Caesar cipher's implementation using Clojure Transducer

caesar-cipher-transducer.clj

;; The Caesar Cipher implementation using Clojure Transducers
;; Author: Abhishek Gupta

;The Caesar Cipher
;Given a string, use transducers to:
;
;Filter out vowels and non-ASCII characters
;Filter out upper-case characters
;Rotate all remaining characters via a Caesar cipher,
;And reduce the rotated characters into a map counting the number of occurrences of each character.
;Example:
;
;(defn caesar-count
;  [string cipher]
;  ???)
;
;(caesar-count "abc" 0)
;; ⇒ {\c 1, \b 1}
;
;(caesar-count "abc" 1)
;; ⇒ {\d 1, \c 1}
;
;(caesar-count "hello world" 0)
;; ⇒ {\d 1, \r 1, \w 1, \l 3, \h 1}
;
;(caesar-count "hello world" 13)
;; ⇒ {\q 1, \e 1, \j 1, \y 3, \u 1}

(defn rotate [cipher]
  (fn [c]
    (let [base-c (- (int c) 97)
          rotated-c (mod (+ base-c cipher) 26)]
      (char (+ rotated-c 97)))))

(defn caesar-xform
  [cipher]
  (comp
    (map int)
    ; only small aplphabets
    (filter #(and (<= % 122) (>= % 97)))
    ; not vowels
    (filter #(not (or (= % 97) (= % 101) (= % 105) (= % 111) (= % 117))))
    ; rotate characters
    (map (rotate cipher))
    ))

(defn caesar-reducing
  ([]
   [])
  ([result]
   result)
  ([result input] (update-in result (str input) (fnil inc 0)))
  ; (fnil + 0) replaces use of #(+ (or % 0) 1), for when the key does not exist
  ; yet (and thus value is nil)
  )

(defn caesar-count
  [string cipher]
  ;(transduce (caesar-xform cipher) caesar-reducing {} string)
  (transduce (caesar-xform cipher) caesar-reducing {} string)
  )

(comment 
(caesar-count "hello world" 13)
;=> {\u 1, \y 3, \j 1, \e 1, \q 1}
(caesar-count "abc" 1)
;=> {\c 1, \d 1})
)