(defn locator [r n]
  (fn [in]
    (->> (iterate #(/ % 2) (/ n 2))
         (map vector in)
         (keep (fn [[ch n]] (when (#{r} ch) n)))
         (apply +))))

(let [get-row (locator \B 128)
      get-col (locator \R 8)]
  (defn get-seat [in]
    (let [[r c] (split-at 7 in)]
      (+ (* 8 (get-row r))
         (get-col c)))))

(let [seats (into (sorted-set) (map get-seat data-5))
      rng (apply range ((juxt first last) seats))]
  [(first (remove seats rng)) (last seats)])