Ruby class for generating valid fake IBAN numbers for testing

iban_generator.rb

# Module for generating random Iban account numbers
module IBAN
  # Calculates the mandatory checksum of 3..4 characters in iban
  def iban_checksum(account, cc)
    # Converts letters to numbers according the iban rules, A=10..Z=35
    iban_to_num = "#{account}#{cc}00".upcase.chars.map { |d|
       d.match(/[A-Z]/) ? (d.ord - 55).to_s : d
    }.join.to_i

    # Calculate checksum with reverse modulo
    # This is answer to (iban_to_num + checksum) % 97 == 1
    checksum = ( 1 - iban_to_num ) % 97

    # Use leftpad to make it's size always 2
    checksum.to_s.rjust(2, '0')
  end

  def generate_iban(size: 20, cc: 'GB', account: '')
    # Generate random account of size if it's not given
    account = (size - 4).times.map { rand(0..9) }.join if account.empty?

    # Return in iban format
    cc + iban_checksum(account,cc) + account
  end
end

20.times { puts IBAN::generate(size: 18, cc: 'FI') }