Subversion Repositories scarts

// -*- C++ -*-

// Copyright (C) 2005 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, 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301,

// 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.

 */

// For cc_hash_assoc_cntnr.

#include <ext/pb_assoc/assoc_cntnr.hpp>

// For hash-related policies.

#include <ext/pb_assoc/hash_policy.hpp>

// For unary_function and binary_function.

#include <functional>

// For assert.

#include <cassert>

// For string.

#include <string>

/**

 * 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 std::unary_function<

  std::string,

                                     size_t>

{

public:

  typedef size_t size_type;

  // Default constructor.

  simple_string_ranged_hash_fn();

  // Called to notify that the size has changed.

  void

  notify_resized(size_t size);

  /*

   * Called for hashing a string into a size_t in a

   *    given range.

   */

  size_t

  operator()(const std::string& r_string);

  // Swaps content.

  void

  swap(simple_string_ranged_hash_fn& r_other);

private:

  // Records the size of the container object.

  size_t m_container_size;

};

simple_string_ranged_hash_fn::

simple_string_ranged_hash_fn() :

  m_container_size(0)

{

}

void

simple_string_ranged_hash_fn::

notify_resized(size_t size)

{

  m_container_size = size;

}

size_t

simple_string_ranged_hash_fn::

operator()(const std::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.

   */

  std::string::const_iterator it = r_string.begin();

  size_t hash = 0;

  if (m_container_size < 100)

    {

      // For this size, perform an std::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);

}

void

simple_string_ranged_hash_fn::

swap(simple_string_ranged_hash_fn& r_other)

{

  std::swap(m_container_size, r_other.m_container_size);

}

int

main()

{

  /*

   * A collision-chaining hash table storing strings.

   */

  typedef

    pb_assoc::cc_hash_assoc_cntnr<

    std::string,

    pb_assoc::null_data_type,

    // Null hash function

    pb_assoc::null_hash_fn,

    // Equivalence function.

    std::equal_to<

    std::string>,

    // Range hashing function.

    simple_string_ranged_hash_fn >

    set_t;

  /*

   * Note that in the above, the library determines a resize policy

   *    appropriate for direct_mod_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());

}