// Trnasfer the node to PNF (the formula is assumed to be in NNF)
// We assume that the formula also has no temporal operators
public FormulaNode PNF()
{
if (this.logicOp == LogicOperator.VAR)
{
return(new FormulaNode(this.name));
}
FormulaNode res = null, left = null, right = null;
ISet <string> leftSet, rightSet;
string varname;
if (this.logicOp == LogicOperator.NOT)
{
res = new FormulaNode(LogicOperator.NOT);
res.SetChildren(new FormulaNode(this.childNodes[0].name), null);
return(res);
}
if (this.logicOp == LogicOperator.EXISTS ||
this.logicOp == LogicOperator.ALL)
{
res = new FormulaNode(this.logicOp, this.name);
res.SetChildren(this.childNodes[0].PNF(), null);
return(res);
}
// Since we assume NNF, all NOT nodes are next to variables
// Thus all of the remaining operators are binary
left = this.childNodes[0].PNF();
right = this.childNodes[1].PNF();
leftSet = left.GetVariables();
rightSet = right.GetVariables();
if (this.logicOp == LogicOperator.AND ||
this.logicOp == LogicOperator.OR)
{
FormulaNode temp;
if (left.logicOp == LogicOperator.EXISTS ||
left.logicOp == LogicOperator.ALL)
{
if (rightSet.Contains(left.name))
{
varname = UniquePNFVariable();
left = left.Substitute(left.name, varname);
}
res = new FormulaNode(left.logicOp, left.name);
temp = new FormulaNode(this.logicOp);
temp.SetChildren(left.childNodes[0], right);
res.SetChildren(temp.PNF(), null);
}
else if (right.logicOp == LogicOperator.EXISTS ||
right.logicOp == LogicOperator.ALL)
{
if (leftSet.Contains(right.name))
{
varname = UniquePNFVariable();
right = right.Substitute(right.name, varname);
}
res = new FormulaNode(right.logicOp, right.name);
temp = new FormulaNode(this.logicOp);
temp.SetChildren(left, right.childNodes[0]);
res.SetChildren(temp.PNF(), null);
}
else
{
res = new FormulaNode(this.logicOp);
res.SetChildren(left, right);
}
}
// For convenience, we get rid of "->" operators
if (this.logicOp == LogicOperator.IMP)
{
res = new FormulaNode(LogicOperator.OR);
res.SetChildren(new FormulaNode(LogicOperator.NOT), right);
res.childNodes[0].SetChildren(left, null);
res = res.NNF().PNF();
}
return(res);
}