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36fed23c | 1 | // -*- C++ -*- |
2 | ||
fbd26352 | 3 | // Copyright (C) 2005-2019 Free Software Foundation, Inc. |
36fed23c | 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 | |
6bc9506f | 8 | // Foundation; either version 3, or (at your option) any later |
36fed23c | 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 | |
6bc9506f | 17 | // along with this library; see the file COPYING3. If not see |
18 | // <http://www.gnu.org/licenses/>. | |
36fed23c | 19 | |
36fed23c | 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; | |
b34535d7 | 57 | using namespace __gnu_pbds; |
36fed23c | 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. | |
b34535d7 | 100 | typedef __gnu_pbds::priority_queue< pq_value, pq_value_cmp> pq_t; |
36fed23c | 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 | ||
7d03ec88 | 143 | if (a_it[neighbor_i] == a_it[0]) |
144 | continue; | |
145 | ||
36fed23c | 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. | |
7d03ec88 | 152 | a_it[node_id] = a_it[0]; |
36fed23c | 153 | p.pop(); |
154 | } | |
155 | ||
156 | return 0; | |
157 | } |