Clone lookaroundNFA when cloning a state; use compiled regex for

lookarounds instead of compiling a new one

nfa.go

@@ -32,6 +32,7 @@ type State struct {
 	allChars        bool             // Whether or not the state represents all characters (eg. a 'dot' metacharacter). A 'dot' node doesn't store any contents directly, as it would take up too much space
 	except          []rune           // Only valid if allChars is true - match all characters _except_ the ones in this block. Useful for inverting character classes.
 	lookaroundRegex string           // Only for lookaround states - Contents of the regex that the lookaround state holds
+	lookaroundNFA   *State           // Holds the NFA of the lookaroundRegex - if it exists
 }
 
 // Clones the NFA starting from the given state.
@@ -47,6 +48,9 @@ func cloneStateHelper(state *State, cloneMap map[*State]*State) *State {
 	if clone, exists := cloneMap[state]; exists {
 		return clone
 	}
+	if state == nil {
+		return nil
+	}
 	// Recursive case - if the clone doesn't exist, create it, add it to the map,
 	// and recursively call for each of the transition states.
 	clone := &State{
@@ -80,6 +84,10 @@ func cloneStateHelper(state *State, cloneMap map[*State]*State) *State {
 			}
 		}
 	}
+	if state.lookaroundNFA == state {
+		clone.lookaroundNFA = clone
+	}
+	clone.lookaroundNFA = cloneStateHelper(state.lookaroundNFA, cloneMap)
 	return clone
 }
 
@@ -104,9 +112,7 @@ func (s State) checkAssertion(str []rune, idx int) bool {
 		// 		1. Compile the regex stored in the state's contents.
 		// 		2. Run it on the test string.
 		// 		3. Based on the kind of lookaround (and the indices we get), determine what action to take.
-		regex := s.lookaroundRegex
-		re_postfix := shuntingYard(regex)
-		startState := thompson(re_postfix)
+		startState := s.lookaroundNFA
 		matchIndices := findAllMatches(startState, str)
 
 		numMatchesFound := 0