base66 encoder/decoder (encodes integer to url-friendly characters)

base66.clj

(def digits
  (seq (str "0123456789"
            "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
            "abcdefghijklmnopqrstuvwxyz"
            ".-_~")))

(def radix (count digits))

(defn encode
  "Encodes Long to String."
  [n]
  (loop [output ""
         n n]
    (if (>= n radix)
      (let [remainder (mod n radix)
            char (nth digits remainder)]
        (recur (str char output) (quot n radix)))
      (str (nth digits n) output))))


(defn decode 
  "Decodes String to Long."
  [s]
  (let [idxs (map #(.indexOf digits %) (seq s))]
    (->> (reverse idxs)
         (map-indexed (fn [power n] (* n (Math/pow radix power))))
         (reduce +)
         (.longValue))))

(encode 0)              ;=> "0"
(encode 65)             ;=> "~"
(encode 66)             ;=> "10"
Long/MAX_VALUE          ;=> 9223372036854775807
(encode Long/MAX_VALUE) ;=> "5w9csrDC~I7"

(decode "0")            ;=> 0
(decode "~")            ;=> 65
(decode "10")           ;=> 66
(decode "5w9csrDC~I7")  ;=> 9223372036854775807