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

NAME
     std::is_sorted_until - std::is_sorted_until

Synopsis
	Defined in header <algorithm>
	template<   class  ForwardIt  > 				  (since
     C++11)
	ForwardIt is_sorted_until( ForwardIt first, ForwardIt	 (1)  (constexpr
     since C++20)
	last );
	template< class ExecutionPolicy, class ForwardIt >

	ForwardIt  is_sorted_until(  ExecutionPolicy&&	policy,      (2)  (since
     C++17)

				   ForwardIt first, ForwardIt
	last );
	template< class ForwardIt, class Compare >

	ForwardIt is_sorted_until(  ForwardIt  first,  ForwardIt     (3)  (since
     C++11)
	last,							      (constexpr
     since C++20)

				   Compare comp );
	template< class ExecutionPolicy, class ForwardIt, class
	Compare >

	ForwardIt  is_sorted_until(  ExecutionPolicy&&	policy,      (4)  (since
     C++17)
				   ForwardIt first, ForwardIt
	last,

				   Compare comp );

	Examines  the  range [first, last) and finds the largest range beginning
     at first in
	which the elements are sorted in non-descending order.

	1) Finds the largest range whether elements are sorted with respect to
	operator<
	(until C++20)
	std::less{}
	(since C++20).
	3) Finds the largest range whether elements are sorted with  respect  to
     comp.
	2,4) Same as (1,3), but executed according to policy.
	These overloads participate in overload resolution only if

	std::is_execution_policy_v<std::decay_t<ExecutionPolicy>>    is    true.
     (until
										  C++20)
	std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>>      is
     true. (since
										  C++20)

Parameters
	first, last - the range of elements to examine
	policy	     - the execution policy to use. See execution policy for de-
     tails.
		      comparison function object (i.e. an object that  satisfies
     the
		      requirements  of	Compare) which returns true if the first
     argument is
		      less than (i.e. is ordered before) the second.

		      The signature of the comparison function should be equiva-
     lent 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
		      ForwardIt can be dereferenced  and  then	implicitly  con-
     verted to both of
		      them.

Type requirements
	-
	ForwardIt must meet the requirements of LegacyForwardIterator.
	-
	Compare must meet the requirements of Compare.

Return value
	The upper bound of the largest range beginning at first in which the el-
     ements are
	sorted in ascending order. That is, the last iterator it for which range
     [first, it)
	is sorted.

	Returns last for empty ranges and ranges of length one.

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

	1,2) \(\scriptsize O(N)\)O(N) comparisons using
	operator<
	(until C++20)
	std::less{}
	(since C++20).
	3,4) \(\scriptsize O(N)\)O(N) applications of the comparator comp.

Exceptions
	The overloads with a template parameter named ExecutionPolicy report er-
     rors as
	follows:

	  *  If  execution of a function invoked as part of the algorithm throws
     an exception
	    and ExecutionPolicy is one of the standard policies,  std::terminate
     is called.
	    For  any  other  ExecutionPolicy, the behavior is implementation-de-
     fined.
	  * If the algorithm fails to allocate memory, std::bad_alloc is thrown.

Possible implementation
	See also the implementations in libstdc++ and libc++.

				  is_sorted_until (1)
	template<class ForwardIt>
	constexpr //< since C++20
	ForwardIt is_sorted_until(ForwardIt first, ForwardIt last)
	{
	    return std::is_sorted_until(first, last, std::less<>());
	}
				  is_sorted_until (2)
	template<class ForwardIt, class Compare>
	constexpr //< since C++20
	ForwardIt is_sorted_until(ForwardIt first, ForwardIt last, Compare comp)
	{
	    if (first != last)
	    {
		ForwardIt next = first;
		while (++next != last)
		{
		    if (comp(*next, *first))
			return next;
		    first = next;
		}
	    }
	    return last;
	}

Example
     // Run this code

      #include <algorithm>
      #include <cassert>
      #include <iostream>
      #include <iterator>
      #include <random>
      #include <string>

      int main()
      {
	  std::random_device rd;
	  std::mt19937 g(rd());
	  const int N = 6;
	  int nums[N] = {3, 1, 4, 1, 5, 9};

	  const int min_sorted_size = 4;

	  for (int sorted_size = 0; sorted_size < min_sorted_size;)
	  {
	      std::shuffle(nums, nums + N, g);
	      int *const sorted_end = std::is_sorted_until(nums, nums + N);
	      sorted_size = std::distance(nums, sorted_end);
	      assert(sorted_size >= 1);

	      for (const auto i : nums)
		  std::cout << i << ' ';
	      std::cout << ": " << sorted_size << " initial sorted elements\n"
			<< std::string(sorted_size * 2 - 1, '^') << '\n';
	  }
      }

Possible output:
      4 1 9 5 1 3 : 1 initial sorted elements
      ^
      4 5 9 3 1 1 : 3 initial sorted elements
      ^^^^^
      9 3 1 4 5 1 : 1 initial sorted elements
      ^
      1 3 5 4 1 9 : 3 initial sorted elements
      ^^^^^
      5 9 1 1 3 4 : 2 initial sorted elements
      ^^^
      4 9 1 5 1 3 : 2 initial sorted elements
      ^^^
      1 1 4 9 5 3 : 4 initial sorted elements
      ^^^^^^^

See also
	is_sorted		checks whether a range is sorted into  ascending
     order
	(C++11) 		(function template)
	ranges::is_sorted_until finds the largest sorted subrange
	(C++20) 		(niebloid)

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