git.m6w6.name Git - m6w6/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 #ifdef BYTEORDER_LITTLE_ENDIAN
  67   if ((u.i & 0x3) == 0)
  68   {
  69     const uint32_t *k = (const uint32_t *)key;         /* read 32-bit chunks */
  70
  71     /*------ all but last block: aligned reads and affect 32 bits of (a,b,c) */
  72     while (length > 12)
  73     {
  74       a += k[0];
  75       b += k[1];
  76       c += k[2];
  77       mix(a,b,c);
  78       length -= 12;
  79       k += 3;
  80     }
  81
  82     /*----------------------------- handle the last (probably partial) block */
  83     /*
  84      * "k[2]&0xffffff" actually reads beyond the end of the string, but
  85      * then masks off the part it's not allowed to read.  Because the
  86      * string is aligned, the masked-off tail is in the same word as the
  87      * rest of the string.  Every machine with memory protection I've seen
  88      * does it on word boundaries, so is OK with this.  But VALGRIND will
  89      * still catch it and complain.  The masking trick does make the hash
  90      * noticably faster for short strings (like English words).
  91      */
  92     switch(length)
  93     {
  94     case 12: c+=k[2]; b+=k[1]; a+=k[0]; break;
  95     case 11: c+=k[2]&0xffffff; b+=k[1]; a+=k[0]; break;
  96     case 10: c+=k[2]&0xffff; b+=k[1]; a+=k[0]; break;
  97     case 9 : c+=k[2]&0xff; b+=k[1]; a+=k[0]; break;
  98     case 8 : b+=k[1]; a+=k[0]; break;
  99     case 7 : b+=k[1]&0xffffff; a+=k[0]; break;
 100     case 6 : b+=k[1]&0xffff; a+=k[0]; break;
 101     case 5 : b+=k[1]&0xff; a+=k[0]; break;
 102     case 4 : a+=k[0]; break;
 103     case 3 : a+=k[0]&0xffffff; break;
 104     case 2 : a+=k[0]&0xffff; break;
 105     case 1 : a+=k[0]&0xff; break;
 106     case 0 : return c;              /* zero length strings require no mixing */
 107     }
 108
 109   }
 110   else if ((u.i & 0x1) == 0)
 111   {
 112     const uint16_t *k = (const uint16_t *)key;         /* read 16-bit chunks */
 113     const uint8_t  *k8;
 114
 115     /*--------------- all but last block: aligned reads and different mixing */
 116     while (length > 12)
 117     {
 118       a += k[0] + (((uint32_t)k[1])<<16);
 119       b += k[2] + (((uint32_t)k[3])<<16);
 120       c += k[4] + (((uint32_t)k[5])<<16);
 121       mix(a,b,c);
 122       length -= 12;
 123       k += 6;
 124     }
 125
 126     /*----------------------------- handle the last (probably partial) block */
 127     k8 = (const uint8_t *)k;
 128     switch(length)
 129     {
 130     case 12: c+=k[4]+(((uint32_t)k[5])<<16);
 131              b+=k[2]+(((uint32_t)k[3])<<16);
 132              a+=k[0]+(((uint32_t)k[1])<<16);
 133              break;
 134     case 11: c+=((uint32_t)k8[10])<<16;     /* fall through */
 135     case 10: c+=k[4];
 136              b+=k[2]+(((uint32_t)k[3])<<16);
 137              a+=k[0]+(((uint32_t)k[1])<<16);
 138              break;
 139     case 9 : c+=k8[8];                      /* fall through */
 140     case 8 : b+=k[2]+(((uint32_t)k[3])<<16);
 141              a+=k[0]+(((uint32_t)k[1])<<16);
 142              break;
 143     case 7 : b+=((uint32_t)k8[6])<<16;      /* fall through */
 144     case 6 : b+=k[2];
 145              a+=k[0]+(((uint32_t)k[1])<<16);
 146              break;
 147     case 5 : b+=k8[4];                      /* fall through */
 148     case 4 : a+=k[0]+(((uint32_t)k[1])<<16);
 149              break;
 150     case 3 : a+=((uint32_t)k8[2])<<16;      /* fall through */
 151     case 2 : a+=k[0];
 152              break;
 153     case 1 : a+=k8[0];
 154              break;
 155     case 0 : return c;                     /* zero length requires no mixing */
 156     }
 157
 158   }
 159   else
 160   {                        /* need to read the key one byte at a time */
 161 #endif /* little endian */
 162     const uint8_t *k = (const uint8_t *)key;
 163
 164     /*--------------- all but the last block: affect some 32 bits of (a,b,c) */
 165     while (length > 12)
 166     {
 167       a += k[0];
 168       a += ((uint32_t)k[1])<<8;
 169       a += ((uint32_t)k[2])<<16;
 170       a += ((uint32_t)k[3])<<24;
 171       b += k[4];
 172       b += ((uint32_t)k[5])<<8;
 173       b += ((uint32_t)k[6])<<16;
 174       b += ((uint32_t)k[7])<<24;
 175       c += k[8];
 176       c += ((uint32_t)k[9])<<8;
 177       c += ((uint32_t)k[10])<<16;
 178       c += ((uint32_t)k[11])<<24;
 179       mix(a,b,c);
 180       length -= 12;
 181       k += 12;
 182     }
 183
 184     /*-------------------------------- last block: affect all 32 bits of (c) */
 185     switch(length)                   /* all the case statements fall through */
 186     {
 187     case 12: c+=((uint32_t)k[11])<<24;
 188     case 11: c+=((uint32_t)k[10])<<16;
 189     case 10: c+=((uint32_t)k[9])<<8;
 190     case 9 : c+=k[8];
 191     case 8 : b+=((uint32_t)k[7])<<24;
 192     case 7 : b+=((uint32_t)k[6])<<16;
 193     case 6 : b+=((uint32_t)k[5])<<8;
 194     case 5 : b+=k[4];
 195     case 4 : a+=((uint32_t)k[3])<<24;
 196     case 3 : a+=((uint32_t)k[2])<<16;
 197     case 2 : a+=((uint32_t)k[1])<<8;
 198     case 1 : a+=k[0];
 199              break;
 200     case 0 : return c;
 201     }
 202 #ifdef BYTEORDER_LITTLE_ENDIAN
 203   }
 204 #endif
 205
 206   final(a,b,c);
 207   return c;
 208 }
 209
 210