/**
 * Plays the home instance in federated sign-in: it lets this instance's users
 * sign in to other gerbeur instances with their account here.
 *
 * The flow is OpenID Connect's authorization code with PKCE, in three steps.
 *
 * 1. Another instance, the client, sends the user to the `/authorize` page
 *    with a request. The page shows who asks ({@link describeClient}) and,
 *    once the user consents, gets a code ({@link createAuthorizationCode}),
 *    which it carries back to the client's callback page.
 * 2. The client trades the code for an identity token, server to server
 *    ({@link exchangeCode}): this user's name, email and avatar, signed with
 *    the instance key.
 * 3. While the user's grant lasts, the client hears of their changes: a new
 *    name, email or avatar, and the end of their sessions after a password
 *    change. {@link notifyClients} queues the signed events in
 *    `federation_outbox`, and {@link outboxJob} delivers them.
 *
 * Only a local account signs in elsewhere: a user who came from another
 * instance signs in from there. `routes/oauth.ts`, `routes/well-known.ts` and
 * the routes that change a user call it.
 *
 * @module
 */

import { createHash, randomBytes } from "node:crypto";
import { APIException } from "../lib/api-exception.ts";
import { db, transaction } from "../db/connection.ts";
import { FRONTEND_URL, OG_SITE_NAME, PUBLIC_URL } from "../config.ts";
import { signInstanceToken } from "../lib/jwt.ts";
import type { Job } from "../lib/scheduler.ts";
import {
  type AuthorizeRequest,
  FEDERATION_VERSION,
  type FederationEvent,
  type FederationGrant,
  type IdentityClaims,
  instanceHost,
  type InstanceMetadata,
  type OAuthClient,
  parseInstance,
} from "../../shared/federation.ts";
import { fetchMetadata, postEvent } from "../remote/gerbeur-instance.ts";
import { isRemoteUser, type User } from "../model/user.ts";
import { getSigningKey } from "./instance-key-service.ts";
import { getUserById } from "./user-service.ts";
import { getPeerStatus } from "./federation-peer-service.ts";

/**
 * The instance's name in every token it signs and every token addressed to
 * it: its public URL, which must be an origin for other instances to find
 * its metadata.
 */
export const ISSUER: string = PUBLIC_URL;

/**
 * The client's page a home instance sends the user back to, on the frontend.
 * Its metadata announces it, and a home accepts no other.
 */
export const REDIRECT_URI = `${FRONTEND_URL}/federation/callback`;

/** How long a code may wait to be traded, from the consent. */
const CODE_TTL_MS = 60_000;

/**
 * How long an identity token is valid: enough to cross from one server to the
 * other, never enough to be replayed later.
 */
const ID_TOKEN_TTL = "5m";

/**
 * How long an event is valid, and so how long the outbox keeps trying to
 * deliver it before giving up.
 */
const EVENT_TTL_MS = 3 * 86_400_000;

/** The longest wait between two tries of an event. */
const MAX_RETRY_MS = 6 * 3_600_000;

/**
 * Returns the instance's metadata, served at
 * `/.well-known/openid-configuration`.
 */
export function getInstanceMetadata(): InstanceMetadata {
  return {
    issuer: ISSUER,
    authorization_endpoint: `${FRONTEND_URL}/authorize`,
    token_endpoint: `${PUBLIC_URL}/api/oauth/tokens`,
    jwks_uri: `${PUBLIC_URL}/.well-known/jwks.json`,
    response_types_supported: ["code"],
    grant_types_supported: ["authorization_code"],
    subject_types_supported: ["public"],
    id_token_signing_alg_values_supported: ["EdDSA"],
    code_challenge_methods_supported: ["S256"],
    scopes_supported: ["openid", "profile", "email"],
    gerbeur_federation: FEDERATION_VERSION,
    gerbeur_site_name: OG_SITE_NAME,
    gerbeur_redirect_uri: REDIRECT_URI,
    gerbeur_events_endpoint: `${PUBLIC_URL}/api/federation/events`,
  };
}

/** Returns the SHA-256 of `value`, as base64url. */
function sha256(value: string): string {
  return createHash("sha256").update(value).digest("base64url");
}

/**
 * Returns what this instance tells another of `user`: their name, email and
 * avatar, whose URL changes with each new one.
 */
export function identityClaims(user: User): IdentityClaims {
  return {
    preferred_username: user.username,
    email: user.email,
    ...(user.avatarMime && {
      picture: `${PUBLIC_URL}/api/users/${user.id}/avatar?v=${
        user.updatedAt?.getTime() ?? 0
      }`,
    }),
  };
}

/**
 * Returns the client `clientId` names, checked: an instance other than this
 * one, not blocked here, answering as a gerbeur. `user` must have a local
 * account, which alone may sign in elsewhere.
 *
 * @throws {APIException} 400 when `clientId` isn't an instance's address, is
 * this instance, or `user` comes from another instance; 403 when the client
 * is blocked here; 502 when it doesn't answer as a gerbeur.
 */
async function checkClient(
  user: User,
  clientId: string,
): Promise<{ clientId: string; metadata: InstanceMetadata }> {
  if (isRemoteUser(user)) {
    throw new APIException(
      400,
      "This account signs in from the instance it comes from",
    );
  }
  if (parseInstance(clientId) !== clientId || clientId === ISSUER) {
    throw new APIException(400, "Invalid client");
  }
  if (getPeerStatus(clientId) === "blocked") {
    throw new APIException(403, "This instance is blocked here");
  }
  try {
    return { clientId, metadata: await fetchMetadata(clientId) };
  } catch (err) {
    console.error(`[federation] client ${clientId} unreachable:`, err);
    throw new APIException(502, "That instance doesn't answer as a gerbeur");
  }
}

/**
 * Returns the instance `clientId` names, for the `/authorize` page's question
 * to user `userId`, and whether they already let it sign them in.
 *
 * @throws {APIException} As {@link checkClient}.
 */
export async function describeClient(
  userId: string,
  clientId: string,
): Promise<OAuthClient> {
  const { metadata } = await checkClient(getUserById(userId), clientId);
  const granted = !!db.prepare(
    `SELECT 1 FROM federation_grants WHERE user_id = ? AND client_id = ?;`,
  ).get(userId, clientId);
  return {
    clientId,
    host: instanceHost(clientId),
    name: metadata.gerbeur_site_name,
    granted,
  };
}

/**
 * Records user `userId`'s consent to sign in to the client of `request`, and
 * returns where to send them: the client's callback page with a code, which
 * the client trades within a minute ({@link exchangeCode}). The grant lasts
 * until they withdraw it, so the page asks no more.
 *
 * @throws {APIException} 400 when the request is malformed or its
 * `redirectUri` isn't the client's own; otherwise as {@link checkClient}.
 */
export async function createAuthorizationCode(
  userId: string,
  request: AuthorizeRequest,
): Promise<string> {
  const user = getUserById(userId);
  const { clientId, metadata } = await checkClient(user, request.clientId);
  if (request.redirectUri !== metadata.gerbeur_redirect_uri) {
    throw new APIException(400, "Invalid redirect URI");
  }
  if (
    request.codeChallengeMethod !== "S256" ||
    !/^[A-Za-z0-9_-]{43}$/.test(request.codeChallenge) ||
    !request.state || request.state.length > 512 ||
    !request.nonce || request.nonce.length > 512
  ) {
    throw new APIException(400, "Invalid request");
  }

  const code = randomBytes(32).toString("base64url");
  const now = new Date();
  transaction(() => {
    db.prepare(`DELETE FROM oauth_codes WHERE expires_at < ?;`).run(
      now.toISOString(),
    );
    db.prepare(
      `INSERT INTO oauth_codes (code_hash, user_id, client_id, redirect_uri, code_challenge, nonce, expires_at)
         VALUES (?, ?, ?, ?, ?, ?, ?);`,
    ).run(
      sha256(code),
      userId,
      clientId,
      request.redirectUri,
      request.codeChallenge,
      request.nonce,
      new Date(now.getTime() + CODE_TTL_MS).toISOString(),
    );
    db.prepare(
      `INSERT INTO federation_grants (user_id, client_id, client_name, granted_at, last_used_at)
         VALUES (?, ?, ?, ?, ?)
       ON CONFLICT (user_id, client_id) DO UPDATE SET client_name = excluded.client_name;`,
    ).run(
      userId,
      clientId,
      metadata.gerbeur_site_name,
      now.toISOString(),
      now.toISOString(),
    );
  });

  const redirect = new URL(request.redirectUri);
  redirect.searchParams.set("code", code);
  redirect.searchParams.set("state", request.state);
  // RFC 9207: tells the client which instance answered, against mix-ups.
  redirect.searchParams.set("iss", ISSUER);
  return redirect.href;
}

/**
 * Trades a code for an identity token, for `POST /api/oauth/tokens`, which
 * the client calls server to server with the form `params`. A code works once,
 * within a minute, for the client and redirect it was made for, and with the
 * verifier whose hash the request carried (PKCE). The token, signed with the
 * instance key and addressed to the client, carries {@link identityClaims}
 * and the request's nonce.
 *
 * @throws {APIException} 400 when anything doesn't match.
 */
export async function exchangeCode(
  params: URLSearchParams,
): Promise<{ id_token: string; token_type: string }> {
  const code = params.get("code");
  const clientId = params.get("client_id");
  const verifier = params.get("code_verifier");
  if (
    params.get("grant_type") !== "authorization_code" || !code || !clientId ||
    !verifier || !/^[A-Za-z0-9._~-]{43,128}$/.test(verifier)
  ) {
    throw new APIException(400, "invalid_request");
  }

  // Taken in the same statement that finds it, so it can't be traded twice.
  const row = db.prepare(
    `DELETE FROM oauth_codes WHERE code_hash = ?
     RETURNING user_id, client_id, redirect_uri, code_challenge, nonce, expires_at;`,
  ).get(sha256(code)) as
    | {
      user_id: string;
      client_id: string;
      redirect_uri: string;
      code_challenge: string;
      nonce: string;
      expires_at: string;
    }
    | undefined;
  if (
    !row || row.expires_at < new Date().toISOString() ||
    row.client_id !== clientId ||
    row.redirect_uri !== params.get("redirect_uri") ||
    row.code_challenge !== sha256(verifier)
  ) {
    throw new APIException(400, "invalid_grant");
  }

  const user = getUserById(row.user_id);
  if (isRemoteUser(user)) throw new APIException(400, "invalid_grant");
  db.prepare(
    `UPDATE federation_grants SET last_used_at = ? WHERE user_id = ? AND client_id = ?;`,
  ).run(new Date().toISOString(), user.id, clientId);

  const idToken = await signInstanceToken(
    { ...identityClaims(user), nonce: row.nonce },
    await getSigningKey(),
    { issuer: ISSUER, audience: clientId, subject: user.id },
    ID_TOKEN_TTL,
  );
  return { id_token: idToken, token_type: "Bearer" };
}

interface GrantRow {
  client_id: string;
  client_name: string | null;
  granted_at: string;
  last_used_at: string;
}

/**
 * Returns the instances user `userId` let their account sign in to, the most
 * recently used first, for the settings page.
 */
export function listGrants(userId: string): FederationGrant[] {
  const rows = db.prepare(
    `SELECT client_id, client_name, granted_at, last_used_at FROM federation_grants
      WHERE user_id = ? ORDER BY last_used_at DESC;`,
  ).all(userId) as unknown as GrantRow[];
  return rows.map((r) => ({
    clientId: r.client_id,
    host: instanceHost(r.client_id),
    name: r.client_name,
    grantedAt: new Date(r.granted_at),
    lastUsedAt: new Date(r.last_used_at),
  }));
}

/**
 * Withdraws user `userId`'s grant to the instance whose host is `host`: it
 * stops answering for them there, and is told to end their sessions.
 *
 * @throws {APIException} 404 when they gave that instance no grant.
 */
export async function revokeGrant(userId: string, host: string): Promise<void> {
  const grant = listGrants(userId).find((g) => g.host === host.toLowerCase());
  if (!grant) throw new APIException(404, "Grant not found");
  await notifyClients(userId, { type: "revoke" }, [grant.clientId]);
  db.prepare(
    `DELETE FROM federation_grants WHERE user_id = ? AND client_id = ?;`,
  ).run(userId, grant.clientId);
}

/**
 * Tells each instance user `userId` signed in to of `event`, or only those of
 * `clientIds`: `update` when their name, email or avatar changed, `revoke`
 * when their sessions there must end. The routes that change a user call it,
 * and for an `update`, `event` may be just the type: the claims are read now.
 * The events are queued in the outbox and their delivery starts at once.
 */
export async function notifyClients(
  userId: string,
  event: FederationEvent | { type: "update" },
  clientIds?: string[],
): Promise<void> {
  const clients = clientIds ?? listGrants(userId).map((g) => g.clientId);
  if (clients.length === 0) return;
  const payload: FederationEvent = event.type === "update"
    ? { type: "update", claims: identityClaims(getUserById(userId)) }
    : event;

  const key = await getSigningKey();
  const now = new Date();
  for (const clientId of clients) {
    const token = await signInstanceToken(
      { ...payload },
      key,
      { issuer: ISSUER, audience: clientId, subject: userId },
      `${EVENT_TTL_MS / 1000}s`,
    );
    db.prepare(
      `INSERT INTO federation_outbox (id, client_id, event, next_attempt_at, created_at)
         VALUES (?, ?, ?, ?, ?);`,
    ).run(
      crypto.randomUUID(),
      clientId,
      token,
      now.toISOString(),
      now.toISOString(),
    );
  }
  deliverOutbox().catch((err) =>
    console.error("[federation] delivery failed:", err)
  );
}

/** Set while {@link deliverOutbox} runs, so that two runs don't overlap. */
let delivering = false;

/**
 * Sends each due event of the outbox to its instance. One it took, or refused
 * for good, leaves the outbox; one that failed waits twice as long as the time
 * before, up to 6 hours, until it expires after 3 days.
 */
export async function deliverOutbox(): Promise<void> {
  if (delivering) return;
  delivering = true;
  try {
    const now = new Date();
    db.prepare(`DELETE FROM federation_outbox WHERE created_at < ?;`).run(
      new Date(now.getTime() - EVENT_TTL_MS).toISOString(),
    );
    const due = db.prepare(
      `SELECT id, client_id, event, attempts FROM federation_outbox
        WHERE next_attempt_at <= ? ORDER BY created_at LIMIT 100;`,
    ).all(now.toISOString()) as {
      id: string;
      client_id: string;
      event: string;
      attempts: number;
    }[];
    for (const row of due) {
      try {
        if (!await postEvent(row.client_id, row.event)) {
          console.error(`[federation] ${row.client_id} refused an event`);
        }
        db.prepare(`DELETE FROM federation_outbox WHERE id = ?;`).run(row.id);
      } catch (err) {
        const wait = Math.min(60_000 * 2 ** row.attempts, MAX_RETRY_MS);
        db.prepare(
          `UPDATE federation_outbox SET attempts = attempts + 1, next_attempt_at = ? WHERE id = ?;`,
        ).run(new Date(Date.now() + wait).toISOString(), row.id);
        console.error(`[federation] ${row.client_id} unreachable:`, err);
      }
    }
  } finally {
    delivering = false;
  }
}

/** Retries the outbox's failed events every minute. */
export const outboxJob: Job = {
  name: "federation",
  everyMs: 60_000,
  run: deliverOutbox,
};