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 //===----------------------------------------------------------------------===//
 //
 // 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___PSTL_BACKENDS_DEFAULT_H
 #define _LIBCPP___PSTL_BACKENDS_DEFAULT_H
 
 #include <__algorithm/copy_n.h>
 #include <__algorithm/equal.h>
 #include <__algorithm/fill_n.h>
 #include <__algorithm/for_each_n.h>
 #include <__config>
 #include <__functional/identity.h>
 #include <__functional/not_fn.h>
 #include <__functional/operations.h>
 #include <__iterator/concepts.h>
 #include <__iterator/iterator_traits.h>
 #include <__pstl/backend_fwd.h>
 #include <__pstl/dispatch.h>
 #include <__utility/empty.h>
 #include <__utility/forward.h>
 #include <__utility/move.h>
 #include <optional>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif
 
 _LIBCPP_PUSH_MACROS
 #include <__undef_macros>
 
 #if _LIBCPP_STD_VER >= 17
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 namespace __pstl {
 
 //
 // This file provides an incomplete PSTL backend that implements all of the PSTL algorithms
 // based on a smaller set of basis operations.
 //
 // It is intended as a building block for other PSTL backends that implement some operations more
 // efficiently but may not want to define the full set of PSTL algorithms.
 //
 // This backend implements all the PSTL algorithms based on the following basis operations:
 //
 // find_if family
 // --------------
 // - find
 // - find_if_not
 // - any_of
 // - all_of
 // - none_of
 // - is_partitioned
 //
 // for_each family
 // ---------------
 // - for_each_n
 // - fill
 // - fill_n
 // - replace
 // - replace_if
 // - generate
 // - generate_n
 //
 // merge family
 // ------------
 // No other algorithms based on merge
 //
 // stable_sort family
 // ------------------
 // - sort
 //
 // transform_reduce and transform_reduce_binary family
 // ---------------------------------------------------
 // - count_if
 // - count
 // - equal(3 legs)
 // - equal
 // - reduce
 //
 // transform and transform_binary family
 // -------------------------------------
 // - replace_copy_if
 // - replace_copy
 // - move
 // - copy
 // - copy_n
 // - rotate_copy
 //
 
 //////////////////////////////////////////////////////////////
 // find_if family
 //////////////////////////////////////////////////////////////
 template <class _ExecutionPolicy>
 struct __find<__default_backend_tag, _ExecutionPolicy> {
   template <class _Policy, class _ForwardIterator, class _Tp>
   [[nodiscard]] _LIBCPP_HIDE_FROM_ABI optional<_ForwardIterator>
   operator()(_Policy&& __policy, _ForwardIterator __first, _ForwardIterator __last, const _Tp& __value) const noexcept {
     using _FindIf = __dispatch<__find_if, __current_configuration, _ExecutionPolicy>;
     return _FindIf()(
         __policy, std::move(__first), std::move(__last), [&](__iter_reference<_ForwardIterator> __element) {
           return __element == __value;
         });
   }
 };
 
 template <class _ExecutionPolicy>
 struct __find_if_not<__default_backend_tag, _ExecutionPolicy> {
   template <class _Policy, class _ForwardIterator, class _Pred>
   [[nodiscard]] _LIBCPP_HIDE_FROM_ABI optional<_ForwardIterator>
   operator()(_Policy&& __policy, _ForwardIterator __first, _ForwardIterator __last, _Pred&& __pred) const noexcept {
     using _FindIf = __dispatch<__find_if, __current_configuration, _ExecutionPolicy>;
     return _FindIf()(__policy, __first, __last, std::not_fn(std::forward<_Pred>(__pred)));
   }
 };
 
 template <class _ExecutionPolicy>
 struct __any_of<__default_backend_tag, _ExecutionPolicy> {
   template <class _Policy, class _ForwardIterator, class _Pred>
   [[nodiscard]] _LIBCPP_HIDE_FROM_ABI optional<bool>
   operator()(_Policy&& __policy, _ForwardIterator __first, _ForwardIterator __last, _Pred&& __pred) const noexcept {
     using _FindIf = __dispatch<__find_if, __current_configuration, _ExecutionPolicy>;
     auto __res    = _FindIf()(__policy, __first, __last, std::forward<_Pred>(__pred));
     if (!__res)
       return nullopt;
     return *__res != __last;
   }
 };
 
 template <class _ExecutionPolicy>
 struct __all_of<__default_backend_tag, _ExecutionPolicy> {
   template <class _Policy, class _ForwardIterator, class _Pred>
   [[nodiscard]] _LIBCPP_HIDE_FROM_ABI optional<bool>
   operator()(_Policy&& __policy, _ForwardIterator __first, _ForwardIterator __last, _Pred&& __pred) const noexcept {
     using _AnyOf = __dispatch<__any_of, __current_configuration, _ExecutionPolicy>;
     auto __res   = _AnyOf()(__policy, __first, __last, [&](__iter_reference<_ForwardIterator> __value) {
       return !__pred(__value);
     });
     if (!__res)
       return nullopt;
     return !*__res;
   }
 };
 
 template <class _ExecutionPolicy>
 struct __none_of<__default_backend_tag, _ExecutionPolicy> {
   template <class _Policy, class _ForwardIterator, class _Pred>
   [[nodiscard]] _LIBCPP_HIDE_FROM_ABI optional<bool>
   operator()(_Policy&& __policy, _ForwardIterator __first, _ForwardIterator __last, _Pred&& __pred) const noexcept {
     using _AnyOf = __dispatch<__any_of, __current_configuration, _ExecutionPolicy>;
     auto __res   = _AnyOf()(__policy, __first, __last, std::forward<_Pred>(__pred));
     if (!__res)
       return nullopt;
     return !*__res;
   }
 };
 
 template <class _ExecutionPolicy>
 struct __is_partitioned<__default_backend_tag, _ExecutionPolicy> {
   template <class _Policy, class _ForwardIterator, class _Pred>
   [[nodiscard]] _LIBCPP_HIDE_FROM_ABI optional<bool>
   operator()(_Policy&& __policy, _ForwardIterator __first, _ForwardIterator __last, _Pred&& __pred) const noexcept {
     using _FindIfNot   = __dispatch<__find_if_not, __current_configuration, _ExecutionPolicy>;
     auto __maybe_first = _FindIfNot()(__policy, std::move(__first), __last, __pred);
     if (__maybe_first == nullopt)
       return nullopt;
 
     __first = *__maybe_first;
     if (__first == __last)
       return true;
     ++__first;
     using _NoneOf = __dispatch<__none_of, __current_configuration, _ExecutionPolicy>;
     return _NoneOf()(__policy, std::move(__first), std::move(__last), __pred);
   }
 };
 
 //////////////////////////////////////////////////////////////
 // for_each family
 //////////////////////////////////////////////////////////////
 template <class _ExecutionPolicy>
 struct __for_each_n<__default_backend_tag, _ExecutionPolicy> {
   template <class _Policy, class _ForwardIterator, class _Size, class _Function>
   [[nodiscard]] _LIBCPP_HIDE_FROM_ABI optional<__empty>
   operator()(_Policy&& __policy, _ForwardIterator __first, _Size __size, _Function __func) const noexcept {
     if constexpr (__has_random_access_iterator_category_or_concept<_ForwardIterator>::value) {
       using _ForEach          = __dispatch<__for_each, __current_configuration, _ExecutionPolicy>;
       _ForwardIterator __last = __first + __size;
       return _ForEach()(__policy, std::move(__first), std::move(__last), std::move(__func));
     } else {
       // Otherwise, use the serial algorithm to avoid doing two passes over the input
       std::for_each_n(std::move(__first), __size, std::move(__func));
       return __empty{};
     }
   }
 };
 
 template <class _ExecutionPolicy>
 struct __fill<__default_backend_tag, _ExecutionPolicy> {
   template <class _Policy, class _ForwardIterator, class _Tp>
   [[nodiscard]] _LIBCPP_HIDE_FROM_ABI optional<__empty>
   operator()(_Policy&& __policy, _ForwardIterator __first, _ForwardIterator __last, _Tp const& __value) const noexcept {
     using _ForEach = __dispatch<__for_each, __current_configuration, _ExecutionPolicy>;
     using _Ref     = __iter_reference<_ForwardIterator>;
     return _ForEach()(__policy, std::move(__first), std::move(__last), [&](_Ref __element) { __element = __value; });
   }
 };
 
 template <class _ExecutionPolicy>
 struct __fill_n<__default_backend_tag, _ExecutionPolicy> {
   template <class _Policy, class _ForwardIterator, class _Size, class _Tp>
   [[nodiscard]] _LIBCPP_HIDE_FROM_ABI optional<__empty>
   operator()(_Policy&& __policy, _ForwardIterator __first, _Size __n, _Tp const& __value) const noexcept {
     if constexpr (__has_random_access_iterator_category_or_concept<_ForwardIterator>::value) {
       using _Fill             = __dispatch<__fill, __current_configuration, _ExecutionPolicy>;
       _ForwardIterator __last = __first + __n;
       return _Fill()(__policy, std::move(__first), std::move(__last), __value);
     } else {
       // Otherwise, use the serial algorithm to avoid doing two passes over the input
       std::fill_n(std::move(__first), __n, __value);
       return optional<__empty>{__empty{}};
     }
   }
 };
 
 template <class _ExecutionPolicy>
 struct __replace<__default_backend_tag, _ExecutionPolicy> {
   template <class _Policy, class _ForwardIterator, class _Tp>
   [[nodiscard]] _LIBCPP_HIDE_FROM_ABI optional<__empty>
   operator()(_Policy&& __policy, _ForwardIterator __first, _ForwardIterator __last, _Tp const& __old, _Tp const& __new)
       const noexcept {
     using _ReplaceIf = __dispatch<__replace_if, __current_configuration, _ExecutionPolicy>;
     using _Ref       = __iter_reference<_ForwardIterator>;
     return _ReplaceIf()(
         __policy, std::move(__first), std::move(__last), [&](_Ref __element) { return __element == __old; }, __new);
   }
 };
 
 template <class _ExecutionPolicy>
 struct __replace_if<__default_backend_tag, _ExecutionPolicy> {
   template <class _Policy, class _ForwardIterator, class _Pred, class _Tp>
   [[nodiscard]] _LIBCPP_HIDE_FROM_ABI optional<__empty> operator()(
       _Policy&& __policy, _ForwardIterator __first, _ForwardIterator __last, _Pred&& __pred, _Tp const& __new_value)
       const noexcept {
     using _ForEach = __dispatch<__for_each, __current_configuration, _ExecutionPolicy>;
     using _Ref     = __iter_reference<_ForwardIterator>;
     return _ForEach()(__policy, std::move(__first), std::move(__last), [&](_Ref __element) {
       if (__pred(__element))
         __element = __new_value;
     });
   }
 };
 
 template <class _ExecutionPolicy>
 struct __generate<__default_backend_tag, _ExecutionPolicy> {
   template <class _Policy, class _ForwardIterator, class _Generator>
   [[nodiscard]] _LIBCPP_HIDE_FROM_ABI optional<__empty>
   operator()(_Policy&& __policy, _ForwardIterator __first, _ForwardIterator __last, _Generator&& __gen) const noexcept {
     using _ForEach = __dispatch<__for_each, __current_configuration, _ExecutionPolicy>;
     using _Ref     = __iter_reference<_ForwardIterator>;
     return _ForEach()(__policy, std::move(__first), std::move(__last), [&](_Ref __element) { __element = __gen(); });
   }
 };
 
 template <class _ExecutionPolicy>
 struct __generate_n<__default_backend_tag, _ExecutionPolicy> {
   template <class _Policy, class _ForwardIterator, class _Size, class _Generator>
   [[nodiscard]] _LIBCPP_HIDE_FROM_ABI optional<__empty>
   operator()(_Policy&& __policy, _ForwardIterator __first, _Size __n, _Generator&& __gen) const noexcept {
     using _ForEachN = __dispatch<__for_each_n, __current_configuration, _ExecutionPolicy>;
     using _Ref      = __iter_reference<_ForwardIterator>;
     return _ForEachN()(__policy, std::move(__first), __n, [&](_Ref __element) { __element = __gen(); });
   }
 };
 
 //////////////////////////////////////////////////////////////
 // stable_sort family
 //////////////////////////////////////////////////////////////
 template <class _ExecutionPolicy>
 struct __sort<__default_backend_tag, _ExecutionPolicy> {
   template <class _Policy, class _RandomAccessIterator, class _Comp>
   _LIBCPP_HIDE_FROM_ABI optional<__empty> operator()(
       _Policy&& __policy, _RandomAccessIterator __first, _RandomAccessIterator __last, _Comp&& __comp) const noexcept {
     using _StableSort = __dispatch<__stable_sort, __current_configuration, _ExecutionPolicy>;
     return _StableSort()(__policy, std::move(__first), std::move(__last), std::forward<_Comp>(__comp));
   }
 };
 
 //////////////////////////////////////////////////////////////
 // transform_reduce family
 //////////////////////////////////////////////////////////////
 template <class _ExecutionPolicy>
 struct __count_if<__default_backend_tag, _ExecutionPolicy> {
   template <class _Policy, class _ForwardIterator, class _Predicate>
   [[nodiscard]] _LIBCPP_HIDE_FROM_ABI optional<__iter_diff_t<_ForwardIterator>> operator()(
       _Policy&& __policy, _ForwardIterator __first, _ForwardIterator __last, _Predicate&& __pred) const noexcept {
     using _TransformReduce = __dispatch<__transform_reduce, __current_configuration, _ExecutionPolicy>;
     using _DiffT           = __iter_diff_t<_ForwardIterator>;
     using _Ref             = __iter_reference<_ForwardIterator>;
     return _TransformReduce()(
         __policy, std::move(__first), std::move(__last), _DiffT{}, std::plus{}, [&](_Ref __element) -> _DiffT {
           return __pred(__element) ? _DiffT(1) : _DiffT(0);
         });
   }
 };
 
 template <class _ExecutionPolicy>
 struct __count<__default_backend_tag, _ExecutionPolicy> {
   template <class _Policy, class _ForwardIterator, class _Tp>
   [[nodiscard]] _LIBCPP_HIDE_FROM_ABI optional<__iter_diff_t<_ForwardIterator>>
   operator()(_Policy&& __policy, _ForwardIterator __first, _ForwardIterator __last, _Tp const& __value) const noexcept {
     using _CountIf = __dispatch<__count_if, __current_configuration, _ExecutionPolicy>;
     using _Ref     = __iter_reference<_ForwardIterator>;
     return _CountIf()(__policy, std::move(__first), std::move(__last), [&](_Ref __element) -> bool {
       return __element == __value;
     });
   }
 };
 
 template <class _ExecutionPolicy>
 struct __equal_3leg<__default_backend_tag, _ExecutionPolicy> {
   template <class _Policy, class _ForwardIterator1, class _ForwardIterator2, class _Predicate>
   [[nodiscard]] _LIBCPP_HIDE_FROM_ABI optional<bool>
   operator()(_Policy&& __policy,
              _ForwardIterator1 __first1,
              _ForwardIterator1 __last1,
              _ForwardIterator2 __first2,
              _Predicate&& __pred) const noexcept {
     using _TransformReduce = __dispatch<__transform_reduce_binary, __current_configuration, _ExecutionPolicy>;
     return _TransformReduce()(
         __policy,
         std::move(__first1),
         std::move(__last1),
         std::move(__first2),
         true,
         std::logical_and{},
         std::forward<_Predicate>(__pred));
   }
 };
 
 template <class _ExecutionPolicy>
 struct __equal<__default_backend_tag, _ExecutionPolicy> {
   template <class _Policy, class _ForwardIterator1, class _ForwardIterator2, class _Predicate>
   [[nodiscard]] _LIBCPP_HIDE_FROM_ABI optional<bool>
   operator()(_Policy&& __policy,
              _ForwardIterator1 __first1,
              _ForwardIterator1 __last1,
              _ForwardIterator2 __first2,
              _ForwardIterator2 __last2,
              _Predicate&& __pred) const noexcept {
     if constexpr (__has_random_access_iterator_category<_ForwardIterator1>::value &&
                   __has_random_access_iterator_category<_ForwardIterator2>::value) {
       if (__last1 - __first1 != __last2 - __first2)
         return false;
       // Fall back to the 3 legged algorithm
       using _Equal3Leg = __dispatch<__equal_3leg, __current_configuration, _ExecutionPolicy>;
       return _Equal3Leg()(
           __policy, std::move(__first1), std::move(__last1), std::move(__first2), std::forward<_Predicate>(__pred));
     } else {
       // If we don't have random access, fall back to the serial algorithm cause we can't do much
       return std::equal(
           std::move(__first1),
           std::move(__last1),
           std::move(__first2),
           std::move(__last2),
           std::forward<_Predicate>(__pred));
     }
   }
 };
 
 template <class _ExecutionPolicy>
 struct __reduce<__default_backend_tag, _ExecutionPolicy> {
   template <class _Policy, class _ForwardIterator, class _Tp, class _BinaryOperation>
   [[nodiscard]] _LIBCPP_HIDE_FROM_ABI optional<_Tp>
   operator()(_Policy&& __policy, _ForwardIterator __first, _ForwardIterator __last, _Tp __init, _BinaryOperation&& __op)
       const noexcept {
     using _TransformReduce = __dispatch<__transform_reduce, __current_configuration, _ExecutionPolicy>;
     return _TransformReduce()(
         __policy,
         std::move(__first),
         std::move(__last),
         std::move(__init),
         std::forward<_BinaryOperation>(__op),
         __identity{});
   }
 };
 
 //////////////////////////////////////////////////////////////
 // transform family
 //////////////////////////////////////////////////////////////
 template <class _ExecutionPolicy>
 struct __replace_copy_if<__default_backend_tag, _ExecutionPolicy> {
   template <class _Policy, class _ForwardIterator, class _ForwardOutIterator, class _Pred, class _Tp>
   [[nodiscard]] _LIBCPP_HIDE_FROM_ABI optional<__empty>
   operator()(_Policy&& __policy,
              _ForwardIterator __first,
              _ForwardIterator __last,
              _ForwardOutIterator __out_it,
              _Pred&& __pred,
              _Tp const& __new_value) const noexcept {
     using _Transform = __dispatch<__transform, __current_configuration, _ExecutionPolicy>;
     using _Ref       = __iter_reference<_ForwardIterator>;
     auto __res =
         _Transform()(__policy, std::move(__first), std::move(__last), std::move(__out_it), [&](_Ref __element) {
           return __pred(__element) ? __new_value : __element;
         });
     if (__res == nullopt)
       return nullopt;
     return __empty{};
   }
 };
 
 template <class _ExecutionPolicy>
 struct __replace_copy<__default_backend_tag, _ExecutionPolicy> {
   template <class _Policy, class _ForwardIterator, class _ForwardOutIterator, class _Tp>
   [[nodiscard]] _LIBCPP_HIDE_FROM_ABI optional<__empty>
   operator()(_Policy&& __policy,
              _ForwardIterator __first,
              _ForwardIterator __last,
              _ForwardOutIterator __out_it,
              _Tp const& __old_value,
              _Tp const& __new_value) const noexcept {
     using _ReplaceCopyIf = __dispatch<__replace_copy_if, __current_configuration, _ExecutionPolicy>;
     using _Ref           = __iter_reference<_ForwardIterator>;
     return _ReplaceCopyIf()(
         __policy,
         std::move(__first),
         std::move(__last),
         std::move(__out_it),
         [&](_Ref __element) { return __element == __old_value; },
         __new_value);
   }
 };
 
 // TODO: Use the std::copy/move shenanigans to forward to std::memmove
 //       Investigate whether we want to still forward to std::transform(policy)
 //       in that case for the execution::par part, or whether we actually want
 //       to run everything serially in that case.
 template <class _ExecutionPolicy>
 struct __move<__default_backend_tag, _ExecutionPolicy> {
   template <class _Policy, class _ForwardIterator, class _ForwardOutIterator>
   [[nodiscard]] _LIBCPP_HIDE_FROM_ABI optional<_ForwardOutIterator>
   operator()(_Policy&& __policy, _ForwardIterator __first, _ForwardIterator __last, _ForwardOutIterator __out_it)
       const noexcept {
     using _Transform = __dispatch<__transform, __current_configuration, _ExecutionPolicy>;
     return _Transform()(__policy, std::move(__first), std::move(__last), std::move(__out_it), [&](auto&& __element) {
       return std::move(__element);
     });
   }
 };
 
 // TODO: Use the std::copy/move shenanigans to forward to std::memmove
 template <class _ExecutionPolicy>
 struct __copy<__default_backend_tag, _ExecutionPolicy> {
   template <class _Policy, class _ForwardIterator, class _ForwardOutIterator>
   [[nodiscard]] _LIBCPP_HIDE_FROM_ABI optional<_ForwardOutIterator>
   operator()(_Policy&& __policy, _ForwardIterator __first, _ForwardIterator __last, _ForwardOutIterator __out_it)
       const noexcept {
     using _Transform = __dispatch<__transform, __current_configuration, _ExecutionPolicy>;
     return _Transform()(__policy, std::move(__first), std::move(__last), std::move(__out_it), __identity());
   }
 };
 
 template <class _ExecutionPolicy>
 struct __copy_n<__default_backend_tag, _ExecutionPolicy> {
   template <class _Policy, class _ForwardIterator, class _Size, class _ForwardOutIterator>
   [[nodiscard]] _LIBCPP_HIDE_FROM_ABI optional<_ForwardOutIterator>
   operator()(_Policy&& __policy, _ForwardIterator __first, _Size __n, _ForwardOutIterator __out_it) const noexcept {
     if constexpr (__has_random_access_iterator_category_or_concept<_ForwardIterator>::value) {
       using _Copy             = __dispatch<__copy, __current_configuration, _ExecutionPolicy>;
       _ForwardIterator __last = __first + __n;
       return _Copy()(__policy, std::move(__first), std::move(__last), std::move(__out_it));
     } else {
       // Otherwise, use the serial algorithm to avoid doing two passes over the input
       return std::copy_n(std::move(__first), __n, std::move(__out_it));
     }
   }
 };
 
 template <class _ExecutionPolicy>
 struct __rotate_copy<__default_backend_tag, _ExecutionPolicy> {
   template <class _Policy, class _ForwardIterator, class _ForwardOutIterator>
   [[nodiscard]] _LIBCPP_HIDE_FROM_ABI optional<_ForwardOutIterator>
   operator()(_Policy&& __policy,
              _ForwardIterator __first,
              _ForwardIterator __middle,
              _ForwardIterator __last,
              _ForwardOutIterator __out_it) const noexcept {
     using _Copy       = __dispatch<__copy, __current_configuration, _ExecutionPolicy>;
     auto __result_mid = _Copy()(__policy, __middle, std::move(__last), std::move(__out_it));
     if (__result_mid == nullopt)
       return nullopt;
     return _Copy()(__policy, std::move(__first), std::move(__middle), *std::move(__result_mid));
   }
 };
 
 } // namespace __pstl
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP_STD_VER >= 17
 
 _LIBCPP_POP_MACROS
 
 #endif // _LIBCPP___PSTL_BACKENDS_DEFAULT_H

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