Check credit card number.

credit-check-sum.clj

(defn- charlist-to-digits [lst]
  (map #(Integer/parseInt %)
    (map #(str (first %))
      (partition 1 2 lst))))

(defn- odd-digits [card-num]
  (charlist-to-digits (reverse card-num)))

(defn- even-digits [card-num]
  (charlist-to-digits (rest (reverse card-num))))

(defn- sum-of-odd-digits [card-num]
  (reduce + (odd-digits card-num)))

(defn- list-of-even-digits [card-num]
  (map #(let [v (* % 2)] (if (>= v 10) (- v 9) v))
    (even-digits card-num)))

(defn credit-check
"if card-num is a valid number, return true. otherwise false."
 [card-num]
  (let [check-sum (mod (reduce + (cons (sum-of-odd-digits card-num) (list-of-even-digits card-num))) 10)]
    (= check-sum 0)))