run-length-encoding.clj

;; Inspired by:
;; https://danielbmarkham.com/fun-with-an-interview-question/
;; https://github.com/smcl/LetterSequencer/blob/master/Interview.LetterSequencer.FSharp/Program.fs

(defn run-length-encode
  "Accepts a string (or any seq of characters).
  Returns its encoded representation as a seq of char-count tuples.
  Example: (aaabb ~> [[a 3][b 2]])"
  ([s]
   (run-length-encode s []))
  ([s encoded]
   (if (empty? s)
     encoded
     (let [[matched rest] (split-with #{(first s)} s)]
       (recur rest (conj encoded [(first s) (count matched)]))))))

(defn run-length-decode
  "Accepts a seq of char-count tuples, returns a string ([[\a 2] [\b 1]] ~> aab)"
  [codes]
  (reduce
   (fn [s [ch n]]
     (str s (str/join (repeat n ch))))
   ""
   codes))

(comment
  (run-length-encode "aaaabbbcca")
  (run-length-decode (run-length-encode "aaaabbbcca"))
  (run-length-decode []))