git.m6w6.name Git - awesomized/libmemcached/blob - libmemcached/jenkins_hash.c

   1 /*
   2 *
   3 * By Bob Jenkins, 2006.  bob_jenkins@burtleburtle.net.  You may use this
   4 * code any way you wish, private, educational, or commercial.  It's free.
   5 * Use for hash table lookup, or anything where one collision in 2^^32 is
   6 * acceptable.  Do NOT use for cryptographic purposes.
   7 * http://burtleburtle.net/bob/hash/index.html
   8 *
   9 * Modified by Brian Pontz for libmemcached
  10 * TODO:
  11 * Add big endian support
  12 */
  13
  14 #include "common.h"
  15
  16 #define hashsize(n) ((uint32_t)1<<(n))
  17 #define hashmask(n) (hashsize(n)-1)
  18 #define rot(x,k) (((x)<<(k)) | ((x)>>(32-(k))))
  19
  20 #define mix(a,b,c) \
  21 { \
  22   a -= c;  a ^= rot(c, 4);  c += b; \
  23   b -= a;  b ^= rot(a, 6);  a += c; \
  24   c -= b;  c ^= rot(b, 8);  b += a; \
  25   a -= c;  a ^= rot(c,16);  c += b; \
  26   b -= a;  b ^= rot(a,19);  a += c; \
  27   c -= b;  c ^= rot(b, 4);  b += a; \
  28 }
  29
  30 #define final(a,b,c) \
  31 { \
  32   c ^= b; c -= rot(b,14); \
  33   a ^= c; a -= rot(c,11); \
  34   b ^= a; b -= rot(a,25); \
  35   c ^= b; c -= rot(b,16); \
  36   a ^= c; a -= rot(c,4);  \
  37   b ^= a; b -= rot(a,14); \
  38   c ^= b; c -= rot(b,24); \
  39 }
  40
  41 /*
  42 jenkins_hash() -- hash a variable-length key into a 32-bit value
  43   k       : the key (the unaligned variable-length array of bytes)
  44   length  : the length of the key, counting by bytes
  45   initval : can be any 4-byte value
  46 Returns a 32-bit value.  Every bit of the key affects every bit of
  47 the return value.  Two keys differing by one or two bits will have
  48 totally different hash values.
  49
  50 The best hash table sizes are powers of 2.  There is no need to do
  51 mod a prime (mod is sooo slow!).  If you need less than 32 bits,
  52 use a bitmask.  For example, if you need only 10 bits, do
  53   h = (h & hashmask(10));
  54 In which case, the hash table should have hashsize(10) elements.
  55 */
  56
  57 uint32_t jenkins_hash(const void *key, size_t length, uint32_t initval)
  58 {
  59   uint32_t a,b,c;                                          /* internal state */
  60   union { const void *ptr; size_t i; } u;     /* needed for Mac Powerbook G4 */
  61
  62   /* Set up the internal state */
  63   a = b = c = 0xdeadbeef + ((uint32_t)length) + initval;
  64
  65   u.ptr = key;
  66   if ((u.i & 0x3) == 0) {
  67     const uint32_t *k = (const uint32_t *)key;         /* read 32-bit chunks */
  68
  69     /*------ all but last block: aligned reads and affect 32 bits of (a,b,c) */
  70     while (length > 12)
  71     {
  72       a += k[0];
  73       b += k[1];
  74       c += k[2];
  75       mix(a,b,c);
  76       length -= 12;
  77       k += 3;
  78     }
  79
  80     /*----------------------------- handle the last (probably partial) block */
  81     /*
  82      * "k[2]&0xffffff" actually reads beyond the end of the string, but
  83      * then masks off the part it's not allowed to read.  Because the
  84      * string is aligned, the masked-off tail is in the same word as the
  85      * rest of the string.  Every machine with memory protection I've seen
  86      * does it on word boundaries, so is OK with this.  But VALGRIND will
  87      * still catch it and complain.  The masking trick does make the hash
  88      * noticably faster for short strings (like English words).
  89      */
  90     switch(length)
  91     {
  92     case 12: c+=k[2]; b+=k[1]; a+=k[0]; break;
  93     case 11: c+=k[2]&0xffffff; b+=k[1]; a+=k[0]; break;
  94     case 10: c+=k[2]&0xffff; b+=k[1]; a+=k[0]; break;
  95     case 9 : c+=k[2]&0xff; b+=k[1]; a+=k[0]; break;
  96     case 8 : b+=k[1]; a+=k[0]; break;
  97     case 7 : b+=k[1]&0xffffff; a+=k[0]; break;
  98     case 6 : b+=k[1]&0xffff; a+=k[0]; break;
  99     case 5 : b+=k[1]&0xff; a+=k[0]; break;
 100     case 4 : a+=k[0]; break;
 101     case 3 : a+=k[0]&0xffffff; break;
 102     case 2 : a+=k[0]&0xffff; break;
 103     case 1 : a+=k[0]&0xff; break;
 104     case 0 : return c;              /* zero length strings require no mixing */
 105     }
 106
 107   } else if ((u.i & 0x1) == 0) {
 108     const uint16_t *k = (const uint16_t *)key;         /* read 16-bit chunks */
 109     const uint8_t  *k8;
 110
 111     /*--------------- all but last block: aligned reads and different mixing */
 112     while (length > 12)
 113     {
 114       a += k[0] + (((uint32_t)k[1])<<16);
 115       b += k[2] + (((uint32_t)k[3])<<16);
 116       c += k[4] + (((uint32_t)k[5])<<16);
 117       mix(a,b,c);
 118       length -= 12;
 119       k += 6;
 120     }
 121
 122     /*----------------------------- handle the last (probably partial) block */
 123     k8 = (const uint8_t *)k;
 124     switch(length)
 125     {
 126     case 12: c+=k[4]+(((uint32_t)k[5])<<16);
 127              b+=k[2]+(((uint32_t)k[3])<<16);
 128              a+=k[0]+(((uint32_t)k[1])<<16);
 129              break;
 130     case 11: c+=((uint32_t)k8[10])<<16;     /* fall through */
 131     case 10: c+=k[4];
 132              b+=k[2]+(((uint32_t)k[3])<<16);
 133              a+=k[0]+(((uint32_t)k[1])<<16);
 134              break;
 135     case 9 : c+=k8[8];                      /* fall through */
 136     case 8 : b+=k[2]+(((uint32_t)k[3])<<16);
 137              a+=k[0]+(((uint32_t)k[1])<<16);
 138              break;
 139     case 7 : b+=((uint32_t)k8[6])<<16;      /* fall through */
 140     case 6 : b+=k[2];
 141              a+=k[0]+(((uint32_t)k[1])<<16);
 142              break;
 143     case 5 : b+=k8[4];                      /* fall through */
 144     case 4 : a+=k[0]+(((uint32_t)k[1])<<16);
 145              break;
 146     case 3 : a+=((uint32_t)k8[2])<<16;      /* fall through */
 147     case 2 : a+=k[0];
 148              break;
 149     case 1 : a+=k8[0];
 150              break;
 151     case 0 : return c;                     /* zero length requires no mixing */
 152     }
 153
 154   } else {                        /* need to read the key one byte at a time */
 155     const uint8_t *k = (const uint8_t *)key;
 156
 157     /*--------------- all but the last block: affect some 32 bits of (a,b,c) */
 158     while (length > 12)
 159     {
 160       a += k[0];
 161       a += ((uint32_t)k[1])<<8;
 162       a += ((uint32_t)k[2])<<16;
 163       a += ((uint32_t)k[3])<<24;
 164       b += k[4];
 165       b += ((uint32_t)k[5])<<8;
 166       b += ((uint32_t)k[6])<<16;
 167       b += ((uint32_t)k[7])<<24;
 168       c += k[8];
 169       c += ((uint32_t)k[9])<<8;
 170       c += ((uint32_t)k[10])<<16;
 171       c += ((uint32_t)k[11])<<24;
 172       mix(a,b,c);
 173       length -= 12;
 174       k += 12;
 175     }
 176
 177     /*-------------------------------- last block: affect all 32 bits of (c) */
 178     switch(length)                   /* all the case statements fall through */
 179     {
 180     case 12: c+=((uint32_t)k[11])<<24;
 181     case 11: c+=((uint32_t)k[10])<<16;
 182     case 10: c+=((uint32_t)k[9])<<8;
 183     case 9 : c+=k[8];
 184     case 8 : b+=((uint32_t)k[7])<<24;
 185     case 7 : b+=((uint32_t)k[6])<<16;
 186     case 6 : b+=((uint32_t)k[5])<<8;
 187     case 5 : b+=k[4];
 188     case 4 : a+=((uint32_t)k[3])<<24;
 189     case 3 : a+=((uint32_t)k[2])<<16;
 190     case 2 : a+=((uint32_t)k[1])<<8;
 191     case 1 : a+=k[0];
 192              break;
 193     case 0 : return c;
 194     }
 195   }
 196
 197   final(a,b,c);
 198   return c;
 199 }
 200
 201