public static NFA Or(NFA a, NFA b)
{
NFA nfa = new NFA();
// Composite NFA contains all the and all edges in both NFAs
nfa.AddAll(a);
nfa.AddAll(b);
// Add a start state, link to both NFAs old start state with
// epsilon links
nfa.StartState = new NFA.State();
nfa.States.Add(nfa.StartState);
nfa.Transitions.Add(new Transition <NFA.State>(nfa.StartState, a.StartState));
nfa.Transitions.Add(new Transition <NFA.State>(nfa.StartState, b.StartState));
// Add a new accept state, link all old accept states to the new accept
// state with an epsilon link and remove the accept flag
NFA.State newAcceptState = new NFA.State {
AcceptState = true
};
foreach (NFA.State oldAcceptState in nfa.States.Where(f => f.AcceptState))
{
oldAcceptState.AcceptState = false;
nfa.Transitions.Add(new Transition <NFA.State>(oldAcceptState, newAcceptState));
}
nfa.States.Add(newAcceptState);
return(nfa);
}
public static NFA RepeatZeroOrMore(NFA input)
{
NFA nfa = new NFA();
// Add everything from the input
nfa.AddAll(input);
// Create a new starting state, link it to the old accept state with Epsilon
nfa.StartState = new NFA.State();
nfa.States.Add(nfa.StartState);
NFA.State oldAcceptState = input.States.First(f => f.AcceptState);
nfa.Transitions.Add(new Transition <NFA.State>(nfa.StartState, oldAcceptState));
// Add epsilon link from old accept state of input to start, to allow for repetition
nfa.Transitions.Add(new Transition <NFA.State>(oldAcceptState, input.StartState));
// Create new accept state, link old accept state to new accept state with epsilon
NFA.State acceptState = new NFA.State {
AcceptState = true
};
nfa.States.Add(acceptState);
oldAcceptState.AcceptState = false;
nfa.Transitions.Add(new Transition <NFA.State>(oldAcceptState, acceptState));
return(nfa);
}
private static NFA NumberedRepeat(NFA nfa, int minRepetitions, int maxRepetitions)
{
// To create a suitable expression, the special case of infinite max repetitions
// must be separately handled.
bool infiniteMax = false;
if (maxRepetitions == int.MaxValue)
{
infiniteMax = true;
maxRepetitions = minRepetitions;
}
else if (maxRepetitions < minRepetitions)
{
maxRepetitions = minRepetitions;
}
// Copy the NFA max repetitions times, link them together.
NFA output = nfa.Copy();
Stack <NFA.State> epsilonLinkStates = new Stack <NFA.State>();
for (int i = 1; i < maxRepetitions; ++i)
{
NFA newNfa = nfa.Copy();
if (i >= minRepetitions || (infiniteMax && i == maxRepetitions - 1))
{
epsilonLinkStates.Push(newNfa.StartState);
}
output = And(output, newNfa);
}
if (infiniteMax)
{
// Use Single to force an exception if this has gone astray
NFA.State finalState = epsilonLinkStates.Single();
// Make a little epsilon loop from the final accept state to the start state of the final state
output.Transitions.Add(new Transition <NFA.State>(output.States.Single(f => f.AcceptState), finalState));
}
else
{
// Add epsilon transitions from accept to beginning states of NFAs in the chain
NFA.State acceptState = output.States.Single(f => f.AcceptState);
while (epsilonLinkStates.Count > 0)
{
output.Transitions.Add(new Transition <NFA.State>(epsilonLinkStates.Pop(), acceptState));
}
}
return(output);
}
private static NFA RepeatOnceOrMore(NFA nfa)
{
// Add an epsilon transition from the accept state back to the start state
NFA.State oldAcceptState = nfa.States.First(f => f.AcceptState);
nfa.Transitions.Add(new Transition <NFA.State>(oldAcceptState, nfa.StartState));
// Add a new accept state, since we cannot have edges exiting the accept state
NFA.State newAcceptState = new NFA.State {
AcceptState = true
};
nfa.Transitions.Add(new Transition <NFA.State>(oldAcceptState, newAcceptState));
nfa.States.Add(newAcceptState);
// Clear the accept flag of the old accept state
oldAcceptState.AcceptState = false;
return(nfa);
}
private static NFA Accept(CharSet acceptCharacters)
{
// Generate a NFA with a simple path with one state transitioning into an accept state.
NFA nfa = new NFA();
NFA.State state = new NFA.State();
nfa.States.Add(state);
NFA.State acceptState = new NFA.State {
AcceptState = true
};
nfa.States.Add(acceptState);
nfa.Transitions.Add(new Transition <NFA.State>(state, acceptState, acceptCharacters));
nfa.StartState = state;
return(nfa);
}
