defprotocol Shard.Manifest do
  @moduledoc"""
  A shard manifest is a data structure that uniquely defines the identity of the shard.

  The hash of the manifest is the unique identifier of that shard on the network.
  
  The Manifest protocol is a protocol implemented by the manifest structs for the
  different shard types. It contains an operation start() that is able to launch the
  correct process for this shard and connect to other peers that use it.
  """

  @doc"""
  Start the corresponding Shard process
  """
  def start(manifest)
end

defmodule Shard.Manager do
  @moduledoc"""
    Maintains several important tables :

    - :shard_db (persistent with DETS)
    
      List of
        { id, manifest, pid | nil }

    - :shard_state (persistent with DETS)

      List of
        { id, state }

    - :peer_db (persistent with DETS)

      Mult-list of
        { shard_id, peer_info }   # TODO: add health info (last seen, ping, etc)

      peer_info := {:inet, ip, port} | {:inet6, ip, port} | {:onion, name}

    - :shard_procs (not persistent)

      List of
        { {id, path}, pid }
  """

  use GenServer

  require Logger

  @shard_db [Application.get_env(:shard, :data_path), "shard_db"] |> Path.join |> String.to_atom
  @shard_state [Application.get_env(:shard, :data_path), "shard_state"] |> Path.join |> String.to_atom
  @peer_db [Application.get_env(:shard, :data_path), "peer_db"] |> Path.join |> String.to_atom

  def start_link(_) do
    GenServer.start_link(__MODULE__, nil, name: __MODULE__)
  end

  def init(_) do
    :dets.open_file(@shard_db, [type: :set])
    for [{id, manifest, _pid}] <- :dets.match @shard_db, :"$1" do
      :dets.insert @shard_db, {id, manifest, nil}
      spawn fn -> Shard.Manifest.start manifest end
    end

    :dets.open_file(@shard_state, [type: :set])
    :dets.open_file(@peer_db, [type: :bag])

    :ets.new(:shard_procs, [:set, :protected, :named_table])

    {:ok, nil}
  end

  def handle_call({:register, shard_id, manifest, pid}, _from, state) do
    will_live = case :dets.lookup(@shard_db, shard_id) do
      [{ ^shard_id, _, pid }] when pid != nil -> not Process.alive?(pid)
      _ -> true
    end
    reply = if will_live do
      Process.monitor(pid)
      :dets.insert(@shard_db, {shard_id, manifest, pid})
      :ok
    else
      :redundant
    end
    {:reply, reply, state}
  end

  def handle_cast({:dispatch_to, shard_id, path, pid}, state) do
    :ets.insert(:shard_procs, { {shard_id, path}, pid })
    Process.monitor(pid)
    {:noreply, state}
  end

  def handle_cast({:interested, conn_pid, peer_info, auth, shards}, state) do
    for shard_id <- shards do
      case :dets.lookup(@shard_db, shard_id) do
        [{ ^shard_id, _, pid }] ->
          :dets.insert(@peer_db, {shard_id, peer_info})
          GenServer.cast(pid, {:interested, conn_pid, auth})
        [] -> nil
      end
    end
    {:noreply, state}
  end

  def handle_cast({:not_interested, peer_info, shard_id}, state) do
    :dets.match_delete(@peer_db, {shard_id, peer_info})
    {:noreply, state}
  end

  def handle_cast({:shard_peer_db_insert, shard_id, peer_info}, state) do
    :dets.insert(@peer_db, {shard_id, peer_info})
    {:noreply, state}
  end

  def handle_info({:DOWN, _, :process, pid, _}, state) do
    :ets.match_delete(:shard_procs, {:_, pid})
    {:noreply, state}
  end


  # ======================
  # CALLED BY SNet.TcpConn
  # ======================

  @doc"""
  Dispatch incoming message to correct shard process
  """
  def incoming(conn_pid, peer_info, auth, {:interested, shards}) do
    GenServer.cast(__MODULE__, {:interested, conn_pid, peer_info, auth, shards})
  end

  def incoming(_conn_pid, peer_info, _auth, {:not_interested, shard}) do
    GenServer.cast(__MODULE__, {:not_interested, peer_info, shard})
  end

  def incoming(conn_pid, peer_info, auth, {shard_id, path, msg}) do
    case :dets.lookup(@shard_db, shard_id) do
      [] ->
        GenServer.cast(conn_pid, {:send_msg, {:not_interested, shard_id}})
      [_] ->
        case :dets.match(@peer_db, {shard_id, peer_info}) do
          [] ->
            GenServer.cast(__MODULE__, {:shard_peer_db_insert, shard_id, peer_info})
          _ -> nil
        end
        case :ets.lookup(:shard_procs, {shard_id, path}) do
          [{ {^shard_id, ^path}, pid }] ->
            GenServer.cast(pid, {:msg, conn_pid, auth, shard_id, path, msg})
          [] ->
            Logger.info("Warning: dropping message for #{inspect shard_id}/#{inspect path}, no handler running.\n\t#{inspect msg}")
        end
    end
  end


  # ================
  # CALLED BY Sapp.*
  # ================

  @doc"""
  Register a process as the main process for a shard.

  Returns either :ok or :redundant, in which case the process must exit.
  """
  def register(shard_id, manifest, pid) do
    GenServer.call(__MODULE__, {:register, shard_id, manifest, pid})
  end

  @doc"""
  Register a process as the handler for shard packets for a given path.
  """
  def dispatch_to(shard_id, path, pid) do
    GenServer.cast(__MODULE__, {:dispatch_to, shard_id, path, pid})
  end

  @doc"""
  Return the list of all peer info for peers that are interested in a certain shard
  """
  def get_shard_peers(shard_id) do
    for {_, peer_info} <- :dets.lookup(@peer_db, shard_id), do: peer_info
  end

  @doc"""
  Return the saved state value for a shard
  """
  def load_state(shard_id) do
    case :dets.lookup(@shard_state, shard_id) do
      [{^shard_id, state}] -> state
      _ -> nil
    end
  end

  @doc"""
  Save a state value for a shard
  """
  def save_state(shard_id, state) do
    :dets.insert(@shard_state, {shard_id, state})
  end


  # ================
  # CALLED BY ANYONE
  # ================

  @doc"""
  Returns the pid for a shard if it exists
  """
  def find_proc(shard_id) do
    case :dets.lookup(@shard_db, shard_id) do
      [{^shard_id, _, pid}] -> pid
      _ -> nil
    end
  end

  @doc"""
  Returns the pid for a shard defined by its manifest.
  Start it if it doesn't exist.
  """
  def find_or_start(manifest) do
    id = SData.term_hash manifest
    case find_proc id do
      nil ->
        case Shard.Manifest.start manifest do
          {:ok, pid} -> pid
          {:error, :redundant} -> find_proc id
        end
      pid -> pid
    end
  end

  @doc"""
  Return the list of all shards.
  """
  def list_shards() do
    for [x] <- :dets.match(@shard_db, :"$1"), do: x
  end
end