/**
 * Talking to the fofos server.
 *
 * A thin client on purpose. fofos's `/api/admin` already answers in the shape a panel wants — every
 * route returns `{ data: … }`, and a mutation returns the whole section it belongs to — so there is
 * nothing here to reshape. What this owns is the credential, the failure vocabulary, and the one
 * decision about what a non-answer means.
 *
 * See `docs/SURFACES.md` in `fofos/platform` for the routes.
 */

export type FofosCredentials = { url: string; token: string };

/** No trailing slash, so joining a path never produces a double one. */
export const normaliseUrl = (url: string): string => url.trim().replace(/\/+$/, '');

/**
 * A failure with the two things a panel needs to say something useful: whether fofos answered at all,
 * and whether it refused the credential.
 *
 * Those are genuinely different problems — one is "the server is down or the URL is wrong", the other
 * is "the token is wrong" — and a panel that cannot tell them apart can only say "something went
 * wrong", which sends someone to check the wrong half.
 */
export class FofosError extends Error {
  constructor(
    message: string,
    readonly status: number | null,
    /** fofos refused the credential. `status` is 401. */
    readonly unauthorised = false,
  ) {
    super(message);
    this.name = 'FofosError';
  }
}

/** `null` status means the request never got an answer — DNS, refused connection, timeout. */
const unreachable = (url: string, cause: unknown) =>
  new FofosError(
    `Could not reach fofos at ${url}. ${cause instanceof Error ? cause.message : String(cause)}`,
    null,
  );

type RequestOptions = {
  method?: string;
  path: string;
  search?: string;
  /** Already-serialised. Forwarded as given so what fofos receives is what the caller sent. */
  body?: string | FormData | null;
  contentType?: string | null;
};

export type FofosResponse = { status: number; body: string; contentType: string | null };

/**
 * One request, returning fofos's own answer verbatim — including the bad ones.
 *
 * It does NOT throw on a 4xx or 5xx. fofos's error envelope is more informative than anything this
 * could substitute (it names the field), and the caller is forwarding to a panel that should see what
 * the server actually said. Only a request that got no answer at all throws, because there is nothing
 * to forward.
 */
export async function fofosRequest(
  credentials: FofosCredentials,
  options: RequestOptions,
): Promise<FofosResponse> {
  const url = `${normaliseUrl(credentials.url)}/api/admin${options.path}${options.search ?? ''}`;

  const headers: Record<string, string> = { Authorization: `Bearer ${credentials.token}` };
  // Left unset for FormData: fetch has to write its own, including the multipart boundary, and a
  // hand-set Content-Type here produces a body the other end cannot parse.
  if (options.contentType) headers['Content-Type'] = options.contentType;

  let response: Response;
  try {
    response = await fetch(url, {
      method: options.method ?? 'GET',
      headers,
      body: options.body ?? undefined,
      // fofos is a small server on the same network and every one of these is a database read. A
      // request still hanging after this is not going to complete.
      signal: AbortSignal.timeout(30_000),
    });
  } catch (cause) {
    throw unreachable(normaliseUrl(credentials.url), cause);
  }

  return {
    status: response.status,
    body: await response.text(),
    contentType: response.headers.get('content-type'),
  };
}

export type ProbeResult = {
  reachable: boolean;
  authenticated: boolean;
  sections?: string[];
  error?: string;
};

/**
 * Is this URL a fofos server, and does it accept this token?
 *
 * Both questions at once, because a person setting this up has one form and should not have to work
 * out which half is wrong. `GET /api/admin` sits behind the guard for exactly this reason: a 200 is
 * both answers, and a 401 separates them.
 */
export async function probe(credentials: FofosCredentials): Promise<ProbeResult> {
  let response: FofosResponse;
  try {
    response = await fofosRequest(credentials, { path: '' });
  } catch (error) {
    return { reachable: false, authenticated: false, error: error instanceof Error ? error.message : String(error) };
  }

  if (response.status === 401) {
    return { reachable: true, authenticated: false, error: 'fofos refused that token.' };
  }

  if (response.status !== 200) {
    return {
      reachable: true,
      authenticated: false,
      // A 404 here is the interesting one: something answered, but it is not serving /api/admin. That
      // is usually a URL pointing at the built static site rather than at the live server.
      error:
        response.status === 404
          ? 'That address answered, but it is not serving /api/admin. Is it pointing at the static site rather than the fofos server?'
          : `fofos answered ${response.status}.`,
    };
  }

  try {
    const parsed = JSON.parse(response.body) as { data?: { service?: string; sections?: string[] } };
    if (parsed.data?.service !== 'fofos') {
      return { reachable: true, authenticated: false, error: 'That address answered, but it is not fofos.' };
    }
    return { reachable: true, authenticated: true, sections: parsed.data.sections ?? [] };
  } catch {
    return { reachable: true, authenticated: false, error: 'That address answered with something that is not JSON.' };
  }
}