cljs.reader/StringPushbackReader

(deftype StringPushbackReader [s buffer ^:mutable idx]
  PushbackReader
  (read-char [reader]
    (if (zero? (alength buffer))
      (do
        (set! idx (inc idx))
        (aget s idx))
      (.pop buffer)))
  (unread [reader ch]
    (.push buffer ch)))