fable/val Public Public: anyone can read this; nobody can change it.

/**

 * The shape of a served match: one world, always running while somebody is

 * connected, and the seats people sit in.

 *

 * Nothing here knows what the game is. The game hands over its state and a

 * handful of answers — how to step it, who is in it, what one viewer may see

 * and what a snapshot looks like — and this module keeps the rules every

 * served game needs and none should re-derive:

 *

 * - The idle gate: nobody watching, nothing happens. The world freezes when

 *   the last connection goes and carries on when someone arrives.

 * - A playing seat is rationed by address, not by cookie: a cookie costs one

 *   HTTP request, an address is the one thing a machine cannot mint two of.

 *   Both claims go in synchronously, before a socket is ever offered a seat.

 * - A connection with no seat watches: it is sent the public state and no

 *   position in it.

 * - Every snapshot is built for one viewer, from the bodies the game says

 *   that viewer can see. The client never learns a position it has no line

 *   of sight to.

 */

export const TICK = 1 / 30;

/**

 * How many ticks apart the snapshots go out.

 *

 * The simulation runs at 30Hz. A small room gets every tick: it is the case

 * that matters (one or two people playing each other), the payload is a

 * couple of kilobytes of JSON a seat, and the arena only runs at all while

 * somebody is connected. Above that the cost multiplies by seats and the

 * older cadence — every other tick — comes back.

 */

export const broadcastEvery = (filled: number): number => (filled <= 2 ? 1 : 2);

export type Seat = {

  playerId: string;

  token: string;

  address: string;

  send: (data: string) => void;

  /** The sticky part of the snapshot as this seat last received it, so it is only resent when it changes. */

  board: string;

};

/**

 * Why this connection is watching. The client says so in words, and the two

 * have different answers — open the tab that is playing, or let the page keep

 * its cookie — so which one it is travels with the snapshot.

 */

export type Why = "address" | "cookie" | "unsaid";

export type Command = { t: string; [key: string]: unknown };

/** What the arena needs of a body: a way to find it, address it, and tell a bot from a person. */

export type Body = { id: string; name: string; bot: boolean };

/** The game, as the arena sees it. */

export type Game<S, P extends Body> = {

  /** The one world. Mutated in place; the arena never replaces it. */

  state: S;

  players: (state: S) => P[];

  /** One fixed step of the world, bots and rules and all. */

  step: (state: S, dt: number) => void;

  /** Hand a bot's seat to a person: the same body, made human. Null when there is none. */

  sit: (state: S) => P | null;

  /** Give a seat back to a bot. */

  stand: (state: S, player: P) => void;

  /** May this viewer be sent this body? The server's answer, never the client's. */

  visible: (state: S, viewer: P, other: P) => boolean;

  /** The snapshot for one viewer, given exactly the bodies they may see. */

  snapshot: (state: S, viewer: P, seen: P[]) => Record<string, unknown>;

  /**

   * A field of the snapshot resent only when it differs from what the seat

   * last received; the client keeps the one it has. Null sends everything.

   */

  sticky: string | null;

  /** The public view for a connection with no seat. Must carry no position. */

  spectatorSnapshot: (state: S, why: Why) => Record<string, unknown>;

  /** Everything a client may ask for, validated by the game and nowhere else. */

  command: (state: S, player: P, message: Command) => void;

};

export function makeArena<S, P extends Body>(game: Game<S, P>) {

  const state = game.state;

  const seats = new Map<string, Seat>();

  // A session is a cookie, and a cookie costs one HTTP request — free for

  // anyone to mint as many of as they like. The address is the one thing here

  // a single machine cannot hand itself two of, so it is what a playing seat

  // is actually rationed by; the session on top of it is only what lets a

  // reload find its way back to the same seat rather than a fresh one.

  const addressSeats = new Map<string, string>();

  let ticks = 0;

  const match = (): S => state;

  const findPlayer = (id: string): P | undefined => game.players(state).find((p) => p.id === id);

  /**

   * A name nobody else in this match is using. Two players under one name

   * make a feed line that reads as a team kill; a person who picks a taken

   * name gets a number after it, inside the sixteen characters a name is

   * allowed.

   */

  function freeName(want: string, exceptId: string): string {

    const taken = new Set(

      game

        .players(state)

        .filter((p) => p.id !== exceptId)

        .map((p) => p.name),

    );

    if (!taken.has(want)) return want;

    for (let n = 2; n < 100; n++) {

      const tried = `${want.slice(0, 13)} ${n}`;

      if (!taken.has(tried)) return tried;

    }

    return want;

  }

  /**

   * Whether this address already plays under a different session. Checked

   * before a socket is even offered a seat, so the caller can hand it a

   * spectator view instead of upgrading it into a second side's vision.

   */

  function addressCapped(address: string, session: string): boolean {

    const holder = addressSeats.get(address);

    return holder !== undefined && holder !== session;

  }

  /**

   * Sit somebody down, replacing a bot where it stands.

   *

   * Both claims go in right here, synchronously, before the caller ever opens

   * a socket. A websocket upgrade completes on a later tick, so two requests

   * arriving close together both used to read the maps before either had

   * written to them — one client, two seats, opposite sides, the vision of

   * both. Claiming inline closes that window regardless of how the two

   * requests were related.

   */

  function takeSeat(address: string, session: string, name: string): P | null {

    const existing = seats.get(session);

    const held = existing ? findPlayer(existing.playerId) : undefined;

    if (held) {

      held.name = freeName(name, held.id);

      return held;

    }

    if (addressCapped(address, session)) return null;

    const seat = game.sit(state);

    if (!seat) return null;

    seat.name = freeName(name, seat.id);

    addressSeats.set(address, session);

    seats.set(session, { playerId: seat.id, token: "", address, send: () => {}, board: "" });

    return seat;

  }

  /** The socket that actually opened fills in how to reach it and its token. */

  function attach(session: string, playerId: string, address: string, token: string, send: (data: string) => void): void {

    seats.set(session, { playerId, token, address, send, board: "" });

  }

  const stillHeld = (playerId: string): boolean => [...seats.values()].some((s) => s.playerId === playerId);

  /**

   * A person leaving gives the seat back to a bot, mid-round and all — but only

   * if this is still the socket the session is on. A reload opens the new

   * socket before the old one's close event arrives; without the token, that

   * late close would delete the reconnect's seat entry and hand a live human's

   * seat to a bot mid-round.

   */

  function release(session: string, token: string): void {

    const seat = seats.get(session);

    if (!seat || seat.token !== token) return;

    seats.delete(session);

    if (addressSeats.get(seat.address) === session) addressSeats.delete(seat.address);

    const player = findPlayer(seat.playerId);

    if (player && !stillHeld(seat.playerId)) game.stand(state, player);

  }

  /**

   * Test-only hygiene: give back whatever this address currently holds. The

   * arena is one match shared by the whole test run, over real HTTP, from one

   * loopback address — this is how a test that needs that address free gets it

   * free, rather than depending on every other test in the file to clean up

   * after itself.

   */

  function releaseAddress(address: string): void {

    const session = addressSeats.get(address);

    if (session !== undefined) release(session, seats.get(session)?.token ?? "");

  }

  const seatCount = (): number => seats.size;

  // ------------------------------------------------------------ spectators

  // A connection that never carried a session the page minted gets no side and

  // no seat: only the public state everyone already sees on the page itself.

  const spectators = new Map<string, { send: (data: string) => void; why: Why }>();

  const spectatorSnapshot = (why: Why = "unsaid"): string =>

    JSON.stringify({ t: "s", spectator: true, why, ...game.spectatorSnapshot(state, why) });

  function watch(token: string, send: (data: string) => void, why: Why = "unsaid"): void {

    spectators.set(token, { send, why });

    send(spectatorSnapshot(why));

  }

  function unwatch(token: string): void {

    spectators.delete(token);

  }

  // ----------------------------------------------------------- the commands

  function command(playerId: string, message: Command): void {

    const player = findPlayer(playerId);

    if (!player || player.bot) return;

    game.command(state, player, message);

  }

  // ----------------------------------------------------------- the snapshot

  function snapshotFor(viewerId: string): Record<string, unknown> {

    const players = game.players(state);

    const me = findPlayer(viewerId) ?? players[0]!;

    const seen = players.filter((p) => game.visible(state, me, p));

    return { t: "s", ...game.snapshot(state, me, seen) };

  }

  /** The full snapshot for one viewer, as one string. */

  const snapshot = (viewerId: string): string => JSON.stringify(snapshotFor(viewerId));

  /**

   * The snapshot a seat is actually sent on this tick: everything, every time,

   * except the sticky field, which goes out only when it differs from what

   * this seat last received.

   */

  function snapshotForSeat(seat: Seat): string {

    const full = snapshotFor(seat.playerId);

    if (game.sticky !== null) {

      const part = JSON.stringify(full[game.sticky]);

      if (part === seat.board) delete full[game.sticky];

      else seat.board = part;

    }

    return JSON.stringify(full);

  }

  // --------------------------------------------------------------- the loop

  /**

   * Nobody watching, nothing happens.

   *

   * An arena that ticks around the clock plays out rounds for an empty room —

   * burning a share of a small box's CPU and filling the record with matches

   * no one saw. The world freezes when the last connection goes and carries on

   * from exactly where it stopped when someone arrives, so a visitor never

   * finds a score that ran up while they were away.

   *

   * A spectator counts as somebody: watching is watching.

   */

  const attended = (): boolean => seats.size > 0 || spectators.size > 0;

  /** How many connections the arena is playing for: seats plus watchers. */

  const attendance = (): number => seats.size + spectators.size;

  function step(): void {

    game.step(state, TICK);

    ticks += 1;

    if (ticks % broadcastEvery(seats.size) !== 0) return;

    for (const seat of seats.values()) {

      try {

        seat.send(snapshotForSeat(seat));

      } catch {

        // A socket that will not take a frame is closed already; the close

        // handler gives the seat back.

      }

    }

    for (const spectator of spectators.values()) {

      try {

        spectator.send(spectatorSnapshot(spectator.why));

      } catch {

        // Same story: the close handler drops it from the set.

      }

    }

  }

  let timer: ReturnType<typeof setInterval> | null = null;

  /**

   * One beat of real time. The gate lives here rather than inside `step` so the

   * tests can still drive the simulation directly with `advance` — a test is an

   * attendance of its own, and making it prove otherwise would only teach the

   * suite to work around the rule.

   */

  function beat(): void {

    if (attended()) step();

  }

  function start(): void {

    if (timer) return;

    timer = setInterval(beat, TICK * 1000);

    // The clock must never hold the process open on its own.

    (timer as unknown as { unref?: () => void }).unref?.();

  }

  function stop(): void {

    if (timer) clearInterval(timer);

    timer = null;

  }

  /** Used by the tests: run the arena forward without waiting for real time. */

  function advance(seconds: number): void {

    for (let i = 0; i < Math.round(seconds / TICK); i++) step();

  }

  return {

    match,

    findPlayer,

    addressCapped,

    takeSeat,

    attach,

    release,

    releaseAddress,

    seatCount,

    watch,

    unwatch,

    spectatorSnapshot,

    command,

    snapshot,

    snapshotForSeat,

    attendance,

    beat,

    start,

    stop,

    advance,

  };

}