Spanish Tax ID Validator

spanish_tax_id_validator.rb

class SpanishTaxIdValidator < ActiveModel::EachValidator
  def validate_each(object, attribute, value)  
    valid_tax_id = case 
    when value.match(/^[0-9]{8}[a-z]/i)
      validate_nif(value.dup)
    when value.match(/[a-w][0-9]{7}[0-9a-z]/i)
      validate_cif(value.dup)
    when value.match(/[xyz][0-9]{7,8}[a-z]/i)
      validate_nie(value.dup)
    else
      false
    end

    object.errors[attribute] << (options[:message] || "is not valid") if not valid_tax_id
  end

  private
  def 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

  def 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)
    calculated_number = 0 if calculated_number==10
    calculated_letter = (64 + calculated_number).chr

    if letter.match(/[KNPQS]/)
      return check.to_s == calculated_letter.to_s
    else
      return check.to_i == calculated_number.to_i
    end
  end

  def validate_nie(value)
    if value[0].chr.upcase == 'X'
      value[0] = '0'
    else
      value[0] = '1'
    end
    value.slice(0) if value.size > 9
    validate_nif(value)
  end
end

Yay Ruby!!