/**
 * Talks to other gerbeur instances for federated sign-in: it reads their
 * metadata and public keys, trades a code for an identity token, delivers
 * events, and fetches a user's avatar. `federation-service.ts`,
 * `identity-provider-service.ts` and `federation-peer-service.ts` reach other
 * instances only through it.
 *
 * An identity is only as safe as the connection it came over, so unlike
 * `fetch.ts` it never falls back to skipping certificate checks, and follows
 * no redirect. It reaches only https instances on public addresses, unless
 * `GERBEUR_FEDERATION_ALLOW_PRIVATE` lets two instances on one machine talk.
 *
 * @module
 */

import type { JSONWebKeySet } from "@panva/jose";
import {
  type InstanceMetadata,
  isInstanceMetadata,
} from "../../shared/federation.ts";
import {
  FEDERATION_ALLOW_PRIVATE,
  MAX_IMAGE_SIZE_BYTES,
  OG_SITE_NAME,
} from "../config.ts";
import { resolvePublicAddress } from "./public-address.ts";
import { readBytesCapped, readTextCapped } from "./fetch.ts";
import { TtlCache } from "../lib/ttl-cache.ts";

/** How long a request to another instance may take, body included. */
const TIMEOUT_MS = 8000;

/** Caps a metadata document, a key set or a token response. */
const MAX_JSON_BYTES = 64 * 1024;

const USER_AGENT = `gerbeur (${OG_SITE_NAME}; federation)`;

/**
 * Keeps each instance's metadata and keys for 10 minutes, so that a sign-in
 * or an event doesn't fetch them each time. A key set is refetched sooner
 * when a token names a key it lacks ({@link fetchKeys} with `fresh`), at most
 * once a minute: anyone can send a token naming a key that doesn't exist.
 */
const metadataCache = new TtlCache<InstanceMetadata>(10 * 60_000, 500);
const keysCache = new TtlCache<JSONWebKeySet>(10 * 60_000, 500);
const keysRefetched = new TtlCache<true>(60_000, 500);

/**
 * Checks that `url` may be reached: https on a public address, or anything
 * under `GERBEUR_FEDERATION_ALLOW_PRIVATE`.
 *
 * @throws {Error} When it may not.
 */
export async function assertReachable(url: URL): Promise<void> {
  if (FEDERATION_ALLOW_PRIVATE) {
    if (url.protocol !== "https:" && url.protocol !== "http:") {
      throw new Error(`Unsupported protocol ${url.protocol}`);
    }
    return;
  }
  if (url.protocol !== "https:") {
    throw new Error(`Refusing to reach ${url.origin} without https`);
  }
  await resolvePublicAddress(url);
}

/**
 * Makes one request to another instance, after {@link assertReachable}, with
 * the deadline covering the body. The caller reads or cancels the body.
 */
async function request(url: string, init: RequestInit = {}): Promise<Response> {
  const target = new URL(url);
  await assertReachable(target);
  return await fetch(target, {
    ...init,
    redirect: "error",
    signal: AbortSignal.timeout(TIMEOUT_MS),
    headers: {
      "User-Agent": USER_AGENT,
      "Accept": "application/json",
      ...init.headers,
    },
  });
}

/**
 * Returns the JSON body of `res`, up to {@link MAX_JSON_BYTES}.
 *
 * @throws {Error} When the status isn't a success or the body isn't JSON.
 */
async function readJson(res: Response, what: string): Promise<unknown> {
  if (!res.ok) {
    await res.body?.cancel();
    throw new Error(`${what} answered ${res.status}`);
  }
  return JSON.parse(await readTextCapped(res, MAX_JSON_BYTES));
}

/**
 * Returns the metadata of the instance at `origin`, from its
 * `/.well-known/openid-configuration`, cached. It must name `origin` as its
 * issuer and speak this protocol's version.
 *
 * @throws {Error} When it can't be reached or isn't a gerbeur instance.
 */
export async function fetchMetadata(origin: string): Promise<InstanceMetadata> {
  const hit = metadataCache.get(origin);
  if (hit) return hit;
  const body = await readJson(
    await request(`${origin}/.well-known/openid-configuration`),
    origin,
  );
  if (!isInstanceMetadata(body)) {
    throw new Error(`${origin} is not a gerbeur instance this one can trust`);
  }
  if (body.issuer !== origin) {
    throw new Error(`${origin} names itself ${body.issuer}`);
  }
  return metadataCache.set(origin, body);
}

/**
 * Returns the public keys of the instance at `origin`, cached unless `fresh`
 * asks to refetch them and they weren't in the last minute.
 *
 * @throws {Error} When they can't be fetched.
 */
export async function fetchKeys(
  origin: string,
  fresh = false,
): Promise<JSONWebKeySet> {
  const refetch = fresh && !keysRefetched.get(origin);
  const hit = refetch ? undefined : keysCache.get(origin);
  if (hit) return hit;
  if (refetch) keysRefetched.set(origin, true);
  const { jwks_uri } = await fetchMetadata(origin);
  const body = await readJson(await request(jwks_uri), `${origin}'s keys`);
  if (
    !body || typeof body !== "object" ||
    !Array.isArray((body as { keys?: unknown }).keys)
  ) {
    throw new Error(`${origin}'s keys are not a key set`);
  }
  return keysCache.set(origin, body as JSONWebKeySet);
}

/**
 * Trades `code` for an identity token at the token endpoint of the instance
 * at `origin`, and returns the token, unverified.
 *
 * @throws {Error} When the instance refuses or answers no token.
 */
export async function exchangeCode(
  origin: string,
  params: {
    code: string;
    clientId: string;
    redirectUri: string;
    codeVerifier: string;
  },
): Promise<string> {
  const { token_endpoint } = await fetchMetadata(origin);
  const body = await readJson(
    await request(token_endpoint, {
      method: "POST",
      headers: { "Content-Type": "application/x-www-form-urlencoded" },
      body: new URLSearchParams({
        grant_type: "authorization_code",
        code: params.code,
        client_id: params.clientId,
        redirect_uri: params.redirectUri,
        code_verifier: params.codeVerifier,
      }),
    }),
    `${origin}'s token endpoint`,
  );
  const idToken = (body as { id_token?: unknown } | null)?.id_token;
  if (typeof idToken !== "string") {
    throw new Error(`${origin} answered no identity token`);
  }
  return idToken;
}

/**
 * Sends the signed event `event` to the events endpoint of the instance at
 * `origin`, and returns whether it took it. A 4xx other than 429 means it never
 * will, which the caller treats as delivered.
 *
 * @throws {Error} When it can't be reached, or answers 429 or a 5xx.
 */
export async function postEvent(
  origin: string,
  event: string,
): Promise<boolean> {
  const { gerbeur_events_endpoint } = await fetchMetadata(origin);
  const res = await request(gerbeur_events_endpoint, {
    method: "POST",
    headers: { "Content-Type": "application/json" },
    body: JSON.stringify({ event }),
  });
  await res.body?.cancel();
  if (res.ok) return true;
  if (res.status === 429 || res.status >= 500) {
    throw new Error(`${origin}'s events endpoint answered ${res.status}`);
  }
  return false;
}

/**
 * Returns the image at `url`, a user's avatar on their home instance, up to
 * the size an avatar may be.
 *
 * @throws {Error} When it can't be fetched.
 */
export async function fetchAvatar(url: string): Promise<Uint8Array> {
  const res = await request(url, { headers: { "Accept": "image/*" } });
  if (!res.ok) {
    await res.body?.cancel();
    throw new Error(`The avatar at ${url} answered ${res.status}`);
  }
  return await readBytesCapped(res, MAX_IMAGE_SIZE_BYTES + 1);
}

/** Empties the caches, for the tests' reset between cases. */
export function clearInstanceCaches(): void {
  metadataCache.clear();
  keysCache.clear();
  keysRefetched.clear();
}