defmodule AttestoPhoenix.Controller.DeviceVerificationController do
  @moduledoc """
  Device verification page (RFC 8628 §3.3).

  The user-facing leg of the device grant: the user opens
  `GET /oauth/device_verification` (optionally with `?user_code=...` from the
  `verification_uri_complete`), authenticates with the host, confirms the
  `user_code`, and approves or denies. This controller owns the protocol — it
  resolves and normalizes the `user_code`, drives the host login, and performs
  the atomic store transition (`Attesto.DeviceCode.approve/3` / `deny/2`) — while
  the host owns the HTML and the login UI through two callbacks:

    * `:authenticate_device_user` — `(conn -> {:ok, subject} | {:halt, conn})`.
      Establishes the resource owner (the host's session/login). `subject` is a
      map with `:subject` (the `sub`) and optional `:scope` / `:claims`
      (carrying e.g. `acr`/`auth_time` for step-up). `{:halt, conn}` lets the
      host take over the connection to render its login UI.
    * `:render_device_verification` — `(conn, view -> conn)`. Renders the page.
      `view` is a map `%{stage, user_code, pending}` where `stage` is
      `:prompt` (ask the user to enter/confirm the code — `pending` is the
      `Attesto.DeviceCodeStore.pending_view()` or `nil`), `:approved`,
      `:denied`, or `:invalid` (unknown/expired/already-decided code).

  ## No auto-approval (RFC 8628 §3.3.1 / §5.4)

  A `user_code` arriving via `verification_uri_complete` is only ever
  pre-filled — approval requires an explicit user POST carrying
  `decision=approve`. The controller never approves from a GET or from the URL
  alone, closing the one-click remote-phishing vector.

  ## Host responsibility: CSRF + session (REQUIRED)

  The approve/deny POST is a state-changing, session-authenticated action. The
  library performs the store transition but does NOT own the browser session or
  CSRF token, so the host MUST mount these routes behind a pipeline that
  enforces CSRF protection (`protect_from_forgery`) and a same-site session —
  otherwise a logged-in victim's browser can be made to POST an attacker's
  `user_code` and approve the attacker's device (a confused-deputy / device
  login CSRF). The controller additionally requires HTTPS (a credential-bearing
  authorization action must not cross a plain-HTTP hop).
  """

  use Phoenix.Controller, formats: [:html, :json]

  alias Attesto.DeviceCode
  alias AttestoPhoenix.{Callback, Config, RequestContext}

  @spec verify(Plug.Conn.t(), map()) :: Plug.Conn.t()
  def verify(conn, params) do
    config = resolve_config()

    with :ok <- require_enabled(config),
         :ok <- check_https(conn, config),
         {:ok, store} <- require_store(config),
         {:ok, conn, subject} <- authenticate(conn, config) do
      handle(conn, config, store, subject, params)
    else
      {:halt, conn} -> conn
      {:error, status, body} -> conn |> put_status(status) |> json(body)
    end
  end

  defp check_https(conn, config) do
    case RequestContext.check_https(conn, config) do
      :ok -> :ok
      {:error, :insecure_transport} -> {:error, 400, %{error: "invalid_request", error_description: "TLS required"}}
    end
  end

  # GET (or POST without a decision) shows the confirm prompt; a POST carrying an
  # explicit `decision` performs the approve/deny store transition. The user_code
  # is normalized in the core before any store lookup (fail-closed).
  defp handle(conn, config, store, subject, params) do
    user_code = string_param(params["user_code"])
    decision = string_param(params["decision"])

    case {conn.method, decision} do
      {"POST", "approve"} -> approve(conn, config, store, subject, user_code)
      {"POST", "deny"} -> deny(conn, config, store, user_code)
      _ -> prompt(conn, config, store, user_code)
    end
  end

  defp approve(conn, config, store, subject, user_code) do
    approval = %{
      subject: Map.get(subject, :subject) || Map.get(subject, :sub),
      # The granted scope is the subject's narrowed scope when the host
      # login/consent layer supplied one, otherwise the originally requested
      # scope bound to the device code — never broader than what was requested.
      scope: Map.get(subject, :scope) || pending_scope(store, user_code),
      claims: Map.get(subject, :claims, %{})
    }

    case DeviceCode.approve(store, user_code, approval) do
      :ok -> render_stage(conn, config, :approved, user_code, nil)
      _error -> render_stage(conn, config, :invalid, user_code, nil)
    end
  end

  defp deny(conn, config, store, user_code) do
    case DeviceCode.deny(store, user_code) do
      :ok -> render_stage(conn, config, :denied, user_code, nil)
      _error -> render_stage(conn, config, :invalid, user_code, nil)
    end
  end

  # No decision yet: show the confirm screen with the pending request the user is
  # about to authorize (or just the code-entry prompt when no/invalid code).
  defp prompt(conn, config, _store, nil), do: render_stage(conn, config, :prompt, nil, nil)

  defp prompt(conn, config, store, user_code) do
    case DeviceCode.lookup(store, user_code) do
      {:ok, %{status: :pending} = view} -> render_stage(conn, config, :prompt, user_code, view)
      {:ok, _decided} -> render_stage(conn, config, :invalid, user_code, nil)
      _ -> render_stage(conn, config, :invalid, user_code, nil)
    end
  end

  defp pending_scope(store, user_code) do
    case DeviceCode.lookup(store, user_code) do
      {:ok, %{scope: scope}} -> scope
      _ -> []
    end
  end

  defp authenticate(conn, config) do
    case config.authenticate_device_user do
      nil ->
        {:error, 500, %{error: "server_error", error_description: "device verification login is not configured"}}

      callback ->
        case Callback.invoke(callback, [conn]) do
          {:ok, subject} when is_map(subject) -> {:ok, conn, subject}
          {:halt, halted} -> {:halt, halted}
          _ -> {:error, 500, %{error: "server_error"}}
        end
    end
  end

  defp render_stage(conn, config, stage, user_code, pending) do
    view = %{stage: stage, user_code: user_code, pending: pending}

    case config.render_device_verification do
      nil -> conn |> put_status(200) |> json(default_body(view))
      callback -> Callback.invoke(callback, [conn, view])
    end
  end

  # When no host renderer is wired, fall back to a minimal JSON body so the
  # endpoint is still functional in tests / API-only deployments.
  defp default_body(%{stage: stage, user_code: user_code}), do: %{stage: stage, user_code: user_code}

  defp require_enabled(config) do
    if Config.device_authorization_enabled?(config),
      do: :ok,
      else: {:error, 404, %{error: "not_found"}}
  end

  defp require_store(config) do
    case Config.device_code_store(config) do
      store when is_atom(store) and not is_nil(store) -> {:ok, store}
      _ -> {:error, 500, %{error: "server_error", error_description: "device authorization is not configured"}}
    end
  end

  defp string_param(value) when is_binary(value) and value != "", do: value
  defp string_param(_value), do: nil

  defp resolve_config do
    otp_app = Application.get_env(:attesto_phoenix, :otp_app)
    Config.from_otp_app(otp_app, Config)
  end
end