;; draws pretty pictures, makes a great gift.

(define draw-every 1000) ; update screen every N points

(define (HOP a b c)
  (let loop ((x 0) (y 0)
             (counter 0)
             (color (next-color)))
    (let ((update? (zero? (modulo counter draw-every))))
      (when update? (xflush *dpy*))
        ; following two lines are the actual Hopalong algorithm:
      (let ((new-x (- y (* (sign x) (sqrt (abs (- (* b x) c))))))
            (new-y (- a x)))
        (draw-point new-x new-y color)
        (loop new-x new-y (modulo (+ counter 1) draw-every)
              (if update? (next-color) color))))))

(define (SIGN x) (if (negative? x) -1 1))