ISIN validation in Ruby

isin_validator.rb

class IsinValidator < ActiveModel::Validator
  CHARS = '0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ'

  def validate(record)
    value = record[:isin_cusip]
    return if value.blank?

    digits = value.upcase.chars.collect { |c| CHARS.index c }
    digits = digits.join.chars.collect { |c| c.to_i }
    actual_check_digit = digits.pop
    odd, even, _ = digits.inject([[], [], true]) do |(odd, even, isodd), digit|
      isodd ? [odd << digit, even, !isodd] : [odd, even << digit, !isodd]
    end
    big, small = (odd.size == even.size) ? [even, odd] : [odd, even]
    big.collect! { |d| d * 2}

    sum = (big + small).inject(0) do |sum, d|
      sum + ((d > 9) ? (d % 10 + d / 10) : d)
    end

    expected_check_digit = (10 - (sum % 10)) % 10

    if actual_check_digit != expected_check_digit
      record.errors.add(:isin_cusip, "The ISIN number seems invalid, check digit is #{actual_check_digit} but should be #{expected_check_digit}")
    end
  end
end