fc8dc48020
git-subtree-dir: third_party/abseil_cpp git-subtree-mainline:ffb2ae54begit-subtree-split:768eb2ca28
96 lines
3.6 KiB
C++
96 lines
3.6 KiB
C++
// Copyright 2018 The Abseil Authors.
|
|
//
|
|
// Licensed under the Apache License, Version 2.0 (the "License");
|
|
// you may not use this file except in compliance with the License.
|
|
// You may obtain a copy of the License at
|
|
//
|
|
// https://www.apache.org/licenses/LICENSE-2.0
|
|
//
|
|
// Unless required by applicable law or agreed to in writing, software
|
|
// distributed under the License is distributed on an "AS IS" BASIS,
|
|
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
|
|
// See the License for the specific language governing permissions and
|
|
// limitations under the License.
|
|
//
|
|
// https://code.google.com/p/cityhash/
|
|
//
|
|
// This file provides a few functions for hashing strings. All of them are
|
|
// high-quality functions in the sense that they pass standard tests such
|
|
// as Austin Appleby's SMHasher. They are also fast.
|
|
//
|
|
// For 64-bit x86 code, on short strings, we don't know of anything faster than
|
|
// CityHash64 that is of comparable quality. We believe our nearest competitor
|
|
// is Murmur3. For 64-bit x86 code, CityHash64 is an excellent choice for hash
|
|
// tables and most other hashing (excluding cryptography).
|
|
//
|
|
// For 32-bit x86 code, we don't know of anything faster than CityHash32 that
|
|
// is of comparable quality. We believe our nearest competitor is Murmur3A.
|
|
// (On 64-bit CPUs, it is typically faster to use the other CityHash variants.)
|
|
//
|
|
// Functions in the CityHash family are not suitable for cryptography.
|
|
//
|
|
// Please see CityHash's README file for more details on our performance
|
|
// measurements and so on.
|
|
//
|
|
// WARNING: This code has been only lightly tested on big-endian platforms!
|
|
// It is known to work well on little-endian platforms that have a small penalty
|
|
// for unaligned reads, such as current Intel and AMD moderate-to-high-end CPUs.
|
|
// It should work on all 32-bit and 64-bit platforms that allow unaligned reads;
|
|
// bug reports are welcome.
|
|
//
|
|
// By the way, for some hash functions, given strings a and b, the hash
|
|
// of a+b is easily derived from the hashes of a and b. This property
|
|
// doesn't hold for any hash functions in this file.
|
|
|
|
#ifndef ABSL_HASH_INTERNAL_CITY_H_
|
|
#define ABSL_HASH_INTERNAL_CITY_H_
|
|
|
|
#include <stdint.h>
|
|
#include <stdlib.h> // for size_t.
|
|
|
|
#include <utility>
|
|
|
|
#include "absl/base/config.h"
|
|
|
|
namespace absl {
|
|
ABSL_NAMESPACE_BEGIN
|
|
namespace hash_internal {
|
|
|
|
typedef std::pair<uint64_t, uint64_t> uint128;
|
|
|
|
inline uint64_t Uint128Low64(const uint128 &x) { return x.first; }
|
|
inline uint64_t Uint128High64(const uint128 &x) { return x.second; }
|
|
|
|
// Hash function for a byte array.
|
|
uint64_t CityHash64(const char *s, size_t len);
|
|
|
|
// Hash function for a byte array. For convenience, a 64-bit seed is also
|
|
// hashed into the result.
|
|
uint64_t CityHash64WithSeed(const char *s, size_t len, uint64_t seed);
|
|
|
|
// Hash function for a byte array. For convenience, two seeds are also
|
|
// hashed into the result.
|
|
uint64_t CityHash64WithSeeds(const char *s, size_t len, uint64_t seed0,
|
|
uint64_t seed1);
|
|
|
|
// Hash function for a byte array. Most useful in 32-bit binaries.
|
|
uint32_t CityHash32(const char *s, size_t len);
|
|
|
|
// Hash 128 input bits down to 64 bits of output.
|
|
// This is intended to be a reasonably good hash function.
|
|
inline uint64_t Hash128to64(const uint128 &x) {
|
|
// Murmur-inspired hashing.
|
|
const uint64_t kMul = 0x9ddfea08eb382d69ULL;
|
|
uint64_t a = (Uint128Low64(x) ^ Uint128High64(x)) * kMul;
|
|
a ^= (a >> 47);
|
|
uint64_t b = (Uint128High64(x) ^ a) * kMul;
|
|
b ^= (b >> 47);
|
|
b *= kMul;
|
|
return b;
|
|
}
|
|
|
|
} // namespace hash_internal
|
|
ABSL_NAMESPACE_END
|
|
} // namespace absl
|
|
|
|
#endif // ABSL_HASH_INTERNAL_CITY_H_
|