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    SubjectRe: more on hash functions
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    "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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