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(at)ludwig-alpha(dot)unil(dot)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(at)zembu(dot)com