git.m6w6.name Git - awesomized/libmemcached/blob - libhashkit/murmur.cc

   1 /*
   2   "Murmur" hash provided by Austin, tanjent@gmail.com
   3   http://murmurhash.googlepages.com/
   4
   5   Note - This code makes a few assumptions about how your machine behaves -
   6
   7   1. We can read a 4-byte value from any address without crashing
   8   2. sizeof(int) == 4
   9
  10   And it has a few limitations -
  11   1. It will not work incrementally.
  12   2. It will not produce the same results on little-endian and big-endian
  13   machines.
  14
  15   Updated to murmur2 hash - BP
  16 */
  17
  18 #include <libhashkit/common.h>
  19
  20 #ifdef HAVE_MURMUR_HASH
  21
  22 uint32_t hashkit_murmur(const char *key, size_t length, void *context)
  23 {
  24   /*
  25     'm' and 'r' are mixing constants generated offline.  They're not
  26     really 'magic', they just happen to work well.
  27   */
  28
  29   const unsigned int m= 0x5bd1e995;
  30   const uint32_t seed= (0xdeadbeef * (uint32_t)length);
  31   const int r= 24;
  32
  33
  34   // Initialize the hash to a 'random' value
  35
  36   uint32_t h= seed ^ (uint32_t)length;
  37
  38   // Mix 4 bytes at a time into the hash
  39
  40   const unsigned char * data= (const unsigned char *)key;
  41   (void)context;
  42
  43   while(length >= 4)
  44   {
  45     unsigned int k = *(unsigned int *)data;
  46
  47     k *= m;
  48     k ^= k >> r;
  49     k *= m;
  50
  51     h *= m;
  52     h ^= k;
  53
  54     data += 4;
  55     length -= 4;
  56   }
  57
  58   // Handle the last few bytes of the input array
  59
  60   switch(length)
  61   {
  62   case 3: h ^= ((uint32_t)data[2]) << 16;
  63   case 2: h ^= ((uint32_t)data[1]) << 8;
  64   case 1: h ^= data[0];
  65           h *= m;
  66   default: break;
  67   };
  68
  69   /*
  70     Do a few final mixes of the hash to ensure the last few bytes are
  71     well-incorporated.
  72   */
  73
  74   h ^= h >> 13;
  75   h *= m;
  76   h ^= h >> 15;
  77
  78   return h;
  79 }
  80
  81 #else
  82 uint32_t hashkit_murmur(const char *, size_t , void *)
  83 {
  84   return 0;
  85 }
  86 #endif