heapSiftDown

    fn heapSiftDown(nodes: []Node, heap: []Node.Index, start: usize) void {
        var i = start;
        while (true) {
            var min = i;
            const l = i * 2 + 1;
            const r = l + 1;
            min = if (l < heap.len and nodes[heap[l]].smaller(nodes[heap[min]])) l else min;
            min = if (r < heap.len and nodes[heap[r]].smaller(nodes[heap[min]])) r else min;
            if (i == min) break;
            mem.swap(Node.Index, &heap[i], &heap[min]);
            i = min;
        }
    }