Re: git behaviour question regarding SHA-1 and commits

From: Jeff King <peff@peff.net>
Date: Nov 14, 2011, 13:04 UTC
Message-ID: <20111114130407.GA24156@sigill.intra.peff.net>
In-Reply-To: <20111114124851.GB21854@victor>

On Mon, Nov 14, 2011 at 01:48:51PM +0100, Victor Engmark wrote:

Show 11 quoted lines

> > Fortunately we have such a thing:
> > 
> >   http://article.gmane.org/gmane.comp.version-control.git/184243
> > 
> > That one actually has 40 bits of hash entropy, so you'd expect to
> > generate 2^20 (about a million) commits before accidentally colliding.
> > If you want an easier experiment, you could truncate it even further.
> 
> Would it be helpful to truncate this to something ludicrous like a
> single byte of entropy, to be able to write tests for the various tools
> and options?

That's probably too small. Obviously any implementation like this is not going to be usable for interacting with existing repositories, but if you have too many collisions, then you won't even be able to create a few new commits for your test.

Something like 20 bits means you can brute-force a collision for a particular blob, commit, tree, or whatever in a few seconds, but you won't be having accidental ones all the time.

-Peff