karpekar.clj

(def ^:const KARPEKAR 6174)

(defn ->number [ds]
  (reduce (fn [n d] (+ d (* n 10))) ds))

(defn ->digits [n]
  (loop [N n
         ds []]
    (if (= (count ds) 4)
      ds
      (recur (quot N 10) (cons (rem N 10) ds)))))

(defn abs [n] (if (< n 0) (- n) n))

(defn kdiff [n]
  (let [ds (->digits n)
        a (->number (sort ds))
        b (->number (reverse (sort ds)))]
    (abs (- a b))))

(def nums (for [a (range 10), b (range 10), c (range 10) d (range 10)]
            [a b c d]))

(defn karpekar-seq [n]
  (loop [n n
         nums []]
    (if (or (= n 0) (= n KARPEKAR))
      (conj nums n)
      (recur (kdiff n) (conj nums n)))))

(doseq [n nums]
  (println (-> n ->number karpekar-seq)))