diff --git a/lib/matrix_server/event.ex b/lib/matrix_server/event.ex
new file mode 100644
index 0000000..1756f40
--- /dev/null
+++ b/lib/matrix_server/event.ex
@@ -0,0 +1,292 @@
+# https://matrix.uhoreg.ca/stateres/reloaded.html
+defmodule MatrixServer.Event do
+  @derive {Inspect, except: [:prev_events, :auth_events]}
+  defstruct [
+    :event_id,
+    :event_type,
+    :timestamp,
+    :state_key,
+    :sender,
+    :content,
+    :prev_events,
+    :auth_events,
+    :power_levels
+  ]
+
+  alias __MODULE__, as: Event
+
+  @type t :: %Event{event_id: String.t(), event_type: Atom.t(), timestamp: Integer.t()}
+
+  def new_state_event, do: %Event{new() | event_type: :state}
+  def new_message_event, do: %Event{new() | event_type: :message}
+
+  def new do
+    %Event{
+      event_id: "",
+      timestamp: 0,
+      state_key: "",
+      sender: "",
+      content: "",
+      prev_events: [],
+      auth_events: [],
+      power_levels: %{}
+    }
+  end
+
+  def join(user), do: %Event{membership(user) | content: "join"}
+  def leave(user), do: %Event{membership(user) | content: "leave"}
+  def invite(actor, subject), do: %Event{membership(actor, subject) | content: "invite"}
+  def kick(actor, subject), do: %Event{membership(actor, subject) | content: "leave"}
+  def ban(actor, subject), do: %Event{membership(actor, subject) | content: "ban"}
+
+  def set_power_levels(user, power_levels) do
+    %Event{new() | event_type: :power_levels, sender: user, power_levels: power_levels}
+  end
+
+  def set_topic(user, topic) do
+    %Event{new() | event_type: :topic, sender: user, content: topic}
+  end
+
+  def get_state_set_from_event_list(events) do
+    Enum.reduce(events, %{}, fn
+      %Event{event_type: event_type, state_key: state_key} = event, acc ->
+        Map.put(acc, {event_type, state_key}, event)
+    end)
+  end
+
+  def auth_chain(event), do: auth_chain(event, MapSet.new())
+
+  def auth_chain(%Event{auth_events: auth_events}, set) do
+    Enum.reduce(auth_events, set, fn event, acc ->
+      event
+      |> auth_chain()
+      |> MapSet.union(acc)
+      |> MapSet.put(event)
+    end)
+  end
+
+  def in_room(user, state_set) when is_map_key(state_set, {:membership, user}) do
+    state_set[{:membership, user}].content == "join"
+  end
+
+  def in_room(_, _), do: false
+
+  def get_power_levels(state_set) when is_map_key(state_set, {:power_levels, ""}) do
+    state_set[{:power_levels, ""}].power_levels
+  end
+
+  def get_power_levels(_), do: nil
+
+  def has_power_level(_, nil, _), do: true
+
+  def has_power_level(user, power_levels, level) do
+    Map.get(power_levels, user, 0) >= level
+  end
+
+  # TODO: join and power levels events
+  def is_authorized(%Event{event_type: :create, prev_events: prev_events}, _),
+    do: prev_events == []
+
+  def is_authorized(%Event{sender: sender} = event, state_set) do
+    in_room(sender, state_set) and
+      has_power_level(sender, get_power_levels(state_set), get_event_power_level(event))
+  end
+
+  def is_authorized2(%Event{auth_events: auth_events} = event, state_set) do
+    state_set =
+      Enum.reduce(auth_events, state_set, fn %Event{event_type: event_type, state_key: state_key} =
+                                               event,
+                                             acc ->
+        Map.put_new(acc, {event_type, state_key}, event)
+      end)
+
+    is_authorized(event, state_set)
+  end
+
+  def iterative_auth_checks(events, state_set) do
+    Enum.reduce(events, state_set, fn event, acc ->
+      if is_authorized2(event, acc), do: insert_event(event, acc), else: acc
+    end)
+  end
+
+  def insert_event(%Event{event_type: event_type, state_key: state_key} = event, state_set) do
+    Map.put(state_set, {event_type, state_key}, event)
+  end
+
+  def is_control_event(%Event{event_type: :power_levels, state_key: state_key})
+      when state_key != "",
+      do: true
+
+  def is_control_event(%Event{event_type: :join_rules, state_key: state_key})
+      when state_key != "",
+      do: true
+
+  def is_control_event(%Event{
+        event_type: :membership,
+        state_key: state_key,
+        sender: sender,
+        content: "ban"
+      }),
+      do: sender != state_key
+
+  def is_control_event(%Event{
+        event_type: :membership,
+        state_key: state_key,
+        sender: sender,
+        content: "leave"
+      }),
+      do: sender != state_key
+
+  def is_control_event(_), do: false
+
+  def calculate_conflict(state_sets) do
+    domain =
+      state_sets
+      |> Enum.map(&Map.keys/1)
+      |> List.flatten()
+      |> MapSet.new()
+
+    full_state_map_list =
+      Enum.map(domain, fn k ->
+        events =
+          Enum.map(state_sets, &Map.get(&1, k))
+          |> MapSet.new()
+
+        {k, events}
+      end)
+
+    {unconflicted, conflicted} =
+      Enum.split_with(full_state_map_list, fn {_k, events} ->
+        MapSet.size(events) == 1
+      end)
+
+    unconflicted_state_map =
+      Enum.map(unconflicted, fn {k, events} ->
+        event =
+          events
+          |> MapSet.to_list()
+          |> hd()
+
+        {k, event}
+      end)
+      |> Enum.into(%{})
+
+    conflicted_state_map =
+      Enum.flat_map(conflicted, fn {_, events} ->
+        events
+        |> MapSet.delete(nil)
+        |> MapSet.to_list()
+      end)
+      |> MapSet.new()
+
+    {unconflicted_state_map, conflicted_state_map}
+  end
+
+  def full_auth_chain(events) do
+    events
+    |> Enum.map(&auth_chain/1)
+    |> Enum.reduce(MapSet.new(), &MapSet.union/2)
+  end
+
+  def auth_difference(state_sets) do
+    full_auth_chains = Enum.map(state_sets, fn state_set ->
+      state_set
+      |> Map.values()
+      |> full_auth_chain()
+    end)
+
+    auth_chain_union = Enum.reduce(full_auth_chains, MapSet.new(), &MapSet.union/2)
+    auth_chain_intersection = Enum.reduce(full_auth_chains, MapSet.new(), &MapSet.intersection/2)
+
+    MapSet.difference(auth_chain_union, auth_chain_intersection)
+  end
+
+  def example1 do
+    create = %Event{new() | event_id: "create", event_type: :create, sender: "@alice:example.com"}
+
+    alice_joins = %Event{
+      join("@alice:example.com")
+      | event_id: "alice joins",
+        prev_events: [create],
+        auth_events: [create]
+    }
+
+    pl = %Event{
+      set_power_levels("@alice:example.com", %{"@alice:example.com" => 100})
+      | event_id: "power level",
+        prev_events: [alice_joins],
+        auth_events: [alice_joins, create]
+    }
+
+    join_rules = %Event{
+      new()
+      | event_id: "join rules",
+        event_type: :join_rules,
+        sender: "@alice:example.com",
+        content: "private",
+        prev_events: [pl],
+        auth_events: [pl, alice_joins, create]
+    }
+
+    invite_bob = %Event{
+      invite("@alice:example.com", "@bob:example.com")
+      | event_id: "invite bob",
+        prev_events: [join_rules],
+        auth_events: [pl, alice_joins, create]
+    }
+
+    invite_carol = %Event{
+      invite("@alice:example.com", "@carol:example.com")
+      | event_id: "invite carol",
+        prev_events: [invite_bob],
+        auth_events: [pl, alice_joins, create]
+    }
+
+    bob_join = %Event{
+      join("@bob:example.com")
+      | event_id: "bob joins",
+        prev_events: [invite_carol],
+        auth_events: [invite_bob, join_rules, create]
+    }
+
+    [create, alice_joins, pl, join_rules, invite_bob, invite_carol, bob_join]
+  end
+
+  def example2 do
+    create = %Event{
+      new_state_event()
+      | event_id: "create",
+        event_type: :create,
+        sender: "@alice:example.com"
+    }
+
+    pl1 = %Event{
+      set_power_levels("@alice:example.com", %{"@alice:example.com" => 100})
+      | event_id: "power levels 1"
+    }
+
+    pl2 = %Event{
+      set_power_levels("@alice:example.com", %{
+        "@alice:example.com" => 100,
+        "@bob:example.com" => 50
+      })
+      | event_id: "power levels 2"
+    }
+
+    topic = %Event{set_topic("@bob:example.com", "This is a topic") | event_id: "topic"}
+
+    state_set1 = get_state_set_from_event_list([create, pl1])
+    state_set2 = get_state_set_from_event_list([create, pl2, topic])
+    state_set3 = get_state_set_from_event_list([create, pl2])
+    [state_set1, state_set2, state_set3]
+  end
+
+  defp membership(user), do: membership(user, user)
+
+  defp membership(actor, subject) do
+    %Event{new() | event_type: :membership, sender: actor, state_key: subject}
+  end
+
+  defp get_event_power_level(%Event{state_key: ""}), do: 0
+  defp get_event_power_level(_), do: 50
+end