/**
* Alternates NFAs r and s
*
* <p>As a side effect, changes all the NFANodes inside each NFA r and s to be no longer accepting
* and instead have epsilon transitions out to a resulting accept state
*
* @param r
* @param s
* @return An NFA that matches either r or s
*/
public static NFA alternation(NFA r, NFA s) {
NFA nfa = new NFA();
nfa.nodes.addAll(r.nodes);
nfa.nodes.addAll(s.nodes);
// create start node and set eps trans to beginning of a and b
NFANode start = new NFANode();
nfa.nodes.add(start);
nfa.initial = start;
start.addEpsilonTransition(r.initial);
start.addEpsilonTransition(s.initial);
// set end node as accepting
NFANode end = new NFANode();
nfa.nodes.add(end);
end.setAccepting(true);
// set all accepting nodes in A and B as non-accepting and
// add eps-trans out to end node
for (NFANode n : r.nodes) {
if (n.isAccepting()) {
n.addEpsilonTransition(end);
n.setAccepting(false);
}
}
for (NFANode n : s.nodes) {
if (n.isAccepting()) {
n.addEpsilonTransition(end);
n.setAccepting(false);
}
}
return nfa;
}
/**
* An NFA that matches the given character or epsilon if c is null
*
* @param c If null, creates an NFA that matches an epsilon transition (always match)
*/
public static NFA basicMatch(Character c) {
NFA nfa = new NFA();
NFANode n1 = new NFANode();
NFANode n2 = new NFANode();
n1.addTransition(c, n2);
n2.setAccepting(true);
nfa.initial = n1;
nfa.nodes.add(n1);
nfa.nodes.add(n2);
return nfa;
}
/**
* Concatenates NFAs r and s
*
* <p>As a side effect, this causes all the NFANodes that were accepting before to become
* non-accepting and instead have epsilon transitions to the start state of s.
*
* @param r
* @param s
* @return A new NFA that is the concatenation of those two
*/
public static NFA concatenation(NFA r, NFA s) {
NFA nfa = new NFA();
nfa.nodes.addAll(r.nodes);
nfa.nodes.addAll(s.nodes);
// add eps-trans from a's accept to b's initial and remove a's accept flags
for (NFANode node : r.nodes) {
if (node.isAccepting()) {
node.addEpsilonTransition(s.initial);
node.setAccepting(false);
}
}
nfa.initial = r.initial;
return nfa;
}