[libcxx-commits] [libcxx] [libc++] implement adjacent_view (PR #165089)

[Louis Dionne via libcxx-commits]

================
@@ -0,0 +1,408 @@
+// -*- 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_ADJACENT_VIEW_H
+#define _LIBCPP___RANGES_ADJACENT_VIEW_H
+
+#include <__config>
+
+#include <__algorithm/min.h>
+#include <__compare/three_way_comparable.h>
+#include <__concepts/constructible.h>
+#include <__concepts/convertible_to.h>
+#include <__concepts/equality_comparable.h>
+#include <__cstddef/size_t.h>
+#include <__functional/invoke.h>
+#include <__functional/operations.h>
+#include <__iterator/concepts.h>
+#include <__iterator/incrementable_traits.h>
+#include <__iterator/iter_move.h>
+#include <__iterator/iter_swap.h>
+#include <__iterator/iterator_traits.h>
+#include <__iterator/next.h>
+#include <__iterator/prev.h>
+#include <__ranges/access.h>
+#include <__ranges/all.h>
+#include <__ranges/concepts.h>
+#include <__ranges/empty_view.h>
+#include <__ranges/enable_borrowed_range.h>
+#include <__ranges/range_adaptor.h>
+#include <__ranges/size.h>
+#include <__ranges/view_interface.h>
+#include <__ranges/zip_utils.h>
+#include <__type_traits/common_type.h>
+#include <__type_traits/is_nothrow_constructible.h>
+#include <__type_traits/make_unsigned.h>
+#include <__type_traits/maybe_const.h>
+#include <__utility/declval.h>
+#include <__utility/forward.h>
+#include <__utility/integer_sequence.h>
+#include <__utility/move.h>
+#include <array>
+#include <tuple>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_PUSH_MACROS
+#include <__undef_macros>
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#if _LIBCPP_STD_VER >= 23
+
+namespace ranges {
+
+template <forward_range _View, size_t _Np>
+  requires view<_View> && (_Np > 0)
+class adjacent_view : public view_interface<adjacent_view<_View, _Np>> {
+private:
+  _LIBCPP_NO_UNIQUE_ADDRESS _View __base_ = _View();
+
+  template <bool>
+  class __iterator;
+
+  template <bool>
+  class __sentinel;
+
+  struct __as_sentinel {};
+
+public:
+  _LIBCPP_HIDE_FROM_ABI adjacent_view()
+    requires default_initializable<_View>
+  = default;
+
+  _LIBCPP_HIDE_FROM_ABI constexpr explicit adjacent_view(_View __base) : __base_(std::move(__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_); }
+
+  _LIBCPP_HIDE_FROM_ABI constexpr auto begin()
+    requires(!__simple_view<_View>)
+  {
+    return __iterator<false>(ranges::begin(__base_), ranges::end(__base_));
+  }
+
+  _LIBCPP_HIDE_FROM_ABI constexpr auto begin() const
+    requires range<const _View> // todo: this seems under-constrained. lwg issue?
+  {
+    return __iterator<true>(ranges::begin(__base_), ranges::end(__base_));
+  }
+
+  _LIBCPP_HIDE_FROM_ABI constexpr auto end()
+    requires(!__simple_view<_View>)
+  {
+    if constexpr (common_range<_View>) {
+      return __iterator<false>(__as_sentinel{}, ranges::begin(__base_), ranges::end(__base_));
+    } else {
+      return __sentinel<false>(ranges::end(__base_));
+    }
+  }
+
+  _LIBCPP_HIDE_FROM_ABI constexpr auto end() const
+    requires range<const _View>
+  {
+    if constexpr (common_range<const _View>) {
+      return __iterator<true>(__as_sentinel{}, ranges::begin(__base_), ranges::end(__base_));
+    } else {
+      return __sentinel<true>(ranges::end(__base_));
+    }
+  }
+
+  _LIBCPP_HIDE_FROM_ABI constexpr auto size()
+    requires sized_range<_View>
+  {
+    using _ST = decltype(ranges::size(__base_));
+    using _CT = common_type_t<_ST, size_t>;
+    auto __sz = static_cast<_CT>(ranges::size(__base_));
+    __sz -= std::min<_CT>(__sz, _Np - 1);
+    return static_cast<_ST>(__sz);
+  }
+
+  _LIBCPP_HIDE_FROM_ABI constexpr auto size() const
+    requires sized_range<const _View>
+  {
+    using _ST = decltype(ranges::size(__base_));
+    using _CT = common_type_t<_ST, size_t>;
+    auto __sz = static_cast<_CT>(ranges::size(__base_));
+    __sz -= std::min<_CT>(__sz, _Np - 1);
+    return static_cast<_ST>(__sz);
+  }
+};
+
+template <forward_range _View, size_t _Np>
+  requires view<_View> && (_Np > 0)
+template <bool _Const>
+class adjacent_view<_View, _Np>::__iterator {
+  friend adjacent_view;
+  using _Base _LIBCPP_NODEBUG              = __maybe_const<_Const, _View>;
+  array<iterator_t<_Base>, _Np> __current_ = array<iterator_t<_Base>, _Np>();
+
+  _LIBCPP_HIDE_FROM_ABI constexpr __iterator(iterator_t<_Base> __first, sentinel_t<_Base> __last) {
+    __current_[0] = __first;
+    for (int __i = 1; __i < static_cast<int>(_Np); ++__i) {
+      __current_[__i] = ranges::next(__current_[__i - 1], 1, __last);
+    }
+  }
+
+  _LIBCPP_HIDE_FROM_ABI constexpr __iterator(__as_sentinel, iterator_t<_Base> __first, iterator_t<_Base> __last) {
+    if constexpr (!bidirectional_range<_Base>) {
+      __current_.fill(__last);
----------------
ldionne wrote:

Why do we bother filling the whole array with `__last` if we can't do anything but `operator==` on this iterator? Can't we just set the last element of `__current_`?

https://github.com/llvm/llvm-project/pull/165089