defmodule Integrations.Smtp do
  @moduledoc """
  SMTP server monitor.

  Opens a TCP connection to an SMTP server, reads the greeting banner, and
  optionally performs an EHLO handshake to confirm the server is fully
  operational. Reports the time to first byte and lists advertised
  capabilities.

  Uses raw `:gen_tcp` — no external dependencies required.

  Collection only — see `Integrations.Smtp.Display` (same package) for the
  dashboard panel. `Display.BundledDefault` auto-hooks it whenever this
  monitor starts.

  ## Params

    * `:host`          — SMTP server hostname or IP. Required.
    * `:port`          — Port number. Defaults to `25`. Common alternatives:
                         `465` (SMTPS), `587` (submission), `1025` (Mailpit).
    * `:helo_domain`   — Domain to send in the EHLO command. Defaults to
                         `"raven.local"`. Set to a blank string to skip EHLO
                         and only read the banner.
    * `:timeout_ms`    — TCP connect and receive timeout in milliseconds.
                         Defaults to `5000`.
    * `:latency_degraded_ms` — Time-to-banner threshold for `:degraded`.
                               Defaults to `1000`.

  ## Health signal

    * `:up`       — Server responded with a `220` banner; EHLO succeeded
                    (if requested); latency within threshold.
    * `:degraded` — Banner received but EHLO failed, or latency exceeded
                    `:latency_degraded_ms`.
    * `:down`     — Connection refused, timeout, or non-220 banner.
  """

  use CodeNameRaven.Monitor

  @default_port            25
  @default_helo_domain     "raven.local"
  @default_timeout_ms      5_000
  @default_lat_degraded_ms 1_000

  @impl true
  def params_template do
    %{host: "", port: "25", helo_domain: "raven.local", timeout_ms: "5000"}
  end

  @impl true
  def params_schema do
    [
      host:                [type: :string,          required: true,  doc: "SMTP server hostname or IP"],
      port:                [type: :non_neg_integer, default: 25,     doc: "Port number (25, 465, 587, 1025)"],
      helo_domain:         [type: :string,          default: "raven.local", doc: "Domain for EHLO command (blank to skip)"],
      timeout_ms:          [type: :non_neg_integer, default: 5_000,  doc: "TCP connect and receive timeout in milliseconds"],
      latency_degraded_ms: [type: :non_neg_integer, default: 1_000,  doc: "Time-to-banner threshold for degraded"]
    ]
  end

  @impl true
  def target_uri(params) do
    host = get_param(params, :host)
    port = parse_int(get_param(params, :port), @default_port)
    if host, do: {:ok, "smtp://#{host}:#{port}"}, else: :none
  end

  @impl true
  def identity_params(params) do
    %{
      host: get_param(params, :host),
      port: parse_int(get_param(params, :port), @default_port)
    }
  end

  # ---------------------------------------------------------------------------
  # Collect
  # ---------------------------------------------------------------------------

  @impl true
  def collect(params, state) do
    host = get_param(params, :host)

    if is_nil(host) do
      {:error, "missing required param :host", state}
    else
      port        = parse_int(get_param(params, :port), @default_port)
      timeout_ms  = parse_int(get_param(params, :timeout_ms), @default_timeout_ms)
      helo_domain = case get_param(params, :helo_domain) do
        nil -> @default_helo_domain
        ""  -> nil
        d   -> d
      end

      started_at = System.monotonic_time(:millisecond)

      tcp_opts = [:binary, packet: :line, active: false, send_timeout: timeout_ms]

      case :gen_tcp.connect(to_charlist(host), port, tcp_opts, timeout_ms) do
        {:ok, socket} ->
          result = run_smtp_check(socket, helo_domain, timeout_ms, started_at)
          :gen_tcp.close(socket)
          case result do
            {:ok, data} -> {:ok, data, state}
            {:error, reason} -> {:error, reason, state}
          end

        {:error, :econnrefused} ->
          {:error, "connection refused on port #{port}", state}

        {:error, :timeout} ->
          {:error, "connection timed out", state}

        {:error, :nxdomain} ->
          {:error, "hostname not found: #{host}", state}

        {:error, reason} ->
          {:error, inspect(reason), state}
      end
    end
  end

  # ---------------------------------------------------------------------------
  # Healthy?
  # ---------------------------------------------------------------------------

  @impl true
  def healthy?(result) do
    lat_deg = result[:latency_degraded_ms] || @default_lat_degraded_ms

    cond do
      result.banner_code != 220    -> :down
      result.ehlo_attempted and
        not result.ehlo_ok         -> :degraded
      result.banner_ms >= lat_deg  -> :degraded
      true                         -> :up
    end
  end

  # ---------------------------------------------------------------------------
  # Metrics
  # ---------------------------------------------------------------------------

  @impl true
  def metrics(result) do
    base = %{
      banner_ms:    result.banner_ms,
      banner_code:  result.banner_code
    }

    if result.ehlo_attempted do
      Map.put(base, :capabilities_count, length(result.capabilities))
    else
      base
    end
  end

  # ---------------------------------------------------------------------------
  # SMTP protocol
  # ---------------------------------------------------------------------------

  defp run_smtp_check(socket, helo_domain, timeout_ms, started_at) do
    # Read greeting — may be multi-line (220-hostname ... \r\n 220 hostname Ready)
    case recv_smtp_response(socket, timeout_ms) do
      {:ok, {code, lines}} ->
        banner_ms   = System.monotonic_time(:millisecond) - started_at
        banner_text = List.first(lines) || ""

        if helo_domain do
          :gen_tcp.send(socket, "EHLO #{helo_domain}\r\n")

          {ehlo_ok, capabilities} =
            case recv_smtp_response(socket, timeout_ms) do
              {:ok, {250, caps}} -> {true, caps}
              _                  -> {false, []}
            end

          # Polite quit
          :gen_tcp.send(socket, "QUIT\r\n")

          {:ok, %{
            banner_code:      code,
            banner:           banner_text,
            banner_ms:        banner_ms,
            ehlo_attempted:   true,
            ehlo_ok:          ehlo_ok,
            capabilities:     capabilities
          }}
        else
          {:ok, %{
            banner_code:      code,
            banner:           banner_text,
            banner_ms:        banner_ms,
            ehlo_attempted:   false,
            ehlo_ok:          false,
            capabilities:     []
          }}
        end

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

  # Read a complete SMTP response (handles multi-line 250-...\r\n250 ...\r\n)
  defp recv_smtp_response(socket, timeout_ms, acc \\ []) do
    case :gen_tcp.recv(socket, 0, timeout_ms) do
      {:ok, line} ->
        trimmed = String.trim_trailing(line)

        case Regex.run(~r/^(\d{3})([ \-])(.*)$/, trimmed) do
          [_, code_str, " ", text] ->
            code = String.to_integer(code_str)
            {:ok, {code, Enum.reverse([text | acc])}}

          [_, _code_str, "-", text] ->
            recv_smtp_response(socket, timeout_ms, [text | acc])

          _ ->
            if acc == [] do
              {:error, "unexpected response: #{trimmed}"}
            else
              {:ok, {0, Enum.reverse(acc)}}
            end
        end

      {:error, :timeout} -> {:error, "timed out waiting for server response"}
      {:error, reason}   -> {:error, inspect(reason)}
    end
  end

  # ---------------------------------------------------------------------------
  # Helpers
  # ---------------------------------------------------------------------------

  defp get_param(params, key) when is_atom(key) do
    v = params[key] || params[to_string(key)]
    if is_binary(v) and String.trim(v) == "", do: nil, else: v
  end

  defp parse_int(nil, default), do: default
  defp parse_int(v, _) when is_integer(v), do: v
  defp parse_int(v, default) when is_binary(v) do
    case Integer.parse(v) do
      {n, _} -> n
      :error -> default
    end
  end
  defp parse_int(_, default), do: default
end