[libcxx-commits] [libcxx] [libcxx] patch for implementing ranges::find_last (PR #67270)

[via libcxx-commits]

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@@ -0,0 +1,104 @@
+//===----------------------------------------------------------------------===//
+//
+// 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_RANGES_FIND_LAST_IF_H
+#define _LIBCPP___ALGORITHM_RANGES_FIND_LAST_IF_H
+
+#include <__concepts/assignable.h>
+#include <__config>
+#include <__functional/identity.h>
+#include <__functional/invoke.h>
+#include <__iterator/concepts.h>
+#include <__iterator/next.h>
+#include <__iterator/projected.h>
+#include <__ranges/access.h>
+#include <__ranges/concepts.h>
+#include <__ranges/dangling.h>
+#include <__ranges/range_adaptor.h>
+#include <__ranges/subrange.h>
+#include <__utility/move.h>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+#if _LIBCPP_STD_VER >= 23
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+namespace ranges {
+
+template <class _Ip, class _Sp, class _Pred, class _Proj>
+_LIBCPP_NODISCARD_EXT _LIBCPP_HIDE_FROM_ABI static constexpr subrange<_Ip>
+__find_last_if_impl(_Ip __first, _Sp __last, _Pred&& __pred, _Proj&& __proj) {
+  if constexpr (common_range<_Ip> && bidirectional_range<_Ip>) {
+    // Optimized for common_range and bidirectional_range
+    _Ip __original_last = __last; // Save the original value of __last
+    while (__first != __last) {
+      --__last;
+      if (std::invoke(__pred, std::invoke(__proj, *__last))) {
+        return {__last, __original_last}; // Directly return the result
+      }
+    }
+    return {__last, __original_last};
+  } else if constexpr (bidirectional_iterator<_Ip> && assignable_from<_Ip&, _Sp&>) {
+    // For non-common but bidirectional ranges
+    _Ip __result        = ranges::next(__first, __last);
----------------
huixie90 wrote:

To make sure `ranges::next` is constant time, I think you can do 
```
(random_access_iterator<_Ip> && sized_sentinel_for<_Sp, _Ip>) || (bidirectional_iterator<_Ip> && assignable_from<_Ip&, _Sp&>)
```


https://github.com/llvm/llvm-project/pull/67270