colors.ex

defmodule Colors do
  @colors ~w(R G B)

  def calculate(string)
      when byte_size(string) in [0, 1],
      do: string

  def calculate(string) do
    string
    |> String.graphemes()
    |> Enum.chunk_every(2, 1, :discard)
    |> Enum.map_join(&mix_colors/1)
    |> calculate()
  end

  defp mix_colors([c, c]), do: c

  defp mix_colors([c1, c2]) do
    @colors
    |> Kernel.--([c1, c2])
    |> List.last()
  end
end