clacks/lib/clacks_web/controllers/web_finger_controller.ex

defmodule ClacksWeb.WebFingerController do
  use ClacksWeb, :controller
  alias Clacks.Actor

  @acct_regex ~r/(?:acct:)?([a-z0-9_]+)(?:@.+)?/i

  def get(conn, %{"resource" => resource}) do
    with [_, nickname] <- Regex.run(@acct_regex, resource),
         %Actor{local: true} = actor <- Actor.get_by_nickname(nickname) do
      url = Application.get_env(:clacks, ClacksWeb.Endpoint)[:url]
      host = url[:host]
      port = url[:port]
      maybe_port = if port == URI.default_port(url[:scheme]), do: "", else: ":#{port}"

      conn
      |> json(%{
        "subject" => "acct:#{actor.nickname}@#{host}#{maybe_port}",
        "links" => [
          %{
            "rel" => "self",
            "type" => "application/activity+json",
            "href" => actor.ap_id
          }
        ]
      })
    else
      _ ->
        conn
        |> put_status(404)
        |> json(%{error: "No such resource"})
    end
  end
end