require 'active_model'

class IbanValidator < ActiveModel::EachValidator

  def validate_each(record, attribute, value)
    record_error(record, attribute, value) unless value =~ IbanValidator.regexp && valid_iban?(value)
  end
  

  private
  
  def record_error(record, attribute, value)
    record.errors.add(attribute, (options[:message] || :invalid_iban))
  end

  def self.regexp
    /[A-Z]{2}[a-zA-Z0-9]{14,}/
  end

  def valid_iban?(iban)
    # Move first four characters to end of string
    dummy_iban = iban[4..-1] + iban[0..3]

    # Substitute all letters with integers
    checksum = dummy_iban.chars.map { |char| ((char =~ /[a-zA-Z]/).present?) ? (char.downcase.ord - 87).to_s : char }.join

    # Check if division by 97 yields a remainder of 1, in which case it could be a valid IBAN
    (checksum.to_i % 97) == 1
  end
end