1 /*******************************************************************************
2 * SAT4J: a SATisfiability library for Java Copyright (C) 2004, 2012 Artois University and CNRS
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 * Contributors:
28 * CRIL - initial API and implementation
29 *******************************************************************************/
30 package org.sat4j.tools;
31
32 import org.sat4j.core.VecInt;
33 import org.sat4j.specs.ISolver;
34 import org.sat4j.specs.IVecInt;
35 import org.sat4j.specs.TimeoutException;
36
37 /**
38 * The aim of this class is to compute efficiently the literals implied by the
39 * set of constraints (also called backbone or unit implicates).
40 *
41 * The work has been done in the context of ANR BR4CP.
42 *
43 * @author leberre
44 *
45 */
46 public class Backbone {
47
48 private Backbone() {
49
50 }
51
52 /**
53 * Computes the backbone of a formula following the algorithm described in
54 * João Marques-Silva, Mikolás Janota, Inês Lynce: On Computing Backbones of
55 * Propositional Theories. ECAI 2010: 15-20
56 *
57 * We use Sat4j's ability to compute prime implicants instead of models to
58 * simplify the model at each step.
59 *
60 * @param solver
61 * @return
62 * @throws TimeoutException
63 */
64 public static IVecInt compute(ISolver solver) throws TimeoutException {
65 return compute(solver, VecInt.EMPTY);
66 }
67
68 /**
69 * Computes the backbone of a formula following the algorithm described in
70 * João Marques-Silva, Mikolás Janota, Inês Lynce: On Computing Backbones of
71 * Propositional Theories. ECAI 2010: 15-20
72 *
73 * We use Sat4j's ability to compute prime implicants instead of models to
74 * simplify the model at each step.
75 *
76 * @param solver
77 * @param assumptions
78 * @return
79 * @throws TimeoutException
80 */
81 public static IVecInt compute(ISolver solver, IVecInt assumptions)
82 throws TimeoutException {
83 boolean result = solver.isSatisfiable(assumptions);
84 if (!result) {
85 return VecInt.EMPTY;
86 }
87 return compute(solver, solver.primeImplicant(), assumptions);
88
89 }
90
91 public static IVecInt compute(ISolver solver, int[] implicant)
92 throws TimeoutException {
93 return compute(solver, implicant, VecInt.EMPTY);
94 }
95
96 public static IVecInt compute(ISolver solver, int[] implicant,
97 IVecInt assumptions) throws TimeoutException {
98 IVecInt litsToTest = new VecInt();
99 for (int p : implicant) {
100 if (!assumptions.contains(p)) {
101 litsToTest.push(-p);
102 }
103 }
104 IVecInt candidates = new VecInt();
105 assumptions.copyTo(candidates);
106 int p;
107 while (!litsToTest.isEmpty()) {
108 p = litsToTest.last();
109 candidates.push(p);
110 litsToTest.pop();
111 if (solver.isSatisfiable(candidates)) {
112 candidates.pop();
113 implicant = solver.primeImplicant();
114 removeVarNotPresentAndSatisfiedLits(implicant, litsToTest,
115 solver.nVars());
116 } else {
117 candidates.pop().push(-p);
118 }
119 }
120 return candidates;
121 }
122
123 private static void removeVarNotPresentAndSatisfiedLits(int[] implicant,
124 IVecInt litsToTest, int n) {
125 int[] marks = new int[n + 1];
126 for (int p : implicant) {
127 marks[p > 0 ? p : -p] = p;
128 }
129 int q, mark;
130 for (int i = 0; i < litsToTest.size();) {
131 q = litsToTest.get(i);
132 mark = marks[q > 0 ? q : -q];
133 if (mark == 0 || mark == q) {
134 litsToTest.delete(i);
135 } else {
136 i++;
137 }
138 }
139 }
140 }