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; o tannenbaum

; rand -- returns integer on range 0 .. 2^32-1
(define rand ; marsaglia linear congruential method
  (let* ((a 69069) (c 1234567) (m 4294967296)) ; due to knuth
    (lambda () (set! seed (modulo (+ (* a seed) c) m)) seed)))

; seed -- number of seconds since the Unix epoch NON-PORTABLE
; please report ports to other Scheme systems to the author
; uncomment exactly one of the next four options
; (define seed (time-second (current-time))) ; chez
(define seed (current-seconds)) ; racket/chicken
; (define seed (current-time)) ; guile
; (define seed (inexact->exact (round
;   (time->seconds (current-time))))) ; gambit

(define (center width str)
  (let* ((len (string-length str))
         (indent (quotient (- width len) 2)))
    (display (make-string indent #\space))
    (display str) (newline)))

(define (row n)
  (let loop ((n n) (str ""))
    (if (zero? n) str
      (loop (- n 1)
            (string-append
              (if (< (rand) 536870912) "*" "O")
              " " str)))))

(define (tannenbaum)
  (center 60 "*")
  (do ((n 2 (+ n 1))) ((< 20 n))
    (center 60 (row n)))
  (do ((n 1 (+ n 1))) ((< 3 n))
    (center 60 "XXX")))

(tannenbaum)

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