cipher.clj

(ns cipher
  (:use [clojure.contrib.def :only (defnk)]))

(def alphabet-nums (apply hash-map (interleave (range 1 27) "abcdefghijklmnopqrstuvwxyz")))
(def alphabet-chars (apply hash-map (interleave "abcdefghijklmnopqrstuvwxyz" (range 1 27))))

(defn is-letter? [s]
  (if (re-find #"(?i)[a-z]" (.toString s)) true false))

(defn shift-char-forward
  "Shifts the given Character down the alphabet by three."
  [char]
  (let [num (+ char 3)
	index (if (and (> num 25) (not= num 26)) (- num 26) num)] 
    (alphabet-nums index)))

(defn shift-char-backward
  "Same as shift-char-forward, only this is for decryption."
  [char]
  (let [num (- char 3)
	index (if (and (< num 2) (not= num 1)) (+ num 26) num)] 
    (alphabet-nums index)))

(defn trans-string
  "Transforms a string into a vector of numbers
  that correspond with their characters."
  [s]
  (for [x s] (alphabet-chars x)))

(defnk cipher
  "Encrypts a string with Caesar's cipher."
  [s :direction :to]
  (apply str 
	 (let [func 
	       (if (= direction :from) shift-char-backward shift-char-forward)]
	   (for [x (trans-string (.toLowerCase (apply str (filter is-letter? s))))] 
	     (func x)))))

(defn to-cleartext
  "Transforms ciphertext into cleartext."
  [s])

(comment
  cipher> (cipher "abd")
  "deg"
  cipher> (cipher "abd" :from)
; Evaluation aborted.
  cipher> (cipher "abd" :direction :from)
  "xya"
  cipher> (cipher "abd" :direction :to)
  "deg"
)