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)