lib/murmurhash2.c

   1 //-----------------------------------------------------------------------------
   2 // MurmurHash2 was written by Austin Appleby, and is placed in the public
   3 // domain. The author hereby disclaims copyright to this source code.
   4
   5 /* Obtained from https://github.com/aappleby/smhasher */
   6
   7 #include <stdint.h>
   8 #include <string.h>
   9
  10 #include "murmurhash2.h"
  11
  12 uint32_t
  13 murmurhash2(const unsigned char * key, int len, uint32_t seed)
  14 {
  15   // 'm' and 'r' are mixing constants generated offline.
  16   // They're not really 'magic', they just happen to work well.
  17
  18   const uint32_t m = 0x5bd1e995;
  19   const int r = 24;
  20
  21   // Initialize the hash to a 'random' value
  22
  23   uint32_t h = seed ^ len;
  24
  25   // Mix 4 bytes at a time into the hash
  26
  27   const unsigned char *data = key;
  28
  29   while(len >= 4)
  30   {
  31     uint32_t k;
  32
  33     memcpy(&k, data, sizeof(k));
  34
  35     k *= m;
  36     k ^= k >> r;
  37     k *= m;
  38
  39     h *= m;
  40     h ^= k;
  41
  42     data += 4;
  43     len -= 4;
  44   }
  45
  46   // Handle the last few bytes of the input array
  47
  48   switch(len)
  49   {
  50   case 3: h ^= data[2] << 16;
  51   case 2: h ^= data[1] << 8;
  52   case 1: h ^= data[0];
  53       h *= m;
  54   };
  55
  56   // Do a few final mixes of the hash to ensure the last few
  57   // bytes are well-incorporated.
  58
  59   h ^= h >> 13;
  60   h *= m;
  61   h ^= h >> 15;
  62
  63   return h;
  64 }