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// -*- C++ -*-

// Copyright (C) 2005, 2006, 2009 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 hash_shift_mask_example.cpp

 * An example showing how to write a range-hasing functor.

 */

/**

 * In some rare cases, advance knowledge of the distribution of keys allows

 * writing more efficient hash-related policies.

 * In the rather simplistic case of the example, it is known in advance that

 * all keys have 0 two lowest bits. The example shows how to write

 * a range-hashing function disregarding the two lowest bits of the hash value.

 */

#include <functional>

#include <ext/pb_ds/assoc_container.hpp>

#include <ext/pb_ds/hash_policy.hpp>

using namespace std;

using namespace __gnu_pbds;

// A simple hash functor. hash could serve instead of this functor,

// but it is not yet standard everywhere.

struct simple_int_hash : public 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 direct_mask_range_hashing<>

{

public:

  typedef size_t size_type;

  // Swaps with a different instant.

  void

  swap(shift_two_mask_range_hashing& other)

  { direct_mask_range_hashing<>::swap(other); }

  // Called by the container when internally resized.

  void

  notify_resized(size_type size)

  { 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 direct_mask_range_hashing<>::operator()(hash >> 2); }

};

int

main()

{

  // A collision-chaining hash table mapping ints to chars.

  typedef

    cc_hash_table<

    int,

    char,

    // Hash function.

    simple_int_hash,

    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';

  return 0;

}