hsieh.c

/* By Paul Hsieh (C) 2004, 2005.  Covered under the Paul Hsieh
 * derivative license.
 * See: http://www.azillionmonkeys.com/qed/weblicense.html for license
 * details.
 * http://www.azillionmonkeys.com/qed/hash.html
*/

#include "common.h"

#undef get16bits
#if (defined(__GNUC__) && defined(__i386__))
#define get16bits(d) (*((const uint16_t *) (d)))
#endif

#if !defined (get16bits)
#define get16bits(d) ((((uint32_t)(((const uint8_t *)(d))[1])) << 8)\
                      +(uint32_t)(((const uint8_t *)(d))[0]) )
#endif

uint32_t hashkit_hsieh(const char *key, size_t key_length, void *context __attribute__((unused)))
{
  uint32_t hash = 0, tmp;
  int rem;

  if (key_length <= 0 || key == NULL)
    return 0;

  rem = key_length & 3;
  key_length >>= 2;

  /* Main loop */
  for (;key_length > 0; key_length--)
  {
    hash  += get16bits (key);
    tmp    = (get16bits (key+2) << 11) ^ hash;
    hash   = (hash << 16) ^ tmp;
    key  += 2*sizeof (uint16_t);
    hash  += hash >> 11;
  }

  /* Handle end cases */
  switch (rem)
  {
  case 3: hash += get16bits (key);
          hash ^= hash << 16;
          hash ^= (uint32_t)key[sizeof (uint16_t)] << 18;
          hash += hash >> 11;
          break;
  case 2: hash += get16bits (key);
          hash ^= hash << 11;
          hash += hash >> 17;
          break;
  case 1: hash += (unsigned char)(*key);
          hash ^= hash << 10;
          hash += hash >> 1;
  default:
          break;
  }

  /* Force "avalanching" of final 127 bits */
  hash ^= hash << 3;
  hash += hash >> 5;
  hash ^= hash << 4;
  hash += hash >> 17;
  hash ^= hash << 25;
  hash += hash >> 6;

  return hash;
}