gearman/libhashkit/murmur.cc

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 *  HashKit library
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/*
  "Murmur" hash provided by Austin, tanjent@gmail.com
  http://murmurhash.googlepages.com/

  Note - This code makes a few assumptions about how your machine behaves -

  1. We can read a 4-byte value from any address without crashing
  2. sizeof(int) == 4

  And it has a few limitations -
  1. It will not work incrementally.
  2. It will not produce the same results on little-endian and big-endian
  machines.

  Updated to murmur2 hash - BP
*/

#include <libhashkit/common.h>

#ifdef HAVE_MURMUR_HASH

uint32_t hashkit_murmur(const char *key, size_t length, void *context)
{
  /*
    'm' and 'r' are mixing constants generated offline.  They're not
    really 'magic', they just happen to work well.
  */

  const unsigned int m= 0x5bd1e995;
  const uint32_t seed= (0xdeadbeef * (uint32_t)length);
  const int r= 24;


  // Initialize the hash to a 'random' value

  uint32_t h= seed ^ (uint32_t)length;

  // Mix 4 bytes at a time into the hash

  const unsigned char * data= (const unsigned char *)key;
  (void)context;

  while(length >= 4)
  {
    unsigned int k = *(unsigned int *)data;

    k *= m;
    k ^= k >> r;
    k *= m;

    h *= m;
    h ^= k;

    data += 4;
    length -= 4;
  }

  // Handle the last few bytes of the input array

  switch(length)
  {
  case 3: h ^= ((uint32_t)data[2]) << 16;
  case 2: h ^= ((uint32_t)data[1]) << 8;
  case 1: h ^= data[0];
          h *= m;
  default: break;
  };

  /*
    Do a few final mixes of the hash to ensure the last few bytes are
    well-incorporated.
  */

  h ^= h >> 13;
  h *= m;
  h ^= h >> 15;

  return h;
}

#else
uint32_t hashkit_murmur(const char *, size_t , void *)
{
  return 0;
}
#endif