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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.core;
29  
30  /**
31   * Utility methods to avoid using bit manipulation inside code. One should use
32   * Java 1.5 import static feature to use it without class qualification inside
33   * the code.
34   * 
35   * In the DIMACS format, the literals are represented by signed integers, 0
36   * denoting the end of the clause. In the solver, the literals are represented
37   * by positive integers, in order to use them as index in arrays for instance.
38   * 
39   * <pre>
40   *  int p : a literal (p&gt;1)
41   *  p &circ; 1 : the negation of the literal
42   *  p &gt;&gt; 1 : the DIMACS number representing the variable.
43   *  int v : a DIMACS variable (v&gt;0)
44   *  v &lt;&lt; 1 : a positive literal for that variable in the solver.
45   *  v &lt;&lt; 1 &circ; 1 : a negative literal for that variable. 
46   * </pre>
47   * 
48   * @author leberre
49   * 
50   */
51  public final class LiteralsUtils {
52  
53      private LiteralsUtils() {
54          // no instance supposed to be created.
55      }
56  
57      /**
58       * Returns the variable associated to the literal
59       * @param p a literal in internal representation
60       * @return the Dimacs variable associated to that literal.
61       */
62      public static int var(int p) {
63          assert p > 1;
64          return p >> 1;
65      }
66  
67      /**
68       * Returns the opposite literal.
69       * 
70       * @param p a literal in internal representation
71       * @return the opposite literal in internal representation
72       */
73      public static int neg(int p) {
74          return p ^ 1;
75      }
76      
77      /**
78       * Returns the positive literal associated with a variable.
79       * @param var a variable in Dimacs format
80       * @return the positive literal associated with this variable in internal representation
81       */
82      public static int posLit(int var) {
83         return var << 1; 
84      }
85      
86      /**
87       * Returns the negative literal associated with a variable.
88       * @param var a variable in Dimacs format
89       * @return the negative literal associated  with this variable in internal representation
90       */
91      public static int negLit(int var) {
92          return (var << 1)^1;
93      }
94  
95  	/**
96  	 * decode the internal representation of a literal in internal representation
97  	 * into Dimacs format.
98  	 * 
99  	 * @param p
100 	 * 		the literal in internal representation
101 	 * @return the literal in dimacs representation
102 	 */
103 	public static int toDimacs(int p) {
104 		return ((p & 1) == 0 ? 1 : -1) * (p >> 1);
105 	}
106     
107 }