--- /dev/null
+// -*- 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 priority_queue_dijkstra_example.cpp
+ * A basic example showing how to cross reference a vector and a
+ * priority-queue for modify.
+ */
+
+/**
+ * This example shows how to cross-reference priority queues
+ * and a vector. I.e., using a vector to
+ * map keys to entries in a priority queue, and using the priority
+ * queue to map entries to the vector. The combination
+ * can be used for fast modification of keys.
+ *
+ * As an example, a very simple form of Diskstra's algorithm is used. The graph
+ * is represented by an adjacency matrix. Nodes and vertices are size_ts, and
+ * it is assumed that the minimal path between any two nodes is less than 1000.
+ */
+
+
+
+#include <vector>
+#include <iostream>
+#include <ext/pb_ds/priority_queue.hpp>
+
+using namespace std;
+using namespace __gnu_pbds;
+
+// The value type of the priority queue.
+// The first entry is the node's id, and the second is the distance.
+typedef std::pair<size_t, size_t> pq_value;
+
+// Comparison functor used to compare priority-queue value types.
+struct pq_value_cmp : public binary_function<pq_value, pq_value, bool>
+{
+ inline bool
+ operator()(const pq_value& r_lhs, const pq_value& r_rhs) const
+ {
+ // Note that a value is considered smaller than a different value
+ // if its distance is* larger*. This is because by STL
+ // conventions, "larger" entries are nearer the top of the
+ // priority queue.
+ return r_rhs.second < r_lhs.second;
+ }
+};
+
+int main()
+{
+ enum
+ {
+ // Number of vertices is hard-coded in this example.
+ num_vertices = 5,
+ // "Infinity".
+ graph_inf = 1000
+ };
+
+ // The edge-distance matrix.
+ // For example, the distance from node 0 to node 1 is 5, and the
+ // distance from node 1 to node 0 is 2.
+ const size_t a_a_edge_legnth[num_vertices][num_vertices] =
+ {
+ {0, 5, 3, 7, 6},
+ {2, 0, 2, 8, 9},
+ {2, 1, 0, 8, 0},
+ {1, 8, 3, 0, 2},
+ {2, 3, 4, 2, 0}
+ };
+
+ // The priority queue type.
+ typedef __gnu_pbds::priority_queue< pq_value, pq_value_cmp> pq_t;
+
+ // The priority queue object.
+ pq_t p;
+
+ // This vector contains for each node, a find-iterator into the
+ // priority queue.
+ vector<pq_t::point_iterator> a_it;
+
+ // First we initialize the data structures.
+
+ // For each node, we push into the priority queue a value
+ // identifying it with a distance of infinity.
+ for (size_t i = 0; i < num_vertices; ++i)
+ a_it.push_back(p.push(pq_value(i, graph_inf)));
+
+ // Now we take the initial node, in this case 0, and modify its
+ // distance to 0.
+ p.modify(a_it[0], pq_value(0, 0));
+
+ // The priority queue contains all vertices whose final distance has
+ // not been determined, so to finish the algorithm, we must loop
+ // until it is empty.
+ while (!p.empty())
+ {
+ // First we find the node whose distance is smallest.
+ const pq_value& r_v = p.top();
+ const size_t node_id = r_v.first;
+ const size_t dist = r_v.second;
+
+ // This is the node's final distance, so we can print it out.
+ cout << "The distance from 0 to " << node_id
+ << " is " << dist << endl;
+
+ // Now we go over the node's neighbors and "relax" the
+ // distances, if applicable.
+ for (size_t neighbor_i = 0; neighbor_i < num_vertices; ++neighbor_i)
+ {
+ // Potentially, the distance to the neighbor is the distance
+ // to the currently-considered node + the distance from this
+ // node to the neighbor.
+ const size_t pot_dist = dist + a_a_edge_legnth[node_id][neighbor_i];
+
+ if (a_it[neighbor_i] == a_it[0])
+ continue;
+
+ // "Relax" the distance (if appropriate) through modify.
+ if (pot_dist < a_it[neighbor_i]->second)
+ p.modify(a_it[neighbor_i], pq_value(neighbor_i, pot_dist));
+ }
+
+ // Done with the node, so we pop it.
+ a_it[node_id] = a_it[0];
+ p.pop();
+ }
+
+ return 0;
+}
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