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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.minisat.constraints.cnf;
31  
32  import org.sat4j.minisat.core.ILits;
33  import org.sat4j.specs.ContradictionException;
34  import org.sat4j.specs.IVecInt;
35  import org.sat4j.specs.UnitPropagationListener;
36  
37  /**
38   * 
39   * @author daniel
40   * @since 2.1
41   */
42  public abstract class Clauses {
43      /**
44       * Perform some sanity check before constructing a clause a) if a literal is
45       * assigned true, return null (the clause is satisfied) b) if a literal is
46       * assigned false, remove it c) if a clause contains a literal and its
47       * opposite (tautology) return null d) remove duplicate literals e) if the
48       * clause is empty, return null f) if the clause if unit, transmit it to the
49       * object responsible for unit propagation
50       * 
51       * @param ps
52       *            the list of literals
53       * @param voc
54       *            the vocabulary used
55       * @param s
56       *            the object responsible for unit propagation
57       * @return null if the clause should be ignored, the (possibly modified)
58       *         list of literals otherwise
59       * @throws ContradictionException
60       *             if discovered by unit propagation
61       */
62      public static IVecInt sanityCheck(IVecInt ps, ILits voc,
63              UnitPropagationListener s) throws ContradictionException {
64          // si un litt???ral de ps est vrai, retourner vrai
65          // enlever les litt???raux falsifi???s de ps
66          for (int i = 0; i < ps.size();) {
67              // on verifie si le litteral est affecte
68              if (voc.isUnassigned(ps.get(i))) {
69                  // on passe au literal suivant
70                  i++;
71              } else {
72                  // Si le litteral est satisfait, la clause est
73                  // satisfaite
74                  if (voc.isSatisfied(ps.get(i))) {
75                      // on retourne la clause
76                      return null;
77                  }
78                  // on enleve le ieme litteral
79                  ps.delete(i);
80  
81              }
82          }
83  
84          // on trie le vecteur ps
85          ps.sortUnique();
86  
87          // ???limine les clauses tautologiques
88          // deux litt???raux de signe oppos???s apparaissent dans la m???me
89          // clause
90          for (int i = 0; i < ps.size() - 1; i++) {
91              if (ps.get(i) == (ps.get(i + 1) ^ 1)) {
92                  // la clause est tautologique
93                  return null;
94              }
95          }
96  
97          propagationCheck(ps, s);
98  
99          return ps;
100     }
101 
102     /**
103      * Check if this clause is null or unit
104      * 
105      * @param p
106      *            the list of literals (supposed to be clean as after a call to
107      *            sanityCheck())
108      * @param s
109      *            the object responsible for unit propagation
110      * @return true iff the clause should be ignored (because it's unit)
111      * @throws ContradictionException
112      *             when detected by unit propagation
113      */
114     static boolean propagationCheck(IVecInt ps, UnitPropagationListener s)
115             throws ContradictionException {
116         if (ps.size() == 0) {
117             throw new ContradictionException("Creating Empty clause ?"); //$NON-NLS-1$
118         } else if (ps.size() == 1) {
119             if (!s.enqueue(ps.get(0))) {
120                 throw new ContradictionException("Contradictory Unit Clauses"); //$NON-NLS-1$
121             }
122             return true;
123         }
124 
125         return false;
126     }
127 
128 }