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std::prev_permutation(3)       C++ Standard Libary	std::prev_permutation(3)

NAME
     std::prev_permutation - std::prev_permutation

Synopsis
	Defined in header <algorithm>
	template<  class BidirIt >				  (1) (constexpr
     since C++20)
	bool prev_permutation( BidirIt first, BidirIt last );
	template< class BidirIt, class Compare >
	bool prev_permutation( BidirIt first, BidirIt last,	 (2)  (constexpr
     since C++20)
	Compare comp );

	Transforms  the  range	[first, last) into the previous permutation. Re-
     turns true if
	such permutation exists, otherwise transforms the range  into  the  last
     permutation
	(as if by std::sort followed by std::reverse) and returns false.

	1) The set of all permutations is ordered lexicographically with respect
     to
	operator<
	(until C++20)
	std::less{}
	(since C++20).
	2) The set of all permutations is ordered lexicographically with respect
     to comp.

	If
	the type of *first is not Swappable
	(until C++11)
	BidirIt is not ValueSwappable
	(since C++11), the behavior is undefined.

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 equiv-
     alent 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  regard-
     less 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 permu-
     tation.

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

Complexity
	Given \(\scriptsize N\)N as std::distance(first, last):

	1,2) At most \(\scriptsize \frac{N}{2}\)

	N
	2

	swaps.

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;
		}
	    }
	}

Notes
	Averaged  over	the entire sequence of permutations, typical implementa-
     tions use about
	3 comparisons and 1.5 swaps per call.

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

Example
	The  following	code  prints all six permutations of the string "cab" in
     reverse order.

     // Run this code

      #include <algorithm>
      #include <iostream>
      #include <string>

      int main()
      {
	  std::string s = "cab";

	  do
	  {
	      std::cout << s << ' ';
	  }
	  while (std::prev_permutation(s.begin(), s.end()));

	  std::cout << s << '\n';
      }

Output:
      cab bca bac acb abc cba

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

http://cppreference.com 	   2024.06.10		std::prev_permutation(3)
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