sequence.clj

(defn happy-next
  [n]
  (let [digits (->> n str seq (map #(Character/digit % 10)))]
    (->> (map * digits digits)
         (apply +))))

(defn happy'
  [n seen]
  (loop [n n seen seen]
    (cond 
      (= 1 n)            true
      (contains? seen n) false
      :else              (recur (happy-next n) (conj seen n)))))

(defn happy?
  [n]
  (happy' n #{}))

(defn happy*
  [n]
  (cons n (lazy-seq (happy* (happy-next n)))))

(defn happy?
  [n]
  (->> (happy* n)
       (reductions conj [])
       (take-while #(= (distinct %) %))
       (last)
       (last)
       (= 1)))

(def happy-seq 
  (->> (range)
       (map inc)
       (filter happy?)))