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AK: Introduce cutoff to insertion sort for Quicksort

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Implement insertion sort in AK. The cutoff value 7 is a magic number
here, values [5, 15] should work well. Main idea of the cutoff is to
reduce recursion performed by quicksort to speed up sorting
of small partitions.
This commit is contained in:
Marc Luqué 2022-03-25 17:06:11 +01:00 • committed by Linus Groh
parent bbb256e8b5
commit 22f472249d
4 changed files with 119 additions and 0 deletions
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16
AK/QuickSort.h
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@ -1,11 +1,13 @@
/*
* Copyright (c) 2018-2020, Andreas Kling <kling@serenityos.org>
* Copyright (c) 2022, Marc Luqué <marc.luque@outlook.com>
*
* SPDX-License-Identifier: BSD-2-Clause
*/
#pragma once
#include <AK/InsertionSort.h>
#include <AK/StdLibExtras.h>
namespace AK {
@ -14,10 +16,24 @@ namespace AK {
* pivot quick_sort below. The other quick_sort below should only be used when
* you are stuck with simple iterators to a container and you don't have access
* to the container itself.
*
* We use a cutoff to insertion sort for partitions of size 7 or smaller.
* The idea is to avoid recursion for small partitions.
* The value 7 here is a magic number. According to princeton's CS algorithm class
* a value between 5 and 15 should work well in most situations:
* https://algs4.cs.princeton.edu/23quicksort/
*/
static constexpr int INSERTION_SORT_CUTOFF = 7;
template<typename Collection, typename LessThan>
void dual_pivot_quick_sort(Collection& col, int start, int end, LessThan less_than)
{
if ((end + 1) - start <= INSERTION_SORT_CUTOFF) {
AK::insertion_sort(col, start, end, less_than);
return;
}
while (start < end) {
int size = end - start + 1;
if (size > 3) {