Added a division by zero error

creature.go

@@ -29,6 +29,7 @@ const (
 	// registered with the same ID as another one.
 	ErrorIDTaken Error = iota
 	ErrorUnknownInstruction
+	ErrorDivideByZero
 )
 
 // Error returns a textual description of the error.
@@ -38,6 +39,8 @@ func (err Error) Error() (description string) {
 		description = "this ID is taken by another function"
 	case ErrorUnknownInstruction:
 		description = "machine encountered unknown instruction"
+	case ErrorDivideByZero:
+		description = "cannot divide by zero"
 	}
 
 	return
@@ -100,7 +103,13 @@ func (machine *Machine) Execute(offset int) (err error) {
 			// DIV
 			// divides the last word on the stack by the second to
 			// last word on the stack
-			machine.Push(machine.Pop() / machine.Pop())
+			left  := machine.Pop()
+			right := machine.Pop()
+			if left == 0 && right == 0 {
+				err = ErrorDivideByZero
+				return
+			}
+			machine.Push(left / right)
 
 		case 0x8:
 			// EQ

creature_test.go

@@ -23,7 +23,12 @@ func TestArithmetic(test *testing.T) {
 		},
 	}
 
-	machine.Execute(0)
+	err := machine.Execute(0)
+	if err != nil {
+		test.Log("machine exited with error:", err)
+		test.Fail()
+	}
+
 	addResult := machine.Pop()
 	subResult := machine.Pop()
 	mulResult := machine.Pop()