[Nottingham] A Little Coding Gem

[nlug at lists.grepular.com]

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On 15/11/12 18:08, Martin wrote:

> 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
> :-) )

Seems to flip the order of bits in a 4 byte int. Eg:

  00000000 10000000 00000000 00001001

Becomes:

  10010000 00000000 00000001 00000000

Here's the C++ code I wrote to figure it out:

  http://pastebin.com/Bc5ngBgp

Be kind, I've not been programming C/C++ for very long. ;)

I can't think of a use case for this, but I'm sure that they exist.
What was the code being used for?

- -- 
Mike Cardwell  https://grepular.com/     http://cardwellit.com/
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