p92.clj

; Project Euler, problem 92
(use 'clojure.contrib.math)

(defn digits-of [n]
  (map read-string (map str (str n))))

(defn chain [n]
  (let [sum (apply + (map #(expt % 2) (digits-of n)))]
    (lazy-seq
      (cons n (chain sum)))))

(defn has-89? [chain]
   (let [f (first chain)]
     (if (= 89 f)
       true
       (if (= 1 f)
         false
         (recur (rest chain))))))

; took 18 minutes, answer: 8581146
(println (count (filter #(has-89? (chain %)) (range 1 10000000))))