#lang typed/racket/base #:optimize
(require racket/future racket/flonum)
(define MAX-ITERS 50)
(define MAX-DIST 2.0)
(define N 512)
(: mandelbrot-point : Integer Integer -> Integer)
(define (mandelbrot-point x y)
  (define ci
    (fl- (fl/ (fl* 2.0 (->fl y))(->fl N)) 1.0))
  (define cr
    (fl- (fl/ (fl* 2.0 (->fl x))(->fl N)) 1.5))
  (let loop ((i 0) (zr 0.0) (zi 0.0))
    (if (> i MAX-ITERS)
        (char->integer #\*)
        (let ((zrq (fl* zr zr))
              (ziq (fl* zi zi)))
          (if (fl> (fl+ zrq ziq)
                   (fl* MAX-DIST MAX-DIST))
              (char->integer #\space)
              (loop (add1 i)
                    (fl+ (fl- zrq ziq) cr)
                    (fl+ (fl* 2.0 (fl* zr zi))
                         ci)))))))

(: fs (Listof (Futureof Bytes)))
(define fs
  (for/list ([y (in-range N)])
    (let ([bstr (make-bytes N)])
      (future
       (lambda ()
         (for ([x (in-range N)])
           (bytes-set! bstr x (mandelbrot-point x y)))
         bstr)))))

(for: ([f : (Futureof Bytes) (in-list fs)])
  (write-bytes (touch f))
  (newline))