Decoding BEncoded Streams in Clojure

Bencode is the encoding used by file sharing system BitTorrent. Torrent files are simply Bencoded dictionaries. This post will walk you through my Bencode decoder.

You can read about the BitTorrent specification here there is also a lot of information on of course don't forget to check out the Wikipedia article.

Now specs out of the way, let's dissect the code,

(defn decode [stream & i]
  (let [indicator (if (nil? i) (.read stream) (first i))]
     (and (>= indicator 48) 
          (<= indicator 57)) (decode-string stream indicator)
          (= (char indicator) \i) (decode-number stream \e)
          (= (char indicator) \l) (decode-list stream)
          (= (char indicator) \d) (decode-map stream))))

decode will read one byte from the stream determine its type and call the appropriate function.

(defn- decode-number [stream delimeter & ch]
  (loop [i (if (nil? ch) (.read stream) (first ch)), result ""]
    (let [c (char i)]
      (if (= c delimeter)
        (BigInteger. result)
        (recur (.read stream) (str result c))))))

decode-number takes the stream that we are processing and a delimiter, when delimiter is read we stop reading, this function is used to decode both numbers formatted as "i23e" and byte string length "10:".

(defn- decode-string [stream ch]
  (let [length (decode-number stream \: ch)
        buffer (make-array Byte/TYPE length)]
    (.read stream buffer)
    (String. buffer "ISO-8859-1")))

decode-string will parse the string length variable and read indicated bytes from the string, i build the string using ISO-8859-1 that way SHA-1 hashes will not be corrupted.

(defn- decode-list [stream]
  (loop [result []]
    (let [c (char (.read stream))]
      (if (= c \e)
        (recur (conj result (decode stream (int c))))))))

decode-list will call decode on the items until the list delimiter "e" is read. Lists are returned as Clojure vectors.

(defn- decode-map [stream] 
  (apply hash-map (decode-list stream)))

decode-map will decode the map as a list then apply hash-map on it producing a Clojure map.