(def- DEFAULT_ALPHABET "_-0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ")

(def- masks [15 31 63 127 255])

(defn gen [&keys {:size size
                  :alphabet alphabet}] 
  (default size 21)
  (default alphabet DEFAULT_ALPHABET)

  (def mask (find |(>= $ (dec (length alphabet))) masks))
  (def step (->> alphabet
                 (length)
                 (/ (* 1.6 mask size))
                 (math/ceil)))
  (def result (array/new size))
  (while (> size (length result))
    (loop [byte :in (os/cryptorand step) 
           :when (> size (length result))]
      (let [mask-byte (band mask byte)
            char (get alphabet mask-byte)]
        (if char
          (array/push result char))
      )))
  (string/from-bytes ;result))