clojure happy number http://www.4clojure.com/problem/86

happy-numbers.clj

(defn happy-numbers
  [n]
  (letfn [(next-happy [n]
            (->> n
                 str
                 (map (comp int
                            #(Math/pow % 2)
                            read-string
                            str))
                 (reduce +)))]
    (loop [n n
           prev #{}]
      (cond
        (= 1 n) true
        (prev n) false
        :else (recur (next-happy n)
                     (conj prev n))))))