(defun game (&optional (width 320) (height 240) (speed 1))
  (let ((altitude (/ height 2)))
    (sdl:with-init ()
    (sdl:window width height :title-caption "My game")
    (setf (sdl:frame-rate) 60)
    (sdl:with-events ()
      (:quit-event () t)
      (:idle ()
        (sdl:clear-display sdl:*white*)
        (setf speed (if (or (< altitude 10) (> altitude (- height 10)))
          (- speed)
              speed))
        (sdl:draw-box (sdl:rectangle-from-midpoint-*
          (/ width 2) (incf altitude speed) 20 20) :color sdl:*red*)
        (sdl:update-display))))))

(defun calculate-speed (speed altitude height)
  (if (or (< altitude 10) (> altitude (- height 10)))
    (- speed)
    speed))

(defun update-altitude (speed altitude height)
  (+ altitude (calculate-speed speed altitude height)))

(defun game (&optional (width 320) (height 240) (speed 1))
  (let ((altitude (/ height 2)))
    (sdl:with-init ()
      (sdl:window width height :title-caption "My game")
      (setf (sdl:frame-rate) 60)
      (sdl:with-events ()
    (:quit-event () t)
    (:idle ()
      (sdl:clear-display sdl:*white*)
      (setf altitude (update-altitude speed altitude height))
      (sdl:draw-box (sdl:rectangle-from-midpoint-*
        150 altitude 20 20)
        :color sdl:*red*)
      (sdl:update-display))))))