mirror of https://github.com/facebook/rocksdb.git
121 lines
4.5 KiB
C++
121 lines
4.5 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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//
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// Common hash functions with convenient interfaces. If hashing a
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// statically-sized input in a performance-critical context, consider
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// calling a specific hash implementation directly, such as
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// XXH3p_64bits from xxhash.h.
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//
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// Since this is a very common header, implementation details are kept
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// out-of-line. Out-of-lining also aids in tracking the time spent in
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// hashing functions. Inlining is of limited benefit for runtime-sized
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// hash inputs.
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#pragma once
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#include <stddef.h>
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#include <stdint.h>
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#include "rocksdb/slice.h"
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namespace rocksdb {
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// Stable/persistent 64-bit hash. Higher quality and generally faster than
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// Hash(), especially for inputs > 24 bytes.
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extern uint64_t Hash64(const char* data, size_t n, uint64_t seed);
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// Specific optimization without seed (same as seed = 0)
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extern uint64_t Hash64(const char* data, size_t n);
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// Non-persistent hash. Must only used for in-memory data structure.
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// The hash results are thus applicable to change. (Thus, it rarely makes
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// sense to specify a seed for this function.)
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inline uint64_t NPHash64(const char* data, size_t n, uint32_t seed) {
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// Currently same as Hash64
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return Hash64(data, n, seed);
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}
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// Specific optimization without seed (same as seed = 0)
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inline uint64_t NPHash64(const char* data, size_t n) {
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// Currently same as Hash64
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return Hash64(data, n);
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}
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// Stable/persistent 32-bit hash. Moderate quality and high speed on
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// small inputs.
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// TODO: consider rename to Hash32
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extern uint32_t Hash(const char* data, size_t n, uint32_t seed);
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// TODO: consider rename to LegacyBloomHash32
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inline uint32_t BloomHash(const Slice& key) {
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return Hash(key.data(), key.size(), 0xbc9f1d34);
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}
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inline uint64_t GetSliceHash64(const Slice& key) {
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return Hash64(key.data(), key.size());
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}
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inline uint64_t GetSliceNPHash64(const Slice& s) {
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return NPHash64(s.data(), s.size());
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}
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// TODO: consider rename to GetSliceHash32
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inline uint32_t GetSliceHash(const Slice& s) {
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return Hash(s.data(), s.size(), 397);
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}
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// Useful for splitting up a 64-bit hash
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inline uint32_t Upper32of64(uint64_t v) {
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return static_cast<uint32_t>(v >> 32);
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}
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inline uint32_t Lower32of64(uint64_t v) { return static_cast<uint32_t>(v); }
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// std::hash compatible interface.
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// TODO: consider rename to SliceHasher32
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struct SliceHasher {
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uint32_t operator()(const Slice& s) const { return GetSliceHash(s); }
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};
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// An alternative to % for mapping a hash value to an arbitrary range. See
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// https://github.com/lemire/fastrange
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inline uint32_t fastrange32(uint32_t hash, uint32_t range) {
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uint64_t product = uint64_t{range} * hash;
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return static_cast<uint32_t>(product >> 32);
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}
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// An alternative to % for mapping a 64-bit hash value to an arbitrary range
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// that fits in size_t. See https://github.com/lemire/fastrange
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// We find size_t more convenient than uint64_t for the range, with side
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// benefit of better optimization on 32-bit platforms.
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inline size_t fastrange64(uint64_t hash, size_t range) {
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#if defined(HAVE_UINT128_EXTENSION)
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// Can use compiler's 128-bit type. Trust it to do the right thing.
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__uint128_t wide = __uint128_t{range} * hash;
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return static_cast<size_t>(wide >> 64);
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#else
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// Fall back: full decomposition.
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// NOTE: GCC seems to fully understand this code as 64-bit x {32 or 64}-bit
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// -> {96 or 128}-bit multiplication and optimize it down to a single
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// wide-result multiplication (64-bit platform) or two wide-result
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// multiplications (32-bit platforms, where range64 >> 32 is zero).
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uint64_t range64 = range; // ok to shift by 32, even if size_t is 32-bit
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uint64_t tmp = uint64_t{range64 & 0xffffFFFF} * uint64_t{hash & 0xffffFFFF};
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tmp >>= 32;
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tmp += uint64_t{range64 & 0xffffFFFF} * uint64_t{hash >> 32};
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// Avoid overflow: first add lower 32 of tmp2, and later upper 32
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uint64_t tmp2 = uint64_t{range64 >> 32} * uint64_t{hash & 0xffffFFFF};
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tmp += static_cast<uint32_t>(tmp2);
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tmp >>= 32;
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tmp += (tmp2 >> 32);
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tmp += uint64_t{range64 >> 32} * uint64_t{hash >> 32};
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return static_cast<size_t>(tmp);
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#endif
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}
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} // namespace rocksdb
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