From 5ee427230c3c00c4b3feabaeb836621c731fc2b9 Mon Sep 17 00:00:00 2001
From: Aadhavan Srinivasan <aadhavan@twomorecents.org>
Date: Sat, 26 Oct 2024 13:05:44 -0400
Subject: [PATCH] Replaced 'pop' with 'mustPop' which panics on error

---
 main.go | 35 ++++++++++++++++++++++-------------
 1 file changed, 22 insertions(+), 13 deletions(-)

diff --git a/main.go b/main.go
index 76661f8..a2920d8 100644
--- a/main.go
+++ b/main.go
@@ -69,12 +69,17 @@ func shuntingYard(re string) string {
 			if len(opStack) == 0 {
 				opStack = append(opStack, c)
 			} else {
-				if priority(c) > priority(peek(opStack)) { // 2a
+				topStack, err := peek(opStack)
+				if err != nil {
+					panic("ERROR: Operator without operand.")
+				}
+				if priority(c) > priority(topStack) { // 2a
 					opStack = append(opStack, c)
 				} else {
-					for len(opStack) > 0 && priority(c) <= priority(peek(opStack)) { // 2b
-						to_append := pop(&opStack)
+					for priority(c) <= priority(topStack) { // 2b
+						to_append := mustPop(&opStack)
 						outQueue = append(outQueue, to_append)
+						topStack, _ = peek(opStack)
 					}
 					opStack = append(opStack, c)
 				}
@@ -84,17 +89,21 @@ func shuntingYard(re string) string {
 			opStack = append(opStack, c)
 		}
 		if c == ')' {
-			for peek(opStack) != '(' {
-				to_append := pop(&opStack)
+			// Keep popping from opStack until we encounter an opening parantheses. Panic if we reach the end of the stack.
+			for val, err := peek(opStack); val != '('; val, err = peek(opStack) {
+				if err != nil {
+					panic("ERROR: Imbalanced parantheses.")
+				}
+				to_append := mustPop(&opStack)
 				outQueue = append(outQueue, to_append)
 			}
-			_ = pop(&opStack) // Get rid of opening parantheses
+			_ = mustPop(&opStack) // Get rid of opening parantheses
 		}
 	}
 
 	// Pop all remaining operators (and append to outQueue)
 	for len(opStack) > 0 {
-		to_append := pop(&opStack)
+		to_append := mustPop(&opStack)
 		outQueue = append(outQueue, to_append)
 	}
 
@@ -118,15 +127,15 @@ func thompson(re string) *State {
 		// Must be an operator if it isn't alphanumeric
 		switch c {
 		case CONCAT:
-			s2 := pop(&nfa)
-			s1 := pop(&nfa)
+			s2 := mustPop(&nfa)
+			s1 := mustPop(&nfa)
 			for i := range s1.output {
 				s1.output[i].transitions[s2.content] = append(s1.output[i].transitions[s2.content], s2)
 			}
 			s1.output = s2.output
 			nfa = append(nfa, s1)
 		case '*': // Create a 0-state, concat the popped state after it, concat the 0-state after the popped state
-			s1 := pop(&nfa)
+			s1 := mustPop(&nfa)
 			stateToAdd := &State{}
 			stateToAdd.transitions = make(map[int][]*State)
 			stateToAdd.content = EPSILON
@@ -139,7 +148,7 @@ func thompson(re string) *State {
 			stateToAdd.transitions[s1.content] = append(stateToAdd.transitions[s1.content], s1)
 			nfa = append(nfa, stateToAdd)
 		case '+':
-			s1 := pop(&nfa)
+			s1 := mustPop(&nfa)
 			for i := range s1.output {
 				s1.output[i].transitions[s1.content] = append(s1.output[i].transitions[s1.content], s1)
 			}
@@ -148,8 +157,8 @@ func thompson(re string) *State {
 			s1.output = append(s1.output, s1)
 			nfa = append(nfa, s1)
 		case '|':
-			s1 := pop(&nfa)
-			s2 := pop(&nfa)
+			s1 := mustPop(&nfa)
+			s2 := mustPop(&nfa)
 			s3 := State{}
 			s3.transitions = make(map[int][]*State)
 			s3.output = append(s3.output, s1.output...)