public Expression CreateChoice(Expression exp1, Expression exp2)
{
if (exp1 == Expression.NotAllowed)
{
return(exp2);
}
if (exp2 == Expression.NotAllowed)
{
return(exp1);
}
if (exp1 == Expression.Empty && exp2.IsNullable)
{
return(exp2);
}
else
if (exp2 == Expression.Empty && exp1.IsNullable)
{
return(exp1);
}
if (exp2 is ChoiceExp)
{
// operatos are left-associative.
ChoiceExp right = (ChoiceExp)exp2;
return(CreateChoice(CreateChoice(exp1, right.exp1), right.exp2));
}
// see if exp1 has already contained exp2
Expression e = exp1;
while (true)
{
if (e == exp2)
{
// exp1 has already contained exp2.
return(exp1);
}
if (!(e is ChoiceExp))
{
break;
}
ChoiceExp left = (ChoiceExp)e;
if (left.exp2 == exp2)
{
// exp1 has already contained exp2.
return(exp1);
}
e = left.exp1;
}
return(Unify(new ChoiceExp(exp1, exp2)));
}
public void OnChoice(ChoiceExp exp)
{
if (exp.exp1 == Expression.Empty)
{
OptimizedChoice(exp.exp2);
return;
}
if (exp.exp2 == Expression.Empty)
{
OptimizedChoice(exp.exp1);
return;
}
OnBinExp(exp, "|");
}