shift_right() 如何在 C++20 中实现？

Question

在 C++20 中，<algorithm>头文件获得了两种新算法：shift_left()和shift_right(). 它们都接受任何 LegacyForwardIterator。对于shift_left()，指定“以i从?0”开始的递增顺序执行移动；对于shift_right()，指定“如果ForwardIt满足 LegacyBidirectionalIterator 要求，则按照从i开始的递减顺序执行移动last - first - n - 1”。

我可以想到一个相当简单的实现方法shift_left()：

template <typename ForwardIt>
constexpr inline ForwardIt shift_left(ForwardIt first, ForwardIt last, typename std::iterator_traits<ForwardIt>::difference_type n) {
    if (n <= 0) return last;
    ForwardIt it = first;
    for (; n > 0; --n, ++it) {
        if (it == last) return first;
    }
    return std::move(it, last, first);
}

如果ForwardIt满足 LegacyBidirectionalIterator 要求，我可以看到shift_right()可以以与shift_left(). 然而，shift_right()对于非双向前向迭代器如何实现这一点根本不清楚。

我已经找到了一种算法，它使用 at 的[first, first+n)空间作为交换元素的临时空间，但它似乎比shift_left()上面的算法更浪费：

template <typename ForwardIt>
constexpr inline ForwardIt shift_right(ForwardIt first, ForwardIt last, typename std::iterator_traits<ForwardIt>::difference_type n) {
    if (n <= 0) return first;
    ForwardIt it = first;
    for (; n > 0; --n, ++it) {
        if (it == last) return last;
    }
    ForwardIt ret = it;
    ForwardIt ret_it = first;
    for (; it != last; ++it) {
        std::iter_swap(ret_it, it);
        ret_it++;
        if (ret_it == ret) ret_it = first;
    }
    return ret;
}

会有更好的或“预期的”实施方式shift_right()吗？

Answer 1

这是班次的示例实现：https : //github.com/danra/shift_proposal/blob/master/shift_proposal.h

链接来自提案文件：http : //www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0769r0.pdf

#include <algorithm>
#include <iterator>
#include <type_traits>
#include <utility>

template<class I>
using difference_type_t = typename std::iterator_traits<I>::difference_type;

template<class I>
using iterator_category_t = typename std::iterator_traits<I>::iterator_category;

template<class I, class Tag, class = void>
constexpr bool is_category = false;
template<class I, class Tag>
constexpr bool is_category<I, Tag, std::enable_if_t<
    std::is_convertible_v<iterator_category_t<I>, Tag>>> = true;

/// Increment (decrement for negative n) i |n| times or until i == bound,
/// whichever comes first. Returns n - the difference between i's final position
/// and its initial position. (Note: "advance" has overloads with this behavior
/// in the Ranges TS.)
template<class I>
constexpr difference_type_t<I> bounded_advance(
    I& i, difference_type_t<I> n, I const bound)
{
    if constexpr (is_category<I, std::bidirectional_iterator_tag>) {
        for (; n < 0 && i != bound; ++n, void(--i)) {
            ;
        }
    }

    for(; n > 0 && i != bound; --n, void(++i)) {
        ;
    }

    return n;
}

template<class ForwardIt>
ForwardIt shift_left(ForwardIt first, ForwardIt last, difference_type_t<ForwardIt> n)
{
    if (n <= 0) {
        return last;
    }

    auto mid = first;
    if (::bounded_advance(mid, n, last)) {
        return first;
    }

    return std::move(std::move(mid), std::move(last), std::move(first));
}

template<class ForwardIt>
ForwardIt shift_right(ForwardIt first, ForwardIt last, difference_type_t<ForwardIt> n)
{
    if (n <= 0) {
        return first;
    }

    if constexpr (is_category<ForwardIt, std::bidirectional_iterator_tag>) {
        auto mid = last;
        if (::bounded_advance(mid, -n, first)) {
            return last;
        }
        return std::move_backward(std::move(first), std::move(mid), std::move(last));
    } else {
        auto result = first;
        if (::bounded_advance(result, n, last)) {
            return last;
        }

        // Invariant: next(first, n) == result
        // Invariant: next(trail, n) == lead

        auto lead = result;
        auto trail = first;

        for (; trail != result; ++lead, void(++trail)) {
            if (lead == last) {
                // The range looks like:
                //
                //   |-- (n - k) elements --|-- k elements --|-- (n - k) elements --|
                //   ^-first          trail-^                ^-result          last-^
                //
                // Note that distance(first, trail) == distance(result, last)
                std::move(std::move(first), std::move(trail), std::move(result));
                return result;
            }
        }

        for (;;) {
            for (auto mid = first; mid != result; ++lead, void(++trail), ++mid) {
                if (lead == last) {
                    // The range looks like:
                    //
                    //   |-- (n - k) elements --|-- k elements --|-- ... --|-- n elements --|
                    //   ^-first            mid-^         result-^         ^-trail     last-^
                    //
                    trail = std::move(mid, result, std::move(trail));
                    std::move(std::move(first), std::move(mid), std::move(trail));
                    return result;
                }
                std::iter_swap(mid, trail);
            }
        }
    }
}