base-62.clj

(def ^{:const true :private true}
  base-62-alphabet
  "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz")

(defn encode
  "Encodes v in a new base of ks."
  [ks v]
  (let [base (count ks)]
    (apply str
           (reduce
             (fn [acc v]
               (conj acc (nth ks (mod v base))))
             nil
             (take-while pos? (iterate #(quot % base) v))))))

(def ^{:doc "Encodes a given value into a base62 representation."}
  encode-base-62
  (partial encode base-62-alphabet))