defmodule S2.S2S.AppendSession do
  @moduledoc """
  Bidirectional streaming append session.

  Opens a streaming POST to `/v1/streams/{stream}/records` with `Content-Type: s2s/proto`.
  Supports multiple sequential `append/2` calls on the same session, each sending a framed
  `AppendInput` and receiving a framed `AppendAck`.

  ## Process affinity

  Sessions are NOT safe to share across processes. The underlying Mint connection
  delivers TCP messages to the owning process's mailbox. Creating a session in one
  process and calling `append/2` from another will not work — the receiving process
  won't see the TCP data.
  """

  require Logger

  alias S2.S2S.Shared

  @typedoc "An open append session."
  @type t :: %__MODULE__{}

  defstruct [
    :conn,
    :request_ref,
    :basin,
    :stream,
    :owner_pid,
    recv_timeout: 5_000,
    compression: :none,
    closed: false,
    data: <<>>
  ]

  @doc """
  Open a new streaming append session.

  ## Options

    * `:token` — Bearer token for authentication.
    * `:recv_timeout` — Timeout in milliseconds for receiving responses (default: 5000).
    * `:compression` — Compression for S2S frames: `:none`, `:gzip`, or `:zstd` (default: `:none`).

  Returns `{:ok, session}` on success or `{:error, reason}` on failure.
  On `Mint.HTTP2.request/5` failure, returns `{:error, reason, conn}` so the
  caller can still manage the connection.
  """
  @spec open(Mint.HTTP2.t(), String.t(), String.t(), keyword()) ::
          {:ok, t()} | {:error, term()} | {:error, term(), Mint.HTTP2.t()}
  def open(conn, basin, stream, opts \\ []) do
    Logger.debug("S2S.AppendSession.open basin=#{basin} stream=#{stream}")
    path = Shared.records_path(stream)
    token = Keyword.get(opts, :token)
    recv_timeout = Keyword.get(opts, :recv_timeout, Shared.default_timeout())
    compression = Keyword.get(opts, :compression, :none)
    headers = Shared.build_headers(basin, token)

    case Mint.HTTP2.request(conn, "POST", path, headers, :stream) do
      {:ok, conn, request_ref} ->
        session = %__MODULE__{
          conn: conn,
          request_ref: request_ref,
          basin: basin,
          stream: stream,
          owner_pid: self(),
          recv_timeout: recv_timeout,
          compression: compression
        }

        wait_for_headers(session)

      {:error, conn, reason} ->
        {:error, reason, conn}
    end
  end

  @doc """
  Append a batch of records to the session.

  Returns `{:ok, ack, session}` on success or `{:error, reason, session}` on failure.
  The session is marked as closed on error and cannot be reused.
  """
  @spec append(t(), S2.V1.AppendInput.t()) ::
          {:ok, S2.V1.AppendAck.t(), t()} | {:error, term(), t()}
  def append(%__MODULE__{closed: true} = session, _input) do
    {:error, :session_closed, session}
  end

  def append(%__MODULE__{} = session, %S2.V1.AppendInput{} = input) do
    check_owner!(session)

    case Shared.encode_framed(input, compression: session.compression) do
      {:error, reason} ->
        {:error, reason, Shared.close_session(session)}

      {:ok, body} ->
        case Mint.HTTP2.stream_request_body(session.conn, session.request_ref, body) do
          {:ok, conn} ->
            session = %{session | conn: conn}
            receive_ack(session)

          {:error, conn, reason} ->
            {:error, reason, Shared.close_session(session, conn)}
        end
    end
  end

  @doc """
  Close the append session gracefully by sending EOF on the request body.

  Returns `{:ok, session}` or `{:error, reason, session}`.
  """
  @spec close(t()) :: {:ok, t()} | {:error, term(), t()}
  def close(%__MODULE__{closed: true} = session), do: {:ok, session}

  def close(%__MODULE__{} = session) do
    case Mint.HTTP2.stream_request_body(session.conn, session.request_ref, :eof) do
      {:ok, conn} ->
        session = %{session | conn: conn, closed: true}
        drain_final_response(session)

      {:error, conn, reason} ->
        {:error, reason, Shared.close_session(session, conn)}
    end
  end

  defp wait_for_headers(session) do
    case Shared.wait_for_headers(session.conn, session.request_ref, session.recv_timeout) do
      {:ok, 200, _data, conn} ->
        {:ok, %{session | conn: conn}}

      {:ok, status, _data, conn} ->
        {:error, %S2.Error{status: status, message: "unexpected status #{status}"}, conn}

      {:error, reason, conn} ->
        {:error, reason, conn}
    end
  end

  defp receive_ack(session) do
    all_data = session.data

    case Shared.decode_frame(all_data, S2.V1.AppendAck) do
      {:ok, ack, rest} ->
        {:ok, ack, %{session | data: rest}}

      {:error, reason} ->
        {:error, reason, Shared.close_session(session)}

      :incomplete ->
        dl = Shared.deadline(session.recv_timeout)
        do_receive_ack(%{session | data: <<>>}, all_data, dl)
    end
  end

  @doc false
  def handle_ack_response({:ok, conn, responses}, session, acc) do
    session = %{session | conn: conn}
    new_data = Shared.extract_data(responses, session.request_ref)
    all_data = acc <> new_data

    case Shared.check_buffer_size(all_data) do
      {:error, :buffer_overflow} ->
        {:error, :buffer_overflow, Shared.close_session(%{session | data: all_data})}

      :ok ->
        done? = Shared.done?(responses, session.request_ref)

        case Shared.decode_frame(all_data, S2.V1.AppendAck) do
          {:ok, ack, rest} ->
            {:ok, ack, %{session | data: rest}}

          {:error, reason} ->
            {:error, reason, Shared.close_session(session)}

          :incomplete when done? ->
            {:error, :incomplete_frame, Shared.close_session(session)}

          :incomplete ->
            {:continue, session, all_data}
        end
    end
  end

  def handle_ack_response({:error, conn, _error, _responses}, session, _acc) do
    {:error, :stream_error, Shared.close_session(session, conn)}
  end

  def handle_ack_response(:unknown, session, acc) do
    {:continue, session, acc}
  end

  defp do_receive_ack(session, acc, dl) do
    receive do
      message ->
        case handle_ack_response(Mint.HTTP2.stream(session.conn, message), session, acc) do
          {:continue, session, acc} -> do_receive_ack(session, acc, dl)
          result -> result
        end
    after
      Shared.remaining(dl) -> {:error, :timeout, Shared.close_session(session)}
    end
  end

  # After sending EOF, drain the server's final response (status/headers/done)
  # so the HTTP/2 stream is fully closed. Best-effort: if it times out or errors,
  # we still return the closed session.
  defp drain_final_response(session) do
    drain_final_response(session, Shared.deadline(session.recv_timeout))
  end

  @doc false
  def handle_drain_response({:ok, conn, responses}, session) do
    session = %{session | conn: conn}

    if Shared.done?(responses, session.request_ref) do
      {:ok, session}
    else
      {:continue, session}
    end
  end

  def handle_drain_response({:error, conn, _error, _responses}, session) do
    {:ok, %{session | conn: conn}}
  end

  def handle_drain_response(:unknown, session) do
    {:continue, session}
  end

  defp drain_final_response(session, dl) do
    receive do
      message ->
        case handle_drain_response(Mint.HTTP2.stream(session.conn, message), session) do
          {:continue, session} -> drain_final_response(session, dl)
          result -> result
        end
    after
      Shared.remaining(dl) -> {:ok, session}
    end
  end

  defp check_owner!(%__MODULE__{owner_pid: pid}) do
    Shared.assert_owner!(pid, "AppendSession")
  end
end