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to ISBN Verifier in the Ruby Track

Published at May 12 2019 · 0 comments
Instructions
Test suite
Solution

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

ISBN

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.

Example

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.

Task

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.

Caveats

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.


For installation and learning resources, refer to the Ruby resources page.

For running the tests provided, you will need the Minitest gem. Open a terminal window and run the following command to install minitest:

gem install minitest

If you would like color output, you can require 'minitest/pride' in the test file, or note the alternative instruction, below, for running the test file.

Run the tests from the exercise directory using the following command:

ruby isbn_verifier_test.rb

To include color from the command line:

ruby -r minitest/pride isbn_verifier_test.rb

Source

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.

isbn_verifier_test.rb

require 'minitest/autorun'
require_relative 'isbn_verifier'

# Common test data version: 2.7.0 3134243
class IsbnVerifierTest < Minitest::Test
  def test_valid_isbn_number
    # skip
    string = "3-598-21508-8"
    assert IsbnVerifier.valid?(string), "Expected true, '#{string}' is a valid isbn"
  end

  def test_invalid_isbn_check_digit
    skip
    string = "3-598-21508-9"
    refute IsbnVerifier.valid?(string), "Expected false, '#{string}' is not a valid isbn"
  end

  def test_valid_isbn_number_with_a_check_digit_of_10
    skip
    string = "3-598-21507-X"
    assert IsbnVerifier.valid?(string), "Expected true, '#{string}' is a valid isbn"
  end

  def test_check_digit_is_a_character_other_than_x
    skip
    string = "3-598-21507-A"
    refute IsbnVerifier.valid?(string), "Expected false, '#{string}' is not a valid isbn"
  end

  def test_invalid_character_in_isbn
    skip
    string = "3-598-P1581-X"
    refute IsbnVerifier.valid?(string), "Expected false, '#{string}' is not a valid isbn"
  end

  def test_x_is_only_valid_as_a_check_digit
    skip
    string = "3-598-2X507-9"
    refute IsbnVerifier.valid?(string), "Expected false, '#{string}' is not a valid isbn"
  end

  def test_valid_isbn_without_separating_dashes
    skip
    string = "3598215088"
    assert IsbnVerifier.valid?(string), "Expected true, '#{string}' is a valid isbn"
  end

  def test_isbn_without_separating_dashes_and_x_as_check_digit
    skip
    string = "359821507X"
    assert IsbnVerifier.valid?(string), "Expected true, '#{string}' is a valid isbn"
  end

  def test_isbn_without_check_digit_and_dashes
    skip
    string = "359821507"
    refute IsbnVerifier.valid?(string), "Expected false, '#{string}' is not a valid isbn"
  end

  def test_too_long_isbn_and_no_dashes
    skip
    string = "3598215078X"
    refute IsbnVerifier.valid?(string), "Expected false, '#{string}' is not a valid isbn"
  end

  def test_too_short_isbn
    skip
    string = "00"
    refute IsbnVerifier.valid?(string), "Expected false, '#{string}' is not a valid isbn"
  end

  def test_isbn_without_check_digit
    skip
    string = "3-598-21507"
    refute IsbnVerifier.valid?(string), "Expected false, '#{string}' is not a valid isbn"
  end

  def test_check_digit_of_x_should_not_be_used_for_0
    skip
    string = "3-598-21515-X"
    refute IsbnVerifier.valid?(string), "Expected false, '#{string}' is not a valid isbn"
  end

  def test_empty_isbn
    skip
    string = ""
    refute IsbnVerifier.valid?(string), "Expected false, '#{string}' is not a valid isbn"
  end

  def test_input_is_9_characters
    skip
    string = "134456729"
    refute IsbnVerifier.valid?(string), "Expected false, '#{string}' is not a valid isbn"
  end

  def test_invalid_characters_are_not_ignored
    skip
    string = "3132P34035"
    refute IsbnVerifier.valid?(string), "Expected false, '#{string}' is not a valid isbn"
  end

  def test_input_is_too_long_but_contains_a_valid_isbn
    skip
    string = "98245726788"
    refute IsbnVerifier.valid?(string), "Expected false, '#{string}' is not a valid isbn"
  end
end
# frozen_string_literal: true

module IsbnVerifier
  CHECK = -1
  private_constant :CHECK
  CONVERT_CHECK = ->(check) { check == "X" ? 10 : check.to_i }
  private_constant :CONVERT_CHECK
  DASHES = "-"
  private_constant :DASHES
  ISBN_10 = /\A\d{9}(\d|X)\z/.freeze
  private_constant :ISBN_10
  MULTIPLE = 11
  private_constant :MULTIPLE
  WEIGHTS = (1..10).to_a.reverse.freeze
  private_constant :WEIGHTS
  WITHOUT_CHECK = (0..-2).freeze
  private_constant :WITHOUT_CHECK

  module_function

  def valid?(string)
    string = string.delete(DASHES)
    return false unless string.match?(ISBN_10)

    string
      .then(&method(:generate_isbn_integers))
      .zip(WEIGHTS)
      .sum(&method(:isbn_formula))
      .modulo(MULTIPLE)
      .zero?
  end

  def generate_isbn_integers(string)
    check = CONVERT_CHECK.call(string[CHECK])

    string
      .chars[WITHOUT_CHECK]
      .map(&:to_i)
      .append(check)
  end
  private_class_method :generate_isbn_integers

  def isbn_formula((integer, weight))
    integer * weight
  end
  private_class_method :isbn_formula
end

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