absl/hash/internal/city.h

// 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_