ISBN13 check digit

The task is to validate the check digit of an ISBN-13 code. Use some good and bad codes of your choosing for testing. See https://isbn-information.com/the-13-digit-isbn.html for details on the method of validation.

langur

<lang langur>val .isbn13checkdigit = f(.n) {

   val .s = replace(.n, "-", "")
   if len(.s) != 13 {
       return false
   }
   # multiply every other number by 3
   val .nums = map(
       f(.i, .d) if(.i rem 2 == 0: .d x 3; .d),
       series(13),
       map f .c-'0', s2cp .s,
   )
   fold(f .x + .y, .nums) rem 10 == 0

}

val .tests = h{

   "978-1734314502": true,
   "978-1734314509": false,
   "978-1788399081": true,
   "978-1788399083": false,

}

for .key in sort(keys .tests) {

   val .pass = .isbn13checkdigit(.key)
   write .key, ": ", .pass
   writeln if(.pass == .tests[.key]: ""; " (ISBN-13 CHECK DIGIT TEST FAILED)")

}</lang>

Output:

978-1734314502: true
978-1734314509: false
978-1788399081: true
978-1788399083: false