API 2: calculate score

[tomfa]

We want to calculate the cumulative scores after each round. In the BowlingRound type, set the correct sum value.

• 1 bowling pin scored = 1 point
• A strike gets double points of two next throws. Example: the throws [10, 6, 3] results in the points 19 and 28.
• A spare gets double points of the next throw. Example: the throws [6, 4, 5, 3] results in the points 15 and 23