    if k == 0 { return BinaryHeap::new(); }

    let mut heap = iter.by_ref().take(k).collect::<BinaryHeap<_>>();

    iter.for_each(|i| {

        debug_assert_eq!(heap.len(), k);

        // Equivalent to heap.push(min(i, heap.pop())) but more efficient.

        // This should be done with a single `.peek_mut().unwrap()` but

        //  `PeekMut` sifts-down unconditionally on Rust 1.46.0 and prior.

        if *heap.peek().unwrap() > i {

            *heap.peek_mut().unwrap() = i;

        }

    });

    heap

}