added modulo function

src/calc.cpp

@@ -7,6 +7,7 @@ int plus(    int a, int b) { return a + b; }
 int minus(   int a, int b) { return a - b; }
 int multiply(int a, int b) { return a * b; }
 int divide(  int a, int b) { return a / b; }
+int modulo(  int a, int b) { return a % b; }
 
 int pop_stack(auto &stack) {
   const auto ret{stack.top()};
@@ -29,6 +30,7 @@ int evaluate(const std::string &s) {
     case '-': change_stack_state(minus,    stack); break;
     case '*': change_stack_state(multiply, stack); break;
     case '/': change_stack_state(divide,   stack); break;
+    case '%': change_stack_state(modulo,   stack); break;
     case '0'...'9': stack.push(c - '0'); break;
     }
   }

test/test.cpp

@@ -5,4 +5,5 @@
 TEST_CASE( "some example calculations", "[calc]") {
   REQUIRE( evaluate("11+") == 2 );
   REQUIRE( evaluate("12+2-3*") == 3 );
+  REQUIRE( evaluate("32%") == 1 );
 }