Bowling Game: State Machine: Std::Variant: Step 30

To remove the if (pins == 10) from multiple places, we can extract a base class, FirstRollUtils, and put a method, IsStrike(int pins) there:

        struct FirstRollUtils
        {
            static bool IsStrike(int pins) { return pins == 10; }
        };

And use it several times:

        struct WaitingForFirstRollWith0Bonuses : FirstRollUtils
        {
            State Update(int pins, int& score, int& frame) const
            {
                score += pins;
                if (IsStrike(pins)) {
                    ++frame;
                    return WaitingForFirstRollWith2Bonuses{};
                }
                return WaitingForSecondRollWith0Bonuses{pins};
            }
        };
  ...
        struct WaitingForFirstRollWith2Bonuses : FirstRollUtils
        {
            State Update(int pins, int& score, int& frame) const
            {
                score += pins*2;
                if (IsStrike(pins)) {
                    ++frame;
                    return WaitingForFirstRollWith3Bonuses{};
                }
                return WaitingForSecondRollWith1Bonus{pins};
            }
        };
        struct WaitingForFirstRollWith3Bonuses : FirstRollUtils
        {
            State Update(int pins, int& score, int& frame) const
            {
                score += pins*3;
                if (IsStrike(pins)) {
                    ++frame;
                    if (frame == 10)
                        return LastFrameWith2Bonuses{};
                    return WaitingForFirstRollWith3Bonuses{};
                }
                return WaitingForSecondRollWith1Bonus{pins};
            }
        };
  ...
        struct LastFrameWith2Bonuses : FirstRollUtils
        {
            State Update(int pins, int& score, int& /*frame*/) const
            {
                score += pins*2;
                if (IsStrike(pins))
                    return LastFrameWith1Bonus{};
                return GameOver{};
            }
        };

Looks good.

However, I notice that the WaitingForFirstRollWith1Bonus class does not derive from FirstRollUtils, and it doesn’t in fact check for a strike anywhere. Maybe we’re missing a test? Let’s try it by writing:

	{"a spare followed by a strike and a two bonus rolls of 1 and 2 means score is 36", []() {
		Game game;
		game.Roll(5);
		game.Roll(5); // spare
		game.Roll(10); // strike
		game.Roll(1);
		game.Roll(2);
		RollMany(game, 14, 0);
		Assert::AreEqual(36, game.Score()); // game is over here
		Assert::ExpectingException<std::invalid_argument>([&game]() { game.Roll(0); });
	}},

And when run, this test fails, throwing an exception with message, sum of rolls in a frame must be <= 10. Clearly, we need that IsStrike check. Let’s do that and then click next.