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))

(defnk shift-char-forward
  "Shifts the given Character down the alphabet by three.
  Has optional arg :d which takes :fw or :bw, so it can 
  be used for deciphering as well as ciphering. If it 
  isn't provided, will default to :fw for ciphering."
  [char :d :fw]
  (let [num (if (= d :fw) (+ char 3) (- char 3))
	pred (if (= d :fw) (> num 25) (< num 2))
	truec (if (= d :fw) (- num 26) (+ num 26))
	index 
	(if (and pred (not= num (if (= d :fw) 26 1))) truec 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 :d :to]
  (apply str 
	 (let [args
	       (if (= d :from) :bw :fw)]
	   (for [x (trans-string (.toLowerCase (apply str (filter is-letter? s))))] 
	     (shift-char-forward x :d args)))))

(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"
)