[Nottingham] A Little Coding Gem

[Nick Leverton]

On Thu, Nov 15, 2012 at 06:08:24PM +0000, Martin wrote:
> Folks,
> 
> Fortune has just served up a little beauty for me just now:
> 
> 
>    n = ((n >>  1) & 0x55555555) | ((n <<  1) & 0xaaaaaaaa);
>    n = ((n >>  2) & 0x33333333) | ((n <<  2) & 0xcccccccc);
>    n = ((n >>  4) & 0x0f0f0f0f) | ((n <<  4) & 0xf0f0f0f0);
>    n = ((n >>  8) & 0x00ff00ff) | ((n <<  8) & 0xff00ff00);
>    n = ((n >> 16) & 0x0000ffff) | ((n << 16) & 0xffff0000);
> 
> -- Yet another mystical 'C' gem. ...
> 
> 
> Anyone like to guess or even work out what it does? :-> (Evil grin :-) )

Far too lazy for the latter here, that would involve the sort of real
work that I'm avoiding by reading the list :-)  I guessed some sort of
CRC but I see others have given a more accurate answer.

> More to the point, why would you want to do that?!

To save space and/or time and/or registers and/or cache and/or brainpower
(by monkey-copying soemone else's code), delete where not applicable ?

There are loads of techniques to achieve the sort of bit-twiddling
where you (or at least I) sometimes think "how can I do that neatly"
on Bithacks:
http://graphics.stanford.edu/~seander/bithacks.html

But on the rare occasions I need one, at least I comment where it came
from and how it (allegedly) works !

Nick