Re: more on hash functions

Horst von Brand (vonbrand@inf.utfsm.cl)
Thu, 15 Apr 1999 08:44:01 -0400

"Paul F. Dietz" <dietz@interaccess.com> said:
> Chuck Lever wrote:
>
> > #define FIRSTBYTE(x) (0xff & (x))
> > #define SECNDBYTE(x) (0xff & ((x)>>8))
> > #define THIRDBYTE(x) (0xff & ((x)>>16))
> >
> > #define hashfn(key) ( (rand1[FIRSTBYTE(key)] << 4) ^ \
> > (rand2[SECNDBYTE(key)] << 2) ^ \
> > (rand1[THIRDBYTE(key)]) )
> >
> > to generate a 12-bit hash table index. this way i don't have to mask off
> > extra bits as the final step, and i can use daintily sized random tables.

> This is also clearly bad. Suppose you're hashing keys that
> have a common third byte? The low two bits of your hash
> values will be constant.

Take a longer random table (12, i.e. 16 bits in this case), and shift _down_.

-- 
Dr. Horst H. von Brand                       mailto:vonbrand@inf.utfsm.cl
Departamento de Informatica                     Fono: +56 32 654431
Universidad Tecnica Federico Santa Maria              +56 32 654239
Casilla 110-V, Valparaiso, Chile                Fax:  +56 32 797513


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