dtstrhash.c

Go to the documentation of this file.

#include        "dthdr.h"

/* Hashing a string into an unsigned integer.
** The basic method is to continuingly 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)
*/

uint dtstrhash(reg uint h, void* args, reg int n)
{
        reg unsigned char*      s = (unsigned char*)args;

        if(n <= 0)
        {       for(; *s != 0; s += s[1] ? 2 : 1)
                        h = (h + (s[0]<<8) + s[1])*DT_PRIME;
                n = s - (unsigned char*)args;
        }
        else
        {       reg unsigned char*      ends;
                for(ends = s+n-1; s < ends; s += 2)
                        h = (h + (s[0]<<8) + s[1])*DT_PRIME;
                if(s <= ends)
                        h = (h + (s[0]<<8))*DT_PRIME;
        }
        return (h+n)*DT_PRIME;
}