std::ranges::rotate() algorithm

od C++20

Simplified

// (1)
constexpr ranges::subrange<I>
    rotate( I first, I middle, S last );

// (2)
constexpr ranges::borrowed_subrange_t<R>
    rotate( R&& r, ranges::iterator_t<R> middle );

The type of arguments are generic and have the following constraints:

• I - std::permutable
• S - std::sentinel_for<I>
• R - std::ranges::forward_range

Additionally, each overload has the following constraints:

• (2) - std::permutable<ranges::iterator_t<R>>

Detailed

// (1)
template< std::permutable I, std::sentinel_for<I> S >
constexpr ranges::subrange<I>
    rotate( I first, I middle, S last );

// (2)
template< ranges::forward_range R >
requires std::permutable<ranges::iterator_t<R>>
constexpr ranges::borrowed_subrange_t<R>
    rotate( R&& r, ranges::iterator_t<R> middle );

Reverses the order of elements.

• (1) Performs 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.

undefined Behaviour

The behavior is undefined

if [first; last) is not a valid range or middle is not in [first; last).

• ** (2)** Same as (1), but uses r as the 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.

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

{
  new_first,
  last
}

Where new_first is ranges::next(first, ranges::distance(middle, last)) and designates a new location of the element pointed by first.

Complexity

Linear at worst: ranges::distance(first, last) swaps.

Exceptions

(none)

Possible implementation

rotate(1) and rotate(2)

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 {};

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.

Examples

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

Main.cpp

#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!