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172 lines
5.2 KiB
C++
172 lines
5.2 KiB
C++
/*
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* Copyright (c) 2018-2020, Andreas Kling <kling@serenityos.org>
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* Copyright (c) 2022, Marc Luqué <marc.luque@outlook.com>
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*
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* SPDX-License-Identifier: BSD-2-Clause
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*/
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#pragma once
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#include <AK/InsertionSort.h>
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#include <AK/StdLibExtras.h>
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namespace AK {
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// This is a dual pivot quick sort. It is quite a bit faster than the single
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// pivot quick_sort below. The other quick_sort below should only be used when
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// you are stuck with simple iterators to a container and you don't have access
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// to the container itself.
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//
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// We use a cutoff to insertion sort for partitions of size 7 or smaller.
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// The idea is to avoid recursion for small partitions.
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// The value 7 here is a magic number. According to princeton's CS algorithm class
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// a value between 5 and 15 should work well in most situations:
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// https://algs4.cs.princeton.edu/23quicksort/
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static constexpr int INSERTION_SORT_CUTOFF = 7;
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template<typename Collection, typename LessThan>
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void dual_pivot_quick_sort(Collection& col, int start, int end, LessThan less_than)
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{
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if ((end + 1) - start <= INSERTION_SORT_CUTOFF) {
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AK::insertion_sort(col, start, end, less_than);
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return;
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}
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while (start < end) {
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int size = end - start + 1;
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if (size > 3) {
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int third = size / 3;
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if (less_than(col[start + third], col[end - third])) {
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swap(col[start + third], col[start]);
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swap(col[end - third], col[end]);
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} else {
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swap(col[start + third], col[end]);
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swap(col[end - third], col[start]);
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}
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} else {
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if (!less_than(col[start], col[end])) {
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swap(col[start], col[end]);
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}
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}
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int j = start + 1;
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int k = start + 1;
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int g = end - 1;
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auto&& left_pivot = col[start];
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auto&& right_pivot = col[end];
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while (k <= g) {
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if (less_than(col[k], left_pivot)) {
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swap(col[k], col[j]);
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j++;
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} else if (!less_than(col[k], right_pivot)) {
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while (!less_than(col[g], right_pivot) && k < g) {
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g--;
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}
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swap(col[k], col[g]);
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g--;
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if (less_than(col[k], left_pivot)) {
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swap(col[k], col[j]);
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j++;
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}
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}
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k++;
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}
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j--;
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g++;
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swap(col[start], col[j]);
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swap(col[end], col[g]);
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int left_pointer = j;
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int right_pointer = g;
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int left_size = left_pointer - start;
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int middle_size = right_pointer - (left_pointer + 1);
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int right_size = (end + 1) - (right_pointer + 1);
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if (left_size >= middle_size && left_size >= right_size) {
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dual_pivot_quick_sort(col, left_pointer + 1, right_pointer - 1, less_than);
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dual_pivot_quick_sort(col, right_pointer + 1, end, less_than);
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end = left_pointer - 1;
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} else if (middle_size >= right_size) {
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dual_pivot_quick_sort(col, start, left_pointer - 1, less_than);
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dual_pivot_quick_sort(col, right_pointer + 1, end, less_than);
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start = left_pointer + 1;
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end = right_pointer - 1;
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} else {
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dual_pivot_quick_sort(col, start, left_pointer - 1, less_than);
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dual_pivot_quick_sort(col, left_pointer + 1, right_pointer - 1, less_than);
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start = right_pointer + 1;
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}
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}
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}
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template<typename Iterator, typename LessThan>
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void single_pivot_quick_sort(Iterator start, Iterator end, LessThan less_than)
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{
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for (;;) {
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int size = end - start;
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if (size <= 1)
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return;
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int pivot_point = size / 2;
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if (pivot_point)
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swap(*(start + pivot_point), *start);
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auto&& pivot = *start;
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int i = 1;
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for (int j = 1; j < size; ++j) {
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if (less_than(*(start + j), pivot)) {
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swap(*(start + j), *(start + i));
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++i;
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}
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}
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swap(*start, *(start + i - 1));
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// Recur into the shorter part of the remaining data
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// to ensure a stack depth of at most log(n).
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if (i > size / 2) {
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single_pivot_quick_sort(start + i, end, less_than);
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end = start + i - 1;
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} else {
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single_pivot_quick_sort(start, start + i - 1, less_than);
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start = start + i;
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}
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}
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}
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template<typename Iterator>
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void quick_sort(Iterator start, Iterator end)
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{
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single_pivot_quick_sort(start, end, [](auto& a, auto& b) { return a < b; });
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}
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template<typename Iterator, typename LessThan>
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void quick_sort(Iterator start, Iterator end, LessThan less_than)
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{
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single_pivot_quick_sort(start, end, move(less_than));
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}
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template<typename Collection, typename LessThan>
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void quick_sort(Collection& collection, LessThan less_than)
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{
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dual_pivot_quick_sort(collection, 0, collection.size() - 1, move(less_than));
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}
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template<typename Collection>
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void quick_sort(Collection& collection)
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{
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dual_pivot_quick_sort(collection, 0, collection.size() - 1,
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[](auto& a, auto& b) { return a < b; });
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}
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}
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#if USING_AK_GLOBALLY
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using AK::quick_sort;
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#endif
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