(def-module luhn)

(use (Strings [reverse split parse])
     (Funcs [map reduce_indexed])
     (Funcs/Results [expect]))

(pub def-fn luhn (:: string bool) [bin]
  (-<>> bin
    (|> reverse)
    (|> split <> "")
    (|> map <> parse<uint32>)
    (|> map <> expect "luhn: BIN (Bank Identification Number) expected")
    (|> reduce_indexed <> luhn-elt 0)
    (|> % <> 10)
    (|> = 0)))

(def-fn luhn-elt (:: int int int int) [acu elt idx]
  (if (= 0 (% idx 2))
    (+ acu elt)
    (+ acu (let [n (* 2 elt)]
             (if (> n 9) (- n 9) n)))))

; vim: ft=lisp