defmodule Plug.AccessLog.DefaultFormatter.ResponseBytes do
  @moduledoc """
  Calculates response size in bytes.
  """

  import Plug.Conn

  @doc """
  Appends to log output.
  """
  @spec append(String.t, Plug.Conn.t, String.t) :: String.t
  def append(message, conn, fallback) do
    bytes = case get_resp_header(conn, "content-length") do
      [ length ] -> length
      _          -> conn.resp_body |> body_length() |> to_string()
    end

    case bytes do
      "0"   -> message <> fallback
      bytes -> message <> to_string(bytes)
    end
  end

  defp body_length(nil), do: 0
  defp body_length(body) when is_binary(body), do: body |> byte_size()
  defp body_length(body), do: body |> to_string() |> byte_size()
end