defmodule Accrue.Webhook.Plug do
  @moduledoc """
  Core webhook ingestion plug.

  Processes incoming webhook requests by:

  1. Extracting the raw body from `conn.assigns[:raw_body]` (populated
     by `Accrue.Webhook.CachingBodyReader` in the pipeline)
  2. Verifying the `Stripe-Signature` header via `Accrue.Webhook.Signature`
  3. Projecting the verified event into `%Accrue.Webhook.Event{}`
  4. Storing the verified event and raw body in `conn.private` for
     downstream processing

  Signature failures raise `Accrue.SignatureError`, rescued to HTTP 400.

  ## Transactional pipeline

  After verification, `Accrue.Webhook.Ingest.run/4` atomically persists the
  webhook event, enqueues an Oban dispatch job, and records an accrue_events
  ledger entry -- all in a single `Ecto.Multi` transaction.
  """

  @behaviour Plug

  import Plug.Conn

  require Logger

  alias Accrue.Webhook.Signature

  @impl true
  def init(opts), do: opts

  @impl true
  def call(conn, opts) do
    processor = Keyword.fetch!(opts, :processor)
    endpoint = Keyword.get(opts, :endpoint)

    :telemetry.span(
      [:accrue, :webhook, :receive],
      %{processor: processor, endpoint: endpoint},
      fn ->
        result = do_call(conn, processor, endpoint)
        {result, %{processor: processor, endpoint: endpoint}}
      end
    )
  rescue
    e in Accrue.SignatureError ->
      Logger.warning("Webhook signature verification failed: #{e.reason}")

      conn
      |> send_resp(400, Jason.encode!(%{error: "signature_verification_failed"}))
      |> halt()

    e in Accrue.ConfigError ->
      Logger.error("Webhook setup error:\n" <> Exception.message(e))

      conn
      |> send_resp(500, Jason.encode!(%{error: "internal_server_error"}))
      |> halt()
  end

  defp do_call(conn, :stripe, endpoint) do
    raw_body = flatten_raw_body(conn)
    sig_header = get_req_header(conn, "stripe-signature") |> List.first()

    unless sig_header do
      raise Accrue.SignatureError, reason: "missing stripe-signature header"
    end

    secrets = resolve_secrets!(endpoint, :stripe)
    stripe_event = Signature.verify!(raw_body, sig_header, secrets)

    # Transactional persist + Oban enqueue.
    Accrue.Webhook.Ingest.run(conn, :stripe, stripe_event, raw_body, endpoint)
  end

  defp do_call(conn, :braintree, endpoint) do
    conn = Plug.Conn.fetch_query_params(conn)

    bt_signature = conn.params["bt_signature"]
    bt_payload = conn.params["bt_payload"]

    unless bt_signature do
      raise Accrue.SignatureError, reason: "missing bt_signature"
    end

    unless bt_payload do
      raise Accrue.SignatureError, reason: "missing bt_payload"
    end

    braintree_event = Signature.parse_braintree!(bt_signature, bt_payload)

    # Braintree webhooks lack a unique event ID, so we hash the payload for idempotency.
    # We construct a %LatticeStripe.Event{} simply to satisfy the Ingest.run signature and DB constraints.
    event_id = "bt_" <> Base.encode16(:crypto.hash(:sha256, bt_payload), case: :lower)
    kind = braintree_event["kind"] || "unknown"

    livemode = Braintree.get_env(:environment) == :production

    subject = braintree_event["subject"] || %{}
    subject_entity = subject |> Map.values() |> Enum.find(&is_map/1) || %{}
    object_id = subject_entity["id"]
    object_data = if object_id, do: Map.put(subject, "id", object_id), else: subject

    pseudo_event = %LatticeStripe.Event{
      id: event_id,
      type: kind,
      livemode: livemode,
      data: %{"object" => object_data}
    }

    Accrue.Webhook.Ingest.run(conn, :braintree, pseudo_event, bt_payload, endpoint)
  end

  # Endpoint-aware secret resolution.
  #
  # Multi-endpoint mode: when `endpoint:` is passed via the plug init opts,
  # look up `[:webhook_endpoints, endpoint, :secret]` from `Accrue.Config`.
  # If the endpoint is missing, raise `Accrue.SignatureError` (rescued to 400)
  # — fail closed, never bypass verification.
  #
  # Legacy mode: when no `endpoint:` is set, fall back to
  # `Accrue.Config.webhook_signing_secrets/1` keyed by processor atom.
  defp resolve_secrets!(nil, processor), do: Accrue.Config.webhook_signing_secrets(processor)

  defp resolve_secrets!(endpoint, _processor) when is_atom(endpoint) do
    endpoints = Accrue.Config.webhook_endpoints()

    case Keyword.get(endpoints, endpoint) do
      nil ->
        raise Accrue.SignatureError,
          reason: "no webhook_endpoints config for endpoint #{inspect(endpoint)}"

      cfg when is_list(cfg) ->
        case Keyword.fetch(cfg, :secret) do
          {:ok, secret} when is_binary(secret) and secret != "" ->
            secret

          {:ok, secrets} when is_list(secrets) and secrets != [] ->
            secrets

          _ ->
            raise Accrue.SignatureError,
              reason: "no :secret in webhook_endpoints[#{inspect(endpoint)}]"
        end
    end
  end

  @doc false
  def flatten_raw_body(conn) do
    case conn.assigns[:raw_body] do
      nil ->
        diagnostic =
          Accrue.SetupDiagnostic.webhook_raw_body(
            details:
              "Expected conn.assigns[:raw_body]; configure body_reader: {Accrue.Webhook.CachingBodyReader, :read_body, []}"
          )

        raise Accrue.ConfigError, key: :webhook_signing_secrets, diagnostic: diagnostic

      chunks when is_list(chunks) ->
        chunks |> Enum.reverse() |> IO.iodata_to_binary()

      binary when is_binary(binary) ->
        binary
    end
  end
end