[libcxx][ranges] Add ranges::reverse_view.

Differential Revision: https://reviews.llvm.org/D107096

Author: zoecarver

libcxx/docs/Status/RangesPaper.csv

@@ -146,4 +146,4 @@ Section,Description,Dependencies,Assignee,Complete
 `[range.split] <http://wg21.link/range.split>`_,split_view,[range.all],Zoe Carver,In Progress
 `[range.counted] <http://wg21.link/range.counted>`_,view::counted,[range.subrange],Zoe Carver,Not started
 `[range.common] <http://wg21.link/range.common>`_,common_view,[range.all],Zoe Carver,✅
-`[range.reverse] <http://wg21.link/range.reverse>`_,reverse_view,[range.all],Unassigned,Not started
+`[range.reverse] <http://wg21.link/range.reverse>`_,reverse_view,[range.all],Zoe Carver,✅

libcxx/include/CMakeLists.txt

@@ -199,6 +199,8 @@ set(files
   __ranges/enable_view.h
   __ranges/non_propagating_cache.h
   __ranges/ref_view.h
+  __ranges/reverse_view.h
+  __ranges/take_view.h
   __ranges/single_view.h
   __ranges/size.h
   __ranges/subrange.h

libcxx/include/__ranges/reverse_view.h

@@ -0,0 +1,112 @@
+// -*- 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_REVERSE_VIEW_H
+#define _LIBCPP___RANGES_REVERSE_VIEW_H
+
+#include <__config>
+#include <__iterator/concepts.h>
+#include <__iterator/next.h>
+#include <__iterator/reverse_iterator.h>
+#include <__ranges/access.h>
+#include <__ranges/all.h>
+#include <__ranges/concepts.h>
+#include <__ranges/enable_borrowed_range.h>
+#include <__ranges/non_propagating_cache.h>
+#include <__ranges/size.h>
+#include <__ranges/view_interface.h>
+#include <concepts>
+#include <type_traits>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#if !defined(_LIBCPP_HAS_NO_RANGES)
+
+namespace ranges {
+  template<view _View>
+    requires bidirectional_range<_View>
+  class reverse_view : public view_interface<reverse_view<_View>> {
+    // We cache begin() whenever ranges::next is not guaranteed O(1) to provide an
+    // amortized O(1) begin() method.
+    static constexpr bool _UseCache = !random_access_range<_View> && !common_range<_View>;
+    using _Cache = _If<_UseCache, __non_propagating_cache<reverse_iterator<iterator_t<_View>>>, __empty_cache>;
+    [[no_unique_address]] _Cache __cached_begin_ = _Cache();
+    [[no_unique_address]] _View __base_ = _View();
+
+  public:
+    _LIBCPP_HIDE_FROM_ABI
+    reverse_view() requires default_initializable<_View> = default;
+
+    _LIBCPP_HIDE_FROM_ABI
+    constexpr explicit reverse_view(_View __view) : __base_(_VSTD::move(__view)) {}
+
+    _LIBCPP_HIDE_FROM_ABI
+    constexpr _View base() const& requires copy_constructible<_View> { return __base_; }
+
+    _LIBCPP_HIDE_FROM_ABI
+    constexpr _View base() && { return _VSTD::move(__base_); }
+
+    _LIBCPP_HIDE_FROM_ABI
+    constexpr reverse_iterator<iterator_t<_View>> begin() {
+      if constexpr (_UseCache)
+        if (__cached_begin_.__has_value())
+          return *__cached_begin_;
+
+      auto __tmp = _VSTD::make_reverse_iterator(ranges::next(ranges::begin(__base_), ranges::end(__base_)));
+      if constexpr (_UseCache)
+        __cached_begin_.__set(__tmp);
+      return __tmp;
+    }
+
+    _LIBCPP_HIDE_FROM_ABI
+    constexpr reverse_iterator<iterator_t<_View>> begin() requires common_range<_View> {
+      return _VSTD::make_reverse_iterator(ranges::end(__base_));
+    }
+
+    _LIBCPP_HIDE_FROM_ABI
+    constexpr auto begin() const requires common_range<const _View> {
+      return _VSTD::make_reverse_iterator(ranges::end(__base_));
+    }
+
+    _LIBCPP_HIDE_FROM_ABI
+    constexpr reverse_iterator<iterator_t<_View>> end() {
+      return _VSTD::make_reverse_iterator(ranges::begin(__base_));
+    }
+
+    _LIBCPP_HIDE_FROM_ABI
+    constexpr auto end() const requires common_range<const _View> {
+      return _VSTD::make_reverse_iterator(ranges::begin(__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_);
+    }
+  };
+
+  template<class _Range>
+  reverse_view(_Range&&) -> reverse_view<views::all_t<_Range>>;
+
+  template<class _Tp>
+  inline constexpr bool enable_borrowed_range<reverse_view<_Tp>> = enable_borrowed_range<_Tp>;
+} // namespace ranges
+
+#endif // !defined(_LIBCPP_HAS_NO_RANGES)
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP___RANGES_REVERSE_VIEW_H

libcxx/include/module.modulemap

@@ -631,6 +631,7 @@ module std [system] {
       module enable_view            { private header "__ranges/enable_view.h"           }
       module non_propagating_cache  { private header "__ranges/non_propagating_cache.h" }
       module ref_view               { private header "__ranges/ref_view.h"              }
+      module reverse_view           { private header "__ranges/reverse_view.h"          }
       module size                   { private header "__ranges/size.h"                  }
       module single_view            { private header "__ranges/single_view.h"           }
       module subrange               { private header "__ranges/subrange.h"              }

libcxx/include/ranges

@@ -154,6 +154,14 @@ namespace std::ranges {
     requires (!common_range<V> && copyable<iterator_t<V>>)
   class common_view;
 
+ // [range.reverse], reverse view
+  template<view V>
+    requires bidirectional_range<V>
+  class reverse_view;
+
+  template<class T>
+    inline constexpr bool enable_borrowed_range<reverse_view<T>> = enable_borrowed_range<T>;
+
   template<class T>
   inline constexpr bool enable_borrowed_range<common_view<T>> = enable_borrowed_range<T>;
 
@@ -188,6 +196,8 @@ namespace std::ranges {
 #include <__ranges/enable_borrowed_range.h>
 #include <__ranges/enable_view.h>
 #include <__ranges/ref_view.h>
+#include <__ranges/reverse_view.h>
+#include <__ranges/take_view.h>
 #include <__ranges/single_view.h>
 #include <__ranges/size.h>
 #include <__ranges/subrange.h>

libcxx/test/libcxx/diagnostics/detail.headers/ranges/reverse_view.module.verify.cpp

@@ -0,0 +1,16 @@
+// -*- 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
+//
+//===----------------------------------------------------------------------===//
+
+// REQUIRES: modules-build
+
+// WARNING: This test was generated by 'generate_private_header_tests.py'
+// and should not be edited manually.
+
+// expected-error@*:* {{use of private header from outside its module: '__ranges/reverse_view.h'}}
+#include <__ranges/reverse_view.h>

libcxx/test/std/ranges/range.adaptors/range.reverse/base.pass.cpp

@@ -0,0 +1,75 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+// UNSUPPORTED: libcpp-no-concepts
+// UNSUPPORTED: gcc-10
+// UNSUPPORTED: libcpp-has-no-incomplete-ranges
+
+// constexpr V base() const& requires copy_constructible<V> { return base_; }
+// constexpr V base() && { return std::move(base_); }
+
+#include <ranges>
+#include <cassert>
+
+#include "test_macros.h"
+#include "types.h"
+
+constexpr bool test() {
+  int buffer[8] = {1, 2, 3, 4, 5, 6, 7, 8};
+
+  // Test common ranges.
+  {
+    // Test non-const.
+    {
+      auto rev = std::ranges::reverse_view(BidirRange{buffer});
+      assert(rev.base().ptr_ == buffer);
+      assert(std::move(rev).base().ptr_ == buffer);
+
+      ASSERT_SAME_TYPE(decltype(rev.base()), BidirRange);
+      ASSERT_SAME_TYPE(decltype(std::move(rev).base()), BidirRange);
+    }
+    // Test const.
+    {
+      const auto rev = std::ranges::reverse_view(BidirRange{buffer});
+      assert(rev.base().ptr_ == buffer);
+      assert(std::move(rev).base().ptr_ == buffer);
+
+      ASSERT_SAME_TYPE(decltype(rev.base()), BidirRange);
+      ASSERT_SAME_TYPE(decltype(std::move(rev).base()), BidirRange);
+    }
+  }
+  // Test non-common ranges.
+  {
+    // Test non-const (also move only).
+    {
+      auto rev = std::ranges::reverse_view(BidirSentRange<MoveOnly>{buffer});
+      assert(std::move(rev).base().ptr_ == buffer);
+
+      ASSERT_SAME_TYPE(decltype(std::move(rev).base()), BidirSentRange<MoveOnly>);
+    }
+    // Test const.
+    {
+      const auto rev = std::ranges::reverse_view(BidirSentRange<Copyable>{buffer});
+      assert(rev.base().ptr_ == buffer);
+      assert(std::move(rev).base().ptr_ == buffer);
+
+      ASSERT_SAME_TYPE(decltype(rev.base()), BidirSentRange<Copyable>);
+      ASSERT_SAME_TYPE(decltype(std::move(rev).base()), BidirSentRange<Copyable>);
+    }
+  }
+
+  return true;
+}
+
+int main(int, char**) {
+  test();
+  static_assert(test());
+
+  return 0;
+}