cif_validator.ex

defmodule CifValidator do
  @test_key "753217532"

  def valid_cif?(cif) do
    well_formed?(cif) && control_sum_matches?(cif)
  end

  defp control_sum_matches?(cif) do
    test_key = @test_key
      |> to_integer_list
      |> Enum.reverse

    [control | rest] = cif
      |> to_integer_list
      |> Enum.reverse

    rest
      |> Enum.zip(test_key)
      |> Enum.map(fn {a, b} -> a * b end)
      |> Enum.sum
      |> Kernel.*(10)
      |> rem(11)
      |> rem(10)
      |> Kernel.===(control)
  end

  defp well_formed?(cif) do
    Regex.match?(~r/^\d{2,10}$/, cif)
  end

  defp to_integer_list(string) do
    string
      |> String.to_char_list
      |> Enum.map(fn(x) -> String.to_integer << x >> end)
  end
end

cif_validator_test.exs

defmodule OpenapiRest.CifValidatorTest do
  use ExUnit.Case

  @valid_cifs [
    "20959993",
    "18834610",
    "14752305",
    "21237710",
    "17732264"
  ]

  @invalid_cifs [
    "z",
    "12345678",
    "RO20959993",
    ""
  ]

  test "valid cifs" do
    assert Enum.all?(@valid_cifs, fn(x) -> OpenapiRest.CifValidator.valid_cif?(x); end)
  end

  test "invalid cifs" do
    assert Enum.all?(@invalid_cifs, fn(x) -> !OpenapiRest.CifValidator.valid_cif?(x); end)
  end
end