// 2nd step
public static Step EliminateImpliesStep(Step step)
{
List <Symbol> symbols = step.Top.ReverseBFS();
step.Title = "Eliminating Implies";
foreach (Symbol symbol in symbols)
{
if (symbol is Operator && symbol.Name == "IMPLIES")
{
var parentSymbol = symbol.Parent;
var leftChildSymbol = symbol.Children[0];
var rightChildSymbol = symbol.Children[1];
Symbol newSymbolOperator = new Operator(name: "OR");
Symbol newChildLeft = new Negation();
// 1st copy left child of IMPLIES statement
newChildLeft.SetChild(0, leftChildSymbol.Clone());
// 2nd asign Negation before copied left child of IMPLIES
newSymbolOperator.SetChild(0, newChildLeft);
// We can only clone because right child if free of change
newSymbolOperator.SetChild(1, rightChildSymbol.Clone());
if (parentSymbol == null)
{
step.Top = newSymbolOperator;
}
else
{
parentSymbol.SetChild(symbol.IndexInParent, newSymbolOperator);
}
}
}
return(step);
}
// 3rd step
public static Step MoveNegationInwardsStep(Step step)
{
step.Title = "Moving Negation Inwards";
bool endConditional;
do
{
endConditional = false;
List <Symbol> symbols = step.Top.ReverseBFS();
foreach (var symbol in symbols)
{
if (symbol is Negation && symbol.Children[0].IsFinal == false)
{
var parentSymbol = symbol.Parent;
var childSymbol = symbol.Children[0];
if (childSymbol is Negation) // Negation case
{
endConditional = true;
if (parentSymbol == null)
{
step.Top = childSymbol.Children[0];
}
else
{
parentSymbol.SetChild(symbol.IndexInParent, childSymbol.Children[0]);
}
}
else if (childSymbol is Operator) // AND and OR case
{
// setting end conditional to true
endConditional = true;
if (childSymbol.Name == "AND")
{
Symbol newOperatorSymbol = new Operator(name: "OR");
var leftChildSymbol = childSymbol.Children[0];
var rightChildSymbol = childSymbol.Children[1];
Symbol newLeftChild = new Negation();
Symbol newRightChild = new Negation();
newLeftChild.SetChild(0, leftChildSymbol.Clone());
newRightChild.SetChild(1, rightChildSymbol.Clone());
newOperatorSymbol.SetChild(0, newLeftChild);
newOperatorSymbol.SetChild(1, newRightChild);
if (parentSymbol == null)
{
step.Top = newOperatorSymbol;
}
else
{
parentSymbol.SetChild(symbol.IndexInParent, newOperatorSymbol);
}
}
else if (childSymbol.Name == "OR")
{
Symbol newOperatorSymbol = new Operator(name: "AND");
var leftChildSymbol = childSymbol.Children[0];
var rightChildSymbol = childSymbol.Children[1];
Symbol newLeftChild = new Negation();
Symbol newRightChild = new Negation();
newLeftChild.SetChild(0, leftChildSymbol.Clone());
newRightChild.SetChild(1, rightChildSymbol.Clone());
newOperatorSymbol.SetChild(0, newLeftChild);
newOperatorSymbol.SetChild(1, newRightChild);
if (parentSymbol == null)
{
step.Top = newOperatorSymbol;
}
else
{
parentSymbol.SetChild(symbol.IndexInParent, newOperatorSymbol);
}
}
}
else if (childSymbol is Quantifier) // FORALL and EXISTS case
{
// setting end conditional to true
endConditional = true;
if (childSymbol.Name == "FORALL")
{
Symbol newOperatorSymbol = new Operator(name: "EXISTS");
var leftChildSymbol = childSymbol.Children[0];
var rightChildSymbol = childSymbol.Children[1];
Symbol newRightChild = new Negation();
newRightChild.SetChild(0, rightChildSymbol.Clone());
newOperatorSymbol.SetChild(0, leftChildSymbol);
newOperatorSymbol.SetChild(1, newRightChild);
if (parentSymbol == null)
{
step.Top = newOperatorSymbol;
}
else
{
parentSymbol.SetChild(symbol.IndexInParent, newOperatorSymbol);
}
}
else if (childSymbol.Name == "EXISTS")
{
Symbol newOperatorSymbol = new Operator(name: "FORALL");
var leftChildSymbol = childSymbol.Children[0];
var rightChildSymbol = childSymbol.Children[1];
Symbol newRightChild = new Negation();
newRightChild.SetChild(0, rightChildSymbol.Clone());
newOperatorSymbol.SetChild(0, leftChildSymbol);
newOperatorSymbol.SetChild(1, newRightChild);
if (parentSymbol == null)
{
step.Top = newOperatorSymbol;
}
else
{
parentSymbol.SetChild(symbol.IndexInParent, newOperatorSymbol);
}
}
}
}
}
} while (endConditional);
return(step);
}