public object VisitConnectedSentence(ConnectedSentence sentence, object arg)
{
ArgData ad = (ArgData)arg;
ISentence first = sentence.First;
ISentence second = sentence.Second;
first.Accept(this, arg);
if (Connectors.IsAnd(sentence.Connector))
{
ad.Clauses.Add(new Clause());
}
second.Accept(this, arg);
return(sentence);
}
public object VisitConnectedSentence(ConnectedSentence sentence, object arg)
{
// Distribute V over ^:
// This will cause flattening out of nested ^s and Vs
var alpha = (ISentence)sentence.First.Accept(this, arg);
var beta = (ISentence)sentence.Second.Accept(this, arg);
// (alpha V (beta ^ gamma)) equivalent to
// ((alpha V beta) ^ (alpha V gamma))
if (Connectors.IsOr(sentence.Connector) && beta is ConnectedSentence)
{
var betaAndGamma = (ConnectedSentence)beta;
if (Connectors.IsAnd(betaAndGamma.Connector))
{
beta = betaAndGamma.First;
var gamma = betaAndGamma.Second;
return(new ConnectedSentence(Connectors.And,
(ISentence)(new ConnectedSentence(Connectors.Or, alpha, beta)).Accept(this, arg),
(ISentence)(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.Connector) && alpha is ConnectedSentence)
{
var alphaAndGamma = (ConnectedSentence)alpha;
if (Connectors.IsAnd(alphaAndGamma.Connector))
{
alpha = alphaAndGamma.First;
var gamma = alphaAndGamma.Second;
return(new ConnectedSentence(
Connectors.And,
(ISentence)(new ConnectedSentence(Connectors.Or, alpha, beta)).Accept(this, arg),
(ISentence)(new ConnectedSentence(Connectors.Or, gamma, beta)).Accept(this, arg)));
}
}
return(new ConnectedSentence(sentence.Connector, alpha, beta));
}
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:
ISentence negated = notSentence.Negated;
// ~(~alpha) equivalent to alpha (double negation elimination)
if (negated is NotSentence)
{
return(((NotSentence)negated).Negated.Accept(this, arg));
}
if (negated is ConnectedSentence)
{
ConnectedSentence negConnected = (ConnectedSentence)negated;
ISentence alpha = negConnected.First;
ISentence beta = negConnected.Second;
// ~(alpha ^ beta) equivalent to (~alpha V ~beta) (De Morgan)
if (Connectors.IsAnd(negConnected.Connector))
{
// I need to ensure the ~s are moved in deeper
ISentence notAlpha = (ISentence)(new NotSentence(alpha)).Accept(
this, arg);
ISentence notBeta = (ISentence)(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.Connector))
{
// I need to ensure the ~s are moved in deeper
ISentence notAlpha = (ISentence)(new NotSentence(alpha)).Accept(
this, arg);
ISentence notBeta = (ISentence)(new NotSentence(beta)).Accept(
this, arg);
return(new ConnectedSentence(Connectors.And, notAlpha, notBeta));
}
}
// in addition, rules for negated quantifiers:
if (negated is QuantifiedSentence)
{
QuantifiedSentence negQuantified = (QuantifiedSentence)negated;
// I need to ensure the ~ is moved in deeper
ISentence notP = (ISentence)(new NotSentence(negQuantified
.Quantified)).Accept(this, arg);
// ~ForAll x p becomes Exists x ~p
if (Quantifiers.IsForall(negQuantified.Quantifier))
{
return(new QuantifiedSentence(Quantifiers.Exists, negQuantified
.Variables, notP));
}
// ~Exists x p becomes ForAll x ~p
if (Quantifiers.IsExists(negQuantified.Quantifier))
{
return(new QuantifiedSentence(Quantifiers.ForAll, negQuantified
.Variables, notP));
}
}
return(new NotSentence((ISentence)negated.Accept(this, arg)));
}