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AK: Change pivot selection of dual_pivot_quick_sort

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Picking the first and last elements as pivots makes it so that
a sorted array is the worst-case input for the algorithm.

This change instead picks pivots at approximately 1/3 and 2/3 in
the array. This results in desired performance for sorted arrays.

Of course this only changes which inputs result in worst-case
performance, but hopefully those inputs occur less frequently than
already sorted arrays.
This commit is contained in:
Mart G 2021-04-28 16:53:10 +02:00 • committed by Andreas Kling
parent edfe81b1ee
commit 67b0d04315
1 changed files with 17 additions and 6 deletions
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23
AK/QuickSort.h
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@ -18,13 +18,24 @@ namespace AK {
template<typename Collection, typename LessThan> template<typename Collection, typename LessThan>
void dual_pivot_quick_sort(Collection& col, int start, int end, LessThan less_than) void dual_pivot_quick_sort(Collection& col, int start, int end, LessThan less_than)
{ {
if (start >= end) { int size = end - start + 1;
if (size <= 1) {
return; return;
} }
int left_pointer, right_pointer; if (size > 3) {
if (!less_than(col[start], col[end])) { int third = size / 3;
swap(col[start], col[end]); if (less_than(col[start + third], col[end - third])) {
swap(col[start + third], col[start]);
swap(col[end - third], col[end]);
} else {
swap(col[start + third], col[end]);
swap(col[end - third], col[start]);
}
} else {
if (!less_than(col[start], col[end])) {
swap(col[start], col[end]);
}
} }
int j = start + 1; int j = start + 1;
@ -57,8 +68,8 @@ void dual_pivot_quick_sort(Collection& col, int start, int end, LessThan less_th
swap(col[start], col[j]); swap(col[start], col[j]);
swap(col[end], col[g]); swap(col[end], col[g]);
left_pointer = j; int left_pointer = j;
right_pointer = g; int right_pointer = g;
dual_pivot_quick_sort(col, start, left_pointer - 1, less_than); dual_pivot_quick_sort(col, start, left_pointer - 1, less_than);
dual_pivot_quick_sort(col, left_pointer + 1, right_pointer - 1, less_than); dual_pivot_quick_sort(col, left_pointer + 1, right_pointer - 1, less_than);