[libcxx-commits] [libcxx] [libcxx] adds ranges::fold_left_with_iter and ranges::fold_left (PR #75259)

[Will Hawkins via libcxx-commits]

================
@@ -0,0 +1,118 @@
+// -*- 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_FOLD_H
+#define _LIBCPP___ALGORITHM_FOLD_H
+
+#include <__concepts/assignable.h>
+#include <__concepts/convertible_to.h>
+#include <__concepts/invocable.h>
+#include <__concepts/movable.h>
+#include <__config>
+#include <__functional/invoke.h>
+#include <__functional/reference_wrapper.h>
+#include <__iterator/concepts.h>
+#include <__iterator/iterator_traits.h>
+#include <__iterator/next.h>
+#include <__ranges/access.h>
+#include <__ranges/concepts.h>
+#include <__ranges/dangling.h>
+#include <__type_traits/decay.h>
+#include <__type_traits/invoke.h>
+#include <__utility/forward.h>
+#include <__utility/move.h>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#if _LIBCPP_STD_VER >= 23
+
+namespace ranges {
+template <class _Ip, class _Tp>
+struct in_value_result {
+  _Ip in;
+  _Tp result;
+};
+
+template <class _Ip, class _Tp>
+using fold_left_with_iter_result = in_value_result<_Ip, _Tp>;
+
+template <class _Fp, class _Tp, class _Ip, class _Rp, class _Up = decay_t<_Rp>>
+concept __indirectly_binary_left_foldable_impl =
+    convertible_to<_Rp, _Up> &&                    //
+    movable<_Tp> &&                                //
+    movable<_Up> &&                                //
+    convertible_to<_Tp, _Up> &&                    //
+    invocable<_Fp&, _Up, iter_reference_t<_Ip>> && //
+    assignable_from<_Up&, invoke_result_t<_Fp&, _Up, iter_reference_t<_Ip>>>;
+
+template <class _Fp, class _Tp, class _Ip>
+concept __indirectly_binary_left_foldable =
+    copy_constructible<_Fp> &&                     //
+    invocable<_Fp&, _Tp, iter_reference_t<_Ip>> && //
+    __indirectly_binary_left_foldable_impl<_Fp, _Tp, _Ip, invoke_result_t<_Fp&, _Tp, iter_reference_t<_Ip>>>;
+
+struct __fold_left_with_iter {
+  template <input_iterator _Ip, sentinel_for<_Ip> _Sp, class _Tp, __indirectly_binary_left_foldable<_Tp, _Ip> _Fp>
+  _LIBCPP_NODISCARD_EXT _LIBCPP_HIDE_FROM_ABI static constexpr auto
+  operator()(_Ip __first, _Sp __last, _Tp __init, _Fp __f) {
+    using _Up = decay_t<invoke_result_t<_Fp&, _Tp, iter_reference_t<_Ip>>>;
+
+    if (__first == __last) {
+      return fold_left_with_iter_result<_Ip, _Up>{std::move(__first), _Up(std::move(__init))};
+    }
+
+    _Up __result = std::invoke(__f, std::move(__init), *__first);
+    for (++__first; __first != __last; ++__first) {
+      __result = std::invoke(__f, std::move(__result), *__first);
+    }
+
+    return fold_left_with_iter_result<_Ip, _Up>{std::move(__first), std::move(__result)};
+  }
+
+  template <input_range _Rp, class _Tp, __indirectly_binary_left_foldable<_Tp, iterator_t<_Rp>> _Fp>
+  _LIBCPP_NODISCARD_EXT _LIBCPP_HIDE_FROM_ABI static constexpr auto operator()(_Rp&& __r, _Tp __init, _Fp __f) {
+    auto __result = operator()(ranges::begin(__r), ranges::end(__r), std::move(__init), std::ref(__f));
----------------
hawkinsw wrote:

It looks like there is a slight difference between the implementation here and the spec:
```C++
ranges::fold_left_with_iter(std::move(first), last, std::move(init), f).value
```
from [[alg.fold.1]](https://eel.is/c++draft/alg.fold#1). I am not nearly enough of an expert to know if you are doing something special here or if it's just a mistake. 

I hope that this is helpful.

https://github.com/llvm/llvm-project/pull/75259