//
// PROTECTED:
//
// Note: Override this method if you wish to change the initial variable
// ordering when dpllSatisfiable is called.
protected ICollection <PropositionSymbol> getPropositionSymbolsInSentence(Sentence s)
{
ICollection <PropositionSymbol> result = CollectionFactory.CreateQueue <PropositionSymbol>(
SymbolCollector.getSymbolsFrom(s));
return(result);
}
/**
* Adds the specified sentence to the knowledge base.
*
* @param aSentence
* a fact to be added to the knowledge base.
*/
public void tell(Sentence aSentence)
{
if (!(sentences.Contains(aSentence)))
{
sentences.Add(aSentence);
_asCNF = _asCNF.extend(ConvertToConjunctionOfClauses.convert(aSentence).getClauses());
symbols.AddAll(SymbolCollector.getSymbolsFrom(aSentence));
}
}
protected IMap <PropositionSymbol, bool?> initializeInferred(KnowledgeBase kb)
{
// inferred <- a table, where inferred[s] is initially false for all
// symbols
IMap <PropositionSymbol, bool?> inferred = CollectionFactory.CreateInsertionOrderedMap <PropositionSymbol, bool?>();
foreach (PropositionSymbol p in SymbolCollector.getSymbolsFrom(kb.asSentence()))
{
inferred.Put(p, false);
}
return(inferred);
}
public void testCollectSymbolsFromComplexSentence()
{
Sentence sentence = (Sentence)parser.parse("(~B11 | P12 | P21) & (B11 | ~P12) & (B11 | ~P21)");
ISet <PropositionSymbol> s = SymbolCollector.getSymbolsFrom(sentence);
Assert.AreEqual(3, s.Size());
Sentence b11 = parser.parse("B11");
Sentence p21 = parser.parse("P21");
Sentence p12 = parser.parse("P12");
Assert.IsTrue(s.Contains(b11 as PropositionSymbol));
Assert.IsTrue(s.Contains(p21 as PropositionSymbol));
Assert.IsTrue(s.Contains(p12 as PropositionSymbol));
}
public void testDPLLReturnsFalseWhenOneClauseFalseInModel()
{
Model model = new Model();
model = model.union(new PropositionSymbol("A"), true).union(
new PropositionSymbol("B"), false);
Sentence sentence = parser.parse("(A | B) & (A => B)");
ISet <Clause> clauses = ConvertToConjunctionOfClauses.convert(sentence)
.getClauses();
ICollection <PropositionSymbol> symbols = CollectionFactory.CreateQueue <PropositionSymbol>(
SymbolCollector.getSymbolsFrom(sentence));
bool satisfiable = dpll.dpll(clauses, symbols, model);
Assert.AreEqual(false, satisfiable);
}
public void testIssue66()
{
// http://code.google.com/p/aima-java/issues/detail?id=66
Model model = new Model();
model = model.union(new PropositionSymbol("A"), false)
.union(new PropositionSymbol("B"), false)
.union(new PropositionSymbol("C"), true);
Sentence sentence = parser.parse("((A | B) | C)");
ISet <Clause> clauses = ConvertToConjunctionOfClauses.convert(sentence)
.getClauses();
ICollection <PropositionSymbol> symbols = CollectionFactory.CreateQueue <PropositionSymbol>(
SymbolCollector.getSymbolsFrom(sentence));
bool satisfiable = dpll.dpll(clauses, symbols, model);
Assert.AreEqual(true, satisfiable);
}
//
// SUPPORTING CODE
//
/**
* Determine if KB |= α, i.e. alpha is entailed by KB.
*
* @param kb
* a Knowledge Base in propositional logic.
* @param alpha
* a propositional sentence.
* @return true, if α is entailed by KB, false otherwise.
*/
public bool isEntailed(KnowledgeBase kb, Sentence alpha)
{
// AIMA3e p.g. 260: kb |= alpha, can be done by testing
// unsatisfiability of kb & ~alpha.
ISet <Clause> kbAndNotAlpha = CollectionFactory.CreateSet <Clause>();
Sentence notQuery = new ComplexSentence(Connective.NOT, alpha);
ISet <PropositionSymbol> symbols = CollectionFactory.CreateSet <PropositionSymbol>();
ICollection <PropositionSymbol> querySymbols = CollectionFactory.CreateQueue <PropositionSymbol>(SymbolCollector.getSymbolsFrom(notQuery));
kbAndNotAlpha.AddAll(kb.asCNF());
kbAndNotAlpha.AddAll(ConvertToConjunctionOfClauses.convert(notQuery).getClauses());
symbols.AddAll(querySymbols);
symbols.AddAll(kb.getSymbols());
return(!dpll(kbAndNotAlpha, CollectionFactory.CreateQueue <PropositionSymbol>(symbols), new Model()));
}
/**
* function TT-ENTAILS?(KB, α) returns true or false.
*
* @param kb
* KB, the knowledge base, a sentence in propositional logic
* @param alpha
* α, the query, a sentence in propositional logic
*
* @return true if KB entails α, false otherwise.
*/
public bool ttEntails(KnowledgeBase kb, Sentence alpha)
{
// symbols <- a list of proposition symbols in KB and &alpha
ICollection <PropositionSymbol> symbols = CollectionFactory.CreateQueue <PropositionSymbol>(SymbolCollector.getSymbolsFrom(kb.asSentence(), alpha));
// return TT-CHECK-ALL(KB, α symbols, {})
return(ttCheckAll(kb, alpha, symbols, new Model()));
}