Re: Uh, this is *not* a 64-bit CRC ...

On Wed, Feb 28, 2001 at 04:53:09PM -0500, Tom Lane wrote:
> I just took a close look at the COMP_CRC64 macro in xlog.c.
> 
> This isn't a 64-bit CRC.  It's two independent 32-bit CRCs, one done
> on just the odd-numbered bytes and one on just the even-numbered bytes
> of the datastream.  That's hardly any stronger than a single 32-bit CRC;
> it's certainly not what I thought we had agreed to implement.
> 
> We can't change this algorithm without forcing an initdb, which would be
> a rather unpleasant thing to do at this late stage of the release cycle.
> But I'm not happy with it.  Comments?

This might be a good time to update:
 The CRC-64 code used in the SWISS-PROT genetic database is (now) at:
   ftp://ftp.ebi.ac.uk/pub/software/swissprot/Swissknife/old/SPcrc.tar.gz
 From the README:
 The code in this package has been derived from the BTLib package obtained from Christian Iseli
<chris@ludwig-alpha.unil.ch>.From his mail:
 
 The reference is: W. H. Press, S. A. Teukolsky, W. T. Vetterling, and B. P.  Flannery, "Numerical recipes in C", 2nd
ed.,Cambridge University Press.  Pages 896ff.
 
 The generator polynomial is x64 + x4 + x3 + x1 + 1.

I would suggest that if you don't change the algorithm, at least change
the name in the sources.  Were you to #ifdef in a real crc-64, and make 
a compile-time option to select the old one, you could allow users who 
wish to avoid the initdb a way to continue with the existing pair of 
CRC-32s.

Nathan Myers
ncm@zembu.com