defmodule RNS.PacketHandler do
  @moduledoc """
  Process that handles packets.
  """
  require Logger

  require RNS.Packet

  @doc """
  Create a new PacketHandler process.
  This is the function an interface should run.
  """
  def handle(data_fn) do
    spawn(__MODULE__, :start, [data_fn])
  end

  @doc """
  This is the actual handler.
  It should NEVER BE RUN BY AN INTERFACE as it does not do it in another process.
  """
  @spec start(fun(), RNS.Interface.t(), RNS.Config.transport()) :: :ok
  def start(packet_fn, interface, transport) do
    case packet_fn.() do
      {:ok, raw} ->
        packet = RNS.Packet.from_raw(raw)
        packet_hash = RNS.Packet.hash(packet)

        if not RNS.PacketHashStore.exists?(packet_hash) do
          RNS.PacketHashStore.add(packet_hash)
          handle(packet, interface, transport)
        end

      {:error, error} ->
        Logger.warning(
          "Failed to pre-handle packet on interface #{inspect(interface)}: #{inspect(error)}."
        )
    end
  end

  @spec handle(RNS.Packet.t(), RNS.Interface.t(), RNS.Config.transport()) :: :ok
  def handle(packet, interface, transport) do
    interface_id = RNS.Interface.id(interface)

    packet_hops = RNS.Packet.hops(packet)
    packet = RNS.Packet.packet(packet, hops: packet_hops + 1)

    case RNS.Packet.packet_type(packet) do
      :announce ->
        RNS.Packet.Announce.handle(packet, interface_id)

      _type ->
        if RNS.Packet.propagation_type(packet) == :transport do
          maybe_route(packet, transport, interface)
        end
    end
  end

  @spec maybe_route(RNS.Packet.t(), RNS.Config.transport(), RNS.Interface.t()) :: :ok
  def maybe_route(packet, transport, source) do
    case transport do
      true ->
        route(packet, source)

      false ->
        destination_hash = RNS.Packet.destination_hash(packet)

        case RNS.DestinationStore.fetch(destination_hash) do
          {:ok, destination} ->
            if RNS.Destination.hops(destination) <= 0 do
              route(packet, source, destination)
            end

          {:error, :not_found} ->
            :ok
        end
    end
  end

  @spec route(RNS.Packet.t(), RNS.Interface.t()) :: :ok
  def route(packet, source) do
    destination_hash = RNS.Packet.destination_hash(packet)

    case RNS.DestinationStore.fetch(destination_hash) do
      {:ok, destination} ->
        route(packet, source, destination)
    end
  end

  @spec route(RNS.Packet.t(), RNS.Interface.t(), RNS.Destination.t()) :: :ok
  def route(packet, source, destination) do
    RNS.Interface.send(packet, RNS.Destination.interface(destination), source)
  end
end