[racket] A puzzle

The following program was a puzzle in the 2011 MIT Mystery Hunt held last
weekend.  (I'm not linking directly to the site because they posted a
solution and I don't want you to be tempted to look at the answer
prematurely...)  I was disappointed that Dr Racket doesn't seem to have a
multi-threaded stepping debugger, at least not in a form that could easily
run this program-- correct me if I'm wrong.

I would be interested to hear what other ideas people have about solving
this puzzle, either automatically or by hand (or some combination).  But, I
haven't solved it myself yet, so I may be avoiding reading replies for a
while...

--Doug



Efficiency

*The Prince is quite proud of his competitive Katamari-rolling time record,
having mastered the art of multitasking.*

All procedure calls are evaluated from left to right.


(define (send x)
  (if (or (integer? x) (char? x))
      (display x)
      (send x))
  (flush-output)
  #t)
(define (sendc x)
  (if (and (>= x 0) (< x 26))
      (send (integer->char (+ 65 x)))

      (sendc x)))

(define a 2)
(define b '(11))
(define c 5)
(define d 30)
(define e 3)

(thread
 (lambda ()
   (set! a (* a (first a)))
   (send a)
   (set! c (lambda () (- b 4)))

   (set! c (lambda () (- b 11)))
   (set! d (rest d))
   (sendc (/ 4 (+ (- c) c)))
   (set! a (sqrt (+ c (* c c))))
   (sendc b)
   (set! b '(((24 12 15 (2))) 1 (5 1) 3 2 8 10 (9 1 (3)) ((3) (1)) 5 14 10))

   (set! d '())
   (sendc (length (append c b)))
   (sendc (length (append a b c c c d)))
   (set! d 2)
   (map (lambda (x)
          (set! c (first b))
          (sendc x)
          (set! b c)) a)

   (send (* (+ 3 (first c)) 100 (- (sqrt a) 1)))
   (set! a (+ a 1))
   (set! e (- e 1))
   ))

(thread
 (lambda ()
   (set! a (list c))
   (set! a (lambda (x) (if x (sendc x) x)))
   (let loop ()

     (set! b (c))
     (if (a b)
         (loop)
         (set! d '(2 4 8 16 32))))
   (set! c (first d))
   (set! c (second d))
   (set! b (- a (sqrt (first d))))
   (set! a (list 14 (- (length b) 1) c))

   (let loop ()
     (set! d b)
     (if (null? d)
         (set! a (+ a 1))
         (begin
           (sendc (first d))
           (set! b (rest d))
           (loop))))
   (set! e (- e 1))
   ))

(thread
 (lambda ()
   (set! b d)
   (set! c (lambda () (/ b 2)))
   (let loop ()
     (set! d (- d 10))
     (if (and (= d 0)
              (= b 0)
	      (< b 0))
         (set! b #f)

         (loop)))
   (set! c (first d))
   (set! a 0)
   (sendc (length b))
   (set! c (list (second b)))
   (sendc (sqrt (+ 12 (length d))))
   (send (+ (/ (foldl * 26 a)
               (+ (length (append a b))

		  (* (length c) (length c))))
            d))
   (set! a (first b))
   (sendc (/ 204 a))
   (set! e (- e 1))
   ))

(let loop ()
  (if (> e 0)
      (loop)
      (newline)))
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