Implement federation room state snapshot endpoint

lib/matrix_server/encodable_map.ex

@@ -1,6 +1,6 @@
 # https://github.com/michalmuskala/jason/issues/69
 defmodule MatrixServer.EncodableMap do
-  alias MatrixServer.{EncodableMap, Event}
+  alias MatrixServer.EncodableMap
   alias MatrixServer.Types.{UserId, RoomId, EventId, GroupId, AliasId}
 
   defstruct pairs: []

lib/matrix_server/room_server.ex

@@ -60,6 +60,10 @@ defmodule MatrixServer.RoomServer do
     GenServer.call(pid, {:server_in_room, domain})
   end
 
+  def get_state_at_event(pid, event) do
+    GenServer.call(pid, {:get_state_at_event, event})
+  end
+
   ### Implementation
 
   @impl true
@@ -104,6 +108,26 @@ defmodule MatrixServer.RoomServer do
     {:reply, result, state}
   end
 
+  def handle_call({:get_state_at_event, %Event{room_id: room_id} = event}, _from, state) do
+    room_events =
+      Event
+      |> where([e], e.room_id == ^room_id)
+      |> select([e], {e.event_id, e})
+      |> Repo.all()
+      |> Enum.into(%{})
+
+    state_set = StateResolution.resolve(event, false)
+    state_events = Map.values(state_set)
+
+    auth_chain =
+      state_set
+      |> Map.values()
+      |> StateResolution.full_auth_chain(room_events)
+      |> Enum.map(&room_events[&1])
+
+    {:reply, {state_events, auth_chain}, state}
+  end
+
   defp create_room_insert_events(room, account, %CreateRoom{
          room_version: room_version,
          preset: preset,

lib/matrix_server/state_resolution.ex

@@ -55,6 +55,12 @@ defmodule MatrixServer.StateResolution do
     |> do_resolve(room_events)
   end
 
+  def full_auth_chain(events, room_events) do
+    events
+    |> Enum.map(&auth_chain(&1, room_events))
+    |> Enum.reduce(MapSet.new(), &MapSet.union/2)
+  end
+
   defp do_resolve([], _), do: %{}
 
   defp do_resolve(state_sets, room_events) do
@@ -150,12 +156,6 @@ defmodule MatrixServer.StateResolution do
     MapSet.difference(auth_chain_union, auth_chain_intersection)
   end
 
-  defp full_auth_chain(events, room_events) do
-    events
-    |> Enum.map(&auth_chain(&1, room_events))
-    |> Enum.reduce(MapSet.new(), &MapSet.union/2)
-  end
-
   defp auth_chain(%Event{auth_events: auth_events}, room_events) do
     auth_events
     |> Enum.map(&room_events[&1])