exercism.elixir.bob.exs

defmodule Teenager do

  def hey(msg) do
    msg |> interpret |> respond
  end


  defp interpret(""), do: :silence

  defp interpret(msg) do
    reversed = MyString.reverse msg
    upcased  = String.upcase reversed

    do_interpret reversed, upcased
  end

  defp do_interpret(upcased, upcased), do: :shout

  defp do_interpret("?" <> _, _), do: :question

  defp do_interpret(_, _), do: :anything


  defp respond(:anything), do: "Whatever."

  defp respond(:shout),    do: "Woah, chill out!"

  defp respond(:question), do: "Sure."

  defp respond(:silence),  do: "Fine. Be that way."

end

# Because there is no String.reverse/1 yet.
defmodule MyString do

  def reverse(string) when is_binary(string), do: do_reverse(String.next_grapheme(string), "")

  defp do_reverse({char, rest}, reversed) do
    do_reverse(String.next_grapheme(rest), char <> reversed)
  end

  defp do_reverse(:no_grapheme, reversed), do: reversed

end