1.18.8 Handling Input

Input handling consists of handling two different kinds of events: key events, and mouse events, plus various helper functions.

The change function handles keyboard input. If the input is n and the game is over, then restart the game. If the input is h then turn on the help and otherwise do nothing.

The release function adjusts the world for a key release event.

The player-moved? predicate returns a posn indicating where the player chose to move when the mouse event corresponds to a player move, and returns #f. It first checks to see if the mouse event is a button up event and that the game is not over, and then it just calls circle-at-point.

The circle-at-point function returns a posn when the coordinate (x,y) is inside an unblocked circle on the given board. Instead of computing the nearest circle to the coordinates, it simply iterates over the cells on the board and returns the posn of the matching cell.

The point-in-this-circle? function returns #t when the point (x,y) on the screen falls within the circle located at the posn p.

This is precise about checking the circles. For example, a point that is (14,14) away from the center of a circle is still in the circle:

but one that is (15,15) away is no longer in the circle, since it crosses the boundary away from a circle of radius 20 at that point.

The implementation of point-in-this-circle? uses complex numbers to represent both points on the screen and directional vectors. In particular, the variable center is a complex number whose real part is the x coordinate of the center of the cell at p, and its imaginary part is y coordinate. Similarly, mp is bound to a complex number corresponding to the position of the mouse, at (x, y). Then, the function computes the vector between the two points by subtracting the complex numbers from each other and extracting the magnitude from that vector.

In the event that player-moved? returns a posn, the clack function blocks the clicked on cell using block-cell/world, which simply calls block-cell.

The move-cat function uses calls build-bfs-table to find the shortest distance from all of the cells to the boundary, and then uses find-best-positions to compute the list of neighbors of the cat that have the shortest distance to the boundary. If that list is empty, then next-cat-position is #f, and otherwise, it is a random element from that list.

Once next-cat-position has been computed, it is used to update the cat and state fields of the world, recording the cat’s new position and whether or not the cat won.

The find-best-positions function accepts two parallel lists, one of posns, and one of scores for those posns, and it returns either a non-empty list of posns that have tied for the best score, or it returns #f, if the best score is '∞.

This is a helper function that behaves like <=, but is extended to deal properly with '∞.

Finally, to complete the mouse event handling, the update-world-posn function is called from clack. It updates the mouse-down field of the world. If the p argument is a posn, it corresponds to the location of the mouse, in graphical coordinates. So, the function converts it to a cell position on the board and uses that. Otherwise, when p is #f, the mouse-down field is just updated to #f.