city.h
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00001 // Copyright 2018 The Abseil Authors.
00002 //
00003 // Licensed under the Apache License, Version 2.0 (the "License");
00004 // you may not use this file except in compliance with the License.
00005 // You may obtain a copy of the License at
00006 //
00007 //      https://www.apache.org/licenses/LICENSE-2.0
00008 //
00009 // Unless required by applicable law or agreed to in writing, software
00010 // distributed under the License is distributed on an "AS IS" BASIS,
00011 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
00012 // See the License for the specific language governing permissions and
00013 // limitations under the License.
00014 //
00015 // https://code.google.com/p/cityhash/
00016 //
00017 // This file provides a few functions for hashing strings.  All of them are
00018 // high-quality functions in the sense that they pass standard tests such
00019 // as Austin Appleby's SMHasher.  They are also fast.
00020 //
00021 // For 64-bit x86 code, on short strings, we don't know of anything faster than
00022 // CityHash64 that is of comparable quality.  We believe our nearest competitor
00023 // is Murmur3.  For 64-bit x86 code, CityHash64 is an excellent choice for hash
00024 // tables and most other hashing (excluding cryptography).
00025 //
00026 // For 32-bit x86 code, we don't know of anything faster than CityHash32 that
00027 // is of comparable quality.  We believe our nearest competitor is Murmur3A.
00028 // (On 64-bit CPUs, it is typically faster to use the other CityHash variants.)
00029 //
00030 // Functions in the CityHash family are not suitable for cryptography.
00031 //
00032 // Please see CityHash's README file for more details on our performance
00033 // measurements and so on.
00034 //
00035 // WARNING: This code has been only lightly tested on big-endian platforms!
00036 // It is known to work well on little-endian platforms that have a small penalty
00037 // for unaligned reads, such as current Intel and AMD moderate-to-high-end CPUs.
00038 // It should work on all 32-bit and 64-bit platforms that allow unaligned reads;
00039 // bug reports are welcome.
00040 //
00041 // By the way, for some hash functions, given strings a and b, the hash
00042 // of a+b is easily derived from the hashes of a and b.  This property
00043 // doesn't hold for any hash functions in this file.
00044 
00045 #ifndef ABSL_HASH_INTERNAL_CITY_H_
00046 #define ABSL_HASH_INTERNAL_CITY_H_
00047 
00048 #include <stdint.h>
00049 #include <stdlib.h>  // for size_t.
00050 #include <utility>
00051 
00052 namespace absl {
00053 namespace hash_internal {
00054 
00055 typedef std::pair<uint64_t, uint64_t> uint128;
00056 
00057 inline uint64_t Uint128Low64(const uint128 &x) { return x.first; }
00058 inline uint64_t Uint128High64(const uint128 &x) { return x.second; }
00059 
00060 // Hash function for a byte array.
00061 uint64_t CityHash64(const char *s, size_t len);
00062 
00063 // Hash function for a byte array.  For convenience, a 64-bit seed is also
00064 // hashed into the result.
00065 uint64_t CityHash64WithSeed(const char *s, size_t len, uint64_t seed);
00066 
00067 // Hash function for a byte array.  For convenience, two seeds are also
00068 // hashed into the result.
00069 uint64_t CityHash64WithSeeds(const char *s, size_t len, uint64_t seed0,
00070                            uint64_t seed1);
00071 
00072 // Hash function for a byte array.  Most useful in 32-bit binaries.
00073 uint32_t CityHash32(const char *s, size_t len);
00074 
00075 // Hash 128 input bits down to 64 bits of output.
00076 // This is intended to be a reasonably good hash function.
00077 inline uint64_t Hash128to64(const uint128 &x) {
00078   // Murmur-inspired hashing.
00079   const uint64_t kMul = 0x9ddfea08eb382d69ULL;
00080   uint64_t a = (Uint128Low64(x) ^ Uint128High64(x)) * kMul;
00081   a ^= (a >> 47);
00082   uint64_t b = (Uint128High64(x) ^ a) * kMul;
00083   b ^= (b >> 47);
00084   b *= kMul;
00085   return b;
00086 }
00087 
00088 }  // namespace hash_internal
00089 }  // namespace absl
00090 
00091 #endif  // ABSL_HASH_INTERNAL_CITY_H_


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autogenerated on Wed Jun 19 2019 19:42:14