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1 | // -*- C++ -*- |
2 | ||
99dee823 | 3 | // Copyright (C) 2005-2021 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/>. | |
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 store_hash_example.cpp | |
34 | * An example showing how to store hash-values with | |
35 | * each entry in a hash-based container. | |
36 | */ | |
37 | ||
38 | /** | |
39 | * This example shows how to configure a hash-based container to store | |
40 | * the hash value of each key along with each entry. This technique | |
41 | * is useful for complex keys (e.g., strings in this example), since | |
42 | * it lowers the cost of collisions. For simpler types (e.g., integers), | |
43 | * where the cost of comparing keys is of the same order as the cost | |
44 | * of comparing hash values, this technique adds unnecessary overhead. | |
45 | */ | |
46 | ||
47 | #include <functional> | |
48 | #include <string> | |
49 | #include <ext/pb_ds/assoc_container.hpp> | |
50 | #include <ext/pb_ds/hash_policy.hpp> | |
51 | ||
52 | using namespace std; | |
5e11f978 | 53 | using namespace __gnu_pbds; |
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54 | |
55 | // A string hash functor. | |
56 | struct string_hash : public unary_function<string, size_t> | |
57 | { | |
58 | inline size_t | |
59 | operator()(string str) const | |
60 | { | |
61 | string::const_iterator b = str.begin(); | |
62 | string::const_iterator e = str.end(); | |
63 | ||
64 | size_t hash = 0; | |
65 | while (b != e) | |
66 | { | |
67 | hash *= 5; | |
68 | hash += static_cast<size_t>(*b); | |
69 | ++b; | |
70 | } | |
71 | return hash; | |
72 | } | |
73 | }; | |
74 | ||
75 | int main() | |
76 | { | |
77 | // A collision-chaining hash table mapping strings to ints. | |
78 | typedef | |
79 | cc_hash_table< | |
80 | string, | |
81 | int, | |
82 | string_hash, | |
83 | equal_to<string>, | |
84 | direct_mask_range_hashing<>, | |
85 | hash_standard_resize_policy<>, | |
86 | true> | |
87 | map_t; | |
88 | ||
89 | map_t h; | |
90 | ||
91 | // Use regularly. | |
92 | h["Hello, "] = 0; | |
93 | h["world"] = 1; | |
94 | ||
95 | return 0; | |
96 | } | |
97 |