lib/lookup.c

   1 /*
   2  * Hash lookup function.
   3  * Copyright (C) 2003, 2004 Mondru AB.
   4  *
   5  * The contents of this file may be used under the terms of the GNU
   6  * General Public License Version 2, provided that the above copyright
   7  * notice and this permission notice is included in all copies or
   8  * substantial portions of the software.
   9  *
  10  */
  11
  12 /**
  13  * lookup()
  14  * Generates a 32 bit hash.
  15  * Based on public domain code by Bob Jenkins
  16  * It should be one of the best hash functions around in terms of both
  17  * statistical properties and speed. It is NOT recommended for cryptographic
  18  * purposes.
  19  **/
  20 unsigned long int lookup(k, length, level)
  21 register unsigned char *k;      /* the key */
  22 register unsigned long int length;      /* the length of the key */
  23 register unsigned long int level;       /* the previous hash, or an arbitrary value */
  24 {
  25
  26 #define mix(a,b,c) \
  27 { \
  28   a -= b; a -= c; a ^= (c>>13); \
  29   b -= c; b -= a; b ^= (a<<8); \
  30   c -= a; c -= b; c ^= (b>>13); \
  31   a -= b; a -= c; a ^= (c>>12);  \
  32   b -= c; b -= a; b ^= (a<<16); \
  33   c -= a; c -= b; c ^= (b>>5); \
  34   a -= b; a -= c; a ^= (c>>3);  \
  35   b -= c; b -= a; b ^= (a<<10); \
  36   c -= a; c -= b; c ^= (b>>15); \
  37 }
  38
  39         typedef unsigned long int ub4;  /* unsigned 4-byte quantities */
  40         register unsigned long int a, b, c, len;
  41
  42         /* Set up the internal state */
  43         len = length;
  44         a = b = 0x9e3779b9;     /* the golden ratio; an arbitrary value */
  45         c = level;              /* the previous hash value */
  46
  47   /*---------------------------------------- handle most of the key */
  48         while (len >= 12) {
  49                 a += (k[0] + ((ub4) k[1] << 8) + ((ub4) k[2] << 16) +
  50                       ((ub4) k[3] << 24));
  51                 b += (k[4] + ((ub4) k[5] << 8) + ((ub4) k[6] << 16) +
  52                       ((ub4) k[7] << 24));
  53                 c += (k[8] + ((ub4) k[9] << 8) + ((ub4) k[10] << 16) +
  54                       ((ub4) k[11] << 24));
  55                 mix(a, b, c);
  56                 k += 12;
  57                 len -= 12;
  58         }
  59
  60   /*------------------------------------- handle the last 11 bytes */
  61         c += length;
  62         switch (len) {          /* all the case statements fall through */
  63         case 11:
  64                 c += ((ub4) k[10] << 24);
  65         case 10:
  66                 c += ((ub4) k[9] << 16);
  67         case 9:
  68                 c += ((ub4) k[8] << 8);
  69                 /* the first byte of c is reserved for the length */
  70         case 8:
  71                 b += ((ub4) k[7] << 24);
  72         case 7:
  73                 b += ((ub4) k[6] << 16);
  74         case 6:
  75                 b += ((ub4) k[5] << 8);
  76         case 5:
  77                 b += k[4];
  78         case 4:
  79                 a += ((ub4) k[3] << 24);
  80         case 3:
  81                 a += ((ub4) k[2] << 16);
  82         case 2:
  83                 a += ((ub4) k[1] << 8);
  84         case 1:
  85                 a += k[0];
  86                 /* case 0: nothing left to add */
  87         }
  88         mix(a, b, c);
  89   /*-------------------------------------------- report the result */
  90         return c;
  91 }