# [swift-evolution] Proposal: Implement a rotate algorithm, equivalent to std::rotate() in C++

Dave Abrahams dabrahams at apple.com
Mon Dec 14 09:48:48 CST 2015

```> On Dec 14, 2015, at 3:59 AM, Sergey Bolshedvorsky <sergey at bolshedvorsky.com> wrote:
>
>
>
> There are 3 main algorithms: forward iteration, random access iteration and bidirectional iteration. All excerpts from the book Alexander A. Stepanov. “From Mathematics to Generic Programming”, Chapters 11.3 - 11.6
>
>
> 1. The forward iteration can be implemented by using Gries-Mills algorithm. This algorithm returns a new middle: a position where the first element moved.
>
> template <ForwardIterator I>
> I rotate(I f, I m, I l, std::forward_iterator_tag) {
>     if (f == m) return l;
>     if (m == l) return f;
>     pair<I, I> p = swap_ranges(f, m, m, l);
>     while (p.first != m || p.second != l) {
>         if (p.second == l) {
>             rotate_unguarded(p.first, m, l);
>             return p.first;
>         }
>         f = m;
>         m = p.second;
>         p = swap_ranges(f, m, m, l);
>     }
>     return m;
> }
>
>
> 2. The random access iteration can be implement in this way:
>
> template <RandomAccessIterator I>
> I rotate(I f, I m, I l, std::random_access_iterator_tag) {
>     if (f == m) return l;
>     if (m == l) return f;
>     DifferenceType<I> cycles = gcd(m - f, l - m);
>     rotate_transform<I> rotator(f, m, l);
>     while (cycles-- > 0) rotate_cycle_from(f + cycles, rotator);
>     return rotator.m1;
> }
>
>
> 3. The bidirectional iteration can be implement by using reverse algorithm in this way:
>
> template <BidirectionalIterator I>
> I rotate(I f, I m, I l, bidirectional_iterator_tag) {
>      reverse(f, m);
>      reverse(m, l);
>      pair<I, I> p = reverse_until(f, m, l);
>      reverse(p.first, p.second);
>      if (m == p.first) return p.second;
>      return p.first;
> }
>
>
> We need to hide the complexity of these algorithms, therefore we need to write a simple version that works for any type of iterations.
>
> Shall I create a formal PR to swift-evolution with a proposed solution and detailed design?

>
> Sergey
>
>
>> On 14 Dec 2015, at 08:51, Dave Abrahams <dabrahams at apple.com <mailto:dabrahams at apple.com>> wrote:
>>
>>>
>>> On Dec 13, 2015, at 2:20 AM, Sergey Bolshedvorsky via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>>
>>> Hi everyone,
>>>
>>> I’ve selected a ticket SR-125 as my first task (https://bugs.swift.org/browse/SR-125 <https://bugs.swift.org/browse/SR-125>).
>>>
>>> I would like to propose an implementation of this method in Swift stdlib.
>>>
>>> std::rotate() method performs a left rotation on a range of elements.
>>> C++ declaration is void rotate (ForwardIterator first, ForwardIterator middle, ForwardIterator last)
>>> Specifically, it 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.
>>> A precondition of this function is that [first, n_first) and [middle, last) are valid ranges.
>>>
>>
>> This is a really important algorithm, with applications even in GUI programming (see slide <http://www.bfilipek.com/2014/12/top-5-beautiful-c-std-algorithms.html#slide> and gather <http://www.bfilipek.com/2014/12/top-5-beautiful-c-std-algorithms.html#gather>), so I'm really happy someone is taking it on. You'll need different implementations depending on the index's protocol conformance <http://stackoverflow.com/questions/21160875/why-is-stdrotate-so-fast>.  C++ implementations can get pretty sophisticated <http://llvm.org/viewvc/llvm-project/libcxx/trunk/include/algorithm?view=markup&pathrev=251836>.  Would you like additional thoughts (and if so, of what nature), or will those do? ;-)
>>
>>
>> -Dave
>

-Dave

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