Validate Spanish NIFs ("CIFs") in Ruby.

spanish_organization_number.rb

# By "organization number" we mean an organization NIF (Spanish tax identification number). We don't mean personal NIFs such as DNI/NIE (see `SpanishIdentityNumber` for that).
#
# This class also supports personal NIFs starting with the letters K, L or M, though it's inaccurate to call these "organization numbers". More about these: https://translate.google.co.uk/translate?sl=auto&tl=en&u=https%3A%2F%2Fes.wikipedia.org%2Fwiki%2FN%25C3%25BAmero_de_identificaci%25C3%25B3n_fiscal
#
# It's easier to support them than not to, and means that any such users can provide this number, even though it's represented a bit inaccurately in our system. They're likely to be very rare. Feel free to think up a way to represent it more accurately. (Renaming `spanish_organization_number` to something like `non_dni_or_nie_spanish_tax_identification_number`?)

class SpanishOrganizationNumber
  # NIFs do not have the letter prefixes "I" and "O" since those can be confused for the digits "1" and "0".
  # Don't know why "T" is not a prefix.
  # "X", "Y" and "Z" are NIE prefixes, so they're not used for NIFs that are not NIEs.
  RE = /\A[ABCDEFGHIJKLMNOPQRSUVW]\d{7}[\dA-Z]\z/

  PREFIXES_THAT_HAVE_A_LETTER_AS_CHECK_SUM = %w[ N P Q R S W ]

  def self.valid?(number)
    number = number.to_s.upcase.strip

    return false unless number.match?(RE)

    central_digits = number[1, 7]

    sum = central_digits.each_char.with_index.sum { |digit_string, zero_based_index|
      digit = digit_string.to_i
      index = zero_based_index + 1

      if index.odd?
        double = digit * 2
        (double / 10) + (double % 10)
      else
        digit
      end
    }

    digit = (10 - sum.to_s.last.to_i)
    digit = 0 if digit == 10

    check_sum =
      if PREFIXES_THAT_HAVE_A_LETTER_AS_CHECK_SUM.include?(number[0])
        "JABCDEFGHI"[digit]
      else
        digit.to_s
      end

    number[-1] == check_sum
  end
end

spanish_organization_number_spec.rb

require "spec_helper"
require "spanish_organization_number"

describe SpanishOrganizationNumber, ".valid?" do
  it "is true for a valid organization NIF" do
    expect(SpanishOrganizationNumber.valid?("B08670408")).to be true
    expect(SpanishOrganizationNumber.valid?("Q5355054G")).to be true
  end

  it "is false for a NIF that doesn't match the checksum" do
    expect(SpanishOrganizationNumber.valid?("B08670409")).to be false
  end

  it "is false for a NIF with an invalid prefix letter" do
    expect(SpanishOrganizationNumber.valid?("T08670408")).to be false
  end

  it "is false for a number that doesn't match the format at all" do
    expect(SpanishOrganizationNumber.valid?("ABC123")).to be false
  end

  it "is false for nil" do
    expect(SpanishOrganizationNumber.valid?(nil)).to be false
  end

  it "is false for the empty string" do
    expect(SpanishOrganizationNumber.valid?("")).to be false
  end
end