checksum.clj

(def check-sum
     (fn [sequence]
       (apply + (map (fn [position digit]
                       (* digit (if (odd? position) 1 3)))
                     (range 1 (inc (count sequence)))
                     sequence))))

(defn check-sum-upc [coll]
 (apply + (map *
               (take (count coll) (cycle '(1 3)))
               coll)))

(defn check-sum-upc [coll]
 (apply + (map * (cycle '(1 3)) coll)))