Schrödinger's Stride

[Tomek Sowiński]

Peter Alexander napisał:

> On 10/10/10 7:34 PM, Tomek Sowiński wrote:
>> Currently the contents of Stride depend on from which end we look at it:
>>
>> auto m = [1, 2, 3, 4, 1, 2, 3, 4, 1, 2, 3, 4]; // 3 rows, 4 columns
>> auto col = stride(m, 4);
>> assert(equal(col, [1, 1, 1]));
>> assert(equal(retro(col), [4, 4, 4]));
>>
>> Is the quantum behavior intended?
> 
> Hmm, that does seem counter-intuitive, but I can't see any other way for
> it to work.
> 
> Consider a doubly-linked list with the same data as above. How could
> Stride implement back() without first computing the length of the list?

I don't know but that wouldn't work now anyway -- popBack() and back() is defined if 
isBidirectionalRange!(R) && hasLength!(R), where R is the underlying.

> You could implement it correctly and efficiently for (non-infinite)
> random access ranges, but then you'd have inconsistent behavior between
> random access ranges and non-RA bidirectional ranges.

It's not about random access, but length. Having length, it's trivial to calculate how many 
popBack()s one needs to prime the stride. For ranges without length it doesn't matter 
because the above.

> The solution I believe is to make Stride a forward range for directional
> ranges, and give it the correct semantics for back() when used on a
> random access range i.e. use ((length-1) - (length-1) % stride) to get
> the back element index so that the retro traversal is actually the
> reverse of the forward traversal.
>
> For bidirectional ranges, people can always just use stride(retro(r), n)
> to get a backwards traversing stride.

Actually I see in Stride's ctor an attempt to pop slack items off the input's back:

    this(R input, size_t n)
    {
        _input = input;
        _n = n;
        static if (hasLength!(R))
        {
            auto slack = _input.length % _n;
            if (slack) slack--;
            if (!slack) return;

Since retro'ing isn't unittested, I can't tell if it was to address this issue or not. I think it's 
just a matter of fixing the 'slack' number. Phobos people?

-- 
Tomek