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
18namespace base {
19
20// WARNING: This hash function should not be used for any cryptographic purpose.
21BASE_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.
25inline 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.
31inline 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 "Über die Komplexität der Multiplikation in
39// eingeschränkten 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@chromium.org for any questions.
44inline 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.
65inline 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
95template <typename T1, typename T2>
96inline 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.
106template <typename T>
107struct IntPairHash;
108
109template <typename Type1, typename Type2>
110struct 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