hash_shift_mask.cc

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// -*- 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 hash_shift_mask_example.cpp
 * An example showing how to write a range-hasing 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>

/*
 * A simple hash functor.
 *	std::hash could serve instead of this functor,
 *	but it is not yet standard everywhere.
 */
struct simple_int_hash : public std::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 pb_assoc::direct_mask_range_hashing<>
{
public:
  typedef size_t size_type;

public:
  /*
   * Swaps with a different instant.
   */
  void
  swap(shift_two_mask_range_hashing& r_other)
  {
    pb_assoc::direct_mask_range_hashing<>::swap(r_other);
  }

  /*
   * Called by the container when internally resized.
   */
  void
  notify_resized(size_type size)
  {
    pb_assoc::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 (pb_assoc::direct_mask_range_hashing<>::operator()(
							      hash >> 2));
  }
};

int
main()
{
  /*
   * A collision-chaining hash table mapping ints to chars.
   */
  typedef
    pb_assoc::cc_hash_assoc_cntnr<
    int,
    char,
    // Hash function.
    simple_int_hash,
    std::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';
}