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85 lines
3.4 KiB
C++
85 lines
3.4 KiB
C++
// SPDX-FileCopyrightText: 2011 Google, Inc.
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// SPDX-FileContributor: Geoff Pike
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// SPDX-FileContributor: Jyrki Alakuijala
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// SPDX-License-Identifier: MIT
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// CityHash, by Geoff Pike and Jyrki Alakuijala
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//
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// http://code.google.com/p/cityhash/
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//
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// This file provides a few functions for hashing strings. All of them are
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// high-quality functions in the sense that they pass standard tests such
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// as Austin Appleby's SMHasher. They are also fast.
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//
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// For 64-bit x86 code, on short strings, we don't know of anything faster than
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// CityHash64 that is of comparable quality. We believe our nearest competitor
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// is Murmur3. For 64-bit x86 code, CityHash64 is an excellent choice for hash
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// tables and most other hashing (excluding cryptography).
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//
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// For 64-bit x86 code, on long strings, the picture is more complicated.
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// On many recent Intel CPUs, such as Nehalem, Westmere, Sandy Bridge, etc.,
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// CityHashCrc128 appears to be faster than all competitors of comparable
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// quality. CityHash128 is also good but not quite as fast. We believe our
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// nearest competitor is Bob Jenkins' Spooky. We don't have great data for
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// other 64-bit CPUs, but for long strings we know that Spooky is slightly
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// faster than CityHash on some relatively recent AMD x86-64 CPUs, for example.
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// Note that CityHashCrc128 is declared in citycrc.h.
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//
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// For 32-bit x86 code, we don't know of anything faster than CityHash32 that
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// is of comparable quality. We believe our nearest competitor is Murmur3A.
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// (On 64-bit CPUs, it is typically faster to use the other CityHash variants.)
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//
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// Functions in the CityHash family are not suitable for cryptography.
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//
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// Please see CityHash's README file for more details on our performance
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// measurements and so on.
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//
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// WARNING: This code has been only lightly tested on big-endian platforms!
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// It is known to work well on little-endian platforms that have a small penalty
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// for unaligned reads, such as current Intel and AMD moderate-to-high-end CPUs.
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// It should work on all 32-bit and 64-bit platforms that allow unaligned reads;
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// bug reports are welcome.
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//
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// By the way, for some hash functions, given strings a and b, the hash
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// of a+b is easily derived from the hashes of a and b. This property
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// doesn't hold for any hash functions in this file.
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#pragma once
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#include <cstddef>
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#include "common/common_types.h"
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namespace Common {
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// Hash function for a byte array.
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[[nodiscard]] u64 CityHash64(const char* buf, size_t len);
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// Hash function for a byte array. For convenience, a 64-bit seed is also
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// hashed into the result.
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[[nodiscard]] u64 CityHash64WithSeed(const char* buf, size_t len, u64 seed);
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// Hash function for a byte array. For convenience, two seeds are also
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// hashed into the result.
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[[nodiscard]] u64 CityHash64WithSeeds(const char* buf, size_t len, u64 seed0, u64 seed1);
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// Hash function for a byte array.
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[[nodiscard]] u128 CityHash128(const char* s, size_t len);
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// Hash function for a byte array. For convenience, a 128-bit seed is also
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// hashed into the result.
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[[nodiscard]] u128 CityHash128WithSeed(const char* s, size_t len, u128 seed);
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// Hash 128 input bits down to 64 bits of output.
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// This is intended to be a reasonably good hash function.
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[[nodiscard]] inline u64 Hash128to64(const u128& x) {
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// Murmur-inspired hashing.
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const u64 mul = 0x9ddfea08eb382d69ULL;
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u64 a = (x[0] ^ x[1]) * mul;
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a ^= (a >> 47);
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u64 b = (x[1] ^ a) * mul;
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b ^= (b >> 47);
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b *= mul;
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return b;
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}
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} // namespace Common
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