1 /*******************************************************************************
2 * SAT4J: a SATisfiability library for Java Copyright (C) 2004-2008 Daniel Le Berre
3 *
4 * All rights reserved. This program and the accompanying materials
5 * are made available under the terms of the Eclipse Public License v1.0
6 * which accompanies this distribution, and is available at
7 * http://www.eclipse.org/legal/epl-v10.html
8 *
9 * Alternatively, the contents of this file may be used under the terms of
10 * either the GNU Lesser General Public License Version 2.1 or later (the
11 * "LGPL"), in which case the provisions of the LGPL are applicable instead
12 * of those above. If you wish to allow use of your version of this file only
13 * under the terms of the LGPL, and not to allow others to use your version of
14 * this file under the terms of the EPL, indicate your decision by deleting
15 * the provisions above and replace them with the notice and other provisions
16 * required by the LGPL. If you do not delete the provisions above, a recipient
17 * may use your version of this file under the terms of the EPL or the LGPL.
18 *
19 * Based on the original MiniSat specification from:
20 *
21 * An extensible SAT solver. Niklas Een and Niklas Sorensson. Proceedings of the
22 * Sixth International Conference on Theory and Applications of Satisfiability
23 * Testing, LNCS 2919, pp 502-518, 2003.
24 *
25 * See www.minisat.se for the original solver in C++.
26 *
27 *******************************************************************************/
28 package org.sat4j.specs;
29
30
31 /**
32 * Represents an optimization problem. The SAT solver will find suboptimal solutions
33 * of the problem until no more solutions are available. The latest solution found
34 * will be the optimal one.
35 *
36 * Such kind of problem is supposed to be handled:
37 * <pre>
38 boolean isSatisfiable = false;
39
40 IOptimizationProblem optproblem = (IOptimizationProblem) problem;
41
42 try {
43 while (optproblem.admitABetterSolution()) {
44 if (!isSatisfiable) {
45 if (optproblem.nonOptimalMeansSatisfiable()) {
46 setExitCode(ExitCode.SATISFIABLE);
47 if (optproblem.hasNoObjectiveFunction()) {
48 return;
49 }
50 log("SATISFIABLE"); //$NON-NLS-1$
51 }
52 isSatisfiable = true;
53 log("OPTIMIZING..."); //$NON-NLS-1$
54 }
55 log("Got one! Elapsed wall clock time (in seconds):" //$NON-NLS-1$
56 + (System.currentTimeMillis() - getBeginTime())
57 / 1000.0);
58 getLogWriter().println(
59 CURRENT_OPTIMUM_VALUE_PREFIX
60 + optproblem.calculateObjective());
61 optproblem.discard();
62 }
63 if (isSatisfiable) {
64 setExitCode(ExitCode.OPTIMUM_FOUND);
65 } else {
66 setExitCode(ExitCode.UNSATISFIABLE);
67 }
68 } catch (ContradictionException ex) {
69 assert isSatisfiable;
70 setExitCode(ExitCode.OPTIMUM_FOUND);
71 }
72 </pre>
73 *
74 * @author leberre
75 *
76 */
77 public interface IOptimizationProblem extends IProblem {
78
79 /**
80 * Look for a solution of the optimization problem.
81 *
82 * @return true if a better solution than current one can be found.
83 * @throws TimeoutException if the solver cannot answer in reasonable time.
84 * @see ISolver#setTimeout(int)
85 */
86 boolean admitABetterSolution() throws TimeoutException;
87
88 /**
89 * If the optimization problem has no objective function, then it is a simple
90 * decision problem.
91 *
92 * @return true if the problem is a decision problem, false if the problem is
93 * an optimization problem.
94 */
95 boolean hasNoObjectiveFunction();
96
97 /**
98 * A suboptimal solution has different meaning depending of the optimization problem
99 * considered.
100 *
101 * For instance, in the case of MAXSAT, a suboptimal solution does not mean that the problem is
102 * satisfiable, while in pseudo boolean optimization, it is true.
103 *
104 * @return true if founding a suboptimal solution means that the problem is satisfiable.
105 */
106 boolean nonOptimalMeansSatisfiable();
107
108 /**
109 * Compute the value of the objective function for the current solution.
110 * A call to that method only makes sense if hasNoObjectiveFunction()==false.
111 *
112 * @return the value of the objective function.
113 */
114 Number calculateObjective();
115
116 /**
117 * Discard the current solution in the optimization problem.
118 *
119 * @throws ContradictionException if a trivial inconsistency is detected.
120 */
121 void discard() throws ContradictionException;
122
123 }