std::ranges::rotate(3)

📦 version 2024.07.05, in nixpkgs unstable, opensuse-leap 16.0

💅 std::ranges::rotate (3) 💅

okay so this is the std::ranges::rotate(3) page from the C++ Standard Libary manual — yes, that’s literally how the header spells it, and I’m not fixing it because character-for-character chaos is the vibe 🫶

✨ NAME ✨

std::ranges::rotate — std::ranges::rotate

bestie the name is the name 💀

🌸 Synopsis 🌸

ok so this lives in header <algorithm>, and here’s the call signature situation:

Defined in header <algorithm>
Call signature
template< std::permutable I, std::sentinel_for<I> S >

constexpr ranges::subrange<I>                         (1) (since C++20)

    rotate( I first, I middle, S last );
template< ranges::forward_range R >

requires std::permutable<ranges::iterator_t<R>>       (2) (since C++20)
constexpr ranges::borrowed_subrange_t<R>

    rotate( R&& r, ranges::iterator_t<R> middle );

So (1) does a left rotation on a range of elements. Specifically, ranges::rotate swaps the elements in the range [first, last) in such a way that the element *middle becomes the first element of the new range and *(middle - 1) becomes the last element. slay 💅

⚠️ Undefined behavior alert, no cap: the behavior is undefined if [first, last) is not a valid range or middle is not in [first, last). like, do not test fate on this one 😭

(2) is the same as (1), but it uses r as the range, as if using ranges::begin(r) as first and ranges::end(r) as last.

🫶 niebloid vibe check 🫶

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. iykyk 🦋

💖 Parameters 💖

  • first, last — the range of elements to rotate.
  • r — the range of elements to rotate.
  • middle — the iterator to the element that should appear at the beginning of the rotated range.

🦋 Return value 🦋

It returns {new_first, last}, where new_first compares equal to ranges::next(first, ranges::distance(middle, last)) and designates a new location of the element pointed by first. cute little subrange moment ✨

🔥 Complexity 🔥

Linear at worst: ranges::distance(first, last) swaps. so like, it’s not doing anything unhinged behind your back 🔥

🎀 Notes 🎀

ranges::rotate has better efficiency on common implementations if I models bidirectional_iterator or (better) random_access_iterator.

Implementations (e.g. MSVC STL) may enable vectorization when the iterator type models contiguous_iterator and swapping its value type calls neither non-trivial special member function nor ADL-found swap. fancy implementation behavior, bestie 🎀

👑 Possible implementation 👑

See also the implementations in libstdc++ and MSVC STL.

struct rotate_fn
{
    template<std::permutable I, std::sentinel_for<I> S>
    constexpr ranges::subrange<I>
        operator()(I first, I middle, S last) const
    {
        if (first == middle)
        {
            auto last_it = ranges::next(first, last);
            return {last_it, last_it};
        }
        if (middle == last)
            return {std::move(first), std::move(middle)};

        if constexpr (std::bidirectional_iterator<I>)
        {
            ranges::reverse(first, middle);
            auto last_it = ranges::next(first, last);
            ranges::reverse(middle, last_it);

            if constexpr (std::random_access_iterator<I>)
            {
                ranges::reverse(first, last_it);
                return {first + (last_it - middle), std::move(last_it)};
            }
            else
            {
                auto mid_last = last_it;
                do
                {
                    ranges::iter_swap(first, --mid_last);
                    ++first;
                }
                while (first != middle && mid_last != middle);
                ranges::reverse(first, mid_last);

                if (first == middle)
                    return {std::move(mid_last), std::move(last_it)};
                else
                    return {std::move(first), std::move(last_it)};
            }
        }
        else
        { // I is merely a forward_iterator
            auto next_it = middle;
            do
            { // rotate the first cycle
                ranges::iter_swap(first, next_it);
                ++first;
                ++next_it;
                if (first == middle)
                    middle = next_it;
            }
            while (next_it != last);

            auto new_first = first;
            while (middle != last)
            { // rotate subsequent cycles
                next_it = middle;
                do
                {
                    ranges::iter_swap(first, next_it);
                    ++first;
                    ++next_it;
                    if (first == middle)
                        middle = next_it;
                }
                while (next_it != last);
            }

            return {std::move(new_first), std::move(middle)};
        }
    }

    template<ranges::forward_range R>
    requires std::permutable<ranges::iterator_t<R>>
    constexpr ranges::borrowed_subrange_t<R>
        operator()(R&& r, ranges::iterator_t<R> middle) const
    {
        return (*this)(ranges::begin(r), std::move(middle), ranges::end(r));
    }
};

inline constexpr rotate_fn rotate {};

🌈 Example 🌈

ranges::rotate is a common building block in many algorithms. This example demonstrates insertion sort.

// Run this code

#include <algorithm>
#include <iostream>
#include <numeric>
#include <string>
#include <vector>

int main()
{
    std::string s(16, ' ');

    for (int k {}; k != 5; ++k)
    {
        std::iota(s.begin(), s.end(), 'A');
        std::ranges::rotate(s, s.begin() + k);
        std::cout << "Rotate left (" << k << "): " << s << '\n';
    }
    std::cout << '\n';

    for (int k {}; k != 5; ++k)
    {
        std::iota(s.begin(), s.end(), 'A');
        std::ranges::rotate(s, s.end() - k);
        std::cout << "Rotate right (" << k << "): " << s << '\n';
    }

    std::cout << "\nInsertion sort using `rotate`, step-by-step:\n";

    s = {'2', '4', '2', '0', '5', '9', '7', '3', '7', '1'};

    for (auto i = s.begin(); i != s.end(); ++i)
    {
        std::cout << "i = " << std::ranges::distance(s.begin(), i) << ": ";
        std::ranges::rotate(std::ranges::upper_bound(s.begin(), i, *i), i, i + 1);
        std::cout << s << '\n';
    }
    std::cout << (std::ranges::is_sorted(s) ? "Sorted!" : "Not sorted.") << '\n';
}

⭐ Output ⭐

Rotate left (0): ABCDEFGHIJKLMNOP
Rotate left (1): BCDEFGHIJKLMNOPA
Rotate left (2): CDEFGHIJKLMNOPAB
Rotate left (3): DEFGHIJKLMNOPABC
Rotate left (4): EFGHIJKLMNOPABCD

Rotate right (0): ABCDEFGHIJKLMNOP
Rotate right (1): PABCDEFGHIJKLMNO
Rotate right (2): OPABCDEFGHIJKLMN
Rotate right (3): NOPABCDEFGHIJKLM
Rotate right (4): MNOPABCDEFGHIJKL

Insertion sort using `rotate`, step-by-step:
i = 0: 2420597371
i = 1: 2420597371
i = 2: 2240597371
i = 3: 0224597371
i = 4: 0224597371
i = 5: 0224597371
i = 6: 0224579371
i = 7: 0223457971
i = 8: 0223457791
i = 9: 0122345779
Sorted!

💜 See also 💜

  • ranges::rotate_copy — copies and rotate a range of elements (C++20) (niebloid).
  • ranges::reverse — reverses the order of elements in a range (C++20) (niebloid).
  • rotate — rotates the order of elements in a range (function template).

source/version: http://cppreference.com, dated 2024.06.10 — page id: std::ranges::rotate(3). byeee bestie, go rotate something cute 🎀🫶

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