Reversi Count Flips

The plan is to count how many pieces will flip in a given direction. The key part of this is a helper function that knows how many pieces would be flipped so far in the analysis and a location (posn) the board to continue analyzing.

The attachments are intended to be used to write check-expects for the count-flip-help.

Count Flip Outline

Counting how many pieces flip is done by summing the numbers of pieces that are flipped in each direction. In a given direction, a helper function examines a line of squares until it has figured out how many pieces will flip in that sequence.

• count-flips-all: board player posn(current) -> number: Considering every direction, how many pieces will a particular play flip?
• count-flips-one: board player posn(current) posn(direction) -> number: Considering only a single direction, how many pieces will flip?
• count-flips-help: board player posn(next) posn(direction) number(flip-so-far) -> number: Given the next square to analyze and a count of the number of pieces that would be flipped so far, return the total number of pieces that would flip (in that given direction).

The count-flips-one is a “wrapper” function that starts with the current position (and direction), and then calls count-flips-help using zero flips and the next square in that direction.

Count Flip Helper

We will design a helper function to examine a sequence of positions in a given direction and count how many pieces would flip in that sequence. Remember: opponent’s pieces only flip if they are “sandwiched” between the player’s pieces. Review the rules of “true reversi” if you are confused!

Basic information:

• board (list of piece)
• player: the current player
• next (posn): a square to examine
• dir (posn): the direction to continue looking
• flips-so-far (number)

In order to understand how this function works, it is critical to work through some examples (check-expects) of particular board setups. See the Reversi flips attachment. Please go write check-expects for those examples now.

In each of these cases for logic in the count-flips-help function, figure out what the correct response should be:

1. The next square is not occupied.
2. A player’s own piece occupies the next square. This means no more pieces flip.
3. The opponent’s piece occupies the next square.