// ball:
//    Bounce a ball round the screen

HGR
xPos = GWIDTH / 2
yPos = GHEIGHT / 2
xSpeed = 4
ySpeed = 4
xDir = 1
yDir = 1
r = GWIDTH / 42
CYCLE
  PROC updateBall
  key = INKEY
  IF key = KeyUp THEN ySpeed = ySpeed + 1
  IF (key = KeyDown) AND (ySpeed > 0) THEN ySpeed = ySpeed - 1
  IF (key = KeyLeft) AND (xSpeed > 0) THEN xSpeed = xSpeed - 1
  IF key = KeyRight THEN xSpeed = xSpeed + 1
REPEAT
END

// PROC ball:
//    Draw our ball on the screen

DEF PROC ball(x, y, r, c)
COLOUR = c
CIRCLE (x, y, r, 1)
ENDPROC

// PROC updateBall:
//    Update the ball position coordinates and
//    it's screen position

DEF PROC updateBall
LOCAL newX, newY
newX = xPos + xSpeed * xDir
IF (newX > (GWIDTH - r)) OR (newX < r) THEN
  newX = xPos
  xDir = -1 * xDir
ENDIF
newY = yPos + ySpeed * yDir
IF (newY > (GHEIGHT - r)) OR (newY < r) THEN
  newY = yPos
  yDir = -1 * yDir
ENDIF

PROC ball(xPos, yPos, r, Black)
PROC ball(newX, newY, r, blue)
xPos = newX
yPos = newY
UPDATE
ENDPROC