MurmurHash1.cpp

//-----------------------------------------------------------------------------
// MurmurHash was written by Austin Appleby, and is placed in the public
// domain. The author hereby disclaims copyright to this source code.

// 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.

#include "MurmurHash1.h"

//-----------------------------------------------------------------------------

uint32_t MurmurHash1 ( const void * key, int len, uint32_t seed )
{
  const unsigned int m = 0xc6a4a793;

  const int r = 16;

  unsigned int h = seed ^ (len * m);

  //----------
  
  const unsigned char * data = (const unsigned char *)key;

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

    h += k;
    h *= m;
    h ^= h >> 16;

    data += 4;
    len -= 4;
  }
  
  //----------
  
  switch(len)
  {
  case 3:
    h += data[2] << 16;
  case 2:
    h += data[1] << 8;
  case 1:
    h += data[0];
    h *= m;
    h ^= h >> r;
  };
 
  //----------

  h *= m;
  h ^= h >> 10;
  h *= m;
  h ^= h >> 17;

  return h;
} 

//-----------------------------------------------------------------------------
// MurmurHash1Aligned, by Austin Appleby

// Same algorithm as MurmurHash1, but only does aligned reads - should be safer
// on certain platforms. 

// Performance should be equal to or better than the simple version.

unsigned int MurmurHash1Aligned ( const void * key, int len, unsigned int seed )
{
  const unsigned int m = 0xc6a4a793;
  const int r = 16;

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

  unsigned int h = seed ^ (len * m);

  int align = (uint64_t)data & 3;

  if(align && (len >= 4))
  {
    // Pre-load the temp registers

    unsigned int t = 0, d = 0;

    switch(align)
    {
      case 1: t |= data[2] << 16;
      case 2: t |= data[1] << 8;
      case 3: t |= data[0];
    }

    t <<= (8 * align);

    data += 4-align;
    len -= 4-align;

    int sl = 8 * (4-align);
    int sr = 8 * align;

    // Mix

    while(len >= 4)
    {
      d = *(unsigned int *)data;
      t = (t >> sr) | (d << sl);
      h += t;
      h *= m;
      h ^= h >> r;
      t = d;

      data += 4;
      len -= 4;
    }

    // Handle leftover data in temp registers

    int pack = len < align ? len : align;

    d = 0;

    switch(pack)
    {
    case 3: d |= data[2] << 16;
    case 2: d |= data[1] << 8;
    case 1: d |= data[0];
    case 0: h += (t >> sr) | (d << sl);
        h *= m;
        h ^= h >> r;
    }

    data += pack;
    len -= pack;
  }
  else
  {
    while(len >= 4)
    {
      h += *(unsigned int *)data;
      h *= m;
      h ^= h >> r;

      data += 4;
      len -= 4;
    }
  }

  //----------
  // Handle tail bytes

  switch(len)
  {
  case 3: h += data[2] << 16;
  case 2: h += data[1] << 8;
  case 1: h += data[0];
      h *= m;
      h ^= h >> r;
  };

  h *= m;
  h ^= h >> 10;
  h *= m;
  h ^= h >> 17;

  return h;
}