git.m6w6.name Git - awesomized/libmemcached/blob - 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 #include <cstring>
  60
  61 uint32_t hashkit_murmur(const char *key, size_t length, void *context)
  62 {
  63   /*
  64     'm' and 'r' are mixing constants generated offline.  They're not
  65     really 'magic', they just happen to work well.
  66   */
  67
  68   const unsigned int m= 0x5bd1e995;
  69   const uint32_t seed= (0xdeadbeef * (uint32_t)length);
  70   const int r= 24;
  71
  72
  73   // Initialize the hash to a 'random' value
  74
  75   uint32_t h= seed ^ (uint32_t)length;
  76
  77   // Mix 4 bytes at a time into the hash
  78
  79   const unsigned char * data= (const unsigned char *)key;
  80   (void)context;
  81
  82   while(length >= 4)
  83   {
  84     unsigned int k;
  85     memcpy(&k, data, sizeof(unsigned int));
  86
  87     k *= m;
  88     k ^= k >> r;
  89     k *= m;
  90
  91     h *= m;
  92     h ^= k;
  93
  94     data += 4;
  95     length -= 4;
  96   }
  97
  98   // Handle the last few bytes of the input array
  99
 100   switch(length)
 101   {
 102   case 3: h ^= ((uint32_t)data[2]) << 16;   /* fall through */
 103   case 2: h ^= ((uint32_t)data[1]) << 8;    /* fall through */
 104   case 1: h ^= data[0];
 105           h *= m;
 106   default: break;
 107   };
 108
 109   /*
 110     Do a few final mixes of the hash to ensure the last few bytes are
 111     well-incorporated.
 112   */
 113
 114   h ^= h >> 13;
 115   h *= m;
 116   h ^= h >> 15;
 117
 118   return h;
 119 }
 120
 121 #else
 122 uint32_t hashkit_murmur(const char *, size_t , void *)
 123 {
 124   return 0;
 125 }
 126 #endif