src/basic/MurmurHash2.c

   1 /* SPDX-License-Identifier: LicenseRef-murmurhash2-public-domain */
   2 //-----------------------------------------------------------------------------
   3 // MurmurHash2 was written by Austin Appleby, and is placed in the public
   4 // domain. The author hereby disclaims copyright to this source code.
   5
   6 // Note - This code makes a few assumptions about how your machine behaves -
   7
   8 // 1. We can read a 4-byte value from any address without crashing
   9 // 2. sizeof(int) == 4
  10
  11 // And it has a few limitations -
  12
  13 // 1. It will not work incrementally.
  14 // 2. It will not produce the same results on little-endian and big-endian
  15 //    machines.
  16
  17 #include "MurmurHash2.h"
  18
  19 #if __GNUC__ >= 7
  20 _Pragma("GCC diagnostic ignored \"-Wimplicit-fallthrough\"")
  21 #endif
  22
  23 //-----------------------------------------------------------------------------
  24 // Platform-specific functions and macros
  25
  26 // Microsoft Visual Studio
  27
  28 #if defined(_MSC_VER)
  29
  30 #define BIG_CONSTANT(x) (x)
  31
  32 // Other compilers
  33
  34 #else        // defined(_MSC_VER)
  35
  36 #define BIG_CONSTANT(x) (x##LLU)
  37
  38 #endif // !defined(_MSC_VER)
  39
  40 //-----------------------------------------------------------------------------
  41
  42 uint32_t MurmurHash2 ( const void * key, int len, uint32_t seed )
  43 {
  44   // 'm' and 'r' are mixing constants generated offline.
  45   // They're not really 'magic', they just happen to work well.
  46
  47   const uint32_t m = 0x5bd1e995;
  48   const int r = 24;
  49
  50   // Initialize the hash to a 'random' value
  51
  52   uint32_t h = seed ^ len;
  53
  54   // Mix 4 bytes at a time into the hash
  55
  56   const unsigned char * data = (const unsigned char *)key;
  57
  58   while (len >= 4)
  59   {
  60     uint32_t k = *(uint32_t*)data;
  61
  62     k *= m;
  63     k ^= k >> r;
  64     k *= m;
  65
  66     h *= m;
  67     h ^= k;
  68
  69     data += 4;
  70     len -= 4;
  71   }
  72
  73   // Handle the last few bytes of the input array
  74
  75   switch(len)
  76   {
  77   case 3: h ^= data[2] << 16; /* fall through */
  78   case 2: h ^= data[1] << 8;  /* fall through */
  79   case 1: h ^= data[0];       /* fall through */
  80       h *= m;
  81   };
  82
  83   // Do a few final mixes of the hash to ensure the last few
  84   // bytes are well-incorporated.
  85
  86   h ^= h >> 13;
  87   h *= m;
  88   h ^= h >> 15;
  89
  90   return h;
  91 }