libstdc++-v3/testsuite/ext/pb_ds/example/priority_queue_xref.cc

   1 // -*- C++ -*-
   2
   3 // Copyright (C) 2005-2020 Free Software Foundation, Inc.
   4 //
   5 // This file is part of the GNU ISO C++ Library.  This library is free
   6 // software; you can redistribute it and/or modify it under the terms
   7 // of the GNU General Public License as published by the Free Software
   8 // Foundation; either version 3, or (at your option) any later
   9 // version.
  10
  11 // This library is distributed in the hope that it will be useful, but
  12 // WITHOUT ANY WARRANTY; without even the implied warranty of
  13 // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  14 // General Public License for more details.
  15
  16 // You should have received a copy of the GNU General Public License
  17 // along with this library; see the file COPYING3.  If not see
  18 // <http://www.gnu.org/licenses/>.
  19
  20
  21 // Copyright (C) 2004 Ami Tavory and Vladimir Dreizin, IBM-HRL.
  22
  23 // Permission to use, copy, modify, sell, and distribute this software
  24 // is hereby granted without fee, provided that the above copyright
  25 // notice appears in all copies, and that both that copyright notice
  26 // and this permission notice appear in supporting documentation. None
  27 // of the above authors, nor IBM Haifa Research Laboratories, make any
  28 // representation about the suitability of this software for any
  29 // purpose. It is provided "as is" without express or implied
  30 // warranty.
  31
  32 /**
  33  * @file priority_queue_xref_example.cpp
  34  * A basic example showing how to cross-reference priority queues and other
  35  *    containers for erase.
  36  */
  37
  38 /**
  39  * This example shows how to cross-reference priority queues
  40  * and other containers. I.e., using an associative container to
  41  * map keys to entries in a priority queue, and using the priority
  42  * queue to map entries to the associative container. The combination
  43  * can be used for fast operations involving both priorities and
  44  * arbitrary keys.
  45  *
  46  * The most useful examples of this technique are usually from the
  47  * field of graph algorithms (where erasing or modifying an arbitrary
  48  * entry of a priority queue is sometimes necessary), but a full-blown
  49  * example would be too long. Instead, this example shows a very simple
  50  * version of Dijkstra's
  51  */
  52
  53 #include <iostream>
  54 #include <cassert>
  55 #include <ext/pb_ds/priority_queue.hpp>
  56 #include <ext/pb_ds/assoc_container.hpp>
  57
  58 using namespace std;
  59 using namespace __gnu_pbds;
  60
  61 // A priority queue of integers, which supports fast pushes,
  62 // duplicated-int avoidance, and arbitrary-int erases.
  63 class mapped_priority_queue
  64 {
  65 public:
  66
  67   // Pushes an int into the container. If the key is already in, this
  68   // is a no-op.
  69   void
  70   push(const int& r_str);
  71
  72   // Returns a const reference to the largest int in the container.
  73   int
  74   top() const
  75   {
  76     assert(!empty());
  77     return m_pq.top();
  78   }
  79
  80   // Erases the largest int in the container.
  81   void
  82   pop();
  83
  84   // Erases an arbitrary int. If the int is not in the container, this
  85   // is a no-op, and the return value is false.
  86   bool
  87   erase(const int& r_str);
  88
  89   bool
  90   empty() const
  91   { return m_pq.empty(); }
  92
  93   size_t
  94   size() const
  95   { return m_pq.size(); }
  96
  97 private:
  98   // A priority queue of strings.
  99   typedef __gnu_pbds::priority_queue< int> pq_t;
 100
 101   // A hash-table mapping strings to point_iterators inside the
 102   // priority queue.
 103   typedef cc_hash_table< int, pq_t::point_iterator> map_t;
 104
 105   pq_t m_pq;
 106   map_t m_map;
 107 };
 108
 109 void
 110 mapped_priority_queue::
 111 push(const int& r_str)
 112 {
 113   // First check if the int is already in the container. If so, just return.
 114   if (m_map.find(r_str) != m_map.end())
 115     return;
 116
 117   // Push the int into the priority queue, and store a point_iterator to it.
 118   pq_t::point_iterator pq_it = m_pq.push(r_str);
 119
 120   try
 121     {
 122       // Now make the map associate the int to the point_iterator.
 123       m_map[r_str] = pq_it;
 124     }
 125   catch(...)
 126     {
 127       // If the above failed, we need to remove the int from the
 128       // priority queue as well.
 129       m_pq.erase(pq_it);
 130       throw;
 131     }
 132 }
 133
 134 void
 135 mapped_priority_queue::
 136 pop()
 137 {
 138   assert(!empty());
 139
 140   // Erase the int from the map.
 141   m_map.erase(m_pq.top());
 142
 143   // ...then from the priority queue.
 144   m_pq.pop();
 145 }
 146
 147 bool
 148 mapped_priority_queue::
 149 erase(const int& r_str)
 150 {
 151   map_t::point_iterator map_it = m_map.find(r_str);
 152
 153   // If the int is not in the map, this is a no-op.
 154   if (map_it == m_map.end())
 155     return false;
 156
 157   // Otherwise, we erase it from the priority queue.
 158   m_pq.erase(map_it->second);
 159
 160   // ...then from the map.
 161   m_map.erase(r_str);
 162
 163   return true;
 164 }
 165
 166 int main()
 167 {
 168   // Push some values into the container object.
 169   mapped_priority_queue m;
 170   m.push(1);
 171   m.push(2);
 172
 173   // The following four operations are no-ops: 2 and 1 are already in
 174   // the container.
 175   m.push(2);
 176   m.push(2);
 177   m.push(2);
 178   m.push(1);
 179
 180   m.push(10);
 181   m.push(11);
 182   m.push(12);
 183
 184   // The size should be 5, since m contains the set {1, 2, 10, 11, 12}.
 185   assert(m.size() == 5);
 186
 187   // The largest value should be 12.
 188   assert(m.top() == 12);
 189
 190   // Now erase some values.
 191
 192   // Erasing 1 actually erases a value.
 193   assert(m.erase(1));
 194
 195   // ...but erasing 1 again is a no-op.
 196   assert(!m.erase(1));
 197
 198   // The size should be 5, since m contains the set {2, 10, 11, 12}.
 199   assert(m.size() == 4);
 200
 201   // Now print the values in the container.
 202   while (!m.empty())
 203     {
 204       cout << m.top() << endl;
 205       m.pop();
 206     }
 207
 208   return 0;
 209 }
 210