// getInstance will return a Node but possibly not a Star
// if the child is an emptyString, return emptyString
public static Node getInstance(Node child) {
if (child == EmptySet.getInstance()) return child;
if (!map.containsKey(child)) {
map.put(child, new Star(child));
}
return map.get(child);
}
@Override
public Node visit(Symbol node) {
// Dc(c) = ""
if (c == node.symbol)
return EmptyString.getInstance(); // Do the same thing for the empty string
// Dc(c') = 0 if c is not c'
else return EmptySet.getInstance();
}
@Override
public Node visit(EmptyString node) {
// Dc("") = 0
return EmptySet.getInstance();
}
