std::is_sorted_until

Defined in header `<algorithm>`
	(1)
template< class ForwardIt > ForwardIt is_sorted_until( ForwardIt first, ForwardIt last );		(since C++11) (until C++20)
template< class ForwardIt > constexpr ForwardIt is_sorted_until( ForwardIt first, ForwardIt last );		(since C++20)
template< class ExecutionPolicy, class ForwardIt > ForwardIt is_sorted_until( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last );	(2)	(since C++17)
	(3)
template< class ForwardIt, class Compare > ForwardIt is_sorted_until( ForwardIt first, ForwardIt last, Compare comp );		(since C++11) (until C++20)
template< class ForwardIt, class Compare > constexpr ForwardIt is_sorted_until( ForwardIt first, ForwardIt last, Compare comp );		(since C++20)
template< class ExecutionPolicy, class ForwardIt, class Compare > ForwardIt is_sorted_until( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last, Compare comp );	(4)	(since C++17)

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

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>> is true

Parameters

first, last	-	the range of elements to examine
policy	-	the execution policy to use. See execution policy for details.
comp	-	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);` 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

First version
template<class ForwardIt> ForwardIt is_sorted_until(ForwardIt first, ForwardIt last) { return is_sorted_until(first, last, std::less<>()); }
Second version
template <class ForwardIt, class Compare> 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; }

First version

template<class ForwardIt>
ForwardIt is_sorted_until(ForwardIt first, ForwardIt last)
{
    return is_sorted_until(first, last, std::less<>());
}

Second version

template <class ForwardIt, class Compare>
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 and std::is_sorted_until both return true for empty ranges and ranges of length one.

Example

#include <iostream>
#include <algorithm>
#include <iterator>
#include <random>
 
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;
    int sorted_size = 0;
    do {
        std::shuffle(nums, nums + N, g);
        int *sorted_end = std::is_sorted_until(nums, nums + N);
        sorted_size = std::distance(nums, sorted_end);
 
        for (auto i : nums) std::cout << i << ' ';
        std::cout << " : " << sorted_size << " initial sorted elements\n";
    } while (sorted_size < min_sorted_size);
}

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