Top Secret - Elixir - Exercism

top_secret.ex

defmodule TopSecret do
  @spec to_ast(String.t()) :: tuple()
  def to_ast(string), do: Code.string_to_quoted!(string)

  @spec decode_secret_message_part(tuple(), list()) :: tuple()
  def decode_secret_message_part(ast, acc) when elem(ast, 0) in [:def, :defp] do
    ast
    |> elem(2)
    |> Enum.at(0)
    |> get_function_ast()
    |> get_function_name_and_arity()
    |> get_sliced_name()
    |> get_secret_message_part(ast, acc)
  end

  def decode_secret_message_part(ast, acc), do: {ast, acc}

  defp get_secret_message_part(name, ast, acc), do: {ast, [name | acc]}

  defp get_function_ast(ast) when elem(ast, 0) == :when do
    ast
    |> elem(2)
    |> Enum.at(0)
  end

  defp get_function_ast(ast), do: ast

  defp get_function_name_and_arity(function_ast) do
    fname =
      function_ast
      |> elem(0)
      |> Atom.to_string()

    {fname, get_arity(function_ast)}
  end

  defp get_arity(function_ast) when elem(function_ast, 2) == nil, do: -1

  defp get_arity(function_ast) do
    function_ast
    |> elem(2)
    |> Enum.count()
    |> Kernel.-(1)
  end

  defp get_sliced_name({_fname, arity}) when arity == -1, do: ""
  defp get_sliced_name({fname, arity}) when arity > -1, do: fname |> String.slice(0..arity)

  @spec decode_secret_message(String.t()) :: String.t()
  def decode_secret_message(string) do
    string
    |> to_ast()
    |> Macro.postwalk([], &decode_secret_message_part/2)
    |> Tuple.to_list()
    |> Enum.at(1)
    |> Enum.reverse()
    |> Enum.join()
  end
end