feat: add current room occupancy endpoint

lib/localiser/web/controllers/tag_controller.ex

@@ -2,12 +2,21 @@ defmodule Localiser.Web.Controllers.TagController do
   use Phoenix.Controller, formats: [:json]
   use OpenApiSpex.ControllerSpecs
 
-  alias Localiser.Domain.Tags
+  alias Localiser.Domain.{Rooms, Tags}
+  alias Localiser.Localisation.Tag.Filter, as: TagFilter
   alias Localiser.Web.Schemas
 
   tags ["Tags"]
   security [%{"bearerAuth" => []}]
 
+  operation :occupancy,
+    summary: "Get every tag's current room",
+    responses: [
+      ok: {"Tag occupancy list", "application/json",
+           %OpenApiSpex.Schema{type: :array, items: Schemas.TagOccupancy}},
+      unauthorized: {"Unauthorized", "application/json", Schemas.Error}
+    ]
+
   operation :index,
     summary: "List all tags",
     responses: [
@@ -50,6 +59,18 @@ defmodule Localiser.Web.Controllers.TagController do
       unauthorized: {"Unauthorized", "application/json", Schemas.Error}
     ]
 
+  def occupancy(conn, _params) do
+    tag_to_room = build_tag_to_room_map()
+
+    result =
+      Tags.list_tags()
+      |> Enum.map(fn tag ->
+        %{id: tag.id, tag_id: tag.tag_id, name: tag.name, room_id: Map.get(tag_to_room, tag.tag_id)}
+      end)
+
+    json(conn, result)
+  end
+
   def index(conn, _params) do
     json(conn, Enum.map(Tags.list_tags(), &render_tag/1))
   end
@@ -94,6 +115,20 @@ defmodule Localiser.Web.Controllers.TagController do
     send_resp(conn, :no_content, "")
   end
 
+  defp build_tag_to_room_map do
+    Rooms.list_rooms()
+    |> Enum.flat_map(fn room ->
+      occupants =
+        case Registry.lookup(Localiser.Registry, {:room, room.id}) do
+          [{pid, _}] -> GenServer.call(pid, :get_occupants)
+          []         -> MapSet.new()
+        end
+
+      Enum.map(MapSet.to_list(occupants), &{&1, room.id})
+    end)
+    |> Map.new()
+  end
+
   defp render_tag(tag) do
     live = TagFilter.get_live_state(tag.tag_id)