001 /*
002 * Created on Feb 9, 2005
003 *
004 */
005 package aima.logic.propositional.algorithms;
006
007 import java.util.Iterator;
008 import java.util.List;
009 import java.util.Random;
010 import java.util.Set;
011
012 import aima.logic.propositional.parsing.PEParser;
013 import aima.logic.propositional.parsing.ast.Sentence;
014 import aima.logic.propositional.parsing.ast.Symbol;
015 import aima.logic.propositional.visitors.CNFClauseGatherer;
016 import aima.logic.propositional.visitors.CNFTransformer;
017 import aima.logic.propositional.visitors.SymbolCollector;
018 import aima.util.Converter;
019 import aima.util.Util;
020
021 /**
022 * @author Ravi Mohan
023 *
024 */
025
026 public class WalkSAT {
027 private Model myModel;
028
029 private Random random = new Random();
030
031 public Model findModelFor(String logicalSentence, int numberOfFlips,
032 double probabilityOfRandomWalk) {
033 myModel = new Model();
034 Sentence s = (Sentence) new PEParser().parse(logicalSentence);
035 CNFTransformer transformer = new CNFTransformer();
036 CNFClauseGatherer clauseGatherer = new CNFClauseGatherer();
037 SymbolCollector sc = new SymbolCollector();
038
039 List symbols = new Converter<Symbol>().setToList(sc.getSymbolsIn(s));
040 Random r = new Random();
041 for (int i = 0; i < symbols.size(); i++) {
042 Symbol sym = (Symbol) symbols.get(i);
043 myModel = myModel.extend(sym, Util.randomBoolean());
044 }
045 List<Sentence> clauses = new Converter<Sentence>()
046 .setToList(clauseGatherer.getClausesFrom(transformer
047 .transform(s)));
048
049 for (int i = 0; i < numberOfFlips; i++) {
050 if (getNumberOfClausesSatisfiedIn(new Converter<Sentence>()
051 .listToSet(clauses), myModel) == clauses.size()) {
052 return myModel;
053 }
054 Sentence clause = clauses.get(random.nextInt(clauses.size()));
055
056 List<Symbol> symbolsInClause = new Converter<Symbol>().setToList(sc
057 .getSymbolsIn(clause));
058 if (random.nextDouble() >= probabilityOfRandomWalk) {
059 Symbol randomSymbol = symbolsInClause.get(random
060 .nextInt(symbolsInClause.size()));
061 myModel = myModel.flip(randomSymbol);
062 } else {
063 Symbol symbolToFlip = getSymbolWhoseFlipMaximisesSatisfiedClauses(
064 new Converter<Sentence>().listToSet(clauses),
065 symbolsInClause, myModel);
066 myModel = myModel.flip(symbolToFlip);
067 }
068
069 }
070 return null;
071 }
072
073 private Symbol getSymbolWhoseFlipMaximisesSatisfiedClauses(
074 Set<Sentence> clauses, List<Symbol> symbols, Model model) {
075 if (symbols.size() > 0) {
076 Symbol retVal = symbols.get(0);
077 int maxClausesSatisfied = 0;
078 for (int i = 0; i < symbols.size(); i++) {
079 Symbol sym = symbols.get(i);
080 if (getNumberOfClausesSatisfiedIn(clauses, model.flip(sym)) > maxClausesSatisfied) {
081 retVal = sym;
082 maxClausesSatisfied = getNumberOfClausesSatisfiedIn(
083 clauses, model.flip(sym));
084 }
085 }
086 return retVal;
087 } else {
088 return null;
089 }
090
091 }
092
093 private int getNumberOfClausesSatisfiedIn(Set clauses, Model model) {
094 int retVal = 0;
095 Iterator i = clauses.iterator();
096 while (i.hasNext()) {
097 Sentence s = (Sentence) i.next();
098 if (model.isTrue(s)) {
099 retVal += 1;
100 }
101 }
102 return retVal;
103 }
104 }