# 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