AK: Introduce cutoff to insertion sort for Quicksort

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
Notes: sideshowbarker 2024-07-17 08:55:54 +09:00
4 changed files with 119 additions and 0 deletions

45
AK/InsertionSort.h Normal file
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@ -0,0 +1,45 @@
/*
* Copyright (c) 2022, Marc Luqué <marc.luque@outlook.com>
*
* SPDX-License-Identifier: BSD-2-Clause
*/
#pragma once
#include <AK/Concepts.h>
#include <AK/StdLibExtras.h>
namespace AK {
// Standard Insertion Sort, with `end` inclusive!
template<typename Collection, typename Comparator, typename T = decltype(declval<Collection>()[declval<int>()])>
void insertion_sort(Collection& col, ssize_t start, ssize_t end, Comparator comparator)
requires(Indexable<Collection, T>)
{
for (ssize_t i = start + 1; i <= end; ++i) {
for (ssize_t j = i; j > 0 && comparator(col[j], col[j - 1]); --j)
swap(col[j], col[j - 1]);
}
}
template<typename Collection, typename Comparator, typename T = decltype(declval<Collection>()[declval<int>()])>
void insertion_sort(Collection& collection, Comparator comparator)
requires(Indexable<Collection, T>)
{
if (collection.size() == 0)
return;
insertion_sort(collection, 0, collection.size() - 1, move(comparator));
}
template<typename Collection, typename T = decltype(declval<Collection>()[declval<int>()])>
void insertion_sort(Collection& collection)
requires(Indexable<Collection, T>)
{
if (collection.size() == 0)
return;
insertion_sort(collection, 0, collection.size() - 1, [](auto& a, auto& b) { return a < b; });
}
}
using AK::insertion_sort;

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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) {

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@ -38,6 +38,7 @@ set(AK_TEST_SOURCES
TestIPv4Address.cpp
TestIPv6Address.cpp
TestIndexSequence.cpp
TestInsertionSort.cpp
TestIntegerMath.cpp
TestIntrusiveList.cpp
TestIntrusiveRedBlackTree.cpp

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@ -0,0 +1,57 @@
/*
* Copyright (c) 2022, Marc Luqué <marc.luque@outlook.com>
*
* SPDX-License-Identifier: BSD-2-Clause
*/
#include <LibTest/TestCase.h>
#include <AK/InsertionSort.h>
#include <AK/Random.h>
#include <AK/Vector.h>
static constexpr int const n = 10;
static constexpr int const iterations = 10;
static constexpr int const seed = 1337;
TEST_CASE(sorts_ascending)
{
srand(seed);
for (int i = 0; i < iterations; ++i) {
Vector<int, n> ints;
for (int j = 0; j < n; ++j)
ints.append(get_random<int>());
Vector<int, n> ints_copy = ints;
insertion_sort(ints);
insertion_sort(ints_copy);
for (int j = 0; j < n - 1; ++j) {
EXPECT(ints[j] <= ints[j + 1]);
EXPECT_EQ(ints[j], ints_copy[j]);
EXPECT_EQ(ints[j + 1], ints_copy[j + 1]);
}
}
}
TEST_CASE(sorts_decending)
{
srand(seed);
for (int i = 0; i < iterations; ++i) {
Vector<int, n> ints;
for (int j = 0; j < n; ++j)
ints.append(get_random<int>());
Vector<int, n> ints_copy = ints;
insertion_sort(ints, [](auto& a, auto& b) { return a > b; });
insertion_sort(ints_copy, [](auto& a, auto& b) { return a > b; });
for (int j = 0; j < n - 1; ++j) {
EXPECT(ints[j] >= ints[j + 1]);
EXPECT_EQ(ints[j], ints_copy[j]);
EXPECT_EQ(ints[j + 1], ints_copy[j + 1]);
}
}
}