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.

1 files changed, 45 insertions, 0 deletions

diff --git a/AK/InsertionSort.h b/AK/InsertionSort.h
new file mode 100644
index 0000000000..2ed2e7cab4
--- /dev/null
+++ b/AK/InsertionSort.h
@@ -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;