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AK: Guarantee a maximum stack depth for dual_pivot_quick_sort
When the two chosen pivots happen to be the smallest and largest elements of the array, three partitions will be created, two of size 0 and one of size n-2. If this happens on each recursive call to dual_pivot_quick_sort, the stack depth will reach approximately n/2. To avoid the stack from deepening, iteration can be used for the largest of the three partitions. This ensures the stack depth will only increase for partitions of size n/2 or smaller, which results in a maximum stack depth of log(n).
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Notes:
sideshowbarker
2024-07-18 19:00:08 +09:00
Author: https://github.com/Maato Commit: https://github.com/SerenityOS/serenity/commit/c9f3cc6dcc5 Pull-request: https://github.com/SerenityOS/serenity/pull/6709 Issue: https://github.com/SerenityOS/serenity/issues/6629
111
AK/QuickSort.h
111
AK/QuickSort.h
@ -18,62 +18,75 @@ namespace AK {
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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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int size = end - start + 1;
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if (size <= 1) {
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return;
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}
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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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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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swap(col[k], col[g]);
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g--;
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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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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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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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dual_pivot_quick_sort(col, right_pointer + 1, end, less_than);
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}
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template<typename Iterator, typename LessThan>
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