mars.clj

(def right
  {:N :E
   :E :S
   :S :W
   :W :N})

(def left
  (clojure.set/map-invert right))

(defn bound [bottom top n]
  (max bottom (min top n)))

(defn run-instruction [[plateau-x plateau-y] [_ _ heading :as pos] instruction]
  (if (= :M instruction)
    (case heading
      ;; Horizontal
      :E (update pos 0 (comp (partial bound 0 plateau-x) inc))
      :W (update pos 0 (comp (partial bound 0 plateau-x) dec))
      ;; Vertical
      :N (update pos 1 (comp (partial bound 0 plateau-y) inc))
      :S (update pos 1 (comp (partial bound 0 plateau-y) dec)))
    (update pos 2 (case instruction
                    :L left
                    :R right))))

(defn move-rover [plateau {:keys [position instructions]}]
  {:position (reduce (partial run-instruction plateau) position instructions)})

(defn go! [{:keys [plateau rovers]}]
  {:rovers (mapv (partial move-rover plateau) rovers)})