Spanish VAT validator for Ruby

validate_spanish_vat.rb

module ValidateSpanishVat
  module ClassMethods
    # Implements model level validator for Spanish's VAT number.
    # Usage:
    #
    # Inside a model with a vat field, simply put
    #
    #   validates_spanish_vat field_name
    #
    def validates_spanish_vat(*attr_names)
      configuration = { :message => :invalid, :on => :save }
      configuration.update(attr_names.pop) if attr_names.last.is_a?(Hash)

      validates_each(attr_names, configuration) do |record, attr_name, value|
        record.errors.add(attr_name, configuration[:message]) unless CustomValidator::SpanishVAT.validate(value)
      end
    end
  end
end

module CustomValidator
  class SpanishVAT
    # Main validate function, it runs several regexp's to determine if value is a nif, cif or nie.
    # If so, it runs specific test, else fails
    def self.validate(model_value)
      return false if model_value.nil?
      value = model_value.clone

      case
        when value.match(/[0-9]{8}[a-z]/i)
          return validate_nif(value)
        when value.match(/[a-wyz][0-9]{7}[0-9a-z]/i)
          return validate_cif(value)
        when value.match(/[x][0-9]{7,8}[a-z]/i)
          return validate_nie(value)
      end
      return false
    end

    # Validates NIF
    def self.validate_nif(value)
      letters = "TRWAGMYFPDXBNJZSQVHLCKE"
      check = value.slice!(value.length - 1..value.length - 1).upcase
      calculated_letter = letters[value.to_i % 23].chr
      return check === calculated_letter
    end

    # Validates CIF
    def self.validate_cif(value)
      letter = value.slice!(0).chr.upcase
      check = value.slice!(value.length - 1).chr.upcase

      n1 = n2 = 0
      for idx in 0..value.length - 1
        number = value.slice!(0).chr.to_i
        if (idx % 2) != 0
          n1 += number
        else
          n2 += ((2*number) % 10) + ((2 * number) / 10)
        end
      end
      calculated_number = (10 - ((n1 + n2) % 10)) % 10
      calculated_letter = (64 + calculated_number).chr

      if letter.match(/[QPNS]/)

        return check.to_s == calculated_letter.to_s
      else

        return check.to_i == calculated_number.to_i
      end
    end

    # Validates NIE, in fact is a fake, a NIE is really a NIF with first number changed to capital 'X' letter, so we change the first X to a 0 and then try to
    # pass the nif validator
    def self.validate_nie(value)
      value[0] = '0'
      value.slice(0) if value.size > 9
      validate_nif(value)
    end
  end
end

validate_spanish_vat_test.rb

require File.expand_path(File.join(File.dirname(__FILE__), '../lib/validate_spanish_vat.rb'))
require 'test/unit'

class ValidateSpanishVatTest < Test::Unit::TestCase
  def test_validate_nif
    assert CustomValidator::SpanishVAT.validate('00000000T') # Correct NIF
    assert !CustomValidator::SpanishVAT.validate('00000000L') # Incorrect NIF
  end
  
  def test_validate_nie
    assert CustomValidator::SpanishVAT.validate('X0000000T') # Correct NIE
    assert !CustomValidator::SpanishVAT.validate('X0000000L') # Incorrect NIE
    assert CustomValidator::SpanishVAT.validate('X00000000T') # Correct NIE
    assert !CustomValidator::SpanishVAT.validate('X00000000L') # Incorrect NIE
  end

  def test_validate_cif
    assert CustomValidator::SpanishVAT.validate('B25630633') # Correct CIF
    assert !CustomValidator::SpanishVAT.validate('B25630634') # Incorrect CIF
    assert CustomValidator::SpanishVAT.validate('Q5355054G') # Correct CIF
    assert !CustomValidator::SpanishVAT.validate('Q5355054R') # Incorrect CIF
  end
end

Thanks!