Re: [PATCH 11/14] object-name: simplify computing common prefixes

[Patrick Steinhardt <ps@pks.im>]

On Fri, Mar 20, 2026 at 03:01:48AM -0700, Karthik Nayak wrote:
> Patrick Steinhardt <ps@pks.im> writes:
> > diff --git a/hash.c b/hash.c
> > index 553f2008ea..e925b9754e 100644
> > --- a/hash.c
> > +++ b/hash.c
> > @@ -317,3 +317,21 @@ const struct git_hash_algo *unsafe_hash_algo(const struct git_hash_algo *algop)
> >  	/* Otherwise use the default one. */
> >  	return algop;
> >  }
> > +
> > +unsigned oid_common_prefix_hexlen(const struct object_id *a,
> > +				  const struct object_id *b)
> > +{
> > +	unsigned rawsz = hash_algos[a->algo].rawsz;
> > +
> > +	for (unsigned i = 0; i < rawsz; i++) {
> > +		if (a->hash[i] == b->hash[i])
> > +			continue;
> > +
> 
> Instead of transforming the bytes into 2 hex components we now compare
> the bytes themselves and perhaps then compare parts of it?

Yes, exactly. It should be more performant overall compared to first
converting to their respective hex presentations, even though I doubt it
really matters in practice.

> > +		if ((a->hash[i] ^ b->hash[i]) & 0xf0)
> 
> Okay so if the 4 MSB are the same then we end up here and return i * 2.
> Makes sense.
> 
> > +			return i * 2;
> > +		else
> > +			return i * 2 + 1;
> 
> If not, its the 4 LSB.

Yup.

Thanks!

Patrick