| // -*- C++ -*- |
| |
| // Copyright (C) 2005, 2006 Free Software Foundation, Inc. |
| // |
| // This file is part of the GNU ISO C++ Library. This library is free |
| // software; you can redistribute it and/or modify it under the terms |
| // of the GNU General Public License as published by the Free Software |
| // Foundation; either version 2, or (at your option) any later |
| // version. |
| |
| // This library is distributed in the hope that it will be useful, but |
| // WITHOUT ANY WARRANTY; without even the implied warranty of |
| // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU |
| // General Public License for more details. |
| |
| // You should have received a copy of the GNU General Public License |
| // along with this library; see the file COPYING. If not, write to |
| // the Free Software Foundation, 59 Temple Place - Suite 330, Boston, |
| // MA 02111-1307, USA. |
| |
| // As a special exception, you may use this file as part of a free |
| // software library without restriction. Specifically, if other files |
| // instantiate templates or use macros or inline functions from this |
| // file, or you compile this file and link it with other files to |
| // produce an executable, this file does not by itself cause the |
| // resulting executable to be covered by the GNU General Public |
| // License. This exception does not however invalidate any other |
| // reasons why the executable file might be covered by the GNU General |
| // Public License. |
| |
| // Copyright (C) 2004 Ami Tavory and Vladimir Dreizin, IBM-HRL. |
| |
| // Permission to use, copy, modify, sell, and distribute this software |
| // is hereby granted without fee, provided that the above copyright |
| // notice appears in all copies, and that both that copyright notice |
| // and this permission notice appear in supporting documentation. None |
| // of the above authors, nor IBM Haifa Research Laboratories, make any |
| // representation about the suitability of this software for any |
| // purpose. It is provided "as is" without express or implied |
| // warranty. |
| |
| /** |
| * @file hash_shift_mask_example.cpp |
| * An example showing how to write a range-hasing functor. |
| */ |
| |
| /** |
| * In some rare cases, advance knowledge of the distribution of keys allows |
| * writing more efficient hash-related policies. |
| * In the rather simplistic case of the example, it is known in advance that |
| * all keys have 0 two lowest bits. The example shows how to write |
| * a range-hashing function disregarding the two lowest bits of the hash value. |
| */ |
| |
| #include <functional> |
| #include <ext/pb_ds/assoc_container.hpp> |
| #include <ext/pb_ds/hash_policy.hpp> |
| |
| using namespace std; |
| using namespace pb_ds; |
| |
| // A simple hash functor. hash could serve instead of this functor, |
| // but it is not yet standard everywhere. |
| struct simple_int_hash : public unary_function<int, size_t> |
| { |
| inline size_t |
| operator()(int i) const |
| { return i; } |
| }; |
| |
| // A range-hashing function which shifts 2 bits right and then masks. |
| class shift_two_mask_range_hashing : private direct_mask_range_hashing<> |
| { |
| public: |
| typedef size_t size_type; |
| |
| // Swaps with a different instant. |
| void |
| swap(shift_two_mask_range_hashing& other) |
| { direct_mask_range_hashing<>::swap(other); } |
| |
| // Called by the container when internally resized. |
| void |
| notify_resized(size_type size) |
| { direct_mask_range_hashing<>::notify_resized(size); } |
| |
| // Given a hash value, returns a number in the range of the internal |
| // size of the container. |
| inline size_type |
| operator()(size_type hash) const |
| { return direct_mask_range_hashing<>::operator()(hash >> 2); } |
| }; |
| |
| int |
| main() |
| { |
| // A collision-chaining hash table mapping ints to chars. |
| typedef |
| cc_hash_table< |
| int, |
| char, |
| // Hash function. |
| simple_int_hash, |
| equal_to<int>, |
| // Range hashing function. |
| shift_two_mask_range_hashing> |
| map_t; |
| |
| map_t h; |
| |
| // Use normally. |
| h[16] = 'a'; |
| h[256] = 'e'; |
| h[4] = 'z'; |
| |
| return 0; |
| } |
| |