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1 // Copyright 2016 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 | |
6 #ifndef BASE_CONTAINERS_PAIR_HASH_H_ | |
7 #define BASE_CONTAINERS_PAIR_HASH_H_ | |
8 | |
9 #include <stdint.h> | |
10 | |
11 #include <cstddef> | |
12 #include <utility> | |
13 | |
14 namespace base { | |
15 | |
16 // Implement hashing for pairs of at-most 32 bit integer values. | |
17 // When size_t is 32 bits, we turn the 64-bit hash code into 32 bits by using | |
18 // multiply-add hashing. This algorithm, as described in | |
19 // Theorem 4.3.3 of the thesis "Über die Komplexität der Multiplikation in | |
20 // eingeschränkten Branchingprogrammmodellen" by Woelfel, is: | |
21 // | |
22 // h32(x32, y32) = (h64(x32, y32) * rand_odd64 + rand16 * 2^16) % 2^64 / 2^32 | |
23 // | |
24 // Contact danakj@chromium.org for any questions. | |
25 inline std::size_t HashInts32(uint32_t value1, uint32_t value2) { | |
26 uint64_t value1_64 = value1; | |
27 uint64_t hash64 = (value1_64 << 32) | value2; | |
28 | |
29 if (sizeof(std::size_t) >= sizeof(uint64_t)) | |
30 return static_cast<std::size_t>(hash64); | |
31 | |
32 uint64_t odd_random = 481046412LL << 32 | 1025306955LL; | |
33 uint32_t shift_random = 10121U << 16; | |
34 | |
35 hash64 = hash64 * odd_random + shift_random; | |
36 std::size_t high_bits = static_cast<std::size_t>( | |
37 hash64 >> (8 * (sizeof(uint64_t) - sizeof(std::size_t)))); | |
38 return high_bits; | |
39 } | |
40 | |
41 // Implement hashing for pairs of up-to 64-bit integer values. | |
42 // We use the compound integer hash method to produce a 64-bit hash code, by | |
43 // breaking the two 64-bit inputs into 4 32-bit values: | |
44 // http://opendatastructures.org/versions/edition-0.1d/ods-java/node33.html#SECT ION00832000000000000000 | |
45 // Then we reduce our result to 32 bits if required, similar to above. | |
46 inline std::size_t HashInts64(uint64_t value1, uint64_t value2) { | |
47 uint32_t short_random1 = 842304669U; | |
48 uint32_t short_random2 = 619063811U; | |
49 uint32_t short_random3 = 937041849U; | |
50 uint32_t short_random4 = 3309708029U; | |
51 | |
52 uint32_t value1a = static_cast<uint32_t>(value1 & 0xffffffff); | |
53 uint32_t value1b = static_cast<uint32_t>((value1 >> 32) & 0xffffffff); | |
54 uint32_t value2a = static_cast<uint32_t>(value2 & 0xffffffff); | |
55 uint32_t value2b = static_cast<uint32_t>((value2 >> 32) & 0xffffffff); | |
56 | |
57 uint64_t product1 = static_cast<uint64_t>(value1a) * short_random1; | |
58 uint64_t product2 = static_cast<uint64_t>(value1b) * short_random2; | |
59 uint64_t product3 = static_cast<uint64_t>(value2a) * short_random3; | |
60 uint64_t product4 = static_cast<uint64_t>(value2b) * short_random4; | |
61 | |
62 uint64_t hash64 = product1 + product2 + product3 + product4; | |
63 | |
64 if (sizeof(std::size_t) >= sizeof(uint64_t)) | |
65 return static_cast<std::size_t>(hash64); | |
66 | |
67 uint64_t odd_random = 1578233944LL << 32 | 194370989LL; | |
68 uint32_t shift_random = 20591U << 16; | |
69 | |
70 hash64 = hash64 * odd_random + shift_random; | |
71 std::size_t high_bits = static_cast<std::size_t>( | |
72 hash64 >> (8 * (sizeof(uint64_t) - sizeof(std::size_t)))); | |
73 return high_bits; | |
74 } | |
75 | |
76 template<typename T1, typename T2> | |
77 inline std::size_t HashPair(T1 value1, T2 value2) { | |
danakj
2016/01/14 21:29:10
These helpers can move into an internal namespace?
davidben
2016/01/14 21:34:34
They actually do get called directly for types tha
danakj
2016/01/14 21:47:12
Oh.. pair-like.. ya. We probably don't want to for
davidben
2016/01/15 00:25:42
I'm not sure. I'll give it a try and see how commo
| |
78 // This condition is expected to be compile-time evaluated and optimised away | |
79 // in release builds. | |
80 if (sizeof(T1) > sizeof(uint32_t) || (sizeof(T2) > sizeof(uint32_t))) | |
81 return HashInts64(value1, value2); | |
82 | |
83 return HashInts32(value1, value2); | |
84 } | |
85 | |
86 // A templated hasher for pairs of integer types. | |
87 template<typename Type1, typename Type2> | |
88 struct PairHash { | |
89 std::size_t operator()(std::pair<Type1, Type2> value) const { | |
90 return base::HashPair(value.first, value.second); | |
91 } | |
92 }; | |
93 | |
94 } // namespace base | |
95 | |
96 #endif // BASE_CONTAINERS_PAIR_HASH_H_ | |
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