std::ranges...orted_until(3)

💖 NAME 💖

std::ranges::is_sorted_until — std::ranges::is_sorted_until (yes bestie, the page literally lists it twice, and we respect the icon behavior).

This is the std::ranges...orted_until(3) page from the C++ Standard Libary manual. Okay so let's get into it 💅

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✨ Synopsis ✨

It's defined in header <algorithm>, and the call signature is giving very much "template bestie" energy:

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
    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<
              std::projected<ranges::iterator_t<R>, Proj>> Comp = ranges::less >
constexpr ranges::borrowed_iterator_t<R>
    is_sorted_until( R&& r, Comp comp = {}, Proj proj = {} );

Both (1) and (2) are since C++20, no cap.

Examines the range [first, last) and finds the largest range beginning at first in which the elements are sorted in non-descending order. It's giving "where does the sorted part end?" energy 🦋

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. That's the whole vibe check. Periodt.

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 cannot be specified when calling any of them.
• None of them are visible to argument-dependent lookup.
• When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.

In practice, they may be implemented as function objects, or with special compiler extensions. Lowkey iconic compiler behavior 😌

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🎀 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.

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🌸 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. Slay.

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🦋 Complexity 🦋

Linear in the distance between first and last. Not doing anything extra, just a cute little scan 🫶

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🧚 Possible implementation 🧚

Here's the possible implementation, bestie — code kept exactly as it should be:

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;

        for (auto next = first; ++next != last; first = next)
            if (std::invoke(comp, std::invoke(proj, *next), std::invoke(proj, *first)))
                return 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;

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💜 Notes 💜

ranges::is_sorted_until returns an iterator equal to last for empty ranges and ranges of length one. Tiny ranges stay unbothered 💀✨

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🍓 Example 🍓

Run this code — it's right here, bestie:

#include <array>
#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)

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🌈 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).

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Source/version and date: http://cppreference.com · 2024.06.10 — bye bestie, go find those sorted upper bounds 💅✨