Dogcows Code - chaz/yoink/blob - src/Moof/Hash.cc

   1
   2 /*******************************************************************************
   3
   4  Copyright (c) 2009, Charles McGarvey
   5  All rights reserved.
   6
   7  Redistribution   and   use  in  source  and  binary  forms,  with  or  without
   8  modification, are permitted provided that the following conditions are met:
   9
  10    * Redistributions  of  source  code  must retain the above copyright notice,
  11      this list of conditions and the following disclaimer.
  12    * Redistributions  in binary form must reproduce the above copyright notice,
  13      this  list of conditions and the following disclaimer in the documentation
  14      and/or other materials provided with the distribution.
  15
  16  THIS  SOFTWARE  IS  PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
  17  AND  ANY  EXPRESS  OR  IMPLIED  WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  18  IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
  19  DISCLAIMED.  IN  NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
  20  FOR  ANY  DIRECT,  INDIRECT,  INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
  21  DAMAGES  (INCLUDING,  BUT  NOT  LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
  22  SERVICES;  LOSS  OF  USE,  DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
  23  CAUSED  AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
  24  OR  TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
  25  OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  26
  27 *******************************************************************************/
  28
  29 #include "Hash.hh"
  30
  31
  32 namespace Mf {
  33
  34
  35 // MurmurHash2, by Austin Appleby
  36 // http://murmurhash.googlepages.com/
  37
  38 // This function is in the public domain.
  39
  40 // Note - This code makes a few assumptions about how your machine behaves -
  41
  42 // 1. We can read a 4-byte value from any address without crashing
  43 // 2. sizeof(int) == 4
  44
  45 // And it has a few limitations -
  46
  47 // 1. It will not work incrementally.
  48 // 2. It will not produce the same results on little-endian and big-endian
  49 //    machines.
  50
  51 unsigned getHash::operator()(const void* key, int len, unsigned int seed) const
  52 {
  53         // 'm' and 'r' are mixing constants generated offline.
  54         // They're not really 'magic', they just happen to work well.
  55
  56         const unsigned int m = 0x5bd1e995;
  57         const int r = 24;
  58
  59         // Initialize the hash to a 'random' value
  60
  61         unsigned int h = seed ^ len;
  62
  63         // Mix 4 bytes at a time into the hash
  64
  65         const unsigned char* data = (const unsigned char*)key;
  66
  67         while (len >= 4)
  68         {
  69                 unsigned int k = *(unsigned int*)data;
  70
  71                 k *= m;
  72                 k ^= k >> r;
  73                 k *= m;
  74
  75                 h *= m;
  76                 h ^= k;
  77
  78                 data += 4;
  79                 len -= 4;
  80         }
  81
  82         // Handle the last few bytes of the input array
  83
  84         switch (len)
  85         {
  86         case 3: h ^= data[2] << 16;
  87         case 2: h ^= data[1] << 8;
  88         case 1: h ^= data[0];
  89                 h *= m;
  90         };
  91
  92         // Do a few final mixes of the hash to ensure the last few
  93         // bytes are well-incorporated.
  94
  95         h ^= h >> 13;
  96         h *= m;
  97         h ^= h >> 15;
  98
  99         return h;
 100 }
 101
 102
 103 } // namespace Mf
 104
 105 /** vim: set ts=4 sw=4 tw=80: *************************************************/
 106