Mike's ActionMaster Script

public class ActionMaster {
    public enum Action { Tidy, Reset, EndTurn, EndGame };

    private int[] bowls = new int[21];
    private int bowl = 1;

    public Action Bowl (int pins) {
        if (pins < 0 || pins > 10 ) { throw new UnityException("Invalid number of pins"); }

        bowls[bowl-1] = pins;

        if (bowl == 21) {
            return Action.EndGame;
        }

        if (bowl >= 19) {
            if (bowl == 20 && IsBowl19Strike()) {
                bowl++;
                return Action.Tidy;
            } else if (Bowl21Awarded()) {
                bowl++;
                return Action.Reset;
            } else if (bowl == 20 && !Bowl21Awarded()) {
                return Action.EndGame;
            }
        }

        if (pins == 10) {
            bowl += 2;
            return Action.EndTurn;
        }

        if (bowl % 2 != 0) { // Mid-Frame (or last frame)
            bowl++;
            return Action.Tidy;
        } else if (bowl % 2 == 0) { // End of Frame
            bowl++;
            return Action.EndTurn;
        }

        throw new UnityException("No action to return");
    }

    private bool Bowl21Awarded() {
        return (bowls[19-1] + bowls [20-1] >= 10);
    }

    private bool IsBowl19Strike() {
        return (bowls[19-1] == 10);
    }
}