defmodule Modbux.Rtu.Framer do
  @moduledoc """
  A framer for Modbus RTU frames. This framer doesn't do anything for the transmit
  direction, but for receives, it will collect bytes that follows the Modbus RTU protocol,
  it also execute the CRC validation and returns the frame or the error.
  """

  @behaviour Circuits.UART.Framing

  require Logger
  alias Modbux.Helper

  defmodule State do
    @moduledoc false
    defstruct behavior: nil,
              max_len: nil,
              expected_length: nil,
              index: 0,
              processed: <<>>,
              in_process: <<>>,
              fc: nil,
              error: nil,
              error_message: nil,
              lines: []
  end

  def init(args) do
    # modbus standard max len
    max_len = Keyword.get(args, :max_len, 255)
    behavior = Keyword.get(args, :behavior, :slave)
    state = %State{max_len: max_len, behavior: behavior}
    {:ok, state}
  end

  # do nothing, we assume this is already in the right form
  def add_framing(data, state) do
    {:ok, data, state}
  end

  def remove_framing(data, state) do
    # Logger.debug("new data #{inspect(state)}")
    new_state = process_data(state.in_process <> data, state.expected_length, state.processed, state)
    rc = if buffer_empty?(new_state), do: :ok, else: :in_frame
    # Logger.debug("new data processed #{inspect(new_state)}, #{inspect(rc)}")

    if has_error?(new_state),
      do: dispatch({:ok, new_state.error_message, new_state}),
      else: dispatch({rc, new_state.lines, new_state})
  end

  def frame_timeout(state) do
    partial_line = {:partial, state.processed <> state.in_process}
    new_state = %State{max_len: state.max_len, behavior: state.behavior}
    {:ok, [partial_line], new_state}
  end

  def flush(direction, state) when direction == :receive or direction == :both do
    %State{max_len: state.max_len, behavior: state.behavior}
  end

  def flush(_direction, state) do
    state
  end

  # helper functions

  # handle empty byte
  defp process_data(<<>>, _len, _in_process, state) do
    # Logger.debug("End #{inspect(state)}")
    state
  end

  # get first byte (index 0)
  defp process_data(<<slave_id::size(8), b_tail::binary>>, nil, processed, %{index: 0} = state) do
    # Logger.debug("line 0")
    new_state = %{state | index: 1, processed: processed <> <<slave_id>>, in_process: <<>>}
    process_data(b_tail, nil, new_state.processed, new_state)
  end

  # get second byte (function code) (index 1)
  defp process_data(<<fc::size(8), b_tail::binary>>, nil, processed, %{index: 1} = state) do
    # Logger.debug("line 1")
    new_state = %{state | fc: fc, index: 2, processed: processed <> <<fc>>}
    process_data(b_tail, nil, new_state.processed, new_state)
  end

  # Clause for functions code => 5, 6
  defp process_data(<<len::size(8), b_tail::binary>>, nil, processed, %{index: 2, fc: fc} = state)
       when fc in [5, 6] do
    # Logger.debug("fc 5 or 6")
    new_state = %{state | expected_length: 8, index: 3, processed: processed <> <<len>>}
    process_data(b_tail, new_state.expected_length, new_state.processed, new_state)
  end

  # Clause for functions code => 15 and 16
  defp process_data(<<len::size(8), b_tail::binary>>, nil, processed, %{index: 2, fc: fc} = state)
       when fc in [15, 16] do
    # Logger.debug("fc 15 or 16")
    new_state =
      if state.behavior == :slave do
        %{state | expected_length: 7, index: 3, processed: processed <> <<len>>}
      else
        %{state | expected_length: 8, index: 3, processed: processed <> <<len>>}
      end

    process_data(b_tail, new_state.expected_length, new_state.processed, new_state)
  end

  defp process_data(<<len::size(8), b_tail::binary>>, _len, processed, %{index: 6, fc: fc} = state)
       when fc in [15, 16] do
    # Logger.debug("fc 15 or 16, len")

    new_state =
      if state.behavior == :slave do
        %{state | expected_length: len + 9, index: 7, processed: processed <> <<len>>}
      else
        %{state | expected_length: 8, index: 7, processed: processed <> <<len>>}
      end

    process_data(b_tail, new_state.expected_length, new_state.processed, new_state)
  end

  # Clause for functions code => 1, 2, 3 and 4
  defp process_data(<<len::size(8), b_tail::binary>>, nil, processed, %{index: 2, fc: fc} = state)
       when fc in 1..4 do
    # Logger.debug("fc 1, 2, 3 or 4")

    new_state =
      if state.behavior == :slave do
        %{state | expected_length: 8, index: 3, processed: processed <> <<len>>}
      else
        %{state | expected_length: len + 5, index: 3, processed: processed <> <<len>>}
      end

    process_data(b_tail, new_state.expected_length, new_state.processed, new_state)
  end

  # Clause for exceptions.
  defp process_data(<<len::size(8), b_tail::binary>>, nil, processed, %{index: 2, fc: fc} = state)
       when fc in 129..144 do
    # Logger.debug("exceptions")

    new_state = %{state | expected_length: 5, index: 3, processed: processed <> <<len>>}

    process_data(b_tail, new_state.expected_length, new_state.processed, new_state)
  end

  # Catch all fc (error)
  defp process_data(_data, nil, processed, %{index: 2, fc: _fc} = state) do
    %{state | error: true, error_message: [{:error, :einval, processed}]}
  end

  defp process_data(<<data::size(8), b_tail::binary>>, len, processed, state) when is_binary(processed) do
    current_data = processed <> <<data>>

    # Logger.info(
    #   "(#{__MODULE__}) data_len: #{byte_size(current_data)}, len: #{inspect(len)}, state: #{inspect(state)}"
    # )

    if len == byte_size(current_data) do
      new_state = %{
        state
        | expected_length: nil,
          lines: [current_data],
          in_process: b_tail,
          index: 0,
          processed: <<>>
      }

      # we got the whole thing in 1 pass, so we're done
      check_crc(new_state)
    else
      new_state = %{state | index: state.index + 1, processed: current_data}
      # need to keep reading
      process_data(b_tail, len, current_data, new_state)
    end
  end

  def buffer_empty?(state) do
    state.processed == <<>> and state.in_process == <<>>
  end

  defp has_error?(state) do
    state.error != nil
  end

  defp dispatch({:in_frame, _lines, _state} = msg), do: msg

  defp dispatch({rc, msg, state}) do
    {rc, msg, %State{max_len: state.max_len, behavior: state.behavior}}
  end

  # once we have the full packet, verify it's CRC16
  defp check_crc(state) do
    [packet] = state.lines
    packet_without_crc = Kernel.binary_part(packet, 0, byte_size(packet) - 2)
    expected_crc = Kernel.binary_part(packet, byte_size(packet), -2)
    <<hi_crc, lo_crc>> = Helper.crc(packet_without_crc)
    real_crc = <<lo_crc, hi_crc>>
    # Logger.info("(#{__MODULE__}) #{inspect(expected_crc)} == #{inspect(real_crc)}")

    if real_crc == expected_crc,
      do: state,
      else: %{state | error: true, error_message: [{:error, :ecrc, "CRC Error"}]}
  end
end