001 /*
002 * Created on Dec 4, 2004
003 *
004 */
005 package aima.logic.propositional.visitors;
006
007 import aima.logic.propositional.parsing.AbstractPLVisitor;
008 import aima.logic.propositional.parsing.ast.BinarySentence;
009 import aima.logic.propositional.parsing.ast.Sentence;
010 import aima.logic.propositional.parsing.ast.UnarySentence;
011
012 /**
013 * @author Ravi Mohan
014 *
015 */
016 public class CNFTransformer extends AbstractPLVisitor {
017 @Override
018 public Object visitBinarySentence(BinarySentence bs, Object arg) {
019 if (bs.isBiconditional()) {
020 return transformBiConditionalSentence(bs);
021 } else if (bs.isImplication()) {
022 return transformImpliedSentence(bs);
023 } else if (bs.isOrSentence()
024 && (bs.firstTermIsAndSentence() || bs.secondTermIsAndSentence())) {
025 return distributeOrOverAnd(bs);
026 } else {
027 return super.visitBinarySentence(bs, arg);
028 }
029 }
030
031 @Override
032 public Object visitNotSentence(UnarySentence us, Object arg) {
033 return transformNotSentence(us);
034 }
035
036 public Sentence transform(Sentence s) {
037 Sentence toTransform = s;
038 while (!(toTransform.equals(step(toTransform)))) {
039 toTransform = step(toTransform);
040 }
041
042 return toTransform;
043 }
044
045 private Sentence step(Sentence s) {
046 return (Sentence) s.accept(this, null);
047 }
048
049 private Sentence transformBiConditionalSentence(BinarySentence bs) {
050 Sentence first = new BinarySentence("=>", (Sentence) bs.getFirst()
051 .accept(this, null), (Sentence) bs.getSecond().accept(this,
052 null));
053 Sentence second = new BinarySentence("=>", (Sentence) bs.getSecond()
054 .accept(this, null), (Sentence) bs.getFirst()
055 .accept(this, null));
056 return new BinarySentence("AND", first, second);
057 }
058
059 private Sentence transformImpliedSentence(BinarySentence bs) {
060 Sentence first = new UnarySentence((Sentence) bs.getFirst().accept(
061 this, null));
062 return new BinarySentence("OR", first, (Sentence) bs.getSecond()
063 .accept(this, null));
064 }
065
066 private Sentence transformNotSentence(UnarySentence us) {
067 if (us.getNegated() instanceof UnarySentence) {
068 return (Sentence) ((UnarySentence) us.getNegated()).getNegated()
069 .accept(this, null);
070 } else if (us.getNegated() instanceof BinarySentence) {
071 BinarySentence bs = (BinarySentence) us.getNegated();
072 if (bs.isAndSentence()) {
073 Sentence first = new UnarySentence((Sentence) bs.getFirst()
074 .accept(this, null));
075 Sentence second = new UnarySentence((Sentence) bs.getSecond()
076 .accept(this, null));
077 return new BinarySentence("OR", first, second);
078 } else if (bs.isOrSentence()) {
079 Sentence first = new UnarySentence((Sentence) bs.getFirst()
080 .accept(this, null));
081 Sentence second = new UnarySentence((Sentence) bs.getSecond()
082 .accept(this, null));
083 return new BinarySentence("AND", first, second);
084 } else {
085 return (Sentence) super.visitNotSentence(us, null);
086 }
087 } else {
088 return (Sentence) super.visitNotSentence(us, null);
089 }
090 }
091
092 private Sentence distributeOrOverAnd(BinarySentence bs) {
093 BinarySentence andTerm = bs.firstTermIsAndSentence() ? (BinarySentence) bs
094 .getFirst()
095 : (BinarySentence) bs.getSecond();
096 Sentence otherterm = bs.firstTermIsAndSentence() ? bs.getSecond() : bs
097 .getFirst();
098 // (alpha or (beta and gamma) = ((alpha or beta) and (alpha or gamma))
099 Sentence alpha = (Sentence) otherterm.accept(this, null);
100 Sentence beta = (Sentence) andTerm.getFirst().accept(this, null);
101 Sentence gamma = (Sentence) andTerm.getSecond().accept(this, null);
102 Sentence distributed = new BinarySentence("AND", new BinarySentence(
103 "OR", alpha, beta), new BinarySentence("OR", alpha, gamma));
104 return distributed;
105 }
106
107 }