opIndex, opSlice, length for joiner?

[Maverick Chardet via Digitalmars-d]

On Thursday, 21 January 2016 at 01:22:43 UTC, Andrei Alexandrescu 
wrote:
>
> Thanks for askin. Consider chain(), which currently DOES offer 
> random access if its constituents to. In that case, the number 
> of ranges chained is a compile-time constant dictated by the 
> source code being compiled; no data dependency.
>
> In contrast, the performance of joiner()'s proposed primitives 
> would be dependent on the data. For your simplest example, 
> length would be linear in the number of ranges. One way to 
> improve on that would be to compute length upon the first call 
> and then memoize (and update) it. There is a risk it could go 
> out of sync if someone changes the ranges involved 
> surreptitiously.
>
> One good thing to do here would be to specialize 
> joiner().walkLength. Then people writing r.walkLength would 
> benefit from the quicker specialized version.
>
> For indexed access, things get a fair amount more complicated. 
> You can't afford linear time, so you need some serious data 
> structure to ensure good performance there. That seems to take 
> joiner() in a design space that makes it less attractive; 
> currently it's a simple spec with a somewhat straightforward 
> implementation.
>
>
> Andrei


Thank you for your remarks!

I just finished to implement the specialized version of 
walkLength (this was a great idea), I just have to write a few 
comments and clever unittests before the PR, I may send it later 
today or tomorrow!


Just a question: is it possible for a range to be considered 
infinite and at the same time have a length? I suppose that if it 
is possible, such a range would be ill-formed...

I'm asking because for my specialized version to compile in the 
case where there is no separator, I require 
hasLength!(ElementType!RoR) (otherwise the specialization is 
useless), and I'm wondering wether testing for 
!isInfinite!(ElementType!RoR) for the version with no "upTo" 
parameter is useful or not at all...

Maverick Chardet