[libcxx-commits] [llvm] [clang-tools-extra] [flang] [libcxx] [lld] [compiler-rt] [clang] [libc++][ranges] P2116R9: Implements `views::enumerate` (PR #73617)

[Christopher Di Bella via libcxx-commits]

================
@@ -0,0 +1,333 @@
+// -*- 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___RANGES_ENUMERATE_VIEW_H
+#define _LIBCPP___RANGES_ENUMERATE_VIEW_H
+
+#include <__concepts/convertible_to.h>
+#include <__config>
+#include <__iterator/concepts.h>
+#include <__iterator/distance.h>
+#include <__iterator/iter_move.h>
+#include <__iterator/iterator_traits.h>
+#include <__ranges/access.h>
+#include <__ranges/all.h>
+#include <__ranges/concepts.h>
+#include <__ranges/enable_borrowed_range.h>
+#include <__ranges/range_adaptor.h>
+#include <__ranges/size.h>
+#include <__ranges/view_interface.h>
+#include <__type_traits/maybe_const.h>
+#include <__utility/forward.h>
+#include <__utility/move.h>
+#include <tuple>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#if _LIBCPP_STD_VER >= 23
+
+namespace ranges {
+
+// [concept.object]
+
+template <class _Rp>
+concept __range_with_movable_references =
+    ranges::input_range<_Rp> && std::move_constructible<ranges::range_reference_t<_Rp>> &&
+    std::move_constructible<ranges::range_rvalue_reference_t<_Rp>>;
+
+// [range.enumerate.view]
+
+template <view _View>
+  requires __range_with_movable_references<_View>
+class enumerate_view : public view_interface<enumerate_view<_View>> {
+  _View __base_ = _View();
+
+  // [range.enumerate.iterator]
+  template <bool _Const>
+  class __iterator;
+
+  // [range.enumerate.sentinel]
+  template <bool _Const>
+  class __sentinel;
+
+  template <bool _AnyConst>
+  _LIBCPP_HIDE_FROM_ABI static constexpr decltype(auto) __get_current(const __iterator<_AnyConst>& __iter) {
+    return (__iter.__current_);
+  }
+
+public:
+  _LIBCPP_HIDE_FROM_ABI constexpr enumerate_view()
+    requires(default_initializable<_View>)
+  = default;
+  _LIBCPP_HIDE_FROM_ABI constexpr explicit enumerate_view(_View __base) : __base_(std::move(__base)){};
+
+  _LIBCPP_HIDE_FROM_ABI constexpr auto begin()
+    requires(!__simple_view<_View>)
+  {
+    return __iterator<false>(ranges::begin(__base_), 0);
+  }
+  _LIBCPP_HIDE_FROM_ABI constexpr auto begin() const
+    requires __range_with_movable_references<const _View>
+  {
+    return __iterator<true>(ranges::begin(__base_), 0);
+  }
+
+  _LIBCPP_HIDE_FROM_ABI constexpr auto end()
+    requires(!__simple_view<_View>)
+  {
+    if constexpr (common_range<_View> && sized_range<_View>)
+      return __iterator<false>(ranges::end(__base_), ranges::distance(__base_));
+    else
+      return __sentinel<false>(ranges::end(__base_));
+  }
+  _LIBCPP_HIDE_FROM_ABI constexpr auto end() const
+    requires __range_with_movable_references<const _View>
+  {
+    if constexpr (common_range<const _View> && sized_range<const _View>)
+      return __iterator<true>(ranges::end(__base_), ranges::distance(__base_));
+    else
+      return __sentinel<true>(ranges::end(__base_));
+  }
+
+  _LIBCPP_HIDE_FROM_ABI constexpr auto size()
+    requires sized_range<_View>
+  {
+    return ranges::size(__base_);
+  }
+  _LIBCPP_HIDE_FROM_ABI constexpr auto size() const
+    requires sized_range<const _View>
+  {
+    return ranges::size(__base_);
+  }
+
+  _LIBCPP_HIDE_FROM_ABI constexpr _View base() const&
+    requires copy_constructible<_View>
+  {
+    return __base_;
+  }
+  _LIBCPP_HIDE_FROM_ABI constexpr _View base() && { return std::move(__base_); }
+};
+
+template <class _Range>
+enumerate_view(_Range&&) -> enumerate_view<views::all_t<_Range>>;
+
+// [range.enumerate.iterator]
+
+template <view _View>
+  requires __range_with_movable_references<_View>
+template <bool _Const>
+class enumerate_view<_View>::__iterator {
+  using _Base = __maybe_const<_Const, _View>;
+
+  static consteval auto __get_iterator_concept() {
+    if constexpr (random_access_range<_Base>) {
+      return random_access_iterator_tag{};
+    } else if constexpr (bidirectional_range<_Base>) {
+      return bidirectional_iterator_tag{};
+    } else if constexpr (forward_range<_Base>) {
+      return forward_iterator_tag{};
+    } else {
+      return input_iterator_tag{};
+    }
+  }
+
+  friend class enumerate_view<_View>;
+
+public:
+  using iterator_category = input_iterator_tag;
+  using iterator_concept  = decltype(__get_iterator_concept());
+  using difference_type   = range_difference_t<_Base>;
+  using value_type        = tuple<difference_type, range_value_t<_Base>>;
+
+private:
+  using __reference_type       = tuple<difference_type, range_reference_t<_Base>>;
+  iterator_t<_Base> __current_ = iterator_t<_Base>();
+  difference_type __pos_       = 0;
+
+  _LIBCPP_HIDE_FROM_ABI constexpr explicit __iterator(iterator_t<_Base> __current, difference_type __pos)
+      : __current_(std::move(__current)), __pos_(__pos) {}
+
+public:
+  _LIBCPP_HIDE_FROM_ABI __iterator()
+    requires(default_initializable<iterator_t<_Base>>)
----------------
cjdb wrote:

```suggestion
    requires default_initializable<iterator_t<_Base>>
```

https://github.com/llvm/llvm-project/pull/73617