doSiftDown

        fn doSiftDown(self: *Self, start_index: usize, target_order: Order) void {
            var index = start_index;
            const half = self.len >> 1;
            while (true) {
                const first_grandchild_index = firstGrandchildIndex(index);
                const last_grandchild_index = first_grandchild_index + 3;

                const elem = self.items[index];

                if (last_grandchild_index < self.len) {
                    // All four grandchildren exist
                    const index2 = first_grandchild_index + 1;
                    const index3 = index2 + 1;

                    // Find the best grandchild
                    const best_left = self.bestItemAtIndices(first_grandchild_index, index2, target_order);
                    const best_right = self.bestItemAtIndices(index3, last_grandchild_index, target_order);
                    const best_grandchild = self.bestItem(best_left, best_right, target_order);

                    // If the item is better than or equal to its best grandchild, we are done
                    if (compareFn(self.context, best_grandchild.item, elem) != target_order) return;

                    // Otherwise, swap them
                    self.items[best_grandchild.index] = elem;
                    self.items[index] = best_grandchild.item;
                    index = best_grandchild.index;

                    // We might need to swap the element with it's parent
                    self.swapIfParentIsBetter(elem, index, target_order);
                } else {
                    // The children or grandchildren are the last layer
                    const first_child_index = firstChildIndex(index);
                    if (first_child_index >= self.len) return;

                    const best_descendent = self.bestDescendent(first_child_index, first_grandchild_index, target_order);

                    // If the item is better than or equal to its best descendant, we are done
                    if (compareFn(self.context, best_descendent.item, elem) != target_order) return;

                    // Otherwise swap them
                    self.items[best_descendent.index] = elem;
                    self.items[index] = best_descendent.item;
                    index = best_descendent.index;

                    // If we didn't swap a grandchild, we are done
                    if (index < first_grandchild_index) return;

                    // We might need to swap the element with it's parent
                    self.swapIfParentIsBetter(elem, index, target_order);
                    return;
                }

                // If we are now in the last layer, we are done
                if (index >= half) return;
            }
        }