defmodule Integrations.Dns do
  @moduledoc """
  DNS resolution monitor.

  Queries a DNS nameserver and checks that a name resolves correctly within
  an acceptable time. Uses OTP's built-in `:inet_res` — no external
  dependencies required.

  Useful for monitoring authoritative nameservers, recursive resolvers,
  cluster-internal DNS (e.g. CoreDNS), and split-horizon configurations.

  Collection only — see `Integrations.Dns.Display` (same package) for the
  dashboard panel. `Display.BundledDefault` auto-hooks it whenever this
  monitor starts, same as a single-module package would; a release without
  `raven_web` simply never compiles the display half and runs this monitor
  headless.

  ## Params

    * `:nameserver`    — IP address of the DNS server to query. Required.
                         Port 53 is used unless `:nameserver_port` is set.
    * `:nameserver_port` — DNS server port. Defaults to `53`.
    * `:query`         — Name to resolve. Required.
    * `:type`          — Record type: `"A"`, `"AAAA"`, `"MX"`, `"TXT"`,
                         `"CNAME"`, `"NS"`, `"PTR"`. Defaults to `"A"`.
    * `:expected`      — Expected value in the answer (optional). For A/AAAA
                         records, an IP string. For MX, the exchange hostname.
                         For TXT, a substring that must appear in any record.
                         When set, a mismatch triggers `:degraded`.
    * `:timeout_ms`    — Query timeout in milliseconds. Defaults to `5000`.
    * `:latency_degraded_ms` — RTT threshold for `:degraded`. Defaults to
                               `200`.

  ## Health signal

    * `:up`       — Query resolved; answer matches `:expected` (if set);
                    latency within threshold.
    * `:degraded` — Query resolved but answer does not match `:expected`,
                    or latency exceeded `:latency_degraded_ms`.
    * `:down`     — Query failed (NXDOMAIN, SERVFAIL, timeout, unreachable).
  """

  use CodeNameRaven.Monitor

  @default_port           53
  @default_type           "A"
  @default_timeout_ms     5_000
  @default_lat_degraded   200

  @type_map %{
    "A"     => :a,
    "AAAA"  => :aaaa,
    "MX"    => :mx,
    "TXT"   => :txt,
    "CNAME" => :cname,
    "NS"    => :ns,
    "PTR"   => :ptr,
    "SOA"   => :soa
  }

  @impl true
  def params_template do
    %{nameserver: "", query: "", type: "A", expected: "", timeout_ms: "5000"}
  end

  @impl true
  def params_schema do
    [
      nameserver:          [type: :string,          required: true,  doc: "IP address of DNS server to query"],
      nameserver_port:     [type: :non_neg_integer, default: 53,     doc: "DNS server port"],
      query:               [type: :string,          required: true,  doc: "Name to resolve"],
      type:                [type: {:in, ["A", "AAAA", "MX", "TXT", "CNAME", "NS", "PTR", "SOA"]}, default: "A", doc: "DNS record type"],
      expected:            [type: :string,          required: false, doc: "Expected answer value (mismatch triggers degraded)"],
      timeout_ms:          [type: :non_neg_integer, default: 5_000,  doc: "Query timeout in milliseconds"],
      latency_degraded_ms: [type: :non_neg_integer, default: 200,    doc: "RTT threshold for degraded"]
    ]
  end

  @impl true
  def target_uri(params) do
    ns    = get_param(params, :nameserver)
    query = get_param(params, :query)
    type  = get_param(params, :type) || @default_type
    if ns && query, do: {:ok, "dns://#{ns}/#{query}?type=#{type}"}, else: :none
  end

  @impl true
  def identity_params(params) do
    %{
      nameserver: get_param(params, :nameserver),
      query:      get_param(params, :query),
      type:       get_param(params, :type) || @default_type
    }
  end

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

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

    cond do
      is_nil(nameserver) -> {:error, "missing required param :nameserver", state}
      is_nil(query)      -> {:error, "missing required param :query", state}
      true               ->
        ns_port    = parse_int(get_param(params, :nameserver_port), @default_port)
        type_str   = get_param(params, :type) || @default_type
        type_atom  = Map.get(@type_map, String.upcase(type_str), :a)
        timeout_ms = parse_int(get_param(params, :timeout_ms), @default_timeout_ms)
        expected   = get_param(params, :expected)

        ns_ip = parse_ip(nameserver)

        opts = [
          nameservers: [{ns_ip, ns_port}],
          timeout:     timeout_ms,
          retry:       1
        ]

        started_at = System.monotonic_time(:millisecond)

        case :inet_res.resolve(to_charlist(query), :in, type_atom, opts) do
          {:ok, dns_record} ->
            latency_ms = System.monotonic_time(:millisecond) - started_at
            answers    = extract_answers(dns_record, type_atom)
            matched    = answers_match?(answers, expected, type_atom)

            result = %{
              latency_ms: latency_ms,
              answers:    answers,
              matched:    matched,
              expected:   expected,
              type:       type_str,
              query:      query
            }

            {:ok, result, state}

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

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

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

    cond do
      result.expected != nil and not result.matched -> :degraded
      result.latency_ms >= lat_degraded             -> :degraded
      true                                          -> :up
    end
  end

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

  @impl true
  def metrics(result) do
    %{
      latency_ms:    result.latency_ms,
      answer_count:  length(result.answers)
    }
  end

  # ---------------------------------------------------------------------------
  # Answer extraction
  # ---------------------------------------------------------------------------

  defp extract_answers(dns_rec, type_atom) do
    # dns_rec is a :dns_rec tuple from :inet_res
    # Answers are in element index 3 (0-indexed: type, id, qdlist, anlist, ...)
    anlist = elem(dns_rec, 3)

    Enum.flat_map(anlist, fn rr ->
      data = elem(rr, 6)
      format_rdata(type_atom, data)
    end)
  end

  defp format_rdata(:a, {a, b, c, d}),                  do: ["#{a}.#{b}.#{c}.#{d}"]
  defp format_rdata(:aaaa, addr) when is_tuple(addr),   do: [:inet.ntoa(addr) |> to_string() |> List.wrap()]
  defp format_rdata(:mx, {pref, name}),                 do: ["#{pref} #{to_string(name)}"]
  defp format_rdata(:txt, data) when is_list(data),     do: [data |> Enum.map(&to_string/1) |> Enum.join()]
  defp format_rdata(:txt, data),                        do: [to_string(data)]
  defp format_rdata(:cname, name),                      do: [to_string(name)]
  defp format_rdata(:ns, name),                         do: [to_string(name)]
  defp format_rdata(:ptr, name),                        do: [to_string(name)]
  defp format_rdata(_, data),                           do: [inspect(data)]

  defp answers_match?(_answers, nil, _type),    do: true
  defp answers_match?(answers, expected, :txt) do
    Enum.any?(answers, &String.contains?(&1, expected))
  end
  defp answers_match?(answers, expected, _type) do
    Enum.any?(answers, &String.contains?(&1, expected))
  end

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

  defp parse_ip(str) do
    case :inet.parse_address(to_charlist(str)) do
      {:ok, ip} -> ip
      {:error, _} -> to_charlist(str)
    end
  end

  defp format_dns_error(:nxdomain),    do: "NXDOMAIN — name does not exist"
  defp format_dns_error(:servfail),    do: "SERVFAIL — server returned failure"
  defp format_dns_error(:timeout),     do: "query timed out"
  defp format_dns_error(:refused),     do: "query refused by server"
  defp format_dns_error({:noquery, reason}), do: "no answer: #{inspect(reason)}"
  defp format_dns_error(reason),       do: inspect(reason)

  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