mandelbrot Racket program

#lang racket/base

;;; The Computer Language Benchmarks Game
;;; https://salsa.debian.org/benchmarksgame-team/benchmarksgame/
;;
;;; Derived from the Chicken Scheme variant by Anthony Borla
;;; contributed by Matthew Flatt

 
(require racket/cmdline
         racket/flonum)

(define +limit-sqr+ 4.0)

(define +iterations+ 50)

;; -------------------------------

(define (mandelbrot x y n ci)
  (let ((cr (fl- (fl/ (fl* 2.0 (->fl x)) (->fl n)) 1.5)))
    (let loop ((i 0) (zr 0.0) (zi 0.0))
      (if (> i +iterations+)
          1
          (cond
           ((fl> (fl+ (fl* zr zr) (fl* zi zi)) +limit-sqr+) 0)
           (else (loop (+ 1 i) 
                       (fl+ (fl- (fl* zr zr) (fl* zi zi)) cr) 
                       (fl+ (fl* 2.0 (fl* zr zi)) ci))))))))

;; -------------------------------

(define (main n)
  (let ((out (current-output-port)))

    (fprintf out "P4\n~a ~a\n" n n)

    (let loop-y ((y 0))

      (when (< y n)
        
        (let ([ci (fl- (fl/ (fl* 2.0 (->fl y)) (->fl n)) 1.0)])
          
          (let loop-x ((x 0) (bitnum 0) (byteacc 0))

            (if (< x n)
                (let ([bitnum (+ 1 bitnum)]
                      [byteacc (+ (arithmetic-shift byteacc 1) 
                                  (mandelbrot x y n ci))])

                  (cond
                   ((= bitnum 8)
                    (write-byte byteacc out)
                    (loop-x (+ 1 x) 0 0))
                   
                   [else (loop-x (+ 1 x) bitnum byteacc)]))

                (begin
                  (when (positive? bitnum)
                    (write-byte (arithmetic-shift byteacc 
                                                  (- 8 (bitwise-and n #x7))) 
                                out))

                  (loop-y (add1 y))))))))))

;; -------------------------------

(command-line #:args (n)
              (main (string->number n)))
    

NOTES:
64-bit Ubuntu quad core
Welcome to Racket v7.7.


Wed, 06 May 2020 17:11:24 GMT

MAKE:
/opt/src/racket-7.7/bin/raco make mandelbrot.racket

3.55s to complete and log all make actions

COMMAND LINE:
/opt/src/racket-7.7/bin/racket mandelbrot.racket 16000

(BINARY) PROGRAM OUTPUT NOT SHOWN