97 lines
3.6 KiB
C
97 lines
3.6 KiB
C
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// Copyright 2018 The Abseil Authors.
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// https://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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//
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// https://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 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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#ifndef ABSL_HASH_INTERNAL_CITY_H_
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#define ABSL_HASH_INTERNAL_CITY_H_
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#include <stdint.h>
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#include <stdlib.h> // for size_t.
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#include <utility>
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#include "absl/base/config.h"
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namespace absl {
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ABSL_NAMESPACE_BEGIN
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namespace hash_internal {
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typedef std::pair<uint64_t, uint64_t> uint128;
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inline uint64_t Uint128Low64(const uint128 &x) { return x.first; }
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inline uint64_t Uint128High64(const uint128 &x) { return x.second; }
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// Hash function for a byte array.
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uint64_t CityHash64(const char *s, 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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uint64_t CityHash64WithSeed(const char *s, size_t len, uint64_t 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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uint64_t CityHash64WithSeeds(const char *s, size_t len, uint64_t seed0,
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uint64_t seed1);
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// Hash function for a byte array. Most useful in 32-bit binaries.
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uint32_t CityHash32(const char *s, size_t len);
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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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inline uint64_t Hash128to64(const uint128 &x) {
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// Murmur-inspired hashing.
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const uint64_t kMul = 0x9ddfea08eb382d69ULL;
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uint64_t a = (Uint128Low64(x) ^ Uint128High64(x)) * kMul;
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a ^= (a >> 47);
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uint64_t b = (Uint128High64(x) ^ a) * kMul;
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b ^= (b >> 47);
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b *= kMul;
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return b;
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}
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} // namespace hash_internal
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ABSL_NAMESPACE_END
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} // namespace absl
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#endif // ABSL_HASH_INTERNAL_CITY_H_
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