// -*- 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_EMPTY_H
#define _LIBCPP___RANGES_EMPTY_H

#include <__config>
#include <__iterator/concepts.h>
#include <__ranges/size.h>
#include <type_traits>

#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
#pragma GCC system_header
#endif

_LIBCPP_PUSH_MACROS
#include <__undef_macros>

_LIBCPP_BEGIN_NAMESPACE_STD

#if !defined(_LIBCPP_HAS_NO_RANGES)

// clang-format off
namespace ranges {
// [range.prim.empty]
namespace __empty {
  template <class _Tp>
  concept __member_empty = requires(_Tp&& __t) {
    bool(_VSTD::forward<_Tp>(__t).empty());
  };

  template<class _Tp>
  concept __can_invoke_size =
    !__member_empty<_Tp> &&
    requires(_Tp&& __t) { ranges::size(_VSTD::forward<_Tp>(__t)); };

  template <class _Tp>
  concept __can_compare_begin_end =
    !__member_empty<_Tp> &&
    !__can_invoke_size<_Tp> &&
    requires(_Tp&& __t) {
      bool(ranges::begin(__t) == ranges::end(__t));
      { ranges::begin(__t) } -> forward_iterator;
    };

  struct __fn {
    template <__member_empty _Tp>
    [[nodiscard]] constexpr bool operator()(_Tp&& __t) const
        noexcept(noexcept(bool(__t.empty()))) {
      return __t.empty();
    }

    template <__can_invoke_size _Tp>
    [[nodiscard]] constexpr bool operator()(_Tp&& __t) const
        noexcept(noexcept(ranges::size(_VSTD::forward<_Tp>(__t)))) {
      return ranges::size(_VSTD::forward<_Tp>(__t)) == 0;
    }

    template<__can_compare_begin_end _Tp>
    [[nodiscard]] constexpr bool operator()(_Tp&& __t) const
        noexcept(noexcept(bool(ranges::begin(__t) == ranges::end(__t)))) {
      return ranges::begin(__t) == ranges::end(__t);
    }
  };
}

inline namespace __cpo {
  inline constexpr auto empty = __empty::__fn{};
} // namespace __cpo
} // namespace ranges
// clang-format off

#endif // !defined(_LIBCPP_HAS_NO_RANGES)

_LIBCPP_END_NAMESPACE_STD

_LIBCPP_POP_MACROS

#endif // _LIBCPP___RANGES_EMPTY_H