defimpl Poison.Encoder, for: Apostle.Mail do
  def encode %Apostle.Mail{}=mail, _opts do
    mail
      |> parse_mail
      |> Poison.encode!
  end

  defp parse_mail mail do
    mail
      |> take_attributes([:from, :name, :reply_to, :template_id, :layout_id, :headers])
      |> stringify_values
      |> merge_data(mail.data)
      |> encode_attachments(mail.attachments)
  end

  defp take_attributes map, attrs do
    map
      |> Map.take(attrs)
      |> Enum.reduce(%{}, fn
        ({_k, nil}, r) -> r
        ({_k, ""},  r) -> r
        ({k, v}, r) when is_number(v) -> Map.put r, k, to_string(v)
        ({_k, v}, r) when is_map(v) and map_size(v) == 0 -> r
        ({k, v}, r)   -> Map.put r, k, v
      end)
  end

  defp stringify_values map do
    map
      |> Enum.reduce %{}, fn
        ({_key, nil}, result)     -> result
        ({key, %{}=value}, result)-> Map.put result, key, stringify_values(value)
        ({key, value}, result)    -> Map.put result, key, to_string(value)
      end
  end

  defp merge_data(%{}=mail, %{}=d) when map_size(d) == 0 do
    mail
  end

  defp merge_data %{}=mail, %{}=data do
    Map.put mail, :data, data
  end

  defp encode_attachments(%{}=mail, %{}=a) when map_size(a) == 0 do
    mail
  end

  defp encode_attachments %{}=mail, %{}=attachments do
    attachments = Enum.reduce attachments, %{}, fn({name, content}, result)->
      Map.put result, to_string(name), Base.encode64(content)
    end
    Map.put mail, :attachments, attachments
  end
end