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+/*
+ * Copyright (C) 2015 Apple Inc. All Rights Reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY APPLE INC. ``AS IS'' AND ANY
+ * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
+ * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL APPLE INC. OR
+ * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
+ * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+ * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
+ * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY
+ * OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. 
+ */
+
+#ifndef BubbleSort_h
+#define BubbleSort_h
+
+namespace WTF {
+
+// Why would you want to use bubble sort? When you know that your input is already mostly
+// sorted! This sort is guaranteed stable (it won't reorder elements that were equal), it
+// doesn't require any scratch memory, and is the fastest available sorting algorithm if your
+// input already happens to be sorted. This sort is also likely to have competetive performance
+// for small inputs, even if they are very unsorted.
+
+// We use this sorting algorithm for compiler insertion sets. An insertion set is usually very
+// nearly sorted. It shouldn't take more than a few bubbles to make it fully sorted. We made
+// this decision deliberately. Here's the performance of the testb3 Complex(64, 384) benchmark
+// with the Air::InsertionSet doing no sorting, std::stable_sorting, and bubbleSorting:
+//
+// no sort:          8.8222 +- 0.1911 ms.
+// std::stable_sort: 9.0135 +- 0.1418 ms.
+// bubbleSort:       8.8457 +- 0.1511 ms.
+//
+// Clearly, bubble sort is superior.
+//
+// Note that the critical piece here is that insertion sets tend to be small, they must be
+// sorted, the sort must be stable, they are usually already sorted to begin with, and when they
+// are unsorted it's usually because of a few out-of-place elements.
+
+template<typename IteratorType, typename LessThan>
+void bubbleSort(IteratorType begin, IteratorType end, const LessThan& lessThan)
+{
+    for (;;) {
+        bool changed = false;
+        ASSERT(end >= begin);
+        size_t limit = end - begin;
+        for (size_t i = limit; i-- > 1;) {
+            if (lessThan(begin[i], begin[i - 1])) {
+                std::swap(begin[i], begin[i - 1]);
+                changed = true;
+            }
+        }
+        if (!changed)
+            return;
+        // After one run, the first element in the list is guaranteed to be the smallest.
+        begin++;
+
+        // Now go in the other direction. This eliminates most sorting pathologies.
+        changed = false;
+        ASSERT(end >= begin);
+        limit = end - begin;
+        for (size_t i = 1; i < limit; ++i) {
+            if (lessThan(begin[i], begin[i - 1])) {
+                std::swap(begin[i], begin[i - 1]);
+                changed = true;
+            }
+        }
+        if (!changed)
+            return;
+        // Now the last element is guaranteed to be the largest.
+        end--;
+    }
+}
+
+template<typename IteratorType>
+void bubbleSort(IteratorType begin, IteratorType end)
+{
+    bubbleSort(
+        begin, end,
+        [](auto& left, auto& right) {
+            return left < right;
+        });
+}
+
+} // namespace WTF
+
+using WTF::bubbleSort;
+
+#endif // BubbleSort_h
+