text: // -*- 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___ALGORITHM_IS_PERMUTATION_H
  #define _LIBCPP___ALGORITHM_IS_PERMUTATION_H
  
  #include <__algorithm/comp.h>
  #include <__config>
  #include <__iterator/distance.h>
  #include <__iterator/iterator_traits.h>
  #include <__iterator/next.h>
  
  #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
  #  pragma GCC system_header
  #endif
  
  _LIBCPP_BEGIN_NAMESPACE_STD
  
  template <class _ForwardIterator1, class _ForwardIterator2, class _BinaryPredicate>
  _LIBCPP_NODISCARD_EXT _LIBCPP_CONSTEXPR_AFTER_CXX17 bool
  is_permutation(_ForwardIterator1 __first1, _ForwardIterator1 __last1, _ForwardIterator2 __first2,
                 _BinaryPredicate __pred) {
    //  shorten sequences as much as possible by lopping of any equal prefix
    for (; __first1 != __last1; ++__first1, (void)++__first2)
      if (!__pred(*__first1, *__first2))
        break;
    if (__first1 == __last1)
      return true;
  
    //  __first1 != __last1 && *__first1 != *__first2
    typedef typename iterator_traits<_ForwardIterator1>::difference_type _D1;
    _D1 __l1 = _VSTD::distance(__first1, __last1);
    if (__l1 == _D1(1))
      return false;
    _ForwardIterator2 __last2 = _VSTD::next(__first2, __l1);
    // For each element in [f1, l1) see if there are the same number of
    //    equal elements in [f2, l2)
    for (_ForwardIterator1 __i = __first1; __i != __last1; ++__i) {
      //  Have we already counted the number of *__i in [f1, l1)?
      _ForwardIterator1 __match = __first1;
      for (; __match != __i; ++__match)
        if (__pred(*__match, *__i))
          break;
      if (__match == __i) {
        // Count number of *__i in [f2, l2)
        _D1 __c2 = 0;
        for (_ForwardIterator2 __j = __first2; __j != __last2; ++__j)
          if (__pred(*__i, *__j))
            ++__c2;
        if (__c2 == 0)
          return false;
        // Count number of *__i in [__i, l1) (we can start with 1)
        _D1 __c1 = 1;
        for (_ForwardIterator1 __j = _VSTD::next(__i); __j != __last1; ++__j)
          if (__pred(*__i, *__j))
            ++__c1;
        if (__c1 != __c2)
          return false;
      }
    }
    return true;
  }
  
  template <class _ForwardIterator1, class _ForwardIterator2>
  _LIBCPP_NODISCARD_EXT inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17 bool
  is_permutation(_ForwardIterator1 __first1, _ForwardIterator1 __last1, _ForwardIterator2 __first2) {
    typedef typename iterator_traits<_ForwardIterator1>::value_type __v1;
    typedef typename iterator_traits<_ForwardIterator2>::value_type __v2;
    return _VSTD::is_permutation(__first1, __last1, __first2, __equal_to<__v1, __v2>());
  }
  
  #if _LIBCPP_STD_VER > 11
  template <class _BinaryPredicate, class _ForwardIterator1, class _ForwardIterator2>
  _LIBCPP_CONSTEXPR_AFTER_CXX17 bool
  __is_permutation(_ForwardIterator1 __first1, _ForwardIterator1 __last1, _ForwardIterator2 __first2,
                   _ForwardIterator2 __last2, _BinaryPredicate __pred, forward_iterator_tag, forward_iterator_tag) {
    //  shorten sequences as much as possible by lopping of any equal prefix
    for (; __first1 != __last1 && __first2 != __last2; ++__first1, (void)++__first2)
      if (!__pred(*__first1, *__first2))
        break;
    if (__first1 == __last1)
      return __first2 == __last2;
    else if (__first2 == __last2)
      return false;
  
    typedef typename iterator_traits<_ForwardIterator1>::difference_type _D1;
    _D1 __l1 = _VSTD::distance(__first1, __last1);
  
    typedef typename iterator_traits<_ForwardIterator2>::difference_type _D2;
    _D2 __l2 = _VSTD::distance(__first2, __last2);
    if (__l1 != __l2)
      return false;
  
    // For each element in [f1, l1) see if there are the same number of
    //    equal elements in [f2, l2)
    for (_ForwardIterator1 __i = __first1; __i != __last1; ++__i) {
      //  Have we already counted the number of *__i in [f1, l1)?
      _ForwardIterator1 __match = __first1;
      for (; __match != __i; ++__match)
        if (__pred(*__match, *__i))
          break;
      if (__match == __i) {
        // Count number of *__i in [f2, l2)
        _D1 __c2 = 0;
        for (_ForwardIterator2 __j = __first2; __j != __last2; ++__j)
          if (__pred(*__i, *__j))
            ++__c2;
        if (__c2 == 0)
          return false;
        // Count number of *__i in [__i, l1) (we can start with 1)
        _D1 __c1 = 1;
        for (_ForwardIterator1 __j = _VSTD::next(__i); __j != __last1; ++__j)
          if (__pred(*__i, *__j))
            ++__c1;
        if (__c1 != __c2)
          return false;
      }
    }
    return true;
  }
  
  template <class _BinaryPredicate, class _RandomAccessIterator1, class _RandomAccessIterator2>
  _LIBCPP_CONSTEXPR_AFTER_CXX17 bool __is_permutation(_RandomAccessIterator1 __first1, _RandomAccessIterator2 __last1,
                                                      _RandomAccessIterator1 __first2, _RandomAccessIterator2 __last2,
                                                      _BinaryPredicate __pred, random_access_iterator_tag,
                                                      random_access_iterator_tag) {
    if (_VSTD::distance(__first1, __last1) != _VSTD::distance(__first2, __last2))
      return false;
    return _VSTD::is_permutation<_RandomAccessIterator1, _RandomAccessIterator2,
                                 _BinaryPredicate&>(__first1, __last1, __first2, __pred);
  }
  
  template <class _ForwardIterator1, class _ForwardIterator2, class _BinaryPredicate>
  _LIBCPP_NODISCARD_EXT inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17 bool
  is_permutation(_ForwardIterator1 __first1, _ForwardIterator1 __last1, _ForwardIterator2 __first2,
                 _ForwardIterator2 __last2, _BinaryPredicate __pred) {
    return _VSTD::__is_permutation<_BinaryPredicate&>(
        __first1, __last1, __first2, __last2, __pred, typename iterator_traits<_ForwardIterator1>::iterator_category(),
        typename iterator_traits<_ForwardIterator2>::iterator_category());
  }
  
  template <class _ForwardIterator1, class _ForwardIterator2>
  _LIBCPP_NODISCARD_EXT inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17 bool
  is_permutation(_ForwardIterator1 __first1, _ForwardIterator1 __last1, _ForwardIterator2 __first2,
                 _ForwardIterator2 __last2) {
    typedef typename iterator_traits<_ForwardIterator1>::value_type __v1;
    typedef typename iterator_traits<_ForwardIterator2>::value_type __v2;
    return _VSTD::__is_permutation(__first1, __last1, __first2, __last2, __equal_to<__v1, __v2>(),
                                   typename iterator_traits<_ForwardIterator1>::iterator_category(),
                                   typename iterator_traits<_ForwardIterator2>::iterator_category());
  }
  #endif
  
  _LIBCPP_END_NAMESPACE_STD
  
  #endif // _LIBCPP___ALGORITHM_IS_PERMUTATION_H
  