;;;;;
;; Calulating ISBN-13 check digit
;;;;;

(defn calc-isbn-check-digit
  ""
  [#^String isbn-code]
  (let [split-code (map #(Character/digit % 10) isbn-code)
        partitioned-code (partition 2 split-code)
        first-sum (reduce + (map #(first %) partitioned-code))
        second-sum (reduce + (map #(* 3 (second %)) partitioned-code))
        final-sum (+ first-sum second-sum)
        check-digit (- 10 (mod final-sum 10))]
    check-digit))

;;; Cleaner/Elegant Solution

(defn calc-isbn-check-digit-redux
  ""
  [#^String isbn-code]
  (let [split-code (map #(Character/digit % 10) isbn-code)
        final-sum (reduce + (map * split-code (cycle [1 3])))]
    (- 10 (mod final-sum 10))))