password.ex

defmodule Password do

  @regex_for_mac ~r/^([0-9A-Fa-f]{2}[:-]){5}([0-9A-Fa-f]{2})$/

  def generate(mac_address) do
    case check_mac(mac_address) do
      {:invalid, message} ->
        message
      {:ok} ->
        password =
          Base.encode16(:crypto.hash(:sha256, mac_address))
            |> String.slice(0..7)
    end
  end

  defp check_mac(mac) do
    cond do
      !Regex.match?(@regex_for_mac, mac) ->
        {:invalid, "Mac Address is Invalid."}
      true ->
        {:ok}
    end
  end
end