Firebase Push ID, in Clojurescript

firebase_push_id.cljs

(def firebase-push-id!
    (let [last-push-time (atom nil)
          saved-random   (atom nil)]
      (fn firebase-push-id-inner
        ([] (firebase-push-id-inner nil))
        ([millis]
         (locking last-push-time
           (let [push-chars  "-0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ_abcdefghijklmnopqrstuvwxyz"
                 now         (or millis (.getTime (js/Date.)))
                 duplicate?  (= now @last-push-time)
                 time-stamp  (->> (range 7)
                                  (reductions #(Math/floor (/ % 64)) now)
                                  (map #(mod % 64))
                                  reverse)
                 inc-vec     (fn [v pos]
                               (let [roll-over? (and (= (get v pos) 63)
                                                     (pos? pos))
                                     bumped     (update v pos #(mod (inc %) 64))]
                                 (if-not roll-over?
                                   bumped
                                   (let [blah bumped]
                                     (recur blah (dec pos))))))
                 last-random (let [new-random (if-not duplicate?
                                                (mapv #(Math/floor (* (Math/random) 64))
                                                      (range 12))
                                                (inc-vec @saved-random 12))]
                               (reset! saved-random new-random)
                               new-random)]
             (reset! last-push-time now)
             (->> (concat time-stamp last-random)
                  (map #(.charAt push-chars %))
                  (str/join ""))))))))