std::ranges::is_sorted_until(3)

NAME
std::ranges::is_sorted_until - std::ranges::is_sorted_until

Synopsis
   Defined in header <algorithm>
   Call signature
   template< std::forward_iterator I, std::sentinel_for<I> S, class Proj =
   std::identity,
   std::indirect_strict_weak_order<std::projected<I, Proj>> Comp = (1) (since
   ranges::less > C++20)
   constexpr I is_sorted_until( I first, S last, Comp comp = {}, Proj proj
   = {} );
   template< std::forward_range R, class Proj = std::identity,
   std::indirect_strict_weak_order< (since
   std::projected<ranges::iterator_t<R>, Proj>> Comp = ranges::less > (2) C++20)
   constexpr ranges::borrowed_iterator_t<R>
   is_sorted_until( R&& r, Comp comp = {}, Proj proj = {} );
   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, std::invoke(comp, std::invoke(proj,
   *(it + n)), std::invoke(proj, *it)) evaluates to false.
   1) Elements are compared using the given binary comparison function comp.
   2) Same as (1), but uses r as the source range, as if using ranges::begin(r) as
   first and ranges::end(r) as last.
   The function-like entities described on this page are niebloids, that is:
     * Explicit template argument lists may not be specified when calling any of them.
     * None of them is visible to argument-dependent lookup.
     * When one of them is found by normal unqualified lookup for the name to the left
       of the function-call operator, it inhibits argument-dependent lookup.
   In practice, they may be implemented as function objects, or with special compiler
   extensions.

Parameters
   first, last - iterator-sentinel defining the range to find its sorted upper bound
   r - the range to find its sorted upper bound
   comp - comparison function to apply to the projected elements
   proj - projection to apply to the elements

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

Complexity
   Linear in the distance between first and last.

Possible implementation
  struct is_sorted_until_fn {
    template<std::forward_iterator I, std::sentinel_for<I> S, class Proj = std::identity,
             std::indirect_strict_weak_order<std::projected<I, Proj>> Comp = ranges::less>
    constexpr I operator()(I first, S last, Comp comp = {}, Proj proj = {}) const
    {
        if (first == last) {
            return first;
        }
        auto next = first;
        while (++next != last) {
            if (std::invoke(comp, std::invoke(proj, *next), std::invoke(proj, *first))) {
                return next;
            }
            first = next;
        }
        return first;
    }
    template< ranges::forward_range R, class Proj = std::identity,
            std::indirect_strict_weak_order<
                std::projected<ranges::iterator_t<R>, Proj>> Comp = ranges::less >
    constexpr ranges::borrowed_iterator_t<R>
    operator()( R&& r, Comp comp = {}, Proj proj = {} ) const
    {
        return (*this)(ranges::begin(r), ranges::end(r),
                       std::ref(comp), std::ref(proj));
    }
  };
  inline constexpr is_sorted_until_fn is_sorted_until;

Notes
   ranges::is_sorted_until returns an iterator equal to last for empty ranges and
   ranges of length one.

Example
// Run this code
 #include <algorithm>
 #include <iostream>
 #include <iterator>
 #include <random>
 int main()
 {
     std::random_device rd;
     std::mt19937 g{rd()};
     std::array nums {3, 1, 4, 1, 5, 9};
     constexpr int min_sorted_size = 4;
     int sorted_size = 0;
     do {
         std::ranges::shuffle(nums, g);
         const auto sorted_end = std::ranges::is_sorted_until(nums);
         sorted_size = std::ranges::distance(nums.begin(), sorted_end);
         std::ranges::copy(nums, std::ostream_iterator<int>(std::cout, " "));
         std::cout << " : " << sorted_size << " leading sorted element(s)\n";
     } while (sorted_size < min_sorted_size);
 }

Possible output:
 4 1 9 5 1 3 : 1 leading sorted element(s)
 4 5 9 3 1 1 : 3 leading sorted element(s)
 9 3 1 4 5 1 : 1 leading sorted element(s)
 1 3 5 4 1 9 : 3 leading sorted element(s)
 5 9 1 1 3 4 : 2 leading sorted element(s)
 4 9 1 5 1 3 : 2 leading sorted element(s)
 1 1 4 9 5 3 : 4 leading sorted element(s)

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