dtstrhash.c

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#include        <assert.h>
#include        <cdt/dthdr.h>
#include        <limits.h>
#include        <string.h>
 
#define DT_PRIME        17109811 /* 2#00000001 00000101 00010011 00110011 */
 
/* Hashing a string into an unsigned integer.
** The basic method is to continuously accumulate bytes and multiply
** with some given prime. The length n of the string is added last.
** The recurrent equation is like this:
**      h[k] = (h[k-1] + bytes)*prime   for 0 <= k < n
**      h[n] = (h[n-1] + n)*prime
** The prime is chosen to have a good distribution of 1-bits so that
** the multiplication will distribute the bits in the accumulator well.
** The below code accumulates 2 bytes at a time for speed.
**
** Written by Kiem-Phong Vo (02/28/03)
*/
 
unsigned dtstrhash(void *args, int n) {
        unsigned h = 0;
        unsigned char *s = args;
 
        if(n <= 0)
        {       for(; *s != 0; s += s[1] ? 2 : 1)
                        h = (h + ((unsigned)s[0] << 8u) + (unsigned)s[1]) * DT_PRIME;
                assert(strlen(args) <= INT_MAX);
                n = (int)(s - (unsigned char*)args);
        }
        else
        {       unsigned char*  ends;
                for(ends = s+n-1; s < ends; s += 2)
                        h = (h + ((unsigned)s[0] << 8u) + (unsigned)s[1]) * DT_PRIME;
                if(s <= ends)
                        h = (h + ((unsigned)s[0] << 8u)) * DT_PRIME;
        }
        assert(n >= 0);
        return (h + (unsigned)n) * DT_PRIME;
}