// -*- 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 <__concepts/constructible.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/range_adaptor.h>
#include <__ranges/size.h>
#include <__ranges/subrange.h>
#include <__ranges/view_interface.h>
#include <__type_traits/conditional.h>
#include <__type_traits/remove_cvref.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 >= 20

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>;
    _LIBCPP_NO_UNIQUE_ADDRESS _Cache __cached_begin_ = _Cache();
    _LIBCPP_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_(std::move(__view)) {}

    _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 reverse_iterator<iterator_t<_View>> begin() {
      if constexpr (_UseCache)
        if (__cached_begin_.__has_value())
          return *__cached_begin_;

      auto __tmp = std::make_reverse_iterator(ranges::next(ranges::begin(__base_), ranges::end(__base_)));
      if constexpr (_UseCache)
        __cached_begin_.__emplace(__tmp);
      return __tmp;
    }

    _LIBCPP_HIDE_FROM_ABI
    constexpr reverse_iterator<iterator_t<_View>> begin() requires common_range<_View> {
      return std::make_reverse_iterator(ranges::end(__base_));
    }

    _LIBCPP_HIDE_FROM_ABI
    constexpr auto begin() const requires common_range<const _View> {
      return std::make_reverse_iterator(ranges::end(__base_));
    }

    _LIBCPP_HIDE_FROM_ABI
    constexpr reverse_iterator<iterator_t<_View>> end() {
      return std::make_reverse_iterator(ranges::begin(__base_));
    }

    _LIBCPP_HIDE_FROM_ABI
    constexpr auto end() const requires common_range<const _View> {
      return std::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 views {
  namespace __reverse {
    template<class _Tp>
    inline constexpr bool __is_reverse_view = false;

    template<class _Tp>
    inline constexpr bool __is_reverse_view<reverse_view<_Tp>> = true;

    template<class _Tp>
    inline constexpr bool __is_sized_reverse_subrange = false;

    template<class _Iter>
    inline constexpr bool __is_sized_reverse_subrange<subrange<reverse_iterator<_Iter>, reverse_iterator<_Iter>, subrange_kind::sized>> = true;

    template<class _Tp>
    inline constexpr bool __is_unsized_reverse_subrange = false;

    template<class _Iter, subrange_kind _Kind>
    inline constexpr bool __is_unsized_reverse_subrange<subrange<reverse_iterator<_Iter>, reverse_iterator<_Iter>, _Kind>> = _Kind == subrange_kind::unsized;

    template<class _Tp>
    struct __unwrapped_reverse_subrange {
      using type = void; // avoid SFINAE-ing out the overload below -- let the concept requirements do it for better diagnostics
    };

    template<class _Iter, subrange_kind _Kind>
    struct __unwrapped_reverse_subrange<subrange<reverse_iterator<_Iter>, reverse_iterator<_Iter>, _Kind>> {
      using type = subrange<_Iter, _Iter, _Kind>;
    };

    struct __fn : __range_adaptor_closure<__fn> {
      template<class _Range>
        requires __is_reverse_view<remove_cvref_t<_Range>>
      [[nodiscard]] _LIBCPP_HIDE_FROM_ABI
      constexpr auto operator()(_Range&& __range) const
        noexcept(noexcept(std::forward<_Range>(__range).base()))
        -> decltype(      std::forward<_Range>(__range).base())
        { return          std::forward<_Range>(__range).base(); }

      template<class _Range,
               class _UnwrappedSubrange = typename __unwrapped_reverse_subrange<remove_cvref_t<_Range>>::type>
        requires __is_sized_reverse_subrange<remove_cvref_t<_Range>>
      [[nodiscard]] _LIBCPP_HIDE_FROM_ABI
      constexpr auto operator()(_Range&& __range) const
        noexcept(noexcept(_UnwrappedSubrange(__range.end().base(), __range.begin().base(), __range.size())))
        -> decltype(      _UnwrappedSubrange(__range.end().base(), __range.begin().base(), __range.size()))
        { return          _UnwrappedSubrange(__range.end().base(), __range.begin().base(), __range.size()); }

      template<class _Range,
               class _UnwrappedSubrange = typename __unwrapped_reverse_subrange<remove_cvref_t<_Range>>::type>
        requires __is_unsized_reverse_subrange<remove_cvref_t<_Range>>
      [[nodiscard]] _LIBCPP_HIDE_FROM_ABI
      constexpr auto operator()(_Range&& __range) const
        noexcept(noexcept(_UnwrappedSubrange(__range.end().base(), __range.begin().base())))
        -> decltype(      _UnwrappedSubrange(__range.end().base(), __range.begin().base()))
        { return          _UnwrappedSubrange(__range.end().base(), __range.begin().base()); }

      template<class _Range>
        requires (!__is_reverse_view<remove_cvref_t<_Range>> &&
                  !__is_sized_reverse_subrange<remove_cvref_t<_Range>> &&
                  !__is_unsized_reverse_subrange<remove_cvref_t<_Range>>)
      [[nodiscard]] _LIBCPP_HIDE_FROM_ABI
      constexpr auto operator()(_Range&& __range) const
        noexcept(noexcept(reverse_view{std::forward<_Range>(__range)}))
        -> decltype(      reverse_view{std::forward<_Range>(__range)})
        { return          reverse_view{std::forward<_Range>(__range)}; }
    };
  } // namespace __reverse

  inline namespace __cpo {
    inline constexpr auto reverse = __reverse::__fn{};
  } // namespace __cpo
  } // namespace views
} // namespace ranges

#endif // _LIBCPP_STD_VER >= 20

_LIBCPP_END_NAMESPACE_STD

#endif // _LIBCPP___RANGES_REVERSE_VIEW_H