Implement subtraction in evaluator

lib/evaluator.c

@@ -6,5 +6,12 @@ int evaluate(const expression_t *expression)
 		return expression->number;
 	const int x = evaluate(expression->application.operands[0]);
 	const int y = evaluate(expression->application.operands[1]);
-	return x + y;
+	switch (expression->application.operator) {
+	case OPERATOR_ADD:
+		return x + y;
+	case OPERATOR_SUBTRACT:
+		return x - y;
+	default:
+		return -1;
+	}
 }

tests/evaluator_tests.c

@@ -116,6 +116,25 @@ static void sum_of_the_sum_of_4_and_66_and_83_evaluates_to_153(void)
 	ASSERT_EQUAL(153, result);
 }
 
+static void subtraction_of_56_from_88_evaluates_to_32(void)
+{
+	init_memory_pool(&pool);
+	expression_t *operand0 = allocate_expression(&pool);
+	operand0->is_number = true;
+	operand0->number = 88;
+	expression_t *operand1 = allocate_expression(&pool);
+	operand1->is_number = true;
+	operand1->number = 56;
+	expression_t *expression = allocate_expression(&pool);
+	expression->is_number = false;
+	expression->application.operator= OPERATOR_SUBTRACT;
+	expression->application.operands[0] = operand0;
+	expression->application.operands[1] = operand1;
+
+	const int result = evaluate(expression);
+	ASSERT_EQUAL(32, result);
+}
+
 int main(void)
 {
 	TESTING_BEGIN();
@@ -125,5 +144,6 @@ int main(void)
 	RUN_TEST(sum_of_72_and_6_evaluates_to_78);
 	RUN_TEST(sum_of_21_and_the_sum_of_9_and_31_evaluates_to_61);
 	RUN_TEST(sum_of_the_sum_of_4_and_66_and_83_evaluates_to_153);
+	RUN_TEST(subtraction_of_56_from_88_evaluates_to_32);
 	TESTING_END();
 }