Prog Challenge, Easy!

Implement https://dis.4chan.org/read/prog/1356634826/25 more elegantly.


(defun 3ml-cgi-decode ()
  (let ((stream (if (string-equal "get" request-method)
            (make-string-input-stream query-string)
          *standard-input*))
    (symbols ()))
    (with-open-stream (output (make-string-output-stream))
      (loop for char = (read-char stream nil nil)
      do (case char
           (#\= (push (intern (string-upcase (get-output-stream-string output)))
              symbols))
           ((nil #\&) (when symbols
                (setf (symbol-value (first symbols)) (get-output-stream-string output))))
           (#\+ (write-char #\space output))
           (#\% (let* ((nib1 (read-char stream nil nil))
               (nib2 (read-char stream nil nil))
               (code (+ (* 16 (digit-char-p nib1 16))
                    (digit-char-p nib2 16))))
              (write-char (code-char code) output)))
           (t (write-char char output)))
      while char))
    (setf *cgi-variables* symbols)))

>>21
So what says which things can be compute trivially, which things are impractical to compute, and which things have too many solutions to be meaningful (if I understood your post correctly)?

The difference between the first two is probably halting-complete, but I'd like to know more about the reduction rules that are used.