[libcxx-commits] [libcxx] [libcxx][algorithm] Optimize std::stable_sort via radix sort algorithm (PR #104683)

[Nikolas Klauser via libcxx-commits]

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In-Reply-To: <llvm.org/llvm/llvm-project/pull/104683 at github.com>


================
@@ -0,0 +1,362 @@
+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP___ALGORITHM_RADIX_SORT_H
+#define _LIBCPP___ALGORITHM_RADIX_SORT_H
+
+// This is an implementation of classic LSD radix sort algorithm, running in linear time and using `O(max(N, M))`
+// additional memory, where `N` is size of an input range, `M` — maximum value of
+// a radix of the sorted integer type. Type of the radix and its maximum value are determined at compile time
+// based on type returned by function `__radix`. The default radix is uint8.
+
+// The algorithm is equivalent to several consecutive calls of counting sort for each
+// radix of the sorted numbers from low to high byte.
+// The algorithm uses a temporary buffer of size equal to size of the input range. Each `i`-th pass
+// of the algorithm sorts values by `i`-th radix and moves values to the temporary buffer (for each even `i`, counted
+// from zero), or moves them back to the initial range (for each odd `i`). It there is only one radix in sorted integers
----------------
philnik777 wrote:

```suggestion
// from zero), or moves them back to the initial range (for each odd `i`). If there is only one radix in sorted integers
```

https://github.com/llvm/llvm-project/pull/104683