// -*- 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; }