1 // Copyright (c) 2011 The Chromium Authors. All rights reserved. 2 // Use of this source code is governed by a BSD-style license that can be 3 // found in the LICENSE file. 4 5 #ifndef BASE_HASH_H_ 6 #define BASE_HASH_H_ 7 8 #include <stddef.h> 9 #include <stdint.h> 10 11 #include <limits> 12 #include <string> 13 #include <utility> 14 15 #include "base/base_export.h" 16 #include "base/logging.h" 17 18 namespace base { 19 20 // WARNING: This hash function should not be used for any cryptographic purpose. 21 BASE_EXPORT uint32_t SuperFastHash(const char* data, size_t length); 22 23 // Computes a hash of a memory buffer |data| of a given |length|. 24 // WARNING: This hash function should not be used for any cryptographic purpose. 25 inline uint32_t Hash(const char* data, size_t length) { 26 return SuperFastHash(data, length); 27 } 28 29 // Computes a hash of a string |str|. 30 // WARNING: This hash function should not be used for any cryptographic purpose. 31 inline uint32_t Hash(const std::string& str) { 32 return Hash(str.data(), str.size()); 33 } 34 35 // Implement hashing for pairs of at-most 32 bit integer values. 36 // When size_t is 32 bits, we turn the 64-bit hash code into 32 bits by using 37 // multiply-add hashing. This algorithm, as described in 38 // Theorem 4.3.3 of the thesis "ber die Komplexitt der Multiplikation in 39 // eingeschrnkten Branchingprogrammmodellen" by Woelfel, is: 40 // 41 // h32(x32, y32) = (h64(x32, y32) * rand_odd64 + rand16 * 2^16) % 2^64 / 2^32 42 // 43 // Contact danakj (at) chromium.org for any questions. 44 inline size_t HashInts32(uint32_t value1, uint32_t value2) { 45 uint64_t value1_64 = value1; 46 uint64_t hash64 = (value1_64 << 32) | value2; 47 48 if (sizeof(size_t) >= sizeof(uint64_t)) 49 return static_cast<size_t>(hash64); 50 51 uint64_t odd_random = 481046412LL << 32 | 1025306955LL; 52 uint32_t shift_random = 10121U << 16; 53 54 hash64 = hash64 * odd_random + shift_random; 55 size_t high_bits = 56 static_cast<size_t>(hash64 >> (8 * (sizeof(uint64_t) - sizeof(size_t)))); 57 return high_bits; 58 } 59 60 // Implement hashing for pairs of up-to 64-bit integer values. 61 // We use the compound integer hash method to produce a 64-bit hash code, by 62 // breaking the two 64-bit inputs into 4 32-bit values: 63 // http://opendatastructures.org/versions/edition-0.1d/ods-java/node33.html#SECTION00832000000000000000 64 // Then we reduce our result to 32 bits if required, similar to above. 65 inline size_t HashInts64(uint64_t value1, uint64_t value2) { 66 uint32_t short_random1 = 842304669U; 67 uint32_t short_random2 = 619063811U; 68 uint32_t short_random3 = 937041849U; 69 uint32_t short_random4 = 3309708029U; 70 71 uint32_t value1a = static_cast<uint32_t>(value1 & 0xffffffff); 72 uint32_t value1b = static_cast<uint32_t>((value1 >> 32) & 0xffffffff); 73 uint32_t value2a = static_cast<uint32_t>(value2 & 0xffffffff); 74 uint32_t value2b = static_cast<uint32_t>((value2 >> 32) & 0xffffffff); 75 76 uint64_t product1 = static_cast<uint64_t>(value1a) * short_random1; 77 uint64_t product2 = static_cast<uint64_t>(value1b) * short_random2; 78 uint64_t product3 = static_cast<uint64_t>(value2a) * short_random3; 79 uint64_t product4 = static_cast<uint64_t>(value2b) * short_random4; 80 81 uint64_t hash64 = product1 + product2 + product3 + product4; 82 83 if (sizeof(size_t) >= sizeof(uint64_t)) 84 return static_cast<size_t>(hash64); 85 86 uint64_t odd_random = 1578233944LL << 32 | 194370989LL; 87 uint32_t shift_random = 20591U << 16; 88 89 hash64 = hash64 * odd_random + shift_random; 90 size_t high_bits = 91 static_cast<size_t>(hash64 >> (8 * (sizeof(uint64_t) - sizeof(size_t)))); 92 return high_bits; 93 } 94 95 template <typename T1, typename T2> 96 inline size_t HashInts(T1 value1, T2 value2) { 97 // This condition is expected to be compile-time evaluated and optimised away 98 // in release builds. 99 if (sizeof(T1) > sizeof(uint32_t) || (sizeof(T2) > sizeof(uint32_t))) 100 return HashInts64(value1, value2); 101 102 return HashInts32(value1, value2); 103 } 104 105 // A templated hasher for pairs of integer types. 106 template <typename T> 107 struct IntPairHash; 108 109 template <typename Type1, typename Type2> 110 struct IntPairHash<std::pair<Type1, Type2>> { 111 size_t operator()(std::pair<Type1, Type2> value) const { 112 return HashInts(value.first, value.second); 113 } 114 }; 115 116 } // namespace base 117 118 #endif // BASE_HASH_H_ 119
