hash_resize.cc   [plain text]

// -*- 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
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// 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
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// 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
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/**
 * @file hash_resize_example.cpp
 * An example of externally resizing a map.
 */

/**
 * This example shows how to externally manipulate the size of a hash-based
 * container object throught its resize-policy object.
 **/

#include <functional>
#include <cassert>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/hash_policy.hpp>

using namespace std;
using namespace pb_ds;

// A simple hash functor.
// hash could serve instead of this functor, but it is not yet
// standard everywhere.
struct int_hash : public unary_function<int, size_t>
{
  inline size_t
  operator()(const int& r_i) const
  { return r_i; }
};

int main()
{
  // A probing hash table mapping ints to chars.
  typedef
    gp_hash_table<
    int,
    char,
    int_hash,
    equal_to<
    int>,
    // Combining function.
    direct_mask_range_hashing<>,
    // Probe function.
    linear_probe_fn<>,
    // Resize policy.
    hash_standard_resize_policy<
    hash_exponential_size_policy<>,
    hash_load_check_resize_trigger<>,
    /* Allow external access to size.
     * Without setting this to true, external resizing
     * is not possible.
     */
    true> >
    map_t;

  map_t g;

  // Check the actual size of the container object. In this case, this
  // should be the initial size given by the size policy object.
  assert(g.get_actual_size() == 8);

  // Insert some elements.
  g[1] = 'a';
  g[2] = 'b';
  g[3] = 'c';

  // Now resize the table upward.
  g.resize(200);

  // Check the actual size of the container object. 
  // For the policy used in this example, the nearest larger size than
  // 200 is 256.
  assert(g.get_actual_size() == 256);

  g[67] = 'g';
  g[22] = 'f';

  // Regardless of the internal size, the logical size should be 5.
  assert(g.size() == 5);

  // Now resize the table downward.
  g.resize(106);

  // Check the actual size of the container object. 
  // For the policy used in this example, the nearest larger size than
  // 106 is 128.
  assert(g.get_actual_size() == 128);

  g[37] = 'f';

  // Regardless of the internal size, the logical size should be 5.
  assert(g.size() == 6);

  return 0;
}