reader.clj

(defprotocol PushbackReader
  (read-char [reader] "Returns the next char from the Reader, nil if the end of stream has been reached")
  (unread [reader ch] "Push back a single character on to the stream"))

(defmacro update! [what f]
  (list 'set! what (list f what)))

;;not thread-safe, but i guess i don't care?
(deftype StringPushbackReader
    [^:unsynchronized-mutable ^String s ^"[C" buf ^:unsynchronized-mutable len ^:unsynchronized-mutable buf?]
  PushbackReader
  (read-char [reader]
    (if buf?
      (do (set! buf? false)
          (aget buf 0))
      (when (> len 0)
        (let [r (.charAt s 0)]
          (update! len dec)
          (set! s (.substring s 1))
          r))))
  (unread [reader ch]
    (if buf? (throw (RuntimeException. "Pushback buffer is full")))
    (aset buf 0 ^char ch)
    (set! buf? true)))

(defn push-back-reader [^String s]
  "Creates a StringPushbackReader from a given string"
  (StringPushbackReader. s (char-array 1) (.length s) false))

(defprotocol LineNumberingReader
  (get-line-number [reader])
  (read-line [reader]))

(deftype LineNumberingPushbackReader
    [spr ^:unsynchronized-mutable line ^:unsynchronized-mutable line-start? ^:unsynchronized-mutable prev]
  PushbackReader
  (read-char [reader]
    (when-let [ch (read-char spr)]
      (let [ch (if (= \return ch)
                  (let [c (read-char spr)]
                    (when-not (= \formfeed ch)
                      (unread spr ch))
                    \newline)
                  ch)]
        (set! prev line-start?)
        (set! line-start? (= ch \newline))
        (when line-start?
          (update! line inc))
        ch)))
  (unread [reader ch]
    (when line-start? (update! line dec))
    (set! line-start? prev)
    (unread spr ch))
  
  LineNumberingReader
  (get-line-number [reader] (inc line))
  (read-line [reader]
    (let [c (read-char reader)]
      (case c
        nil nil
        \newline ""
        (let [rest (read-line reader)]
          (set! prev false)
          (set! line-start? true)
          (str c rest))))))

(defn line-numbering-push-back-reader [s]
  (LineNumberingPushbackReader. (push-back-reader s) 0 true nil))