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

[Christopher Di Bella via libcxx-commits]

================
@@ -0,0 +1,87 @@
+//===----------------------------------------------------------------------===//
+//
+// 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_H
+#define _LIBCPP___ALGORITHM_RANGES_FIND_LAST_H
+
+#include <__config>
+#include <__concepts/assignable.h>
+#include <__functional/identity.h>
+#include <__functional/invoke.h>
+#include <__functional/ranges_operations.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 {
+namespace __find_last {
+struct __fn {
+  template <forward_iterator _Ip, sentinel_for<_Ip> _Sp, class _Tp, class _Proj = identity>
+    requires indirect_binary_predicate<ranges::equal_to, projected<_Ip, _Proj>, const _Tp*>
+  _LIBCPP_NODISCARD_EXT _LIBCPP_HIDE_FROM_ABI constexpr subrange<_Ip>
+  operator()(_Ip __first, _Sp __last, const _Tp& __value, _Proj __proj = {}) const {
+    if constexpr ((bidirectional_range<_Ip> && common_range<_Ip>) ||
+                  (bidirectional_iterator<_Ip> && assignable_from<_Ip&, _Sp&>)) {
+      // Implement optimized bidirectional range and common range version.
+      // Perform a reverse search from the end.
+      _Ip __original_last = __last;                        // Save the original value of __last
+      _Ip __result        = ranges::next(__first, __last); // Set __result to the end of the range
+      while (__first != __last) {
+        --__last;
+        if (std::invoke(__proj, *__last) == __value) {
+          __result = __last;
+          break;
+        }
+      }
+      return {__result, __original_last};
+    } else {
+      _Ip __original_first = __first;
+      _Ip __result         = __first;
+      while (__first != __last) {
+        if (std::invoke(__proj, *__first) == __value) {
+          __result = __first;
+        }
+        ++__first;
+      }
+      return {__result, __original_first};
+    }
+  }
+
+  template <forward_range _Rp, class _Tp, class _Proj = identity>
+    requires indirect_binary_predicate<ranges::equal_to, projected<iterator_t<_Rp>, _Proj>, const _Tp*>
+  _LIBCPP_NODISCARD_EXT _LIBCPP_HIDE_FROM_ABI constexpr borrowed_subrange_t<_Rp>
+  operator()(_Rp&& __r, const _Tp& __value, _Proj __proj = {}) const {
+    return operator()(ranges::begin(__r), ranges::end(__r), __value, __proj);
----------------
cjdb wrote:

```suggestion
    [[clang::always_inline]] return operator()(ranges::begin(__r), ranges::end(__r), __value, std::ref(__proj));
```

If we're to keep this as a function call, then we should inline it, so as to not penalise the caller. We also need to not copy `__proj`.

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