std::is_sorted_until(3)

💖 std::is_sorted_until — a C++ Standard Library moment 💖

okay bestie, so this one lives in the C++ Standard Libary manual (yes, that's the manual name, we're rolling with it) and honestly? it's giving "find where the sorted part ends" energy. no cap. let's get into it 🫶

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✨ NAME ✨

std::is_sorted_until — literally std::is_sorted_until, that's the whole name, she's not hiding anything 💅

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🌸 Synopsis 🌸

Defined in header <algorithm>:

template< class ForwardIt >                                  (since C++11)
ForwardIt is_sorted_until( ForwardIt first, ForwardIt    (1)
(constexpr since C++20)
last );
template< class ExecutionPolicy, class ForwardIt >

ForwardIt is_sorted_until( ExecutionPolicy&& policy,     (2) (since C++17)

                               ForwardIt first, ForwardIt
last );
template< class ForwardIt, class Compare >

ForwardIt is_sorted_until( ForwardIt first, ForwardIt    (3) (since C++11)
last,
(constexpr since C++20)

                               Compare comp );
template< class ExecutionPolicy, class ForwardIt, class
Compare >

ForwardIt is_sorted_until( ExecutionPolicy&& policy,     (4) (since C++17)
                               ForwardIt first, ForwardIt
last,

                               Compare comp );

So what does she do? She examines the range [first, last) and finds the largest range beginning at first in which the elements are sorted in non-descending order. iconic behavior honestly ⭐

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.

Now the fine print 🎀 — these overloads participate in overload resolution only if:

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)

like, that's the vibe check the compiler runs on your ExecutionPolicy. periodt 🔥

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💜 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. 🧚

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🦋 Type requirements 🦋

• ForwardIt must meet the requirements of LegacyForwardIterator.
• Compare must meet the requirements of Compare.

no shortcuts bestie, them's the rules 💀

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🌈 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. 🎯

Returns last for empty ranges and ranges of length one. (short ranges get a free pass, we love a low-effort queen 👑)

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⭐ Complexity ⭐

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

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

linear, bestie, she's not doing anything extra 🙈

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💀 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.

⚠️ real talk: that std::terminate is not a vibe. if your functor throws under a standard policy, it's over over. 😭

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🎀 Possible implementation 🎀

See also the implementations in libstdc++ and libc++. but here's the whole mood, written out:

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;
}

she literally just walks forward until something's out of order and points at it. main character energy, minimal effort, maximum results 💅✨

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🍓 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
^^^^^^^

the lil ^ caret moment? that's her pointing at the sorted part like "this you?" 🫵😌

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👀 See also 👀

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

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okay that's the whole page, bestie, you're basically a ForwardIt now 🦋💖

sign-off time: this little masterpiece is from http://cppreference.com, dated 2024.06.10, and it's the std::is_sorted_until(3) page. slay, go sort something 💅🎀