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to ISBN Verifier in the Crystal Track

Published at Feb 24 2020 · 0 comments
Test suite

The ISBN-10 verification process is used to validate book identification numbers. These normally contain dashes and look like: 3-598-21508-8


The ISBN-10 format is 9 digits (0 to 9) plus one check character (either a digit or an X only). In the case the check character is an X, this represents the value '10'. These may be communicated with or without hyphens, and can be checked for their validity by the following formula:

(x1 * 10 + x2 * 9 + x3 * 8 + x4 * 7 + x5 * 6 + x6 * 5 + x7 * 4 + x8 * 3 + x9 * 2 + x10 * 1) mod 11 == 0

If the result is 0, then it is a valid ISBN-10, otherwise it is invalid.


Let's take the ISBN-10 3-598-21508-8. We plug it in to the formula, and get:

(3 * 10 + 5 * 9 + 9 * 8 + 8 * 7 + 2 * 6 + 1 * 5 + 5 * 4 + 0 * 3 + 8 * 2 + 8 * 1) mod 11 == 0

Since the result is 0, this proves that our ISBN is valid.


Given a string the program should check if the provided string is a valid ISBN-10. Putting this into place requires some thinking about preprocessing/parsing of the string prior to calculating the check digit for the ISBN.

The program should be able to verify ISBN-10 both with and without separating dashes.


Converting from strings to numbers can be tricky in certain languages. Now, it's even trickier since the check digit of an ISBN-10 may be 'X' (representing '10'). For instance 3-598-21507-X is a valid ISBN-10.

Bonus tasks

  • Generate a valid ISBN-13 from the input ISBN-10 (and maybe verify it again with a derived verifier).

  • Generate valid ISBN, maybe even from a given starting ISBN.


Follow the setup instructions for Crystal here:


More help installing can be found here:


Making the Test Suite Pass

Execute the tests with:

$ crystal spec

In each test suite all but the first test have been skipped.

Once you get a test passing, you can unskip the next one by changing pending to it.


Converting a string into a number and some basic processing utilizing a relatable real world example. https://en.wikipedia.org/wiki/International_Standard_Book_Number#ISBN-10_check_digit_calculation

Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.


require "spec"
require "../src/*"

describe "IsbnVerifier" do
  it "valid isbn number" do
    IsbnVerifier.valid?("3-598-21508-8").should eq(true)

  pending "invalid isbn check digit" do
    IsbnVerifier.valid?("3-598-21508-9").should eq(false)

  pending "valid isbn number with a check digit of 10" do
    IsbnVerifier.valid?("3-598-21507-X").should eq(true)

  pending "check digit is a character other than X" do
    IsbnVerifier.valid?("3-598-21507-A").should eq(false)

  pending "invalid character in isbn" do
    IsbnVerifier.valid?("3-598-P1581-X").should eq(false)

  pending "X is only valid as a check digit" do
    IsbnVerifier.valid?("3-598-2X507-9").should eq(false)

  pending "valid isbn without separating dashes" do
    IsbnVerifier.valid?("3598215088").should eq(true)

  pending "isbn without separating dashes and X as check digit" do
    IsbnVerifier.valid?("359821507X").should eq(true)

  pending "isbn without check digit and dashes" do
    IsbnVerifier.valid?("359821507").should eq(false)

  pending "too long isbn and no dashes" do
    IsbnVerifier.valid?("3598215078X").should eq(false)

  pending "too short isbn" do
    IsbnVerifier.valid?("00").should eq(false)

  pending "isbn without check digit" do
    IsbnVerifier.valid?("3-598-21507").should eq(false)

  pending "check digit of X should not be used for 0" do
    IsbnVerifier.valid?("3-598-21515-X").should eq(false)

  pending "empty isbn" do
    IsbnVerifier.valid?("").should eq(false)

  pending "input is 9 characters" do
    IsbnVerifier.valid?("134456729").should eq(false)

  pending "invalid characters are not ignored" do
    IsbnVerifier.valid?("3132P34035").should eq(false)

  pending "input is too long but contains a valid isbn" do
    IsbnVerifier.valid?("98245726788").should eq(false)
module IsbnVerifier
  def self.valid?(isbn_number : String)
    nth_digit = 0
    isbn_number.each_char.map_with_index { |digit, i|
      if digit.whitespace? || digit == '-'
      elsif ((digit == 'X') && (nth_digit != 9)) || (digit != 'X' && !digit.to_i?)
        return false
        (digit == 'X' ? 10 : digit.to_i) * (11 - (nth_digit += 1))
    }.sum.to_i % 11 == 0 && nth_digit == 10

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