Class: Pubid::Isbn::CheckDigit

Inherits:
Object
  • Object
show all
Defined in:
lib/pubid/isbn/check_digit.rb

Overview

Pure-functional check digit math for ISBN-10 and ISBN-13 per ISO 2108.

ISBN-10: weighted sum with weights 10..1, mod 11. Check digit 10 is rendered as "X". ISBN-13: weighted sum with alternating weights 1/3, mod 10. Check digit is 0-9.

Class Method Summary collapse

Class Method Details

.compute_isbn10(payload) ⇒ String

Compute the correct ISBN-10 check digit for a 9-digit payload.

Returns:

  • (String) —

    "0".."9" or "X"

Raises:

  • (ArgumentError) 


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# File 'lib/pubid/isbn/check_digit.rb', line 14

def self.compute_isbn10(payload)
  raise ArgumentError, "ISBN-10 payload must be 9 digits" unless payload.to_s.match?(/\A\d{9}\z/)

  sum = payload.to_s.each_char.with_index.sum { |c, i| c.to_i * (10 - i) }
  digit = (11 - sum % 11) % 11
  digit == 10 ? "X" : digit.to_s
end

.compute_isbn13(payload) ⇒ String

Compute the correct ISBN-13 check digit for a 12-digit payload.

Returns:

  • (String) —

    "0".."9"

Raises:

  • (ArgumentError) 


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# File 'lib/pubid/isbn/check_digit.rb', line 24

def self.compute_isbn13(payload)
  raise ArgumentError, "ISBN-13 payload must be 12 digits" unless payload.to_s.match?(/\A\d{12}\z/)

  sum = payload.to_s.each_char.with_index.sum do |c, i|
    c.to_i * (i.even? ? 1 : 3)
  end
  ((10 - sum % 10) % 10).to_s
end

.valid?(full) ⇒ Boolean

Verify the check digit on a full 10- or 13-digit ISBN string.

Parameters:

  • full (String) —

    10 or 13 chars; ISBN-10 may end with X.

Returns:

  • (Boolean) 


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# File 'lib/pubid/isbn/check_digit.rb', line 36

def self.valid?(full)
  digits = full.to_s.upcase.delete("^0-9X")
  case digits.length
  when 10
    payload = digits[0, 9]
    actual = digits[9]
    compute_isbn10(payload) == actual
  when 13
    payload = digits[0, 12]
    actual = digits[12]
    compute_isbn13(payload) == actual
  else
    false
  end
end