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1 | // -*- C++ -*- |
2 | ||
5624e564 | 3 | // Copyright (C) 2005-2015 Free Software Foundation, Inc. |
4569a895 AT |
4 | // |
5 | // This file is part of the GNU ISO C++ Library. This library is free | |
6 | // software; you can redistribute it and/or modify it under the terms | |
7 | // of the GNU General Public License as published by the Free Software | |
748086b7 | 8 | // Foundation; either version 3, or (at your option) any later |
4569a895 AT |
9 | // version. |
10 | ||
11 | // This library is distributed in the hope that it will be useful, but | |
12 | // WITHOUT ANY WARRANTY; without even the implied warranty of | |
13 | // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU | |
14 | // General Public License for more details. | |
15 | ||
16 | // You should have received a copy of the GNU General Public License | |
748086b7 JJ |
17 | // along with this library; see the file COPYING3. If not see |
18 | // <http://www.gnu.org/licenses/>. | |
4569a895 | 19 | |
4569a895 AT |
20 | |
21 | // Copyright (C) 2004 Ami Tavory and Vladimir Dreizin, IBM-HRL. | |
22 | ||
23 | // Permission to use, copy, modify, sell, and distribute this software | |
24 | // is hereby granted without fee, provided that the above copyright | |
25 | // notice appears in all copies, and that both that copyright notice | |
26 | // and this permission notice appear in supporting documentation. None | |
27 | // of the above authors, nor IBM Haifa Research Laboratories, make any | |
28 | // representation about the suitability of this software for any | |
29 | // purpose. It is provided "as is" without express or implied | |
30 | // warranty. | |
31 | ||
32 | /** | |
33 | * @file hash_shift_mask_example.cpp | |
34 | * An example showing how to write a range-hasing functor. | |
35 | */ | |
36 | ||
37 | /** | |
38 | * In some rare cases, advance knowledge of the distribution of keys allows | |
39 | * writing more efficient hash-related policies. | |
40 | * In the rather simplistic case of the example, it is known in advance that | |
41 | * all keys have 0 two lowest bits. The example shows how to write | |
42 | * a range-hashing function disregarding the two lowest bits of the hash value. | |
43 | */ | |
44 | ||
45 | #include <functional> | |
46 | #include <ext/pb_ds/assoc_container.hpp> | |
47 | #include <ext/pb_ds/hash_policy.hpp> | |
48 | ||
49 | using namespace std; | |
5e11f978 | 50 | using namespace __gnu_pbds; |
4569a895 AT |
51 | |
52 | // A simple hash functor. hash could serve instead of this functor, | |
53 | // but it is not yet standard everywhere. | |
54 | struct simple_int_hash : public unary_function<int, size_t> | |
55 | { | |
56 | inline size_t | |
57 | operator()(int i) const | |
58 | { return i; } | |
59 | }; | |
60 | ||
61 | // A range-hashing function which shifts 2 bits right and then masks. | |
62 | class shift_two_mask_range_hashing : private direct_mask_range_hashing<> | |
63 | { | |
64 | public: | |
65 | typedef size_t size_type; | |
66 | ||
67 | // Swaps with a different instant. | |
68 | void | |
69 | swap(shift_two_mask_range_hashing& other) | |
70 | { direct_mask_range_hashing<>::swap(other); } | |
71 | ||
72 | // Called by the container when internally resized. | |
73 | void | |
74 | notify_resized(size_type size) | |
75 | { direct_mask_range_hashing<>::notify_resized(size); } | |
76 | ||
77 | // Given a hash value, returns a number in the range of the internal | |
78 | // size of the container. | |
79 | inline size_type | |
80 | operator()(size_type hash) const | |
81 | { return direct_mask_range_hashing<>::operator()(hash >> 2); } | |
82 | }; | |
83 | ||
84 | int | |
85 | main() | |
86 | { | |
87 | // A collision-chaining hash table mapping ints to chars. | |
88 | typedef | |
89 | cc_hash_table< | |
90 | int, | |
91 | char, | |
92 | // Hash function. | |
93 | simple_int_hash, | |
94 | equal_to<int>, | |
95 | // Range hashing function. | |
96 | shift_two_mask_range_hashing> | |
97 | map_t; | |
98 | ||
99 | map_t h; | |
100 | ||
101 | // Use normally. | |
102 | h[16] = 'a'; | |
103 | h[256] = 'e'; | |
104 | h[4] = 'z'; | |
105 | ||
106 | return 0; | |
107 | } | |
108 |