defmodule IsbnVerifier do
  @doc """
    Checks if a string is a valid ISBN-10 identifier

    ## Examples

      iex> IsbnVerifier.isbn?("3-598-21507-X")
      true

      iex> IsbnVerifier.isbn?("3-598-2K507-0")
      false

  """

  @regex ~r/^(?<digits>\d{9})(?<check_digit>[0-9X])$/

  @spec isbn?(String.t()) :: boolean
  def isbn?(isbn) do
    with isbn_without_dashes <- String.replace(isbn, "-", ""),
         %{"digits" => digits, "check_digit" => check_digit} <-
           Regex.named_captures(@regex, isbn_without_dashes) do
      rem(check_sum(digits) + check_digit(check_digit), 11) == 0
    else
      _ -> false
    end
  end

  defp check_sum(digits) do
    {1, check_sum} =
      digits
      |> String.graphemes()
      |> Enum.reduce({10, 0}, fn digit, {index, acc} ->
        {index - 1, acc + String.to_integer(digit) * index}
      end)

    check_sum
  end

  defp check_digit("X"), do: 10
  defp check_digit(string), do: String.to_integer(string)
end