fix: prevent stack corruption in Divide() on numerator pop failure

Divide() popped the denominator (b) first, validated it, then popped
the numerator (a). If the second pop failed (stack had only 1 element),
the denominator was already removed — leaving the stack corrupted.

Now uses popTwo() to atomically pop both operands before any validation,
matching how other binary operators (Add, Subtract, Multiply, etc.)
handle the error case.

1 files changed, 1 insertions, 6 deletions

diff --git a/internal/rpn/operations_arithmetic.go b/internal/rpn/operations_arithmetic.go
index 96676b8..8504d34 100644
--- a/internal/rpn/operations_arithmetic.go
+++ b/internal/rpn/operations_arithmetic.go
@@ -107,7 +107,7 @@ func (o *Operations) Multiply(stack *Stack) error {
 
 // Divide pops two values from stack, divides (a / b), and pushes result.
 func (o *Operations) Divide(stack *Stack) error {
-	b, err := popStack(stack, "/")
+	a, b, err := popTwo(stack, "/")
 	if err != nil {
 		return err
 	}
@@ -120,11 +120,6 @@ func (o *Operations) Divide(stack *Stack) error {
 		return buildError("/", fmt.Errorf("division by zero"))
 	}
 
-	a, err := popStack(stack, "/")
-	if err != nil {
-		return err
-	}
-
 	aM, err := resolveMetric(o.metricRegistry, a)
 	if err != nil {
 		return buildError("/", err)