pb_ds/example/priority_queue_xref.cc

// -*- C++ -*-

// Copyright (C) 2005-2021 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 priority_queue_xref_example.cpp
 * A basic example showing how to cross-reference priority queues and other
 *    containers for erase.
 */

/**
 * This example shows how to cross-reference priority queues
 * and other containers. I.e., using an associative container to
 * map keys to entries in a priority queue, and using the priority
 * queue to map entries to the associative container. The combination
 * can be used for fast operations involving both priorities and
 * arbitrary keys.
 *
 * The most useful examples of this technique are usually from the
 * field of graph algorithms (where erasing or modifying an arbitrary
 * entry of a priority queue is sometimes necessary), but a full-blown
 * example would be too long. Instead, this example shows a very simple
 * version of Dijkstra's
 */

#include <iostream>
#include <cassert>
#include <ext/pb_ds/priority_queue.hpp>
#include <ext/pb_ds/assoc_container.hpp>

using namespace std;
using namespace __gnu_pbds;

// A priority queue of integers, which supports fast pushes,
// duplicated-int avoidance, and arbitrary-int erases.
class mapped_priority_queue
{
public:

  // Pushes an int into the container. If the key is already in, this
  // is a no-op.
  void
  push(const int& r_str);

  // Returns a const reference to the largest int in the container.
  int
  top() const
  {
    assert(!empty());
    return m_pq.top();
  }

  // Erases the largest int in the container.
  void
  pop();

  // Erases an arbitrary int. If the int is not in the container, this
  // is a no-op, and the return value is false.
  bool
  erase(const int& r_str);

  bool
  empty() const
  { return m_pq.empty(); }

  size_t
  size() const
  { return m_pq.size(); }

private:
  // A priority queue of strings.
  typedef __gnu_pbds::priority_queue< int> pq_t;

  // A hash-table mapping strings to point_iterators inside the
  // priority queue.
  typedef cc_hash_table< int, pq_t::point_iterator> map_t;

  pq_t m_pq;
  map_t m_map;
};

void
mapped_priority_queue::
push(const int& r_str)
{
  // First check if the int is already in the container. If so, just return.
  if (m_map.find(r_str) != m_map.end())
    return;

  // Push the int into the priority queue, and store a point_iterator to it.
  pq_t::point_iterator pq_it = m_pq.push(r_str);

  try
    {
      // Now make the map associate the int to the point_iterator.
      m_map[r_str] = pq_it;
    }
  catch(...)
    {
      // If the above failed, we need to remove the int from the
      // priority queue as well.
      m_pq.erase(pq_it);
      throw;
    }
}

void
mapped_priority_queue::
pop()
{
  assert(!empty());

  // Erase the int from the map.
  m_map.erase(m_pq.top());

  // ...then from the priority queue.
  m_pq.pop();
}

bool
mapped_priority_queue::
erase(const int& r_str)
{
  map_t::point_iterator map_it = m_map.find(r_str);

  // If the int is not in the map, this is a no-op.
  if (map_it == m_map.end())
    return false;

  // Otherwise, we erase it from the priority queue.
  m_pq.erase(map_it->second);

  // ...then from the map.
  m_map.erase(r_str);

  return true;
}

int main()
{
  // Push some values into the container object.
  mapped_priority_queue m;
  m.push(1);
  m.push(2);

  // The following four operations are no-ops: 2 and 1 are already in
  // the container.
  m.push(2);
  m.push(2);
  m.push(2);
  m.push(1);

  m.push(10);
  m.push(11);
  m.push(12);

  // The size should be 5, since m contains the set {1, 2, 10, 11, 12}.
  assert(m.size() == 5);

  // The largest value should be 12.
  assert(m.top() == 12);

  // Now erase some values.

  // Erasing 1 actually erases a value.
  assert(m.erase(1));

  // ...but erasing 1 again is a no-op.
  assert(!m.erase(1));

  // The size should be 5, since m contains the set {2, 10, 11, 12}.
  assert(m.size() == 4);

  // Now print the values in the container.
  while (!m.empty())
    {
      cout << m.top() << endl;
      m.pop();
    }

  return 0;
}