From 213da40c3ba75a270b09abab5da6d8ea41d67da0 Mon Sep 17 00:00:00 2001
From: Aadhavan Srinivasan <aadhavan@twomorecents.org>
Date: Tue, 22 Oct 2024 14:35:03 -0400
Subject: [PATCH] Allow one state to map to multiple states with the same
 transition eg. ab|aa

---
 main.go | 21 ++++++++++-----------
 nfa.go  | 20 +++++++++++---------
 2 files changed, 21 insertions(+), 20 deletions(-)

diff --git a/main.go b/main.go
index a04a100..cc62206 100644
--- a/main.go
+++ b/main.go
@@ -106,7 +106,7 @@ func thompson(re string) State {
 	for _, c := range re {
 		if isAlphaNum(c) {
 			state := State{}
-			state.transitions = make(map[int]*State)
+			state.transitions = make(map[int][]*State)
 			state.content = int(c)
 			state.output = make([]*State, 0)
 			state.output = append(state.output, &state)
@@ -119,14 +119,14 @@ func thompson(re string) State {
 			s2 := pop(&nfa)
 			s1 := pop(&nfa)
 			for i := range s1.output {
-				s1.output[i].transitions[s2.content] = &s2
+				s1.output[i].transitions[s2.content] = append(s1.output[i].transitions[s2.content], &s2)
 			}
 			s1.output = s2.output
 			nfa = append(nfa, s1)
 		case '*':
 			s1 := pop(&nfa)
 			for i := range s1.output {
-				s1.output[i].transitions[s1.content] = &s1
+				s1.output[i].transitions[s1.content] = append(s1.output[i].transitions[s1.content], &s1)
 			}
 			// Reset output to s1 (in case s1 was a union operator state, which has multiple outputs)
 			s1.output = nil
@@ -136,10 +136,10 @@ func thompson(re string) State {
 			s1 := pop(&nfa)
 			s2 := pop(&nfa)
 			s3 := State{}
-			s3.transitions = make(map[int]*State)
+			s3.transitions = make(map[int][]*State)
 			s3.output = append(s3.output, &s1, &s2)
-			s3.transitions[s1.content] = &s1
-			s3.transitions[s2.content] = &s2
+			s3.transitions[s1.content] = append(s3.transitions[s1.content], &s1)
+			s3.transitions[s2.content] = append(s3.transitions[s2.content], &s2)
 			s3.content = UNION
 			s3.isEmpty = true
 
@@ -159,12 +159,11 @@ func thompson(re string) State {
 func main() {
 	var re string
 	//	fmt.Scanln(&re)
-	re = "a(b|c)*d"
+	re = "abab|abbb"
 	re_postfix := shuntingYard(re)
 	fmt.Println(re_postfix)
 	start := thompson(re_postfix)
-
-	assert(len(start.transitions) == 1)
-	assert(len(start.transitions[UNION].transitions) == 2)
-
+	UNUSED(start)
 }
+
+func UNUSED[T any](val T) {}
diff --git a/nfa.go b/nfa.go
index 70bd855..030d21c 100644
--- a/nfa.go
+++ b/nfa.go
@@ -3,11 +3,11 @@ package main
 const EPSILON int = 0
 
 type State struct {
-	content     int            // Contents of current state
-	isEmpty     bool           // If it is empty - Union operator states will be empty
-	isLast      bool           // If it is the last state (acept state)
-	output      []*State       // The outputs of the current state ie. the 'outward arrows'. A union operator state will have more than one of these.
-	transitions map[int]*State // Transitions to different states (can be associated with an int, representing content of destination state)
+	content     int              // Contents of current state
+	isEmpty     bool             // If it is empty - Union operator states will be empty
+	isLast      bool             // If it is the last state (acept state)
+	output      []*State         // The outputs of the current state ie. the 'outward arrows'. A union operator state will have more than one of these.
+	transitions map[int][]*State // Transitions to different states (maps a character (int representation) to a _list of states. This is useful if one character can lead multiple states eg. ab|aa)
 }
 
 type NFA struct {
@@ -21,7 +21,7 @@ func verifyLastStatesHelper(state *State, visited map[*State]bool) {
 		state.isLast = true
 		return
 	}
-	if state.transitions[state.content] == state { // Eg. a*
+	if len(state.transitions) == 1 && len(state.transitions[state.content]) == 1 && state.transitions[state.content][0] == state { // Eg. a*
 		state.isLast = true
 		return
 	}
@@ -29,9 +29,11 @@ func verifyLastStatesHelper(state *State, visited map[*State]bool) {
 		return
 	}
 	visited[state] = true
-	for k := range state.transitions {
-		if state.transitions[k] != state {
-			verifyLastStatesHelper(state.transitions[k], visited)
+	for _, states := range state.transitions {
+		for i := range states {
+			if states[i] != state {
+				verifyLastStatesHelper(states[i], visited)
+			}
 		}
 	}
 }