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// 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_ |