001 /*
002 * Created on Sep 20, 2004
003 *
004 */
005 package aima.logic.fol;
006
007 import java.util.ArrayList;
008 import java.util.List;
009 import java.util.Map;
010
011 import aima.logic.fol.kb.data.Literal;
012 import aima.logic.fol.parsing.AbstractFOLVisitor;
013 import aima.logic.fol.parsing.ast.AtomicSentence;
014 import aima.logic.fol.parsing.ast.Function;
015 import aima.logic.fol.parsing.ast.QuantifiedSentence;
016 import aima.logic.fol.parsing.ast.Sentence;
017 import aima.logic.fol.parsing.ast.Term;
018 import aima.logic.fol.parsing.ast.Variable;
019
020 /**
021 * @author Ravi Mohan
022 * @author Ciaran O'Reilly
023 */
024 public class SubstVisitor extends AbstractFOLVisitor {
025
026 public SubstVisitor() {
027 }
028
029 /**
030 * Note: Refer to Artificial Intelligence A Modern Approach (2nd Edition):
031 * page 273.
032 *
033 * @param theta
034 * a substitution.
035 * @param aSentence
036 * the substitution has been applied to.
037 * @return a new Sentence representing the result of applying the
038 * substitution theta to aSentence.
039 *
040 */
041 public Sentence subst(Map<Variable, Term> theta, Sentence aSentence) {
042 return (Sentence) aSentence.accept(this, theta);
043 }
044
045 public Term subst(Map<Variable, Term> theta, Term aTerm) {
046 return (Term) aTerm.accept(this, theta);
047 }
048
049 public Function subst(Map<Variable, Term> theta, Function aFunction) {
050 return (Function) aFunction.accept(this, theta);
051 }
052
053 public Literal subst(Map<Variable, Term> theta, Literal aLiteral) {
054 return aLiteral.newInstance(
055 (AtomicSentence) aLiteral.getAtomicSentence().accept(this,
056 theta));
057 }
058
059 @SuppressWarnings("unchecked")
060 @Override
061 public Object visitVariable(Variable variable, Object arg) {
062 Map<Variable, Term> substitution = (Map<Variable, Term>) arg;
063 if (substitution.containsKey(variable)) {
064 return substitution.get(variable).copy();
065 }
066 return variable.copy();
067 }
068
069 @SuppressWarnings("unchecked")
070 @Override
071 public Object visitQuantifiedSentence(QuantifiedSentence sentence,
072 Object arg) {
073
074 Map<Variable, Term> substitution = (Map<Variable, Term>) arg;
075
076 Sentence quantified = sentence.getQuantified();
077 Sentence quantifiedAfterSubs = (Sentence) quantified.accept(this, arg);
078
079 List<Variable> variables = new ArrayList<Variable>();
080 for (Variable v : sentence.getVariables()) {
081 Term st = substitution.get(v);
082 if (null != st) {
083 if (st instanceof Variable) {
084 // Only if it is a variable to I replace it, otherwise
085 // I drop it.
086 variables.add((Variable) st.copy());
087 }
088 } else {
089 // No substitution for the quantified variable, so
090 // keep it.
091 variables.add(v.copy());
092 }
093 }
094
095 // If not variables remaining on the quantifier, then drop it
096 if (variables.size() == 0) {
097 return quantifiedAfterSubs;
098 }
099
100 return new QuantifiedSentence(sentence.getQuantifier(), variables,
101 quantifiedAfterSubs);
102 }
103 }