Schrödinger's Stride

[Peter Alexander]

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? 
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.

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.