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>1)
41 * p ˆ 1 : the negation of the literal
42 * p >> 1 : the DIMACS number representing the variable.
43 * int v : a DIMACS variable (v>0)
44 * v << 1 : a positive literal for that variable in the solver.
45 * v << 1 ˆ 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 *
60 * @param p
61 * a literal in internal representation
62 * @return the Dimacs variable associated to that literal.
63 */
64 public static int var(int p) {
65 assert p > 1;
66 return p >> 1;
67 }
68
69 /**
70 * Returns the opposite literal.
71 *
72 * @param p
73 * a literal in internal representation
74 * @return the opposite literal in internal representation
75 */
76 public static int neg(int p) {
77 return p ^ 1;
78 }
79
80 /**
81 * Returns the positive literal associated with a variable.
82 *
83 * @param var
84 * a variable in Dimacs format
85 * @return the positive literal associated with this variable in internal
86 * representation
87 */
88 public static int posLit(int var) {
89 return var << 1;
90 }
91
92 /**
93 * Returns the negative literal associated with a variable.
94 *
95 * @param var
96 * a variable in Dimacs format
97 * @return the negative literal associated with this variable in internal
98 * representation
99 */
100 public static int negLit(int var) {
101 return (var << 1) ^ 1;
102 }
103
104 /**
105 * decode the internal representation of a literal in internal
106 * representation into Dimacs format.
107 *
108 * @param p
109 * the literal in internal representation
110 * @return the literal in dimacs representation
111 */
112 public static int toDimacs(int p) {
113 return ((p & 1) == 0 ? 1 : -1) * (p >> 1);
114 }
115
116 /**
117 * encode the classical Dimacs representation (negated integers for negated
118 * literals) into the internal format.
119 *
120 * @param x
121 * the literal in Dimacs format
122 * @return the literal in internal format.
123 * @since 2.2
124 */
125 public static int toInternal(int x) {
126 return ((x < 0) ? ((-x) << 1) ^ 1 : (x << 1));
127 }
128 }