gistfile1.clj

(defn drive-subleq
  "Runs an OISC SUBLEQ program. Given the program (as a list of integers) and the input (in the same way), it runs the program and returns the output."
  [program input]
  (let [-out- (count program)
        -in- (inc -out-)
        input-len (count input)]
    (loop [mem (vec (concat program
                            [0 (if (zero? input-len) -1 (nth input 0))] ; fill in the first char of the input now, unless there is no input, in which case give EOF right away
                            (vec (range 0 1023)))) ; memory starts out as the program, then the special output cell, then the special input cell, followed by 1024 memory cells filled with the numbers from 0 to 1023
           regs {:ip 0}
           output ""]
      (let [[mem regs] (subleq -in- -out- mem regs)]
        (if (and (:in regs)
                 (neg? (mem -in-)))
          [output (- -1 (mem -in-))] ; if wrote a negative value to -in-, exit with one less than its absolute value (so write -1 to exit with code 0)
          (let [output (str output
                            (if (:out regs) ; if there is new output, append it
                              (char (mem -out-))))
                mem (assoc mem -in- (if (:in regs) ; if something was written to IN, honor it
                                      (if (or (>= (mem -in-) input-len)
                                              (neg? (mem -in-)))
                                        -1
                                        (nth input (mem -in-)))
                                      (mem -in-))
                               -out- 0) ; OUT should always contain zero
                regs (dissoc regs :in :out)]
          (recur mem regs output)))))))