defmodule Symbiont.Telepathy.SMTP do
  @moduledoc """
  Muse Inbox SMTP receiver — gen_smtp_server_session behaviour.

  Listens on port 25 (configurable via `:smtp_port`), accepts mail addressed
  to `muse@hydrascale.net` or `muse@cortex.hydrascale.net`, and hands off
  each accepted message to `Symbiont.Telepathy.Pipeline` for async processing.

  ## Data flow

      SMTP client
        → handle_RCPT/2   — validate recipient
        → handle_DATA/4   — parse MIME via :mimemail, log to inbox.jsonl,
                            spawn Pipeline task via Task.Supervisor
        → "250 OK"        — response returned to sender immediately

  Processing (dispatch to Claude, send reply, mark read) happens asynchronously
  under `Symbiont.TaskSupervisor` so the SMTP response is never delayed by
  upstream latency. See `Symbiont.Telepathy.Pipeline` for the pipeline logic.

  ## Supervision

  `Symbiont.Telepathy.Supervisor` starts this module as a `:supervisor`-type
  child (gen_smtp_server is itself a supervisor internally).

  ## Inbox log

  Every accepted message is appended to `/data/muse/inbox.jsonl` in the same
  schema used by the Python `smtp_handler.py`, ensuring backward compatibility
  with any tooling that reads that file. The `processed` flag is flipped to
  `true` by `Pipeline` after successful dispatch.
  """

  @behaviour :gen_smtp_server_session

  require Logger

  @accepted_recipients ["muse@hydrascale.net", "muse@cortex.hydrascale.net"]
  @inbox_log "/data/muse/inbox.jsonl"
  @domain "cortex.hydrascale.net"

  defstruct from: nil, rcpt_to: []

  # ---------------------------------------------------------------------------
  # Supervision interface
  # ---------------------------------------------------------------------------

  @doc """
  Returns a child spec suitable for `Supervisor.start_link/2`.

  Passes `:smtp_port` through to `start_link/1`.
  """
  def child_spec(opts) do
    %{
      id: __MODULE__,
      start: {__MODULE__, :start_link, [opts]},
      type: :supervisor,
      restart: :permanent
    }
  end

  @doc """
  Starts the gen_smtp_server listener process.

  `opts` must include `:smtp_port` (integer). Falls back to 25 if absent.
  """
  def start_link(opts) do
    port = Keyword.get(opts, :smtp_port, 25)
    Logger.info("Telepathy SMTP: starting listener on port #{port}")

    :gen_smtp_server.start(__MODULE__, [
      [port: port, domain: @domain, address: {0, 0, 0, 0}]
    ])
  end

  # ---------------------------------------------------------------------------
  # gen_smtp_server_session callbacks
  # ---------------------------------------------------------------------------

  @impl :gen_smtp_server_session
  def init(_hostname, _session_count, _peer_info, _options) do
    banner = "#{@domain} ESMTP Muse Inbox ready"
    {:ok, banner, %__MODULE__{}}
  end

  @impl :gen_smtp_server_session
  def handle_HELO(_hostname, state) do
    {:ok, state}
  end

  @impl :gen_smtp_server_session
  def handle_EHLO(_hostname, extensions, state) do
    # Return the default extension list unchanged. SIZE and 8BITMIME are
    # included by gen_smtp by default; we don't advertise AUTH.
    {:ok, extensions, state}
  end

  @impl :gen_smtp_server_session
  def handle_MAIL(from, state) do
    {:ok, %{state | from: from}}
  end

  @impl :gen_smtp_server_session
  def handle_MAIL_extension(_extension, state) do
    {:ok, state}
  end

  @impl :gen_smtp_server_session
  def handle_RCPT(to, state) do
    addr =
      to
      |> to_string()
      |> String.trim()
      |> String.trim_leading("<")
      |> String.trim_trailing(">")
      |> String.downcase()

    if addr in @accepted_recipients do
      {:ok, %{state | rcpt_to: [to | state.rcpt_to]}}
    else
      Logger.info("Telepathy SMTP: rejected recipient #{addr}")
      {:error, "550 Recipient #{addr} not accepted", state}
    end
  end

  @impl :gen_smtp_server_session
  def handle_RCPT_extension(_extension, state) do
    {:ok, state}
  end

  @doc """
  Core DATA handler. Called once the entire message has been received.

  1. Parses the raw SMTP payload via `:mimemail.decode/1`.
  2. Appends a raw entry (processed: false) to `inbox.jsonl`.
  3. Fires a `Symbiont.Telepathy.Pipeline.process/4` task under
     `Symbiont.TaskSupervisor` — returns 250 immediately without waiting.
  """
  @impl :gen_smtp_server_session
  def handle_DATA(from, to, data, state) do
    case parse_email(data) do
      {:ok, parsed} ->
        entry = %{
          "timestamp" => DateTime.utc_now() |> DateTime.to_iso8601(),
          "from" => parsed.from,
          "to" => Enum.join(to, ", "),
          "subject" => parsed.subject,
          "message_id" => parsed.message_id,
          "body" => String.slice(parsed.body, 0, 5000),
          "processed" => false
        }

        append_inbox_log(entry)

        Task.Supervisor.start_child(Symbiont.TaskSupervisor, fn ->
          Symbiont.Telepathy.Pipeline.process(
            parsed.from,
            parsed.subject,
            parsed.body,
            parsed.message_id
          )
        end)

        ref = :crypto.strong_rand_bytes(4) |> Base.encode16(case: :lower)
        Logger.info("Telepathy SMTP: accepted #{ref} from #{from} (#{parsed.subject})")
        {:ok, ref, state}

      {:error, reason} ->
        Logger.error("Telepathy SMTP: parse error from #{from}: #{inspect(reason)}")
        {:error, "451 Error processing message", state}
    end
  end

  @impl :gen_smtp_server_session
  def handle_RSET(state) do
    {:ok, %{state | from: nil, rcpt_to: []}}
  end

  @impl :gen_smtp_server_session
  def handle_VRFY(_address, state) do
    {:error, "252 Cannot VRFY user, but will accept message and attempt delivery", state}
  end

  @impl :gen_smtp_server_session
  def handle_other(_verb, _args, state) do
    {"500 Command unrecognized", state}
  end

  @impl :gen_smtp_server_session
  def handle_AUTH(_type, _username, _password, state) do
    {:error, state}
  end

  @impl :gen_smtp_server_session
  def handle_STARTTLS(state) do
    {:ok, state}
  end

  @impl :gen_smtp_server_session
  def code_change(_old_vsn, state, _extra) do
    {:ok, state}
  end

  @impl :gen_smtp_server_session
  def terminate(reason, state) do
    if reason not in [:normal, :shutdown] do
      Logger.debug("Telepathy SMTP: session terminated (#{inspect(reason)})")
    end

    {:ok, state}
  end

  # ---------------------------------------------------------------------------
  # Private: MIME parsing via :mimemail
  # ---------------------------------------------------------------------------

  defp parse_email(data) do
    try do
      {type, subtype, headers, _params, body} = :mimemail.decode(data)

      from = find_header(headers, "From") || "unknown"
      subject = find_header(headers, "Subject") || "(no subject)"
      message_id = find_header(headers, "Message-ID") || ""

      text_body = extract_body(type, subtype, headers, body)

      {:ok, %{from: from, subject: subject, message_id: message_id, body: text_body}}
    rescue
      e -> {:error, Exception.message(e)}
    catch
      kind, value -> {:error, {kind, value}}
    end
  end

  # Plain text — use directly.
  defp extract_body("text", "plain", _headers, body) when is_binary(body) do
    String.trim(body)
  end

  # HTML with no plain-text alternative — return a placeholder matching the
  # Python smtp_handler.py behaviour.
  defp extract_body("text", "html", headers, _body) do
    from = find_header(headers, "From") || "sender"
    "[HTML content from #{from}]"
  end

  # Multipart — walk parts, prefer text/plain, fall back to HTML placeholder.
  defp extract_body("multipart", _subtype, _headers, parts) when is_list(parts) do
    plain = find_part(parts, "text", "plain")
    html = find_part(parts, "text", "html")

    cond do
      plain != nil ->
        {_, _, _, _, body} = plain
        String.trim(body)

      html != nil ->
        {_, _, part_headers, _, _} = html
        from = find_header(part_headers, "From") || "sender"
        "[HTML content from #{from}]"

      true ->
        ""
    end
  end

  defp extract_body(_type, _subtype, _headers, body) when is_binary(body) do
    String.trim(body)
  end

  defp extract_body(_type, _subtype, _headers, _body), do: ""

  defp find_part(parts, type, subtype) do
    Enum.find(parts, fn
      {^type, ^subtype, _, _, _} -> true
      _ -> false
    end)
  end

  defp find_header(headers, name) do
    name_lower = String.downcase(name)

    Enum.find_value(headers, nil, fn
      {n, v} ->
        n_str = to_string(n)
        if String.downcase(n_str) == name_lower, do: to_string(v), else: nil

      _ ->
        nil
    end)
  end

  # ---------------------------------------------------------------------------
  # Private: inbox.jsonl logging
  # ---------------------------------------------------------------------------

  defp append_inbox_log(entry) do
    File.mkdir_p!(Path.dirname(@inbox_log))
    File.write!(@inbox_log, Jason.encode!(entry) <> "\n", [:append])
  rescue
    e -> Logger.warning("Telepathy SMTP: inbox log write failed: #{Exception.message(e)}")
  end
end