Alphabet Cipher solution

coder.clj

;; Solution to https://github.com/gigasquid/wonderland-clojure-katas/tree/master/alphabet-cipher
(ns alphabet-cipher.coder)

(defn crypt [op keyword message]
  (letfn [
      (tochar [idx] (char (+ idx (int \a))))
      (toidx [ch] (- (int ch) (int \a)))
      (encode [keychar msgchar] (tochar (mod (op (toidx msgchar) (toidx keychar)) 26)))]
    (->> message
         (map encode (cycle keyword))
         (apply str))))

(defn encode [keyword message] (crypt + keyword message))

(defn decode [keyword message] (crypt - keyword message))