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

   1 // -*- C++ -*-
   2
   3 // Copyright (C) 2005-2019 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_dijkstra_example.cpp
  34  * A basic example showing how to cross reference a vector and a
  35  * priority-queue for modify.
  36  */
  37
  38 /**
  39  * This example shows how to cross-reference priority queues
  40  * and a vector. I.e., using a vector to
  41  * map keys to entries in a priority queue, and using the priority
  42  * queue to map entries to the vector. The combination
  43  * can be used for fast modification of keys.
  44  *
  45  * As an example, a very simple form of Diskstra's algorithm is used. The graph
  46  * is represented by an adjacency matrix. Nodes and vertices are size_ts, and
  47  * it is assumed that the minimal path between any two nodes is less than 1000.
  48  */
  49
  50
  51
  52 #include <vector>
  53 #include <iostream>
  54 #include <ext/pb_ds/priority_queue.hpp>
  55
  56 using namespace std;
  57 using namespace __gnu_pbds;
  58
  59 // The value type of the priority queue.
  60 // The first entry is the node's id, and the second is the distance.
  61 typedef std::pair<size_t, size_t> pq_value;
  62
  63 // Comparison functor used to compare priority-queue value types.
  64 struct pq_value_cmp : public binary_function<pq_value, pq_value, bool>
  65 {
  66   inline bool
  67   operator()(const pq_value& r_lhs, const pq_value& r_rhs) const
  68   {
  69     // Note that a value is considered smaller than a different value
  70     // if its distance is* larger*. This is because by STL
  71     // conventions, "larger" entries are nearer the top of the
  72     // priority queue.
  73     return r_rhs.second < r_lhs.second;
  74   }
  75 };
  76
  77 int main()
  78 {
  79   enum
  80     {
  81       // Number of vertices is hard-coded in this example.
  82       num_vertices = 5,
  83       // "Infinity".
  84       graph_inf = 1000
  85     };
  86
  87   // The edge-distance matrix.
  88   // For example, the distance from node 0 to node 1 is 5, and the
  89   // distance from node 1 to node 0 is 2.
  90   const size_t a_a_edge_legnth[num_vertices][num_vertices] =
  91     {
  92       {0, 5, 3, 7, 6},
  93       {2, 0, 2, 8, 9},
  94       {2, 1, 0, 8, 0},
  95       {1, 8, 3, 0, 2},
  96       {2, 3, 4, 2, 0}
  97     };
  98
  99   // The priority queue type.
 100   typedef __gnu_pbds::priority_queue< pq_value, pq_value_cmp> pq_t;
 101
 102   // The priority queue object.
 103   pq_t p;
 104
 105   // This vector contains for each node, a find-iterator into the
 106   // priority queue.
 107   vector<pq_t::point_iterator> a_it;
 108
 109   // First we initialize the data structures.
 110
 111   // For each node, we push into the priority queue a value
 112   // identifying it with a distance of infinity.
 113   for (size_t i = 0; i < num_vertices; ++i)
 114     a_it.push_back(p.push(pq_value(i, graph_inf)));
 115
 116   // Now we take the initial node, in this case 0, and modify its
 117   // distance to 0.
 118   p.modify(a_it[0], pq_value(0, 0));
 119
 120   // The priority queue contains all vertices whose final distance has
 121   // not been determined, so to finish the algorithm, we must loop
 122   // until it is empty.
 123   while (!p.empty())
 124     {
 125       // First we find the node whose distance is smallest.
 126       const pq_value& r_v = p.top();
 127       const size_t node_id = r_v.first;
 128       const size_t dist = r_v.second;
 129
 130       // This is the node's final distance, so we can print it out.
 131       cout << "The distance from 0 to " << node_id
 132            << " is " << dist << endl;
 133
 134       // Now we go over the node's neighbors and "relax" the
 135       // distances, if applicable.
 136       for (size_t neighbor_i = 0; neighbor_i < num_vertices; ++neighbor_i)
 137         {
 138           // Potentially, the distance to the neighbor is the distance
 139           // to the currently-considered node + the distance from this
 140           // node to the neighbor.
 141           const size_t pot_dist = dist + a_a_edge_legnth[node_id][neighbor_i];
 142
 143           if (a_it[neighbor_i] == a_it[0])
 144             continue;
 145
 146           // "Relax" the distance (if appropriate) through modify.
 147           if (pot_dist < a_it[neighbor_i]->second)
 148             p.modify(a_it[neighbor_i], pq_value(neighbor_i, pot_dist));
 149         }
 150
 151       // Done with the node, so we pop it.
 152       a_it[node_id] = a_it[0];
 153       p.pop();
 154     }
 155
 156   return 0;
 157 }