defmodule Timber.Plug do
  @moduledoc """
  Documentation for Timber.Plug.
  """

  @doc false
  # Fetches the request ID from the connection using the given header name
  #
  # The request ID may be added to the connection in a number of ways which
  # complicates how we retrieve it. It is usually set by calling the
  # Plug.RequestId module on the connection which sets a request ID only if one
  # hasn't already been set. If the request ID is set by a service prior to
  # Plug, it will be present as a request header. If Plug.RequestId generates a
  # request ID, that request ID is only present in the response headers. The
  # request headers should always take precedent in this function, though.
  #
  # This function will return either a single element list containing a
  # two-element tuple of the form:
  #
  #   {"x-request-id", "myrequestid91391"}
  #
  # or an empty list. This normalizes the expectation of the header name for
  # future processing.
  #
  # Note: Plug.RequestId will change an existing request ID if it doesn't think
  # the request ID is valid. See
  # [request_id.ex](https://github.com/elixir-lang/plug/blob/v1.2.2/lib/plug/request_id.ex#L62).
  @spec get_request_id(Plug.Conn.t(), String.t()) :: [{String.t(), String.t()}] | []
  def get_request_id(conn, header_name) do
    headers =
      case Plug.Conn.get_req_header(conn, header_name) do
        [] -> Plug.Conn.get_resp_header(conn, header_name)
        values -> values
      end

    handle_request_id(headers)
  end

  # Helper function to take the result of the header retrieval function and
  # change it to the desired response format for get_request_id/2
  @spec handle_request_id([] | [String.t()]) :: [{String.t(), String.t()}] | []
  defp handle_request_id([]) do
    []
  end

  defp handle_request_id([request_id | _]) do
    [{"x-request-id", request_id}]
  end

  @doc false
  @spec get_client_ip(Plug.Conn.t()) :: String.t() | nil
  def get_client_ip(%{remote_ip: nil}) do
    nil
  end

  def get_client_ip(%{remote_ip: remote_ip}) do
    remote_ip
    |> :inet.ntoa()
    |> List.to_string()
  end
end