std.string.reverse() for mutable array of chars

[Jonathan M Davis]

On Friday, December 09, 2011 12:44:37 bearophile wrote:
> Jonathan M Davis:
> > It sounded like you were,
> 
> Right, I was :-) But you have changed my mind when you have explained me
> that nothing in std.algorithm is grapheme-aware. So I have reduced the
> amount of what I am asking for.

So, now you're asking that char and wchar arrays be reversible with reverse 
such that their code points are reversed (i.e. the result is the same as if 
you reversed an array of dchar). Well, I'm not sure that you can actually do 
that with the same efficiency. I'd have to think about it more. Regardless, the 
implementation would be _really_ complicated in comparison to how reverse 
works right now. char[] and wchar[] don't work with reverse, because their 
elements aren't swappable. So, you can't just swap elements as you iterate in 
from both ends. You'd have to be moving stuff off into temporaries as you 
swapped them, because the code point on one side wouldn't necessarily fit where 
the code point on the other side was, and in the worst case (i.e. all of the 
code points on one half of the string are multiple code units and all of those 
on the other side are single code units), you'd pretty much end up having to 
copy half the array while you waited for enough space to open up on one side 
to fit the characters from the other side. So, regardless of whether it has the 
same computational complexity as the current reverse, its memory requirements 
would be far more.

I don't think that the request is completely unreasonable, but also I'm not 
sure it's acceptable for reverse to change its performance characteristics as 
much as would be required for it to work with arrays of char or wchar - 
particularly with regards to how much memory would be required. In general, 
the performance characteristics of the algorithms in Phobos don't vary much 
with regards to the type that that's used. I'm pretty sure that in terms of 
big-o notation, the memory complexity wouldn't match (though I don't recall 
exactly how big-o notation works with memory rather than computational 
complexity).

- Jonathan M Davis