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

[Sergey Bolshedvorsky]

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 <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.
>> 
>> 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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