defmodule Memcache.Connection do
  @moduledoc """
  This module provides low level API to connect and execute commands
  on a memcached server.
  """
  require Logger
  use Connection
  alias Memcache.Protocol
  alias Memcache.Utils
  alias Memcache.Receiver

  defmodule State do
    @moduledoc false

    defstruct opts: nil, sock: nil, backoff_current: nil, receiver: nil, receiver_queue: nil
  end

  @doc """
  Starts a connection to memcached server.

  The actual TCP connection to the memcached server is established
  asynchronously after the `start_link/2` returns.

  This function accepts two option lists. The first one specifies the
  options to connect to the memcached server. The second option is
  passed directly to the underlying `GenServer.start_link/3`, so it
  can be used to create named process.

  Memcachex automatically tries to reconnect in case of tcp connection
  failures. It starts with a `:backoff_initial` wait time and
  increases the wait time exponentially on each successive failures
  until it reaches the `:backoff_max`. After that, it waits for
  `:backoff_max` after each failure.

  ## Connection Options

  * `:hostname` - (string) hostname of the memcached server. Defaults
    to `"localhost"`
  * `:port` - (integer) port on which the memcached server is
    listening. Defaults to `11211`
  * `:backoff_initial` - (integer) initial backoff (in milliseconds)
    to be used in case of connection failure. Defaults to `500`
  * `:backoff_max` - (integer) maximum allowed interval between two
    connection attempt. Defaults to `30_000`

  * `:auth` - (tuple) only plain authentication method is
    supported. It is specified using the following format `{:plain,
    "username", "password"}`. Defaults to `nil`.

  ## Example

      {:ok, pid} = Memcache.Connection.start_link()

  """
  @spec start_link(Keyword.t, Keyword.t) :: GenServer.on_start
  def start_link(connection_options \\ [], options \\ []) do
    connection_options = with_defaults(connection_options)
    Connection.start_link(__MODULE__, connection_options, options)
  end

  @default_opts [
    backoff_initial: 500,
    backoff_max: 30_000,
    hostname: 'localhost',
    port: 11211
  ]

  defp with_defaults(opts) do
    Keyword.merge(@default_opts, opts)
    |> Keyword.update!(:hostname, (&if is_binary(&1), do: String.to_char_list(&1), else: &1))
  end

  @doc """
  Executes the command with the given args

  ## options

  * `:cas` - (boolean) returns the CAS value associated with the
    data. This value will be either in second or third position
    of the returned tuple depending on the command. Defaults to `false`

  ## Example

      iex> {:ok, pid} = Memcache.Connection.start_link()
      iex> {:ok} = Memcache.Connection.execute(pid, :SET, ["hello", "world"])
      iex> {:ok, "world"} = Memcache.Connection.execute(pid, :GET, ["hello"])
      {:ok, "world"}

  """
  @spec execute(GenServer.server, atom, [binary], Keyword.t) :: Memcache.result
  def execute(pid, command, args, options \\ []) do
    Connection.call(pid, { :execute, command, args, %{cas: Keyword.get(options, :cas, false)} })
  end

  @doc """
  Executes the list of quiet commands

  ## Example

      iex> {:ok, pid} = Memcache.Connection.start_link()
      iex> {:ok, [{:ok}, {:ok}]} = Memcache.Connection.execute_quiet(pid, [{:SETQ, ["1", "one"]}, {:SETQ, ["2", "two"]}])
      iex> Memcache.Connection.execute_quiet(pid, [{:GETQ, ["1"]}, {:GETQ, ["2"]}])
      {:ok, [{:ok, "one"}, {:ok, "two"}]}

  """
  @spec execute_quiet(GenServer.server, [{atom, [binary]} | {atom, [binary], Keyword.t}]) :: {:ok, [Memcache.result]} | {:error, atom}
  def execute_quiet(pid, commands) do
    Connection.call(pid, { :execute_quiet, commands })
  end

  @doc """
  Closes the connection to the memcached server.

  ## Example
      iex> {:ok, pid} = Memcache.Connection.start_link()
      iex> Memcache.Connection.close(pid)
      {:ok}
  """
  @spec close(GenServer.server) :: {:ok}
  def close(pid) do
    :ok = GenServer.stop(pid)
    {:ok}
  end

  def init(opts) do
    { :connect, :init, %State{opts: opts} }
  end

  def connect(info, %State{opts: opts} = s) do
    sock_opts = [:binary, active: false, packet: :raw]
    case connect_and_authenticate(opts[:hostname], opts[:port], sock_opts, s) do
      { :ok, sock } ->
        _ = if info == :backoff || info == :reconnect do
            Logger.info(["Reconnected to Memcache (", Utils.format_host(opts), ")"])
          end
        { :ok, receiver } = Receiver.start_link([sock, self()])
        receiver_queue = MapSet.new()
        state = %{s | sock: sock, backoff_current: nil, receiver: receiver, receiver_queue: receiver_queue }
        { :ok, state }
      { :error, reason } ->
        backoff = get_backoff(s)
        _ = Logger.error(["Failed to connect to Memcache (", Utils.format_host(opts), "): ", Utils.format_error(reason), ". Sleeping for ", to_string(backoff), "ms."])
        { :backoff, backoff, %{s | backoff_current: backoff} }
      { :stop, reason } -> { :stop, reason, s }
    end
  end

  def disconnect({ :error, reason }, %State{ opts: opts } = s) do
    _ = Logger.error(["Disconnected from Memcache (", Utils.format_host(opts), "): ", Utils.format_error(reason)])
    cleanup(s)
    {:connect, :reconnect, %{s | sock: nil, backoff_current: nil, receiver: nil, receiver_queue: nil}}
  end

  def handle_call({ :execute, _command, _args, _opts }, _from, %State{ sock: nil } = s) do
    {:reply, {:error, :closed}, s}
  end

  def handle_call({ :execute, command, args, opts }, from, s) do
    with :ok <- maybe_deactivate_sock(s) do
      send_and_receive(s, from, command, args, opts)
    end
  end

  def handle_call({ :execute_quiet, _commands }, _from, %State{ sock: nil } = s) do
    {:reply, {:error, :closed}, s}
  end

  def handle_call({ :execute_quiet, commands }, from, s) do
    with :ok <- maybe_deactivate_sock(s) do
      send_and_receive_quiet(s, from, commands)
    end
  end

  def handle_info({:tcp_closed, _socket}, state) do
    error = {:error, :closed}
    { :disconnect, error, state }
  end

  def handle_info({:tcp_error, _socket, reason}, state) do
    error = {:error, reason}
    { :disconnect, error, state }
  end

  def handle_info({:receiver, :disconnect, error, receiver}, %State{receiver: receiver} = state) do
    { :disconnect, error, state }
  end

  def handle_info({:receiver, :done, client, receiver}, %State{receiver: receiver} = state) do
    receiver_queue = MapSet.delete(state.receiver_queue, client)
    state = %{state | receiver_queue: receiver_queue}
    maybe_activate_sock(state)
  end

  def handle_info(msg, state) do
    _ = Logger.warn(["Unknown message: ", inspect(msg)])
    {:noreply, state}
  end

  def terminate(_reason, state) do
    cleanup(state)
  end

  ## Private ##

  def cleanup(%State{ sock: sock, receiver: receiver, receiver_queue: receiver_queue}) do
    if sock do
      :ok = :gen_tcp.close(sock)
    end
    if receiver do
      try do
        Receiver.stop(receiver)
      catch
        :exit, _ -> :ok
      end
    end
    if receiver_queue do
      Enum.each(receiver_queue, fn from ->
        Connection.reply(from, {:error, :closed})
      end)
    end
    :ok
  end

  defp maybe_activate_sock(state) do
    if Enum.empty?(state.receiver_queue) do
      case :inet.setopts(state.sock, [active: :once]) do
        :ok -> { :noreply, state }
        error -> { :disconnect, error, state }
      end
    else
      { :noreply, state }
    end
  end

  defp maybe_deactivate_sock(state) do
    if Enum.empty?(state.receiver_queue) do
      case :inet.setopts(state.sock, [active: false]) do
        :ok -> :ok
        error -> { :disconnect, error, {:error, :closed}, state }
      end
    else
      :ok
    end
  end

  defp send_and_receive(%State{ sock: sock } = s, from, command, args, opts) do
    packet = serialize(command, args)
    case :gen_tcp.send(sock, packet) do
      :ok ->
        s = enqueue_receiver(s, from)
        :ok = Receiver.read(s.receiver, from, command, opts)
        {:noreply, s}
      { :error, _reason } = error -> { :disconnect, error, error, s }
    end
  end

  defp send_and_receive_quiet(%State{ sock: sock } = s, from, commands) do
    { packet, commands, i } = Enum.reduce(commands, { [], [], 1 }, &accumulate_commands/2)
    packet = [packet | serialize(:NOOP, [], i)]
    case :gen_tcp.send(sock, packet) do
      :ok ->
        s = enqueue_receiver(s, from)
        :ok = Receiver.read_quiet(s.receiver, from, Enum.reverse([ { i, :NOOP, [], [] } | commands]))
        {:noreply, s}
      { :error, _reason } = error -> { :disconnect, error, error, s }
    end
  end

  defp enqueue_receiver(state, from) do
    receiver_queue = MapSet.put(state.receiver_queue, from)
    %{state| receiver_queue: receiver_queue}
  end

  defp accumulate_commands({ command, args }, { packet, commands, i }) do
    { [packet | serialize(command, args, i)], [{ i, command, args, %{cas: false} } | commands], i + 1 }
  end
  defp accumulate_commands({ command, args, options }, { packet, commands, i }) do
    { [packet | serialize(command, args, i)], [{ i, command, args, %{cas: Keyword.get(options, :cas, false)}} | commands], i + 1 }
  end

  defp get_backoff(s) do
    if !s.backoff_current do
      s.opts[:backoff_initial]
    else
      Utils.next_backoff(s.backoff_current, s.opts[:backoff_max])
    end
  end

  defp connect_and_authenticate(host, port, sock_opts, state) do
    case :gen_tcp.connect(host, port, sock_opts) do
      {:ok, sock} ->
        with {:ok} <- authenticate(sock, state.opts),
             # Make sure the socket is usable
             {:ok, _} <- execute_command(sock, :NOOP, []),
             :ok <- :inet.setopts(sock, [active: :once]) do
          {:ok, sock}
        else
          error ->
            :gen_tcp.close(sock)
            error
        end
      error -> error
    end
  end

  defp authenticate(sock, opts) do
    case opts[:auth] do
      nil -> {:ok}
      {:plain, username, password} -> auth_plain(sock, username, password)
      _ -> {:stop, "Memcachex client only supports :plain authentication type"}
    end
  end

  defp execute_command(sock, command, args) do
    packet = serialize(command, args)
    case :gen_tcp.send(sock, packet) do
      :ok -> recv_response(sock, command)
      error -> error
    end
  end

  defp auth_plain(sock, username, password) do
    case execute_command(sock, :AUTH_LIST, []) do
      {:ok, {:ok, list}} ->
        supported = String.split(list, " ")
        if !Enum.member?(supported, "PLAIN") do
          {:stop, "Server doesn't support PLAIN authentication"}
        else
          auth_plain_continue(sock, username, password)
        end
      {:ok, {:error, "Unknown command"}} ->
        _ = Logger.warn "Authentication not required/supported by server"
        {:ok}
      {:ok, {:error, reason}} -> {:stop, reason}
      error -> error
    end
  end

  defp auth_plain_continue(sock, username, password) do
    case execute_command(sock, :AUTH_START, ["PLAIN", "\0#{username}\0#{password}"]) do
      {:ok, {:ok}} -> {:ok}
      {:ok, {:error, reason}} ->
        {:stop, reason}
      error -> error
    end
  end

  defp recv_response(sock, command) do
    case Receiver.recv_response(command, sock, <<>>, %{cas: false}) do
      {:ok, response, <<>>} -> {:ok, response}
      {:error, reason} -> {:error, reason}
    end
  end

  defp serialize(command, args, opaque \\ 0) do
    apply(Protocol, :to_binary, [command | [opaque | args]])
  end
end