solution to http://www.4clojure.com/problem/94

game-of-life-step.clj

(defn board-step [board]
  (let [live-cell \#
        dead-cell \space
        live-cell? (fn [cell] (= cell live-cell))
        transit-cell (fn [cell live-neib]
                       (if (live-cell? cell)
                         (cond
                          (< live-neib 2) dead-cell
                          (> live-neib 3) dead-cell
                          :else live-cell)
                         (cond
                          (= live-neib 3) live-cell
                          :else dead-cell)))
        align-subvec (fn [v f t]
                       (let [af (if (< f 0) 0 f)
                             inc-t (inc t)
                             at (if (>= (count v) inc-t) inc-t (count v))]
                         (subvec v af at)))
        mutate-cell (fn [board r c acc-board]
                      (let [cell ((board r) c)
                            hor-zone (align-subvec board (dec r) (inc r))
                            zone (mapcat #(align-subvec % (dec c) (inc c)) hor-zone)
                            live-cells-count (count (filter live-cell? zone))
                            live-neibs (if (live-cell? cell) (dec live-cells-count) live-cells-count)
                            mutated-cell (transit-cell cell live-neibs)]
                        (assoc acc-board r (assoc (acc-board r) c mutated-cell))))]
    (let [vec-board (vec (map vec board)) 
          rc (count board)
          cc (count (first board))]
      (loop [r 0 c 0 acc-board vec-board]
        (let [next-board (mutate-cell vec-board r c acc-board)]
          (cond
           (> cc (inc c)) (recur r (inc c) next-board)
           (> rc (inc r)) (recur (inc r) 0 next-board)
           :else (map #(apply str %) next-board)))))))