5.8: Sliding Tiles with the Mouse

                else:
                    # check if the clicked tile was next to the blank spot

                    blankx, blanky = getBlankPosition(mainBoard)
                    if spotx == blankx + 1 and spoty == blanky:
                        slideTo = LEFT
                    elif spotx == blankx - 1 and spoty == blanky:
                        slideTo = RIGHT
                    elif spotx == blankx and spoty == blanky + 1:
                        slideTo = UP
                    elif spotx == blankx and spoty == blanky - 1:
                        slideTo = DOWN

If getSpotClicked() did not return (None, None), then it will have returned a tuple of two integer values that represent the X and Y coordinate of the spot on the board that was clicked. Then the if and elif statements on lines 5 [89] to 12 [96] check if the spot that was clicked is a tile that is next to the blank spot (otherwise the tile will have no place to slide).

Our getBlankPosition() function will take the board data structure and return the X and Y board coordinates of the blank spot, which we store in the variables blankx and blanky. If the spot the user clicked on was next to the blank space, we set the slideTo variable with the value that the tile should slide.