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AIMA-Java Core Algorithms from the book Artificial Intelligence a Modern Approach 3rd Ed.
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package aima.core.logic.fol;
import java.util.ArrayList;
import java.util.LinkedHashMap;
import java.util.LinkedHashSet;
import java.util.List;
import java.util.Map;
import java.util.Set;
import aima.core.logic.fol.kb.data.CNF;
import aima.core.logic.fol.kb.data.Clause;
import aima.core.logic.fol.parsing.FOLParser;
import aima.core.logic.fol.parsing.FOLVisitor;
import aima.core.logic.fol.parsing.ast.ConnectedSentence;
import aima.core.logic.fol.parsing.ast.Constant;
import aima.core.logic.fol.parsing.ast.Function;
import aima.core.logic.fol.parsing.ast.NotSentence;
import aima.core.logic.fol.parsing.ast.Predicate;
import aima.core.logic.fol.parsing.ast.QuantifiedSentence;
import aima.core.logic.fol.parsing.ast.Sentence;
import aima.core.logic.fol.parsing.ast.Term;
import aima.core.logic.fol.parsing.ast.TermEquality;
import aima.core.logic.fol.parsing.ast.Variable;
/**
* Artificial Intelligence A Modern Approach (3rd Edition): page 345.
*
* Every sentence of first-order logic can be converted into an inferentially
* equivalent CNF sentence.
*
* Note: Transformation rules extracted from 346 and 347, which are
* essentially the INSEADO method outlined in: INSEADO Rules
*
* @author Ciaran O'Reilly
* @author Mike Stampone
*/
public class CNFConverter {
private FOLParser parser = null;
private SubstVisitor substVisitor;
public CNFConverter(FOLParser parser) {
this.parser = parser;
this.substVisitor = new SubstVisitor();
}
/**
* Returns the specified sentence as a list of clauses, where each clause is
* a disjunction of literals.
*
* @param aSentence
* a sentence in first order logic (predicate calculus)
*
* @return the specified sentence as a list of clauses, where each clause is
* a disjunction of literals.
*/
public CNF convertToCNF(Sentence aSentence) {
// I)mplications Out:
Sentence implicationsOut = (Sentence) aSentence.accept(
new ImplicationsOut(), null);
// N)egations In:
Sentence negationsIn = (Sentence) implicationsOut.accept(
new NegationsIn(), null);
// S)tandardize variables:
// For sentences like:
// (FORALL x P(x)) V (EXISTS x Q(x)),
// which use the same variable name twice, change the name of one of the
// variables.
Sentence saQuantifiers = (Sentence) negationsIn.accept(
new StandardizeQuantiferVariables(substVisitor),
new LinkedHashSet());
// Remove explicit quantifiers, by skolemizing existentials
// and dropping universals:
// E)xistentials Out
// A)lls Out:
Sentence andsAndOrs = (Sentence) saQuantifiers.accept(
new RemoveQuantifiers(parser), new LinkedHashSet());
// D)istribution
// V over ^:
Sentence orDistributedOverAnd = (Sentence) andsAndOrs.accept(
new DistributeOrOverAnd(), null);
// O)perators Out
return (new CNFConstructor()).construct(orDistributedOverAnd);
}
}
class ImplicationsOut implements FOLVisitor {
public ImplicationsOut() {
}
public Object visitPredicate(Predicate p, Object arg) {
return p;
}
public Object visitTermEquality(TermEquality equality, Object arg) {
return equality;
}
public Object visitVariable(Variable variable, Object arg) {
return variable;
}
public Object visitConstant(Constant constant, Object arg) {
return constant;
}
public Object visitFunction(Function function, Object arg) {
return function;
}
public Object visitNotSentence(NotSentence notSentence, Object arg) {
Sentence negated = notSentence.getNegated();
return new NotSentence((Sentence) negated.accept(this, arg));
}
public Object visitConnectedSentence(ConnectedSentence sentence, Object arg) {
Sentence alpha = (Sentence) sentence.getFirst().accept(this, arg);
Sentence beta = (Sentence) sentence.getSecond().accept(this, arg);
// Eliminate <=>, bi-conditional elimination,
// replace (alpha <=> beta) with (~alpha V beta) ^ (alpha V ~beta).
if (Connectors.isBICOND(sentence.getConnector())) {
Sentence first = new ConnectedSentence(Connectors.OR,
new NotSentence(alpha), beta);
Sentence second = new ConnectedSentence(Connectors.OR, alpha,
new NotSentence(beta));
return new ConnectedSentence(Connectors.AND, first, second);
}
// Eliminate =>, implication elimination,
// replacing (alpha => beta) with (~alpha V beta)
if (Connectors.isIMPLIES(sentence.getConnector())) {
return new ConnectedSentence(Connectors.OR, new NotSentence(alpha),
beta);
}
return new ConnectedSentence(sentence.getConnector(), alpha, beta);
}
public Object visitQuantifiedSentence(QuantifiedSentence sentence,
Object arg) {
return new QuantifiedSentence(sentence.getQuantifier(),
sentence.getVariables(), (Sentence) sentence.getQuantified()
.accept(this, arg));
}
}
class NegationsIn implements FOLVisitor {
public NegationsIn() {
}
public Object visitPredicate(Predicate p, Object arg) {
return p;
}
public Object visitTermEquality(TermEquality equality, Object arg) {
return equality;
}
public Object visitVariable(Variable variable, Object arg) {
return variable;
}
public Object visitConstant(Constant constant, Object arg) {
return constant;
}
public Object visitFunction(Function function, Object arg) {
return function;
}
public Object visitNotSentence(NotSentence notSentence, Object arg) {
// CNF requires NOT (~) to appear only in literals, so we 'move ~
// inwards' by repeated application of the following equivalences:
Sentence negated = notSentence.getNegated();
// ~(~alpha) equivalent to alpha (double negation elimination)
if (negated instanceof NotSentence) {
return ((NotSentence) negated).getNegated().accept(this, arg);
}
if (negated instanceof ConnectedSentence) {
ConnectedSentence negConnected = (ConnectedSentence) negated;
Sentence alpha = negConnected.getFirst();
Sentence beta = negConnected.getSecond();
// ~(alpha ^ beta) equivalent to (~alpha V ~beta) (De Morgan)
if (Connectors.isAND(negConnected.getConnector())) {
// I need to ensure the ~s are moved in deeper
Sentence notAlpha = (Sentence) (new NotSentence(alpha)).accept(
this, arg);
Sentence notBeta = (Sentence) (new NotSentence(beta)).accept(
this, arg);
return new ConnectedSentence(Connectors.OR, notAlpha, notBeta);
}
// ~(alpha V beta) equivalent to (~alpha ^ ~beta) (De Morgan)
if (Connectors.isOR(negConnected.getConnector())) {
// I need to ensure the ~s are moved in deeper
Sentence notAlpha = (Sentence) (new NotSentence(alpha)).accept(
this, arg);
Sentence notBeta = (Sentence) (new NotSentence(beta)).accept(
this, arg);
return new ConnectedSentence(Connectors.AND, notAlpha, notBeta);
}
}
// in addition, rules for negated quantifiers:
if (negated instanceof QuantifiedSentence) {
QuantifiedSentence negQuantified = (QuantifiedSentence) negated;
// I need to ensure the ~ is moved in deeper
Sentence notP = (Sentence) (new NotSentence(
negQuantified.getQuantified())).accept(this, arg);
// ~FORALL x p becomes EXISTS x ~p
if (Quantifiers.isFORALL(negQuantified.getQuantifier())) {
return new QuantifiedSentence(Quantifiers.EXISTS,
negQuantified.getVariables(), notP);
}
// ~EXISTS x p becomes FORALL x ~p
if (Quantifiers.isEXISTS(negQuantified.getQuantifier())) {
return new QuantifiedSentence(Quantifiers.FORALL,
negQuantified.getVariables(), notP);
}
}
return new NotSentence((Sentence) negated.accept(this, arg));
}
public Object visitConnectedSentence(ConnectedSentence sentence, Object arg) {
return new ConnectedSentence(sentence.getConnector(),
(Sentence) sentence.getFirst().accept(this, arg),
(Sentence) sentence.getSecond().accept(this, arg));
}
public Object visitQuantifiedSentence(QuantifiedSentence sentence,
Object arg) {
return new QuantifiedSentence(sentence.getQuantifier(),
sentence.getVariables(), (Sentence) sentence.getQuantified()
.accept(this, arg));
}
}
class StandardizeQuantiferVariables implements FOLVisitor {
// Just use a localized indexical here.
private StandardizeApartIndexical quantifiedIndexical = new StandardizeApartIndexical() {
private int index = 0;
public String getPrefix() {
return "q";
}
public int getNextIndex() {
return index++;
}
};
private SubstVisitor substVisitor = null;
public StandardizeQuantiferVariables(SubstVisitor substVisitor) {
this.substVisitor = substVisitor;
}
public Object visitPredicate(Predicate p, Object arg) {
return p;
}
public Object visitTermEquality(TermEquality equality, Object arg) {
return equality;
}
public Object visitVariable(Variable variable, Object arg) {
return variable;
}
public Object visitConstant(Constant constant, Object arg) {
return constant;
}
public Object visitFunction(Function function, Object arg) {
return function;
}
public Object visitNotSentence(NotSentence sentence, Object arg) {
return new NotSentence((Sentence) sentence.getNegated().accept(this,
arg));
}
public Object visitConnectedSentence(ConnectedSentence sentence, Object arg) {
return new ConnectedSentence(sentence.getConnector(),
(Sentence) sentence.getFirst().accept(this, arg),
(Sentence) sentence.getSecond().accept(this, arg));
}
@SuppressWarnings("unchecked")
public Object visitQuantifiedSentence(QuantifiedSentence sentence,
Object arg) {
Set seenSoFar = (Set) arg;
// Keep track of what I have to subst locally and
// what my renamed variables will be.
Map localSubst = new LinkedHashMap();
List replVariables = new ArrayList();
for (Variable v : sentence.getVariables()) {
// If local variable has be renamed already
// then I need to come up with own name
if (seenSoFar.contains(v)) {
Variable sV = new Variable(quantifiedIndexical.getPrefix()
+ quantifiedIndexical.getNextIndex());
localSubst.put(v, sV);
// Replacement variables should contain new name for variable
replVariables.add(sV);
} else {
// Not already replaced, this name is good
replVariables.add(v);
}
}
// Apply the local subst
Sentence subst = substVisitor.subst(localSubst,
sentence.getQuantified());
// Ensure all my existing and replaced variable
// names are tracked
seenSoFar.addAll(replVariables);
Sentence sQuantified = (Sentence) subst.accept(this, arg);
return new QuantifiedSentence(sentence.getQuantifier(), replVariables,
sQuantified);
}
}
class RemoveQuantifiers implements FOLVisitor {
private FOLParser parser = null;
private SubstVisitor substVisitor = null;
public RemoveQuantifiers(FOLParser parser) {
this.parser = parser;
substVisitor = new SubstVisitor();
}
public Object visitPredicate(Predicate p, Object arg) {
return p;
}
public Object visitTermEquality(TermEquality equality, Object arg) {
return equality;
}
public Object visitVariable(Variable variable, Object arg) {
return variable;
}
public Object visitConstant(Constant constant, Object arg) {
return constant;
}
public Object visitFunction(Function function, Object arg) {
return function;
}
public Object visitNotSentence(NotSentence sentence, Object arg) {
return new NotSentence((Sentence) sentence.getNegated().accept(this,
arg));
}
public Object visitConnectedSentence(ConnectedSentence sentence, Object arg) {
return new ConnectedSentence(sentence.getConnector(),
(Sentence) sentence.getFirst().accept(this, arg),
(Sentence) sentence.getSecond().accept(this, arg));
}
@SuppressWarnings("unchecked")
public Object visitQuantifiedSentence(QuantifiedSentence sentence,
Object arg) {
Sentence quantified = sentence.getQuantified();
Set universalScope = (Set) arg;
// Skolemize: Skolemization is the process of removing existential
// quantifiers by elimination. This is done by introducing Skolem
// functions. The general rule is that the arguments of the Skolem
// function are all the universally quantified variables in whose
// scope the existential quantifier appears.
if (Quantifiers.isEXISTS(sentence.getQuantifier())) {
Map skolemSubst = new LinkedHashMap();
for (Variable eVar : sentence.getVariables()) {
if (universalScope.size() > 0) {
// Replace with a Skolem Function
String skolemFunctionName = parser.getFOLDomain()
.addSkolemFunction();
skolemSubst.put(eVar, new Function(skolemFunctionName,
new ArrayList(universalScope)));
} else {
// Replace with a Skolem Constant
String skolemConstantName = parser.getFOLDomain()
.addSkolemConstant();
skolemSubst.put(eVar, new Constant(skolemConstantName));
}
}
Sentence skolemized = substVisitor.subst(skolemSubst, quantified);
return skolemized.accept(this, arg);
}
// Drop universal quantifiers.
if (Quantifiers.isFORALL(sentence.getQuantifier())) {
// Add to the universal scope so that
// existential skolemization may be done correctly
universalScope.addAll(sentence.getVariables());
Sentence droppedUniversal = (Sentence) quantified.accept(this, arg);
// Enusre my scope is removed before moving back up
// the call stack when returning
universalScope.removeAll(sentence.getVariables());
return droppedUniversal;
}
// Should not reach here as have already
// handled the two quantifiers.
throw new IllegalStateException("Unhandled Quantifier:"
+ sentence.getQuantifier());
}
}
class DistributeOrOverAnd implements FOLVisitor {
public DistributeOrOverAnd() {
}
public Object visitPredicate(Predicate p, Object arg) {
return p;
}
public Object visitTermEquality(TermEquality equality, Object arg) {
return equality;
}
public Object visitVariable(Variable variable, Object arg) {
return variable;
}
public Object visitConstant(Constant constant, Object arg) {
return constant;
}
public Object visitFunction(Function function, Object arg) {
return function;
}
public Object visitNotSentence(NotSentence sentence, Object arg) {
return new NotSentence((Sentence) sentence.getNegated().accept(this,
arg));
}
public Object visitConnectedSentence(ConnectedSentence sentence, Object arg) {
// Distribute V over ^:
// This will cause flattening out of nested ^s and Vs
Sentence alpha = (Sentence) sentence.getFirst().accept(this, arg);
Sentence beta = (Sentence) sentence.getSecond().accept(this, arg);
// (alpha V (beta ^ gamma)) equivalent to
// ((alpha V beta) ^ (alpha V gamma))
if (Connectors.isOR(sentence.getConnector())
&& ConnectedSentence.class.isInstance(beta)) {
ConnectedSentence betaAndGamma = (ConnectedSentence) beta;
if (Connectors.isAND(betaAndGamma.getConnector())) {
beta = betaAndGamma.getFirst();
Sentence gamma = betaAndGamma.getSecond();
return new ConnectedSentence(Connectors.AND,
(Sentence) (new ConnectedSentence(Connectors.OR, alpha,
beta)).accept(this, arg),
(Sentence) (new ConnectedSentence(Connectors.OR, alpha,
gamma)).accept(this, arg));
}
}
// ((alpha ^ gamma) V beta) equivalent to
// ((alpha V beta) ^ (gamma V beta))
if (Connectors.isOR(sentence.getConnector())
&& ConnectedSentence.class.isInstance(alpha)) {
ConnectedSentence alphaAndGamma = (ConnectedSentence) alpha;
if (Connectors.isAND(alphaAndGamma.getConnector())) {
alpha = alphaAndGamma.getFirst();
Sentence gamma = alphaAndGamma.getSecond();
return new ConnectedSentence(Connectors.AND,
(Sentence) (new ConnectedSentence(Connectors.OR, alpha,
beta)).accept(this, arg),
(Sentence) (new ConnectedSentence(Connectors.OR, gamma,
beta)).accept(this, arg));
}
}
return new ConnectedSentence(sentence.getConnector(), alpha, beta);
}
public Object visitQuantifiedSentence(QuantifiedSentence sentence,
Object arg) {
// This should not be called as should have already
// removed all of the quantifiers.
throw new IllegalStateException(
"All quantified sentences should have already been removed.");
}
}
class CNFConstructor implements FOLVisitor {
public CNFConstructor() {
}
public CNF construct(Sentence orDistributedOverAnd) {
ArgData ad = new ArgData();
orDistributedOverAnd.accept(this, ad);
return new CNF(ad.clauses);
}
public Object visitPredicate(Predicate p, Object arg) {
ArgData ad = (ArgData) arg;
if (ad.negated) {
ad.clauses.get(ad.clauses.size() - 1).addNegativeLiteral(p);
} else {
ad.clauses.get(ad.clauses.size() - 1).addPositiveLiteral(p);
}
return p;
}
public Object visitTermEquality(TermEquality equality, Object arg) {
ArgData ad = (ArgData) arg;
if (ad.negated) {
ad.clauses.get(ad.clauses.size() - 1).addNegativeLiteral(equality);
} else {
ad.clauses.get(ad.clauses.size() - 1).addPositiveLiteral(equality);
}
return equality;
}
public Object visitVariable(Variable variable, Object arg) {
// This should not be called
throw new IllegalStateException("visitVariable() should not be called.");
}
public Object visitConstant(Constant constant, Object arg) {
// This should not be called
throw new IllegalStateException("visitConstant() should not be called.");
}
public Object visitFunction(Function function, Object arg) {
// This should not be called
throw new IllegalStateException("visitFunction() should not be called.");
}
public Object visitNotSentence(NotSentence sentence, Object arg) {
ArgData ad = (ArgData) arg;
// Indicate that the enclosed predicate is negated
ad.negated = true;
sentence.getNegated().accept(this, arg);
ad.negated = false;
return sentence;
}
public Object visitConnectedSentence(ConnectedSentence sentence, Object arg) {
ArgData ad = (ArgData) arg;
Sentence first = sentence.getFirst();
Sentence second = sentence.getSecond();
first.accept(this, arg);
if (Connectors.isAND(sentence.getConnector())) {
ad.clauses.add(new Clause());
}
second.accept(this, arg);
return sentence;
}
public Object visitQuantifiedSentence(QuantifiedSentence sentence,
Object arg) {
// This should not be called as should have already
// removed all of the quantifiers.
throw new IllegalStateException(
"All quantified sentences should have already been removed.");
}
class ArgData {
public List clauses = new ArrayList();
public boolean negated = false;
public ArgData() {
clauses.add(new Clause());
}
}
}