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// -*- C++ -*-
// Copyright (C) 2005-2015 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 3, 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 COPYING3. If not see
// <http://www.gnu.org/licenses/>.
// 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 __gnu_pbds;
/**
* 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_type,
null_type,
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;
}
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