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

   1 /*  vim:expandtab:shiftwidth=2:tabstop=2:smarttab:
   2  *
   3  *  HashKit library
   4  *
   5  *  Copyright (C) 2011-2012 Data Differential, http://datadifferential.com/
   6  *  Copyright (C) 2006-2009 Brian Aker All rights reserved.
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  37
  38 /*
  39   "Murmur" hash provided by Austin, tanjent@gmail.com
  40   http://murmurhash.googlepages.com/
  41
  42   Note - This code makes a few assumptions about how your machine behaves -
  43
  44   1. We can read a 4-byte value from any address without crashing
  45   2. sizeof(int) == 4
  46
  47   And it has a few limitations -
  48   1. It will not work incrementally.
  49   2. It will not produce the same results on little-endian and big-endian
  50   machines.
  51
  52   Updated to murmur2 hash - BP
  53 */
  54
  55 #include "libhashkit/common.h"
  56
  57 #ifdef HAVE_MURMUR_HASH
  58
  59 #ifdef BYTESWAP_HEADER
  60 # include BYTESWAP_HEADER
  61 #endif
  62
  63 #include <cstring>
  64
  65 uint32_t hashkit_murmur(const char *key, size_t length, void *context)
  66 {
  67   /*
  68     'm' and 'r' are mixing constants generated offline.  They're not
  69     really 'magic', they just happen to work well.
  70   */
  71
  72   const unsigned int m= 0x5bd1e995;
  73   const uint32_t seed= (0xdeadbeef * (uint32_t)length);
  74   const int r= 24;
  75
  76
  77   // Initialize the hash to a 'random' value
  78
  79   uint32_t h= seed ^ (uint32_t)length;
  80
  81   // Mix 4 bytes at a time into the hash
  82
  83   const unsigned char * data= (const unsigned char *)key;
  84   (void)context;
  85
  86   while(length >= 4)
  87   {
  88     uint32_t k;
  89     memcpy(&k, data, sizeof(k));
  90 #if WORDS_BIGENDIAN
  91     k = BYTESWAP_32(k);
  92 #endif
  93
  94     k *= m;
  95     k ^= k >> r;
  96     k *= m;
  97
  98     h *= m;
  99     h ^= k;
 100
 101     data += 4;
 102     length -= 4;
 103   }
 104
 105   // Handle the last few bytes of the input array
 106   if (length) {
 107     uint32_t k = 0;
 108     memcpy(&k, data, length);
 109 #if WORDS_BIGENDIAN
 110     k = BYTESWAP_32(k);
 111 #endif
 112     h ^= k;
 113     h *= m;
 114   }
 115
 116   /*
 117     Do a few final mixes of the hash to ensure the last few bytes are
 118     well-incorporated.
 119   */
 120
 121   h ^= h >> 13;
 122   h *= m;
 123   h ^= h >> 15;
 124
 125   return h;
 126 }
 127
 128 #else
 129 uint32_t hashkit_murmur(const char *, size_t , void *)
 130 {
 131   return 0;
 132 }
 133 #endif