validate_bank_account.rb

class NorwegianBankAccount
  # Pass me any kind of string representing an account number
  def initialize(str)
    @numbers = str.gsub(/[^\d]/,'')
  end

  # Prettified output
  def to_s
    begin
      parts = @numbers.split(//)
      formatted = [parts[0,4].join(""), parts[4,2].join(""), parts[6,5].join("")].join(".")
      formatted
    rescue => e
      "##{self.class.name}<#{@numbers}> (#{e})"
    end
  end

  # Validation combo. Using a tuple as return value, in true Golang fashion
  def valid?
    if @numbers.length != 11
      return false, "Needs to be 11 digits"
    end
    if !mod_11_control(@numbers, [5,4,3,2,7,6,5,4,3,2])
      return false, "Invalid checksum"
    end
    return true, nil
  end


  # Validate an account number using mod 11 with multiplication factors.
  # Includes check digit for simplicity (ie 11 digits as input)
  def mod_11_control(account_number, factors)
    account_parts = account_number.split(//)
    account_digits = account_parts[0,10].map(&:to_i)
    sum = 0
    account_digits.each_with_index do |n, i|
      s = n * factors[i]
      sum += (n * factors[i])
    end
    check_digit = account_parts[10].to_i

    remainder = sum % 11
    expected_check_digit = case remainder
                           when 0 then remainder
                           when 1 then nil
                           else 11- remainder
                           end
    return check_digit == expected_check_digit
  end
end

# Unittest built in, skip for production
require 'minitest/autorun'
class NorwegianBankAccountTest < Minitest::Test
  def test_invalid_length
    @v = NorwegianBankAccount.new("1234.56.789")
    valid, message = @v.valid?
    refute valid, "#{@v} has an incorrect length"
  end

  def test_invalid_checksum
    @v = NorwegianBankAccount.new("1234.56.78901")
    valid, message = @v.valid?
    refute valid, "#{@v} has an incorrect checksum"
  end

  def test_valid_account_number
    @v = NorwegianBankAccount.new("1299.14.91285")
    valid, message = @v.valid?
    assert valid, "#{@v} should be a valid account number"
  end

  def test_crazy_input
    @v = NorwegianBankAccount.new("Morten Bankebiff")
    valid, message = @v.valid?
    refute valid, "#{@v} is wrong in so many ways"
  end
end