/**
 * Plays the client in federated sign-in: it lets users of other gerbeur
 * instances sign in here with their account there, their home.
 *
 * 1. The login or register page names the user's home, and
 *    {@link startLogin} answers the home's `/authorize` page with a request:
 *    a random state, the hash of a PKCE verifier, and a nonce, kept in
 *    `federation_logins` for 10 minutes.
 * 2. The home sends the user back to the `/federation/callback` page with a
 *    code. {@link completeLogin} trades it for the home's identity token,
 *    checks its signature against the home's keys, and finds or creates the
 *    local account linked to it (`user_identities`), named `<name>@<host>`.
 * 3. The home's events ({@link receiveEvent}) keep the account's name, email
 *    and avatar in step, and end its sessions when the home says so.
 *
 * Whether a user of another instance may join is the admin's call
 * (`federation-peer-service.ts`): from a trusted instance they may, from a
 * blocked one they may not, and from any other only with an invite, which the
 * register page carries. None may while the deployment turns sign-ins from
 * other instances off (`FEDERATION.incoming`). Once here, an account works like any other, but its
 * password, email, avatar and name are its home's (`assertLocalUser`).
 *
 * @module
 */

import { decodeJwt, type JSONWebKeySet } from "@panva/jose";
import { createHash, randomBytes } from "node:crypto";
import { APIException } from "../lib/api-exception.ts";
import { FEDERATION } from "../config.ts";
import { db, transaction } from "../db/connection.ts";
import { verifyInstanceToken } from "../lib/jwt.ts";
import { type RateLimit, takeToken } from "../lib/rate-limit.ts";
import {
  type FederationLoginRequest,
  type FederationSessionRequest,
  type IdentityClaims,
  instanceHost,
  type InstanceMetadata,
  isFederationEvent,
  isIdentityClaims,
  parseInstance,
} from "../../shared/federation.ts";
import { EMAIL_RE, USERNAME_RE } from "../../shared/validation.ts";
import { toPublicUser, type User } from "../model/user.ts";
import {
  exchangeCode,
  fetchAvatar,
  fetchKeys,
  fetchMetadata,
} from "../remote/gerbeur-instance.ts";
import { ISSUER, REDIRECT_URI } from "./identity-provider-service.ts";
import { getPeerStatus } from "./federation-peer-service.ts";
import {
  claimInvite,
  releaseInvite,
  validateInvite,
} from "./invite-service.ts";
import {
  createRemoteUser,
  getUserById,
  revokeSessions,
  sendWelcomeEmail,
  setUserAvatar,
  setUsername,
  updateUser,
} from "./user-service.ts";
import { broadcastUserUpdated, updateClientUsername } from "./ws-service.ts";

/** How long a sign-in may take, from the login page to the callback. */
const LOGIN_TTL_MS = 10 * 60_000;

/**
 * Allows an instance a burst of 120 events, as when many of its users change
 * at once, then one a second. Each may fetch its keys, so a stranger posting
 * events in an instance's name can't make this one hammer it.
 */
const EVENT_LIMIT: RateLimit = { capacity: 120, refill: 1 };

/**
 * Refuses a sign-in from another instance while the deployment turns them off
 * (`FEDERATION.incoming`), as {@link startLogin} and {@link completeLogin}
 * must first.
 *
 * @throws {APIException} 403 when they are off.
 */
function assertIncoming(): void {
  if (!FEDERATION.incoming) {
    throw new APIException(
      403,
      "This instance doesn't take accounts from other instances",
    );
  }
}

/**
 * Returns the origin of the instance `instance` names, checked as a home: not
 * this instance, nor a blocked one.
 *
 * @throws {APIException} 400 when it names no instance or this one, 403 when
 * it is blocked.
 */
function homeOf(instance: string): string {
  const issuer = parseInstance(instance);
  if (!issuer) throw new APIException(400, "Not an instance's address");
  if (issuer === ISSUER) throw new APIException(400, "That is this instance");
  if (getPeerStatus(issuer) === "blocked") {
    throw new APIException(403, "This instance is blocked here");
  }
  return issuer;
}

/**
 * Starts a sign-in with an account on the instance `request.instance` names,
 * and returns its `/authorize` page with the request, for
 * `POST /api/federation/logins`. Unless that instance is trusted, or someone
 * here already comes from it, the request must carry a valid invite, which is
 * claimed only once the account is made; {@link completeLogin} makes no
 * account without one.
 *
 * @throws {APIException} 400 when the instance isn't one; 403 when sign-ins
 * from other instances are off, or it is blocked, neither trusted nor
 * invited, or keeps its users from signing in elsewhere; 404 or 409 when the
 * invite doesn't work; 502 when it doesn't answer as a gerbeur.
 */
export async function startLogin(
  request: FederationLoginRequest,
): Promise<string> {
  assertIncoming();
  const issuer = homeOf(request.instance);
  if (request.inviteToken) {
    validateInvite(request.inviteToken);
  } else if (
    getPeerStatus(issuer) !== "trusted" &&
    // Someone who joined with an invite signs in again without one; whether
    // this user is one of them is only known once their home answers.
    !db.prepare(`SELECT 1 FROM user_identities WHERE issuer = ? LIMIT 1;`)
      .get(issuer)
  ) {
    throw new APIException(
      403,
      "Users of this instance need an invite to join",
    );
  }

  let metadata: InstanceMetadata;
  try {
    metadata = await fetchMetadata(issuer);
  } catch (err) {
    console.error(`[federation] ${issuer} unreachable:`, err);
    throw new APIException(502, "That instance doesn't answer as a gerbeur");
  }
  if (metadata.gerbeur_outgoing === false) {
    throw new APIException(
      403,
      "Accounts on that instance don't sign in to other instances",
    );
  }

  const state = randomBytes(24).toString("base64url");
  const verifier = randomBytes(48).toString("base64url");
  const nonce = randomBytes(24).toString("base64url");
  const now = new Date();
  transaction(() => {
    db.prepare(`DELETE FROM federation_logins WHERE created_at < ?;`).run(
      new Date(now.getTime() - LOGIN_TTL_MS).toISOString(),
    );
    db.prepare(
      `INSERT INTO federation_logins (state, issuer, code_verifier, nonce, invite_token, created_at)
         VALUES (?, ?, ?, ?, ?, ?);`,
    ).run(
      state,
      issuer,
      verifier,
      nonce,
      request.inviteToken ?? null,
      now.toISOString(),
    );
  });

  const url = new URL(metadata.authorization_endpoint);
  url.search = new URLSearchParams({
    response_type: "code",
    scope: "openid profile email",
    client_id: ISSUER,
    redirect_uri: REDIRECT_URI,
    state,
    code_challenge: createHash("sha256").update(verifier).digest("base64url"),
    code_challenge_method: "S256",
    nonce,
  }).toString();
  return url.href;
}

/**
 * Returns the keys of the instance at `issuer`, refetched when `fresh` asks.
 *
 * @throws {APIException} 502 when they can't be fetched: the sender may try
 * again, which it wouldn't after a 4xx.
 */
async function keysOf(issuer: string, fresh: boolean): Promise<JSONWebKeySet> {
  try {
    return await fetchKeys(issuer, fresh);
  } catch (err) {
    console.error(`[federation] ${issuer}'s keys failed:`, err);
    throw new APIException(502, "That instance's keys can't be fetched");
  }
}

/**
 * Returns the payload of `token`, an identity token or an event from the
 * instance at `issuer`, once its signature checks against that instance's
 * keys and it is addressed to this one. The keys are refetched once when the
 * token names one they lack.
 *
 * @throws {APIException} 401 when it doesn't verify, 502 when the keys can't
 * be fetched.
 */
async function verifyFrom(
  token: string,
  issuer: string,
): Promise<Record<string, unknown>> {
  const claims = { issuer, audience: ISSUER };
  let failure: unknown;
  for (const fresh of [false, true]) {
    if (
      fresh &&
      (failure as { code?: string }).code !== "ERR_JWKS_NO_MATCHING_KEY"
    ) {
      break;
    }
    const keys = await keysOf(issuer, fresh);
    try {
      return await verifyInstanceToken(token, keys, claims);
    } catch (err) {
      failure = err;
    }
  }
  console.error(`[federation] a token from ${issuer} failed:`, failure);
  throw new APIException(401, "Invalid token");
}

/**
 * Returns `claims` from the instance at `issuer` once checked: a name a local
 * user could have, an email, and an avatar on that instance, so that it can't
 * make this server fetch from elsewhere. Returns null when they aren't.
 */
function checkClaims(claims: unknown, issuer: string): IdentityClaims | null {
  if (
    !isIdentityClaims(claims) ||
    !USERNAME_RE.test(claims.preferred_username) ||
    !EMAIL_RE.test(claims.email)
  ) {
    return null;
  }
  if (claims.picture !== undefined) {
    try {
      if (new URL(claims.picture).origin !== issuer) return null;
    } catch {
      return null;
    }
  }
  return claims;
}

/**
 * Finishes a sign-in begun by {@link startLogin}, for
 * `POST /api/federation/sessions`, and returns the account it signs in to:
 * the one linked to the identity, brought up to date, or a new one. A login
 * works once. A new account needs the home to be trusted, or the invite the
 * login was started with, claimed now.
 *
 * @throws {APIException} 400 when the login is unknown or expired; 401 when
 * the identity token doesn't verify; 403 when sign-ins from other instances
 * are off, or the account may not be created; 409 when its handle is
 * someone's; 502 when the home fails.
 */
export async function completeLogin(
  request: FederationSessionRequest,
): Promise<User> {
  assertIncoming();
  const login = db.prepare(
    `DELETE FROM federation_logins WHERE state = ?
     RETURNING issuer, code_verifier, nonce, invite_token, created_at;`,
  ).get(request.state) as
    | {
      issuer: string;
      code_verifier: string;
      nonce: string;
      invite_token: string | null;
      created_at: string;
    }
    | undefined;
  if (
    !login ||
    Date.now() - new Date(login.created_at).getTime() > LOGIN_TTL_MS
  ) {
    throw new APIException(400, "This sign-in has expired, start again");
  }
  const issuer = login.issuer;

  let idToken: string;
  try {
    idToken = await exchangeCode(issuer, {
      code: request.code,
      clientId: ISSUER,
      redirectUri: REDIRECT_URI,
      codeVerifier: login.code_verifier,
    });
  } catch (err) {
    console.error(`[federation] ${issuer} refused a code:`, err);
    throw new APIException(502, "The instance refused the sign-in");
  }
  const payload = await verifyFrom(idToken, issuer);
  if (payload.nonce !== login.nonce || typeof payload.sub !== "string") {
    throw new APIException(401, "Invalid token");
  }
  const claims = checkClaims(payload, issuer);
  if (!claims) {
    throw new APIException(502, "The instance sent an invalid identity");
  }
  const subject = payload.sub;

  const identity = db.prepare(
    `SELECT user_id FROM user_identities WHERE issuer = ? AND subject = ?;`,
  ).get(issuer, subject) as { user_id: string } | undefined;
  if (getPeerStatus(issuer) === "blocked") {
    throw new APIException(403, "This instance is blocked here");
  }
  if (identity) {
    return await applyClaims(identity.user_id, issuer, claims);
  }

  const trusted = getPeerStatus(issuer) === "trusted";
  if (!trusted && !login.invite_token) {
    throw new APIException(
      403,
      "Users of this instance need an invite to join",
    );
  }
  const inviterId = trusted ? null : claimInvite(login.invite_token!);
  let user: User;
  try {
    user = transaction(() => {
      const created = createRemoteUser({
        username: `${claims.preferred_username}@${instanceHost(issuer)}`,
        email: claims.email,
      }, inviterId);
      db.prepare(
        `INSERT INTO user_identities (user_id, issuer, subject, synced_at) VALUES (?, ?, ?, ?);`,
      ).run(created.id, issuer, subject, new Date().toISOString());
      return created;
    });
  } catch (err) {
    if (!trusted) releaseInvite(login.invite_token!);
    throw err;
  }
  sendWelcomeEmail(user);
  return await applyClaims(user.id, issuer, claims);
}

/**
 * Brings the account `userId`, from the instance at `issuer`, in step with
 * `claims` and returns it: a new name at home renames it, the old handle
 * staying its alias, and a new email or avatar replaces its own. A failed
 * avatar fetch is logged, and tried again at the next change or sign-in.
 *
 * @throws {APIException} 409 when the new handle is someone else's.
 */
async function applyClaims(
  userId: string,
  issuer: string,
  claims: IdentityClaims,
): Promise<User> {
  let user = getUserById(userId);
  const handle = `${claims.preferred_username}@${instanceHost(issuer)}`;
  if (handle !== user.username) {
    user = setUsername(userId, handle);
    updateClientUsername(userId, handle);
    broadcastUserUpdated(toPublicUser(user));
  }
  if (claims.email !== user.email) {
    user = await updateUser(userId, { email: claims.email });
  }

  const { picture } = db.prepare(
    `SELECT picture FROM user_identities WHERE user_id = ?;`,
  ).get(userId) as { picture: string | null };
  let synced = claims.picture ?? null;
  if (claims.picture && claims.picture !== picture) {
    try {
      user = await setUserAvatar(userId, await fetchAvatar(claims.picture));
    } catch (err) {
      console.error(`[federation] ${handle}'s avatar failed:`, err);
      synced = picture;
    }
  }
  db.prepare(
    `UPDATE user_identities SET picture = ?, synced_at = ? WHERE user_id = ?;`,
  ).run(synced, new Date().toISOString(), userId);
  return user;
}

/**
 * Applies the signed event `token` from a user's home, for
 * `POST /api/federation/events`: `update` brings their account in step,
 * `revoke` ends their sessions here. Only an instance some account here comes
 * from, and not blocked, may send one, about one of those accounts, and at
 * most {@link EVENT_LIMIT}'s rate. The sender gives up on an event at a 4xx
 * other than 429, and tries it again later otherwise.
 *
 * @throws {APIException} 400 when it isn't an event of a known instance about
 * one of its users; 401 when it doesn't verify; 403 when the instance is
 * blocked; 429 when it sends too many; 502 when its keys can't be fetched.
 */
export async function receiveEvent(token: string): Promise<void> {
  let issuer: unknown;
  try {
    issuer = decodeJwt(token).iss;
  } catch {
    throw new APIException(400, "Invalid event");
  }
  if (
    typeof issuer !== "string" ||
    !db.prepare(`SELECT 1 FROM user_identities WHERE issuer = ? LIMIT 1;`)
      .get(issuer)
  ) {
    throw new APIException(400, "Unknown instance");
  }
  if (getPeerStatus(issuer) === "blocked") {
    throw new APIException(403, "This instance is blocked here");
  }
  if (!takeToken(`federation-events:${issuer}`, EVENT_LIMIT)) {
    throw new APIException(429, "Too many events");
  }
  const payload = await verifyFrom(token, issuer);
  const identity = db.prepare(
    `SELECT user_id FROM user_identities WHERE issuer = ? AND subject = ?;`,
  ).get(issuer, String(payload.sub)) as { user_id: string } | undefined;
  if (!identity || !isFederationEvent(payload)) {
    throw new APIException(400, "Invalid event");
  }
  if (payload.type === "revoke") {
    revokeSessions(identity.user_id, new Date(), "Signed out by home");
    return;
  }
  const claims = checkClaims(payload.claims, issuer);
  if (!claims) throw new APIException(400, "Invalid event");
  await applyClaims(identity.user_id, issuer, claims);
}