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RocksDB for Erlang

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rocksdb deps rocksdb util hash.cc
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deps/rocksdb/util/hash.cc 

//  Copyright (c) 2011-present, Facebook, Inc.  All rights reserved.

//  This source code is licensed under both the GPLv2 (found in the

//  COPYING file in the root directory) and Apache 2.0 License

//  (found in the LICENSE.Apache file in the root directory).

//

// Copyright (c) 2011 The LevelDB Authors. All rights reserved.

// Use of this source code is governed by a BSD-style license that can be

// found in the LICENSE file. See the AUTHORS file for names of contributors.



#include <string.h>

#include "util/coding.h"

#include "util/hash.h"

#include "util/util.h"



namespace rocksdb {



uint32_t Hash(const char* data, size_t n, uint32_t seed) {

  // Similar to murmur hash

  const uint32_t m = 0xc6a4a793;

  const uint32_t r = 24;

  const char* limit = data + n;

  uint32_t h = static_cast<uint32_t>(seed ^ (n * m));



  // Pick up four bytes at a time

  while (data + 4 <= limit) {

    uint32_t w = DecodeFixed32(data);

    data += 4;

    h += w;

    h *= m;

    h ^= (h >> 16);

  }



  // Pick up remaining bytes

  switch (limit - data) {

    // Note: The original hash implementation used data[i] << shift, which

    // promotes the char to int and then performs the shift. If the char is

    // negative, the shift is undefined behavior in C++. The hash algorithm is

    // part of the format definition, so we cannot change it; to obtain the same

    // behavior in a legal way we just cast to uint32_t, which will do

    // sign-extension. To guarantee compatibility with architectures where chars

    // are unsigned we first cast the char to int8_t.

    case 3:

      h += static_cast<uint32_t>(static_cast<int8_t>(data[2])) << 16;

      FALLTHROUGH_INTENDED;

    case 2:

      h += static_cast<uint32_t>(static_cast<int8_t>(data[1])) << 8;

      FALLTHROUGH_INTENDED;

    case 1:

      h += static_cast<uint32_t>(static_cast<int8_t>(data[0]));

      h *= m;

      h ^= (h >> r);

      break;

  }

  return h;

}



}  // namespace rocksdb