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rocksdb/util/hash.cc

67 lines
2.5 KiB

// Copyright (c) 2011-present, Facebook, Inc. All rights reserved.
// This source code is licensed under the BSD-style license found in the
// LICENSE file in the root directory of this source tree. An additional grant
// of patent rights can be found in the PATENTS file in the same 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"
namespace rocksdb {
// This function may intentionally do a left shift on a -ve number
#if defined(__clang__)
__attribute__((__no_sanitize__("undefined")))
#elif __GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 9)
__attribute__((__no_sanitize_undefined__))
#endif
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: It would be better if this was cast to unsigned char, but that
// would be a disk format change since we previously didn't have any cast
// at all (so gcc used signed char).
// To understand the difference between shifting unsigned and signed chars,
// let's use 250 as an example. unsigned char will be 250, while signed char
// will be -6. Bit-wise, they are equivalent: 11111010. However, when
// converting negative number (signed char) to int, it will be converted
// into negative int (of equivalent value, which is -6), while converting
// positive number (unsigned char) will be converted to 250. Bitwise,
// this looks like this:
// signed char 11111010 -> int 11111111111111111111111111111010
// unsigned char 11111010 -> int 00000000000000000000000011111010
case 3:
h += static_cast<uint32_t>(static_cast<signed char>(data[2]) << 16);
// fall through
case 2:
h += static_cast<uint32_t>(static_cast<signed char>(data[1]) << 8);
// fall through
case 1:
h += static_cast<uint32_t>(static_cast<signed char>(data[0]));
h *= m;
h ^= (h >> r);
break;
}
return h;
}
} // namespace rocksdb