defmodule MvWeb.Plugs.SetLocale do
  @moduledoc """
  Plug to set the locale for the application.
  Defaults to German if no locale is specified.
  """
  import Plug.Conn

  @supported_locales ["de", "en"]
  @default_locale "de"

  def init(_), do: nil

  def call(conn, _) do
    case fetch_locale(conn) do
      nil ->
        # Set default locale if none found
        Gettext.put_locale(MvWeb.Gettext, @default_locale)
        persist(@default_locale, conn)

      locale ->
        # Set and persist locale
        IO.inspect(locale, label: "locale")

        Gettext.put_locale(MvWeb.Gettext, locale)
        |> persist(conn)
    end
  end

  defp fetch_locale(conn) do
    conn.params["locale"] ||
      get_session(conn, :locale) ||
      parse_accept_language(conn) ||
      @default_locale
  end

  defp parse_accept_language(conn) do
    case get_req_header(conn, "accept-language") do
      [accept_language | _] ->
        parse_accept_language_header(accept_language)

      _ ->
        nil
    end
  end

  defp parse_accept_language_header(accept_language) do
    accept_language
    |> String.split(",")
    |> Enum.map(&parse_language_tag/1)
    |> Enum.find(&(&1 in @supported_locales))
  end

  defp parse_language_tag(tag) do
    tag
    |> String.trim()
    |> String.split(";")
    |> List.first()
    |> String.split("-")
    |> List.first()
    |> String.downcase()
  end

  defp persist(locale, conn) do
    IO.inspect(locale, label: "persist_locale")

    # Ensure locale is a string before setting cookie
    locale_string = to_string(locale)

    put_session(conn, :locale, locale_string)
    |> put_resp_cookie("locale", locale_string)
  end
end