hsieh_hash.cc

nandflash文件系统源代码

//---------------------------------------------------------- -*- Mode: C++ -*-
// $Id: hsieh_hash.h $
//
// Created 2008/07/17
//
// 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
//
// This file is part of Kosmos File System (KFS).
//
// Licensed under the Apache License, Version 2.0
// (the "License"); you may not use this file except in compliance with
// the License. You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
// implied. See the License for the specific language governing
// permissions and limitations under the License.
//
// 
//----------------------------------------------------------------------------

#include "hsieh_hash.h"

#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

using std::string;

using namespace KFS;

uint32_t 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;
            hash ^= data[sizeof (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;
    }

    /* 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;
}

// the wrappers we need
size_t Hsieh_hash_fcn::operator() (const char * data, int len) const
{
    return SuperFastHash(data, len);
}

size_t Hsieh_hash_fcn::operator() (const string &data) const
{
    return SuperFastHash(data.c_str(), data.size());
}