base/hash.cc

// From http://www.azillionmonkeys.com/qed/hash.html

#include "base/hash.h"

typedef uint32 uint32_t;
typedef uint16 uint16_t;

namespace base {

#undef get16bits
#if (defined(__GNUC__) && defined(__i386__)) || defined(__WATCOMC__) \
  || defined(_MSC_VER) || defined (__BORLANDC__) || defined (__TURBOC__)
#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 SuperFastHash(const char * data, int len) {
  uint32_t hash = len, tmp;
  int rem;

  if (len <= 0 || data == NULL)
    return 0;

  rem = len & 3;
  len >>= 2;

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

  /* Handle end cases */
  switch (rem) {
    case 3:
      hash += get16bits(data);
      hash ^= hash << 16;

      // Treat the final character as signed. This ensures all platforms behave
      // consistently with the original x86 code.
      hash ^= static_cast<signed char>(data[sizeof(uint16_t)]) << 18;
      hash += hash >> 11;
      break;
    case 2:
      hash += get16bits(data);
      hash ^= hash << 11;
      hash += hash >> 17;
      break;
    case 1:
      hash += static_cast<signed char>(*data);
      hash ^= hash << 10;
      hash += hash >> 1;
  }

  /* 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;
}

}  // namespace base