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, (until C++20)
   ForwardIt last );
   template< class ForwardIt >
   constexpr ForwardIt is_sorted_until( ForwardIt (since C++20)
   first, ForwardIt last );
   template< class ExecutionPolicy, class ForwardIt
   >
   ForwardIt is_sorted_until( ExecutionPolicy&& (2) (since C++17)
   policy,
   ForwardIt first, ForwardIt last );
   template< class ForwardIt, class Compare >
   ForwardIt is_sorted_until( ForwardIt first, (1) (since C++11)
   ForwardIt last, (until C++20)
   Compare comp );
   template< class ForwardIt, class Compare >
   constexpr ForwardIt is_sorted_until( ForwardIt (since C++20)
   first, ForwardIt last, (3)
   Compare comp );
   template< class ExecutionPolicy, class
   ForwardIt, class Compare >
   ForwardIt is_sorted_until( ExecutionPolicy&& (4) (since C++17)
   policy,
   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.
   A sequence is sorted with respect to a comparator comp if for any iterator it
   pointing to the sequence and any non-negative integer n such that it + n is a valid
   iterator pointing to an element of the sequence, comp(*(it + n), *it) evaluates to
   false.
   1) Elements are compared using operator<.
   3) Elements are compared using the given binary comparison function comp.
   2,4) Same as (1,3), but executed according to policy. These overloads do not
   participate in overload resolution unless
   std::is_execution_policy_v<std::decay_t<ExecutionPolicy>>
   (until C++20)
   std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>>
   (since C++20) is true.

Parameters
   first, last - the range of elements to examine
   policy - the execution policy to use. See execution policy for details.
                 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 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
                 ForwardIt can be dereferenced and then implicitly converted to both of
                 them.

Type requirements
   -
   ForwardIt must meet the requirements of LegacyForwardIterator.

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

Complexity
   Linear in the distance between first and last.

Exceptions
   The overloads with a template parameter named ExecutionPolicy report errors 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-defined.
     * If the algorithm fails to allocate memory, std::bad_alloc is thrown.

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

First version
   template<class ForwardIt>
   constexpr //< since C++20
   ForwardIt is_sorted_until(ForwardIt first, ForwardIt last)
   {
       return std::is_sorted_until(first, last, std::less<>());
   }

Second version
   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;
   }

Notes
   std::is_sorted_until returns last for empty ranges and ranges of length one.

Example
// Run this code
 #include <iostream>
 #include <algorithm>
 #include <iterator>
 #include <random>
 #include <string>
 #include <cassert>
 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 (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)