From 67b0d04315449dd4f780b21c4f156f89e7b635ef Mon Sep 17 00:00:00 2001
From: Mart G <martg_@hotmail.com>
Date: Wed, 28 Apr 2021 16:53:10 +0200
Subject: [PATCH] AK: Change pivot selection of dual_pivot_quick_sort

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.
---
 AK/QuickSort.h | 23 +++++++++++++++++------
 1 file changed, 17 insertions(+), 6 deletions(-)

diff --git a/AK/QuickSort.h b/AK/QuickSort.h
index 2112be95bf..cef2a001fe 100644
--- a/AK/QuickSort.h
+++ b/AK/QuickSort.h
@@ -18,13 +18,24 @@ namespace AK {
 template<typename Collection, typename LessThan>
 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;
     }
 
-    int left_pointer, right_pointer;
-    if (!less_than(col[start], col[end])) {
-        swap(col[start], col[end]);
+    if (size > 3) {
+        int third = size / 3;
+        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;
@@ -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[end], col[g]);
 
-    left_pointer = j;
-    right_pointer = g;
+    int left_pointer = j;
+    int right_pointer = g;
 
     dual_pivot_quick_sort(col, start, left_pointer - 1, less_than);
     dual_pivot_quick_sort(col, left_pointer + 1, right_pointer - 1, less_than);