clacks/lib/clacks_web/controllers/actor_controller.ex

defmodule ClacksWeb.ActorController do
  use ClacksWeb, :controller
  alias Clacks.Actor
  import Ecto.Query

  @context "https://www.w3.org/ns/activitystreams"

  plug :get_actor

  defp get_actor(%Plug.Conn{path_params: %{"nickname" => nickname}} = conn, _opts) do
    case Actor.get_by_nickname(nickname) do
      nil ->
        conn
        |> put_status(404)
        |> halt()

      actor ->
        assign(conn, :actor, actor)
    end
  end

  defp get_actor(conn, _opts), do: conn

  def get(conn, _params) do
    case conn.assigns[:actor] do
      %Actor{local: true, data: data} ->
        conn
        |> put_resp_header("content-type", "application/activity+json")
        |> json(data)

      %Actor{local: false, ap_id: ap_id} ->
        conn
        |> redirect(external: ap_id)
    end
  end

  def followers(conn, %{"page" => page}) do
    {page, _} = Integer.parse(page)

    followers = conn.assigns[:actor].followers

    data =
      collection_page(conn, followers, page)
      |> Map.put("@context", @context)

    conn
    |> put_resp_header("content-type", "application/activity+json")
    |> json(data)
  end

  def followers(conn, _params) do
    %Actor{followers: followers} = conn.assigns[:actor]

    data = %{
      "@context" => @context,
      "type" => "OrderedCollection",
      "id" => current_url(conn, %{}),
      "totalItems" => length(followers),
      "first" => collection_page(conn, followers, 1)
    }

    conn
    |> put_resp_header("content-type", "application/activity+json")
    |> json(data)
  end

  defp collection_page(conn, collection, page) do
    chunks = Enum.chunk_every(collection, 20)
    # page is 1 indexed, so subtract 1 to get the current chunk
    current_chunk = Enum.at(chunks, page - 1)

    data = %{
      "type" => "OrderedCollectionPage",
      "totalItems" => length(collection),
      "partOf" => current_url(conn, %{}),
      "id" => current_url(conn, %{page: page}),
      "orderedItems" => current_chunk || []
    }

    if page < length(chunks) do
      Map.put(data, "next", current_url(conn, %{page: page + 1}))
    else
      data
    end
  end
end