std::is_sorted_until

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C++
Algorithm library
Constrained algorithms and algorithms on ranges (C++20)
Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
Execution policies (C++17)
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Batch operations
(C++17)
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(C++11)                (C++11)(C++11)

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(C++11)
(C++11)
(C++11)
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Generation operations
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Order-changing operations
(until C++17)(C++11)
(C++20)(C++20)
Sampling operations
(C++17)

Sorting and related operations
Partitioning operations
Sorting operations
(C++11)
is_sorted_until
(C++11)

Binary search operations
(on partitioned ranges)
Set operations (on sorted ranges)
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(C++11)
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(C++17)
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(C++20)
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(C++11)


C library
Numeric operations
Operations on uninitialized memory
Defined in header <algorithm>
template < class ForwardIt >
ForwardIt is_sorted_until( ForwardIt first, ForwardIt last ) ;
(1) (since C++11)
(constexpr since C++20)
template < class ExecutionPolicy, class ForwardIt >

ForwardIt is_sorted_until( ExecutionPolicy&& policy,

                           ForwardIt first, ForwardIt last ) ;
(2) (since C++17)
template < class ForwardIt, class Compare >

ForwardIt is_sorted_until( ForwardIt first, ForwardIt last,

                           Compare comp ) ;
(3) (since C++11)
(constexpr 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

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 all following conditions are satisfied:

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
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

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 elements are sorted in ascending order. That is, the last iterator it for which range [first, it)

Returns last for empty ranges and ranges of length one.

Complexity

Given N as std::distance(first, last)

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

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
  • 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

(C++11)
 checks whether a range is sorted into ascending order
(function template)
(C++20)
 finds the largest sorted subrange
(algorithm function object)
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