CIS194_card.clj

(defn digits [x]
  (loop [r x, digits []]
    (if (== 0 r)
      digits
      (recur (quot r 10) (conj digits (rem r 10))))))

(defn double-odd-digits [digits]
  (map-indexed (fn [i d] (if (odd? i) (* 2 d) d)) digits))

(defn valid? [x]
  (let [sum (->> (digits x)
                 (double-odd-digits)
                 (mapcat digits)
                 (reduce + 0))]
    (== 0 (rem sum 10))))

(use 'clojure.test)

(is (valid? 4012888888881881))
(is (not (valid? 4012888888881882)))