mirror of https://github.com/facebook/rocksdb.git
84 lines
2.9 KiB
C++
84 lines
2.9 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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#include "util/util.h"
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#include "util/xxhash.h"
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namespace rocksdb {
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uint32_t Hash(const char* data, size_t n, uint32_t seed) {
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// MurmurHash1 - fast but mediocre quality
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// https://github.com/aappleby/smhasher/wiki/MurmurHash1
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//
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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: The original hash implementation used data[i] << shift, which
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// promotes the char to int and then performs the shift. If the char is
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// negative, the shift is undefined behavior in C++. The hash algorithm is
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// part of the format definition, so we cannot change it; to obtain the same
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// behavior in a legal way we just cast to uint32_t, which will do
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// sign-extension. To guarantee compatibility with architectures where chars
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// are unsigned we first cast the char to int8_t.
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case 3:
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h += static_cast<uint32_t>(static_cast<int8_t>(data[2])) << 16;
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FALLTHROUGH_INTENDED;
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case 2:
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h += static_cast<uint32_t>(static_cast<int8_t>(data[1])) << 8;
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FALLTHROUGH_INTENDED;
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case 1:
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h += static_cast<uint32_t>(static_cast<int8_t>(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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// We are standardizing on a preview release of XXH3, because that's
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// the best available at time of standardizing.
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//
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// In testing (mostly Intel Skylake), this hash function is much more
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// thorough than Hash32 and is almost universally faster. Hash() only
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// seems faster when passing runtime-sized keys of the same small size
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// (less than about 24 bytes) thousands of times in a row; this seems
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// to allow the branch predictor to work some magic. XXH3's speed is
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// much less dependent on branch prediction.
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//
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// Hashing with a prefix extractor is potentially a common case of
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// hashing objects of small, predictable size. We could consider
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// bundling hash functions specialized for particular lengths with
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// the prefix extractors.
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uint64_t Hash64(const char* data, size_t n, uint64_t seed) {
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return XXH3p_64bits_withSeed(data, n, seed);
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}
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uint64_t Hash64(const char* data, size_t n) {
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// Same as seed = 0
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return XXH3p_64bits(data, n);
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}
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} // namespace rocksdb
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