rebase

gistfile1.txt

(ns qrfd.hash)

(def charset "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ")

(def num->char
  (into {} (map-indexed vector charset)))

(def char->num
  (into {} (map-indexed (comp vec reverse vector) charset)))

(def base (count charset))

(def divmod (juxt quot rem))

(defn exp [x n]
  (loop [acc 1 n n]
    (if (zero? n) acc
        (recur (* x acc) (dec n)))))

(defn encode [n]
  (loop [n n a ""]
    (let [[div mod] (divmod n base)]
      (if (zero? div)
        (str (get num->char mod) a)
        (recur div (str (get num->char mod) a))))))

(defn decode-pair [idx chr]
  (* (get char->num chr)
     (exp base idx)))

(def sum (partial reduce +))

(defn decode [s]
  (sum (map-indexed decode-pair (reverse s))))