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/* This file is part of the Spring engine (GPL v2 or later), see LICENSE.html */
#ifndef HSIEH_HASH_H
#define HSIEH_HASH_H
#include <boost/cstdint.hpp> /* Replace with <stdint.h> if appropriate */
#undef get16bits
#define get16bits(d) (*reinterpret_cast<const boost::uint16_t*>(d))
/** @brief a fast hash function
*
* This hash function is roughly 4x as fast as CRC32, but even that is too slow.
* We use a very simplistic add/xor feedback scheme when not debugging.
*/
static inline boost::uint32_t HsiehHash (const void* data_, int len, boost::uint32_t hash)
{
const boost::uint8_t* data = reinterpret_cast<const boost::uint8_t*>(data_);
assert(data != nullptr || len == 0);
int rem = len & 3;
len >>= 2;
/* Main loop */
for (;len > 0; len--) {
hash += get16bits (data);
boost::uint32_t tmp = (get16bits (data+2) << 11) ^ hash;
hash = (hash << 16) ^ tmp;
hash += hash >> 11;
data += 2*sizeof (boost::uint16_t);
}
/* Handle end cases */
switch (rem) {
case 3:
hash += get16bits (data);
hash ^= hash << 16;
hash ^= data[sizeof (boost::uint16_t)] << 18;
hash += hash >> 11;
break;
case 2:
hash += get16bits (data);
hash ^= hash << 11;
hash += hash >> 17;
break;
case 1:
hash += *data;
hash ^= hash << 10;
hash += hash >> 1;
break;
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;
}
#endif // !defined(HSIEH_HASH_H)
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