rle.clj

(ns lab.rle)

(->> (partition-by identity "abbasdfrressaaaa")
     (mapcat (fn [r]
              [(first r) (count r)]))
     (apply str))

(->>
  (concat "abbasdfrressaaaa" [nil]) ;; XXX To help writing last chunk
  (reduce (fn [[result last-char run-count] ch]
                (if (or (= last-char ch)
                        (nil? last-char))
                  [result ch (inc run-count)]
                  [(conj result last-char run-count) ch 1]))
                [[] nil 0])
  (first)
  (apply str))

(loop [src "abbasdfrressaaaa"
       result []
       run-count 1]
  (if (empty? src)
    (apply str result)
    (let [[ch next-ch] src
          tail (rest src)]
    (if (= ch next-ch)
      (recur tail result (inc run-count))
      (recur tail (conj result ch run-count) 1)))))