1 // Copyright 2014 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 #include "base/hash.h"
6
7 #include <functional>
8
SuperFastHash(const char * data,size_t len)9 uint32_t SuperFastHash(const char* data, size_t len) {
10 std::hash<std::string> hash_fn;
11 return hash_fn(std::string(data, len));
12 }
13
14 namespace base {
15
Hash(const void * data,size_t length)16 uint32_t Hash(const void* data, size_t length) {
17 // Currently our in-memory hash is the same as the persistent hash. The
18 // split between in-memory and persistent hash functions is maintained to
19 // allow the in-memory hash function to be updated in the future.
20 return PersistentHash(data, length);
21 }
22
Hash(const std::string & str)23 uint32_t Hash(const std::string& str) {
24 return PersistentHash(str.data(), str.size());
25 }
26
Hash(const string16 & str)27 uint32_t Hash(const string16& str) {
28 return PersistentHash(str.data(), str.size() * sizeof(char16));
29 }
30
PersistentHash(const void * data,size_t length)31 uint32_t PersistentHash(const void* data, size_t length) {
32 // This hash function must not change, since it is designed to be persistable
33 // to disk.
34 if (length > static_cast<size_t>(std::numeric_limits<int>::max())) {
35 NOTREACHED();
36 return 0;
37 }
38 return ::SuperFastHash(reinterpret_cast<const char*>(data),
39 static_cast<int>(length));
40 }
41
PersistentHash(const std::string & str)42 uint32_t PersistentHash(const std::string& str) {
43 return PersistentHash(str.data(), str.size());
44 }
45
46 // Implement hashing for pairs of at-most 32 bit integer values.
47 // When size_t is 32 bits, we turn the 64-bit hash code into 32 bits by using
48 // multiply-add hashing. This algorithm, as described in
49 // Theorem 4.3.3 of the thesis "Über die Komplexität der Multiplikation in
50 // eingeschränkten Branchingprogrammmodellen" by Woelfel, is:
51 //
52 // h32(x32, y32) = (h64(x32, y32) * rand_odd64 + rand16 * 2^16) % 2^64 / 2^32
53 //
54 // Contact danakj@chromium.org for any questions.
HashInts32(uint32_t value1,uint32_t value2)55 size_t HashInts32(uint32_t value1, uint32_t value2) {
56 uint64_t value1_64 = value1;
57 uint64_t hash64 = (value1_64 << 32) | value2;
58
59 if (sizeof(size_t) >= sizeof(uint64_t))
60 return static_cast<size_t>(hash64);
61
62 uint64_t odd_random = 481046412LL << 32 | 1025306955LL;
63 uint32_t shift_random = 10121U << 16;
64
65 hash64 = hash64 * odd_random + shift_random;
66 size_t high_bits =
67 static_cast<size_t>(hash64 >> (8 * (sizeof(uint64_t) - sizeof(size_t))));
68 return high_bits;
69 }
70
71 // Implement hashing for pairs of up-to 64-bit integer values.
72 // We use the compound integer hash method to produce a 64-bit hash code, by
73 // breaking the two 64-bit inputs into 4 32-bit values:
74 // http://opendatastructures.org/versions/edition-0.1d/ods-java/node33.html#SECTION00832000000000000000
75 // Then we reduce our result to 32 bits if required, similar to above.
HashInts64(uint64_t value1,uint64_t value2)76 size_t HashInts64(uint64_t value1, uint64_t value2) {
77 uint32_t short_random1 = 842304669U;
78 uint32_t short_random2 = 619063811U;
79 uint32_t short_random3 = 937041849U;
80 uint32_t short_random4 = 3309708029U;
81
82 uint32_t value1a = static_cast<uint32_t>(value1 & 0xffffffff);
83 uint32_t value1b = static_cast<uint32_t>((value1 >> 32) & 0xffffffff);
84 uint32_t value2a = static_cast<uint32_t>(value2 & 0xffffffff);
85 uint32_t value2b = static_cast<uint32_t>((value2 >> 32) & 0xffffffff);
86
87 uint64_t product1 = static_cast<uint64_t>(value1a) * short_random1;
88 uint64_t product2 = static_cast<uint64_t>(value1b) * short_random2;
89 uint64_t product3 = static_cast<uint64_t>(value2a) * short_random3;
90 uint64_t product4 = static_cast<uint64_t>(value2b) * short_random4;
91
92 uint64_t hash64 = product1 + product2 + product3 + product4;
93
94 if (sizeof(size_t) >= sizeof(uint64_t))
95 return static_cast<size_t>(hash64);
96
97 uint64_t odd_random = 1578233944LL << 32 | 194370989LL;
98 uint32_t shift_random = 20591U << 16;
99
100 hash64 = hash64 * odd_random + shift_random;
101 size_t high_bits =
102 static_cast<size_t>(hash64 >> (8 * (sizeof(uint64_t) - sizeof(size_t))));
103 return high_bits;
104 }
105
106 } // namespace base
107