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% LinkedList NYC Solution % silly, scheme, hack, brainfuck, #pinned % 2012-11-05

Though I hate competition1, I love ice cream, and so the [LinkedList’s programming contest][2] was a worth while endeavor.

I didn’t spend much time on it (I planned about 20 minutes since I had a already planned out what I was going to)–it ended up taking about 45 minutes. Phooey.

My play was simple. Create a program which writes a simple [brainfuck][3] program which is executed on a 6 instruction subset of brainfuck. If you ignore the looping construct, which is the only complicated part of a simple brainfuck system, then an interpreter is about 6 lines of code.

To add a few lines of code for my final trick, I enlisted the help of [Run Length Encoding][4]. The proverbial cherry on top, was formatting the code like a bowl of ice cream (I didn’t think to actually put a cherry on top–I probably would have won if I had).

                      (define (ntbf-eval

                  x h d) (unless (null? x) (case

              (car x) ((#\>) (ntbf-eval (cdr x) h

             (+ d 1))) ((#\<) (ntbf-eval (cdr x) h

            (- d 1))) ((#\+) (begin (vector-set! h d

           (+ 1 (vector-ref h d))) (ntbf-eval (cdr x)

            h d))) ((#\-) (begin (vector-set! h d (-

            1 (vector-ref h d))) (ntbf-eval (cdr x)

           h d))) ((#\.) (begin (format #t "~a" (

          integer->char (vector-ref h d))) (ntbf-eval

         (cdr x) h d))) (else (error "invalid non turing

        brainfuck statement"))))) (define (prog-table) (let

       loop ((i 0) (accum '())) (if (< i 128) (loop (+ 1 i)

       (cons `(,(integer->char i) (,i #\+) (1 #\.)) accum))

        accum))) (define (rld ps) (let ((char (cadr ps)))

          (let loop ((i (car ps)) (accum '())) (if (> i

            0) (loop (- i 1) (cons char accum)) accum)

            ))) (define fragments (prog-table))(define

            (make-prog s) (let ((ls (string->list s)))

              (let loop ((ls ls) (accum  '()))  (if

                (not (null? ls))  (let ((p (assoc

                 (car ls) fragments))) (if (not

                  (null? p)) (loop (cdr ls) (

                    append accum (append (cdr

                     p) '((1 #\>))))) (error

                    "I only do ASCII"))) accum

                  )))) (define (decode ls) (apply

               append (map rld ls))) (begin(ntbf-eval

              (decode    (make-prog "LinkedList NYC"))

              (make-vector 100         0) 0) (newline))

                    ;; LinkedList Ice Cream Bowl

                    ;; By Andrew Gwozdziweycz

The code runs in Guile 2.0+, or a sane scheme implementation that supports format.

BTW, some of the [other entries][5] were pretty cool. If only they’d use their time working on free software…

  1. In fact, I’ve rated it [1 star][6] on [Combosaurus][7]. Why? Well, that’s a long story (I suppose), but it doesn’t bring out the best (in some ways, it does of course, but not the side I’d like to see) in people. 

    [1]: #hate-competition