Re: heads-up: git-index-pack in "next" is broken

[Davide Libenzi <davidel@xmailserver.org>]

On Wed, 18 Oct 2006, Linus Torvalds wrote:

> 
> 
> On Wed, 18 Oct 2006, Davide Libenzi wrote:
> > 
> > The hash value (hence the hash bucket index) simply directs you to the 
> > bucket where a real record-compare loop is performed.
> 
> As far as I can tell not all loops do a real "record-compare" thing.
> 
> Some of the hash loops _only_ look at the hash, and as such a bad hash 
> will do more than just cause bad performance, it will actually degrade the 
> diff itself. Isn't that what XDL_MAX_EQLIMIT effectively does?

The XDL_MAX_EQLIMIT is used to limit the search for equal records, in the 
record-discard phase. Note though, that at that point that "ha" value is a 
record-class ID (every different record/line in the input has a unique ID).
Look at what xdl_classify_record() does. So in that case, XDL_HASHLONG can 
really simply be a bitmask. So comparing "ha" in the loop in there, does 
actually the right thing in any case (equal "ha" means really equal 
record).



> Btw, the binary delta generator doesn't seem to have this issue at all: it 
> uses "unsigned int" for the hash values, so the xdiff delta generation 
> will give the same exact results on 32-bit and 64-bit architectures.
> 
> Or was that one of the changes by Nico? (I only looked at the git version 
> of that code)

The binary diff in libxdiff uses a chaining hash, so even in that case it 
wouldn't have made a difference. I think Nico changed the hash to be a 
coalesced hash, and in that case it does change the output.




- Davide