68 lines
2.5 KiB
C++
68 lines
2.5 KiB
C++
// Copyright (c) 2011-present, Facebook, Inc. All rights reserved.
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// This source code is licensed under both the GPLv2 (found in the
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// COPYING file in the root directory) and Apache 2.0 License
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// (found in the LICENSE.Apache file in the root directory).
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//
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// Copyright (c) 2011 The LevelDB Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style license that can be
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// found in the LICENSE file. See the AUTHORS file for names of contributors.
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#include <string.h>
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#include "util/coding.h"
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#include "util/hash.h"
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namespace rocksdb {
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// This function may intentionally do a left shift on a -ve number
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#if __clang_major__ > 3 || (__clang_major__ == 3 && __clang_minor__ >= 9)
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__attribute__((__no_sanitize__("undefined")))
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#elif __GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 9)
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__attribute__((__no_sanitize_undefined__))
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#endif
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uint32_t Hash(const char* data, size_t n, uint32_t seed) {
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// Similar to murmur hash
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const uint32_t m = 0xc6a4a793;
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const uint32_t r = 24;
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const char* limit = data + n;
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uint32_t h = static_cast<uint32_t>(seed ^ (n * m));
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// Pick up four bytes at a time
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while (data + 4 <= limit) {
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uint32_t w = DecodeFixed32(data);
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data += 4;
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h += w;
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h *= m;
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h ^= (h >> 16);
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}
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// Pick up remaining bytes
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switch (limit - data) {
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// Note: It would be better if this was cast to unsigned char, but that
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// would be a disk format change since we previously didn't have any cast
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// at all (so gcc used signed char).
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// To understand the difference between shifting unsigned and signed chars,
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// let's use 250 as an example. unsigned char will be 250, while signed char
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// will be -6. Bit-wise, they are equivalent: 11111010. However, when
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// converting negative number (signed char) to int, it will be converted
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// into negative int (of equivalent value, which is -6), while converting
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// positive number (unsigned char) will be converted to 250. Bitwise,
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// this looks like this:
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// signed char 11111010 -> int 11111111111111111111111111111010
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// unsigned char 11111010 -> int 00000000000000000000000011111010
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case 3:
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h += static_cast<uint32_t>(static_cast<signed char>(data[2]) << 16);
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// fall through
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case 2:
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h += static_cast<uint32_t>(static_cast<signed char>(data[1]) << 8);
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// fall through
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case 1:
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h += static_cast<uint32_t>(static_cast<signed char>(data[0]));
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h *= m;
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h ^= (h >> r);
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break;
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}
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return h;
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}
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} // namespace rocksdb
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