neiam /gitgud

defmodule GitGudWeb.Plugs.ApContentNegotiation do
  @moduledoc """
  Detects ActivityPub clients by Accept header. When they ask for AP
  JSON, we short-circuit to a dedicated controller; otherwise the
  request flows through to the normal LiveView.

  Activated content types:

    * `application/activity+json`
    * `application/ld+json` (any profile)
    * `application/json` (only as a fallback — not officially AP but
      tolerant to spec-loose clients)
  """

  @behaviour Plug

  import Plug.Conn

  @ap_types ~w(
    application/activity+json
    application/ld+json
  )

  @impl true
  def init(opts), do: opts

  @impl true
  def call(conn, _opts) do
    if ap_requested?(conn) do
      assign(conn, :ap_response?, true)
    else
      assign(conn, :ap_response?, false)
    end
  end

  @doc "Pure helper exposed for controllers that decide manually."
  def ap_requested?(conn) do
    accept_header(conn)
    |> Enum.any?(&ap_type?/1)
  end

  defp accept_header(conn) do
    case get_req_header(conn, "accept") do
      [] -> []
      [h | _] -> String.split(h, ",", trim: true) |> Enum.map(&String.trim/1)
    end
  end

  defp ap_type?(value) do
    base = value |> String.split(";", parts: 2) |> List.first() |> String.downcase()
    base in @ap_types
  end
end