| // -*- 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 ranged_hash_example.cpp |
| * A basic example showing how to write a ranged-hash functor. |
| */ |
| |
| /** |
| * In some cases it is beneficial to write a hash function which determines |
| * hash values based on the size of the container object. |
| * The example shows an example of a string-hashing function which |
| * uses a fast method for hashing strings when the number of strings |
| * in the container object is small, and a slower but more careful method |
| * for hashing strings when the number of strings in the container object |
| * is large. |
| */ |
| |
| #include <functional> |
| #include <cassert> |
| #include <string> |
| #include <ext/pb_ds/assoc_container.hpp> |
| #include <ext/pb_ds/hash_policy.hpp> |
| |
| using namespace std; |
| using namespace pb_ds; |
| using namespace pb_ds; |
| |
| /** |
| * A (somewhat simplistic) ranged-hash function for strings. |
| * It uses the size of the container object to determine |
| * the hashing method. For smaller sizes it uses a simple hash function; |
| * for larger sizes it uses a more complicated hash function. |
| */ |
| class simple_string_ranged_hash_fn |
| : public unary_function<string, size_t> |
| { |
| public: |
| typedef size_t size_type; |
| |
| simple_string_ranged_hash_fn() : m_container_size(0) { } |
| |
| // Called to notify that the size has changed. |
| void |
| notify_resized(size_t size) |
| { m_container_size = size; } |
| |
| // Called for hashing a string into a size_t in a given range. |
| size_t |
| operator()(const string& r_string) |
| { |
| /* |
| * This (simplified) hash algorithm decides that if there are |
| * fewer than 100 strings in the container it will hash |
| * a string by summing its characters; otherwise, it will |
| * perform a more complicated operation in order to produce |
| * hash values with fewer collisions. |
| */ |
| string::const_iterator it = r_string.begin(); |
| size_t hash = 0; |
| if (m_container_size < 100) |
| { |
| // For this size, perform an accumulate type of operation. |
| while (it != r_string.end()) |
| hash += static_cast<size_t>(*it++); |
| } |
| else |
| { |
| // For this size, perform a different operation. |
| while (it != r_string.end()) |
| { |
| hash += static_cast<size_t>(*it++); |
| hash *= 5; |
| } |
| } |
| |
| // The function must, by whatever means, return a size in the |
| // range 0 to m_container_size. |
| return hash % m_container_size; |
| } |
| |
| // Swaps content. |
| void |
| swap(simple_string_ranged_hash_fn& other) |
| { |
| std::swap(m_container_size, other.m_container_size); |
| } |
| |
| private: |
| // Records the size of the container object. |
| size_t m_container_size; |
| }; |
| |
| int |
| main() |
| { |
| // A collision-chaining hash table storing strings. |
| typedef |
| cc_hash_table< |
| string, |
| null_mapped_type, |
| null_hash_fn, |
| equal_to<string>, |
| simple_string_ranged_hash_fn> |
| set_t; |
| |
| // Note that in the above, the library determines a resize policy |
| // appropriate for modulo-based range hashing. |
| set_t h; |
| |
| // Use the table normally. |
| h.insert("Hello, "); |
| h.insert("world"); |
| |
| assert(h.size() == 2); |
| |
| assert(h.find("Hello, ") != h.end()); |
| assert(h.find("world") != h.end()); |
| |
| assert(h.find("Goodbye, oh cruel world!") == h.end()); |
| |
| return 0; |
| } |
| |