std::prev_permutation(3)

NAME
std::prev_permutation - std::prev_permutation

Synopsis
   Defined in header <algorithm>
   template< class BidirIt >
   bool prev_permutation( BidirIt first, BidirIt (until C++20)
   last );
   template< class BidirIt >
   constexpr bool prev_permutation( BidirIt first, (since C++20)
   BidirIt last ); (1)
   template< class BidirIt, class Compare >
   bool prev_permutation( BidirIt first, BidirIt (until C++20)
   last, Compare comp ); (2)
   template< class BidirIt, class Compare >
   constexpr bool prev_permutation( BidirIt first, (since C++20)
   BidirIt last, Compare comp );
   Transforms the range [first, last) into the previous permutation from the set of all
   permutations that are lexicographically ordered with respect to operator< or comp.
   Returns true if such permutation exists, otherwise transforms the range into the
   last permutation (as if by std::sort(first, last); std::reverse(first, last);) and
   returns false.

Parameters
   first, last - the range of elements to permute
                  comparison function object (i.e. an object that satisfies the
                  requirements of Compare) which returns true if the first argument
                  is less than the second.
                  The signature of the comparison function should be equivalent to the
                  following:
                  bool cmp(const Type1 &a, const Type2 &b);
   comp -
                  While the signature does not need to have const &, the function must
                  not modify the objects passed to it and must be able to accept all
                  values of type (possibly const) Type1 and Type2 regardless of value
                  category (thus, Type1 & is not allowed
                  , nor is Type1 unless for Type1 a move is equivalent to a copy
                  (since C++11)).
                  The types Type1 and Type2 must be such that an object of type BidirIt
                  can be dereferenced and then implicitly converted to both of them.

Type requirements
   -
   BidirIt must meet the requirements of ValueSwappable and
   LegacyBidirectionalIterator.

Return value
   true if the new permutation precedes the old in lexicographical order. false if the
   first permutation was reached and the range was reset to the last permutation.

Exceptions
   Any exceptions thrown from iterator operations or the element swap.

Complexity
   At most (last-first)/2 swaps. Averaged over the entire sequence of permutations,
   typical implementations use about 3 comparisons and 1.5 swaps per call.

Notes
   Implementations (e.g. MSVC STL) may enable vectorization when the iterator type
   satisfies LegacyContiguousIterator and swapping its value type calls neither
   non-trivial special member function nor ADL-found swap.

Possible implementation
   template<class BidirIt>
   bool prev_permutation(BidirIt first, BidirIt last)
   {
       if (first == last) return false;
       BidirIt i = last;
       if (first == --i) return false;
       while (1) {
           BidirIt i1, i2;
           i1 = i;
           if (*i1 < *--i) {
               i2 = last;
               while (!(*--i2 < *i))
                   ;
               std::iter_swap(i, i2);
               std::reverse(i1, last);
               return true;
           }
           if (i == first) {
               std::reverse(first, last);
               return false;
           }
       }
   }

Example
   The following code prints all six permutations of the string "abc" in reverse order // Run this code
 #include <algorithm>
 #include <string>
 #include <iostream>
 #include <functional>
 int main()
 {
     std::string s="abc";
     std::sort(s.begin(), s.end(), std::greater<char>());
     do {
         std::cout << s << ' ';
     } while(std::prev_permutation(s.begin(), s.end()));
     std::cout << '\n';
 }

Output:
 cba cab bca bac acb abc

See also
   is_permutation determines if a sequence is a permutation of another
   (C++11) sequence
                        (function template)
                        generates the next greater lexicographic permutation of a
   next_permutation range of elements
                        (function template)
   ranges::prev_permutation generates the next smaller lexicographic permutation of a
   (C++20) range of elements
                        (niebloid)