From: | Martijn van Oosterhout <kleptog(at)svana(dot)org> |
---|---|
To: | Ben <bench(at)silentmedia(dot)com> |
Cc: | Postgresql-General mailing list <pgsql-general(at)postgresql(dot)org> |
Subject: | Re: scoring differences between bitmasks |
Date: | 2005-10-02 12:39:51 |
Message-ID: | 20051002123951.GC30492@svana.org |
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Lists: | pgsql-general |
On Sat, Oct 01, 2005 at 07:12:13PM -0700, Ben wrote:
> I'm looking for a quick way to find the number of bits that are
> different between two bitmasks of the same size. I'll be doing this a
> lot, so speed is desirable.... some kind of indexing would be even
> better. It seems like this problem must have been solved before....
Step 1: Use XOR to get the bits that are different.
Step 2: Count the bits.
Something like x & ((~x) +1) will give you the value of the last
bit that is set, mask it out and repeat. If you need to do it a lot,
build a table of 256 values and then process 8 bits at a time. Should
be fairly straight forward...
Hope this helps,
--
Martijn van Oosterhout <kleptog(at)svana(dot)org> http://svana.org/kleptog/
> Patent. n. Genius is 5% inspiration and 95% perspiration. A patent is a
> tool for doing 5% of the work and then sitting around waiting for someone
> else to do the other 95% so you can sue them.
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