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    SubjectRe: more on hash functions
    >You really have to make every bit of the result depend on
    >every bit in the key.

    What about:

    unsigned int rand1[256];
    #define HASH1(x) ( ((x)<<8)^rand1[(x)>>24] )
    #define hashfn(key) ( { int hashx=HASH1((key)<<8);\
    hashx=HASH1(hashx);\
    HASH1(hashx);\
    } )


    And to initialize the table:
    >----------------------------------------------
    void genrand(void) {
    unsigned int i,j,k;
    for(i=0;i<256;i++){
    k=i<<16;
    for(j=0;j<8;j++) {
    if (k<0x00800000)
    k<<=1;
    else
    k=((k<<1)&0xFFFFFF)^0x14A445;
    }
    k+=i<<24;
    rand1[i]=k;
    }
    }
    >----------------------------------------------

    The key may be up to 24 bits. The result looks like this:

    movl %edx,%eax
    sall $8,%eax
    sall $16,%edx
    sarl $24,%eax
    xorl rand1(,%eax,4),%edx
    movl %edx,%eax
    sall $8,%edx
    sarl $24,%eax
    xorl rand1(,%eax,4),%edx
    movl %edx,%eax
    sall $8,%edx
    sarl $24,%eax
    xorl rand1(,%eax,4),%edx
    movl %edx,%eax

    Is that still fast enough?

    Peter

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