1// -*- C++ -*-
2
3// Copyright (C) 2005, 2006, 2009 Free Software Foundation, Inc.
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
8// Foundation; either version 3, or (at your option) any later
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
17// along with this library; see the file COPYING3.  If not see
18// <http://www.gnu.org/licenses/>.
19
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
49using namespace std;
50using namespace __gnu_pbds;
51
52// A simple hash functor. hash could serve instead of this functor,
53// but it is not yet standard everywhere.
54struct 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.
62class shift_two_mask_range_hashing : private direct_mask_range_hashing<>
63{
64public:
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
84int
85main()
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
109