challenge.clj

(ns test.encoder
    (:require
      [clojure.string :as string]))

(def alphabet "ABCDEFGHIJKLMNOPQRSTUVWXYZ")
(def alphabet? (set alphabet))

(def a->i (zipmap alphabet (range 1 27)))
(def i->a (zipmap (range 1 27) alphabet))

(defn- c>> [previous curr]
  (let [shift (if previous (a->i previous) 0)]
    (-> curr a->i (+ shift) (mod 26) i->a)))
(defn- c<< [previous curr]
  (let [shift (if previous (a->i previous) 0)]
    (-> curr a->i (- shift) (mod 26) i->a)))

(defn- encode* [previous [char & rest]]
  (when char
        (lazy-seq
         (if (alphabet? char)
           (cons (c>> previous char) (encode* char rest))
           (cons char (encode* previous rest))))))
(defn encode [s]
  (->> s .toUpperCase (encode* nil) string/join))

(defn- decode* [previous [char & rest]]
  (when char
        (lazy-seq
         (if (alphabet? char)
           (cons (c<< previous char) (decode* (c<< previous char) rest))
           (cons char (decode* previous rest))))))
(defn decode [s]
  (->> s .toUpperCase (decode* nil) string/join))