Finnish bank reference calculator in ruby

finnish_bank_reference.rb

# Array#sum is needed for nice code outside rails projects
require 'active_support'
require 'active_support/core_ext'

# Module for creating valid Finnish bank references
module FinnishBankRefence

  # Generates reference from a input or a random number
  def self.generate( base = rand(10**3..(10**19-1)) )
    if base < 10**3 or base >= 10**19
      raise ArgumentError, "Valid reference has 3..19 digits, #{base} has #{base.to_s.length}"
    end
    "#{base}#{checksum(base)}"
  end

  # Calculates Finnish bank reference checksum.
  # Checksum is calculated from a sum where all digits are weighted with coefficient
  # The coefficient is from 7,3,1 sequence starting from the smallest digit
  # Checksum is the reverse of modulo10 which can be done with x = -y % 10
  # For example: "617435 4"
  # 6  1   7  4  3  5  4
  # 1  3   7  1  3  7
  # 6  3  49  4  9 35  = 106 → 4
  #
  # Source http://tarkistusmerkit.teppovuori.fi/tarkmerk.htm#viitenumero
  def self.checksum(base)
    weighted_sum = base.to_s.reverse.each_char.with_index.map { |digit,index| digit.to_i * [7,3,1].at(index%3) }.sum
    -weighted_sum % 10
  end
end

20.times { puts FinnishBankRefence::generate }