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

[Marc Knaup]

This is a language restriction/limitation which libraries can't fix.

On Mon, Dec 14, 2015 at 2:50 PM, Tal Atlas via swift-evolution <
swift-evolution at swift.org> wrote:

> This seems like the sort of think that a third-party library could supply.
> Have you considered making a new package for it yourself?
> On Mon, Dec 14, 2015 at 7:00 AM Sergey Bolshedvorsky via swift-evolution <
> swift-evolution at swift.org> 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> wrote:
>>
>>
>> On Dec 13, 2015, at 2:20 AM, Sergey Bolshedvorsky via swift-evolution <
>> 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).
>>
>> 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.
>>
>> What are your thoughts?
>>
>>
>> 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
>>
>>
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