Re: Object hash (was: Re: [ANNOUNCE] git-rev-size: calculate sizes of repository)

On Sunday 20 August 2006 18:51, Johannes Schindelin wrote:
> > > +static unsigned int hash_index(struct hash_map *hash, const char *sha1)
> > > +{
> > > +	unsigned int index = *(unsigned int *)sha1;
> > 
> > If you have the same SHA1, stored at different addresses, you get different
> > indexes for the same SHA1. Index probably should be calculated from the
> > SHA1 string.
> 
> Actually, it does! "*(unsigned int *)sha1" means that the first 4 bytes 
> of the sha1 are interpreted as a number.

Ah, yes. That's fine.

> > > +void hash_put(struct hash_map *hash, struct object *obj)
> > > +{
> > > +	if (++hash->nr > hash->alloc / 2)
> > > +		grow_hash(hash);
> > 
> > If you insert the same object multiple times, hash->nr will get too big.
> 
> First, you cannot put the same object multiple times. That is not what a  
> hash does (at least in this case): it stores unique objects (identified by 
> their sha1 in this case).

I put it the wrong way; I should have said "if you call hash_put() multiple
times with the same object". You get the same index, and nothing should
change. However, you still increment hash->nr, but this error is not really
important as you correct it in grow_hash().

So... sorry for the noise ;-)

Josef
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