ISBN 10 and 13 verifier

isbn-verifier.clj

(ns isbn-verifier)

(defn- digits
  "Splits ISBN 10 string into collection of digits"
  [col]
  (keep #(cond
          (Character/isDigit %) (Character/getNumericValue %)
          (= \X %) 10)
        col))

(defn- isbn10-checksum
  "Calculates ISBN 10 checksum"
  [digits]
  (->> digits
       (map * (range 10 1 -1))
       (apply +)
       (#(- 11 (mod % 11)))))

(defn- isbn13-checksum
  "Calculates ISBN 13 checksum"
  [digits]
  (->> digits
       (map * (flatten (repeat 6 '(1 3))))
       (apply +)
       (#(- 10 (mod % 10)))))

(defn- isbn10?
  "Validates ISBN 10 number"
  [string]
  (let [checksum? #(= (isbn10-checksum %) (last %))]
    (and (not= nil (re-matches #"\d[-]?\d{3}[-]?\d{5}[-]?[\d|X]" string))
         (checksum? (digits string)))))

(defn- isbn13?
  "Validates ISBN 13 number"
  [string]
  (let [checksum? #(= (isbn13-checksum %) (last %))]
    (and (not= nil (re-matches #"978[-]?\d[-]?\d{3}[-]?\d{5}[-]?[\d|X]" string))
         (checksum? (digits string)))))

(defn- isbn10->13
  "Converts ISBN 10 string to ISBN 13 one"
  [isbn10]
  (str "978-" isbn10))

(defn isbn?
  "Validates ISBN number"
  [isbn]
  (and (isbn10? isbn)
       (isbn13? (isbn10->13 isbn))))