station-master/src/server/lobby.ts

/**
 * The lobby — Phase 4 of `docs/architecture/multiplayer.md` (§12 steps 17-20), fully specified in
 * `docs/architecture/lobby-and-sessions.md`.
 *
 * Pure logic, no sockets, no filesystem, no in-memory registry — same split `session.ts` already
 * draws (`http.ts` wires connections and persistence to this; `persistence.ts` is the only thing
 * that touches disk). A `Lobby` exists only BEFORE `Lobby.Start`: once the config locks and the
 * `GameSession` is built, this module is done with that game — everything after is `session.ts`.
 *
 * WHY A SEPARATE FILE FROM `session.ts`. `session.ts` is already the "running game" module and is
 * large; a lobby's concerns — seating, host rights, the join secret's door versus a game code's
 * discovery, config locking — share almost no code with running a Stage clock, and mixing them would
 * make both harder to read for no reuse gained.
 */

import { randomUUID } from 'node:crypto';

import type { GameConfig, PlayerIndex } from '../engine/state.ts';

/** `lobby-and-sessions.md` §1 — names the PLAYER, not the seat. Issued once, at join, and never
 * reissued: it is what makes reconnection work, so it must outlive the connection that first used
 * it. */
export type PlayerSession = {
  token: string;
  gameId: string;
  player: PlayerIndex;
  displayName: string;
};

export type LobbySeat = { kind: 'human'; token: string; displayName: string } | { kind: 'bot' } | null;

/** A game that has not started. `hostToken` ends its meaning at `Lobby.Start` — nothing here
 * survives into the running game except the seat list and the locked `config`. */
export type Lobby = {
  gameId: string;
  gameCode: string;
  hostToken: string;
  config: GameConfig;
  seats: LobbySeat[];
  /** Tokens in the order they joined — human seats only, host first — so a departed host's
   * replacement is unambiguous (`lobby-and-sessions.md` §2: "earliest-joined remaining player"). */
  joinOrder: string[];
  createdAt: number;
};

export type CreateResult = { lobby: Lobby; session: PlayerSession };
export type JoinResult = { ok: true; lobby: Lobby; session: PlayerSession } | { ok: false; code: 'LOBBY_FULL' | 'ALREADY_STARTED' };
export type StartResult = { ok: true; playerNames: string[]; botSeats: PlayerIndex[] } | { ok: false; code: 'NOT_HOST' | 'BAD_PLAYER_COUNT' };

/**
 * `lobby-and-sessions.md` §2 — short and speakable, not an account name and not a UUID. A small
 * fixed word list rather than a dictionary: this is read aloud across a table or a call, not typed
 * from memory, so a handful of unambiguous railroad words serve better than a large random one.
 */
const CODE_WORDS = [
  'RAIL', 'YARD', 'DEPOT', 'COAL', 'MAIL', 'CABOOSE', 'SIGNAL', 'FREIGHT',
  'TRESTLE', 'HOPPER', 'TENDER', 'SIDING', 'GRADE', 'SPUR', 'WHISTLE', 'TROLLEY',
];

function randomCode(): string {
  const word = CODE_WORDS[Math.floor(Math.random() * CODE_WORDS.length)]!;
  const digits = Math.floor(Math.random() * 10_000)
    .toString()
    .padStart(4, '0');
  return `${word}-${digits}`;
}

/** Collision-checked against whatever codes the caller already knows about — `http.ts` holds the
 * live registry, so the check lives there rather than this module owning a set of its own. */
export function freshGameCode(taken: (code: string) => boolean): string {
  let code = randomCode();
  while (taken(code)) code = randomCode();
  return code;
}

/** `lobby-and-sessions.md` §2 — solitaire is exactly 1, competitive/coop are 2-4. Not a rules
 * limit: the engine will build a Division for any seat count. This is the lobby judgment, sized to
 * what has actually been played and tested (`test/multiplayer.test.ts` exercises 2, 3 and 4). */
export function playerCountAllowed(mode: GameConfig['mode'], count: number): boolean {
  return mode === 'solitaire' ? count === 1 : count >= 2 && count <= 4;
}

/** The creating player is the host and takes seat 0 (`lobby-and-sessions.md` §2). */
/**
 * THE TABLE SIZE IS FIXED WHEN THE GAME IS CREATED, and `seats.length` is it.
 *
 * The host says how many are playing, so the seats array is built at full length with the host in
 * chair 0 and the rest empty. Nothing ever grows or shrinks it, which is what makes a gap
 * impossible to express rather than merely illegal — and that matters more than it looks: seats
 * used to be appended as people joined, so a bot dropped into a later chair padded the array with
 * a hole that silently blocked Start. It also removes any need to compact the seats at
 * `Lobby.Start`, and compaction would have shifted the `player` index every `PlayerSession`
 * already carries (`joinLobby` stamps it at join time, and `/api/stream` and `/api/intent` route
 * by it) — quietly handing a player somebody else's railroad.
 *
 * Knowing the count this early has one more consequence, and it is a bug fix: the config's
 * `minCombinedRevenue` is derived from the player count, and the lobby previously had to guess it
 * as 4 before anyone had sat down.
 */
export function createLobby(
  config: GameConfig,
  hostDisplayName: string,
  gameCode: string,
  players: number,
): CreateResult {
  const gameId = randomUUID();
  const token = randomUUID();
  const session: PlayerSession = { token, gameId, player: 0, displayName: hostDisplayName };
  const seats: LobbySeat[] = Array.from({ length: players }, (_, i) =>
    i === 0 ? { kind: 'human', token, displayName: hostDisplayName } : null,
  );
  const lobby: Lobby = {
    gameId,
    gameCode,
    hostToken: token,
    config,
    seats,
    joinOrder: [token],
    createdAt: Date.now(),
  };
  return { lobby, session };
}

/** Joins the first empty seat, or extends the seat list if every seat so far is filled and the
 * mode's cap allows one more (`lobby-and-sessions.md` §2's 2-4 is enforced at `Lobby.Start`, not
 * here — a 5th join before anyone starts is refused outright since there is no seat it could ever
 * play in, but the running count is checked against `playerCountAllowed` at every join too, so a
 * lobby can never grow the seats array past what could legally start). */
export function joinLobby(lobby: Lobby, displayName: string): JoinResult {
  // The table was sized at creation, so joining takes an empty chair or none at all — there is no
  // longer an "append another seat" path for a late arrival to grow the game through.
  const seatIndex = lobby.seats.findIndex((s) => s === null);
  if (seatIndex < 0) return { ok: false, code: 'LOBBY_FULL' };

  const token = randomUUID();
  const session: PlayerSession = { token, gameId: lobby.gameId, player: seatIndex, displayName };
  const seats = [...lobby.seats];
  seats[seatIndex] = { kind: 'human', token, displayName };
  return {
    ok: true,
    lobby: { ...lobby, seats, joinOrder: [...lobby.joinOrder, token] },
    session,
  };
}

/** Host-only in effect (`http.ts` checks the caller's token against `hostToken` before calling
 * this) — marks an empty seat as bot-filled, or clears one back to empty. Never touches an occupied
 * human seat; the host removes a person by them leaving, not by overwriting their seat. */
export function setBotSeat(lobby: Lobby, seat: PlayerIndex, filled: boolean): Lobby {
  const seats = [...lobby.seats];
  // No padding: a seat outside the table the host chose is not a seat, and inventing one is how
  // the old array grew holes in it.
  if (seat < 0 || seat >= seats.length) return lobby;
  if (filled) {
    if (seats[seat] !== null) return lobby;
    seats[seat] = { kind: 'bot' };
  } else {
    if (seats[seat]?.kind !== 'bot') return lobby;
    seats[seat] = null;
  }
  return { ...lobby, seats };
}

/**
 * The host's connection closed before `Lobby.Start`. `lobby-and-sessions.md` §2: rights pass to the
 * earliest-joined remaining player — remaining means still occupying a seat, not necessarily still
 * connected, since a momentary drop should not also cost the NEXT person the host chair. Returns
 * the lobby unchanged if the departing token was not the host, or if no other human seat exists.
 */
export function reassignHost(lobby: Lobby, departingToken: string): Lobby {
  if (lobby.hostToken !== departingToken) return lobby;
  const stillSeated = new Set(
    lobby.seats.flatMap((s) => (s?.kind === 'human' ? [s.token] : [])),
  );
  const next = lobby.joinOrder.find((t) => t !== departingToken && stillSeated.has(t));
  return next ? { ...lobby, hostToken: next } : lobby;
}

/**
 * Locks the config, checks the player count, and hands back exactly what `session.ts`'s
 * `createSession` needs — this function does not call it, so `lobby.ts` never depends on
 * `session.ts` (the dependency runs the other way: `http.ts` calls both).
 */
export function startLobby(lobby: Lobby, callerToken: string): StartResult {
  if (callerToken !== lobby.hostToken) return { ok: false, code: 'NOT_HOST' };
  // Every chair at the table must be taken. The size itself was validated at creation and cannot
  // have moved since, so this is only ever waiting on the last empty seat to fill.
  if (lobby.seats.some((s) => s === null) || !playerCountAllowed(lobby.config.mode, lobby.seats.length)) {
    return { ok: false, code: 'BAD_PLAYER_COUNT' };
  }
  // Seat index IS player index — no compaction, because there is nothing to compact past.
  const taken = lobby.seats as Exclude<LobbySeat, null>[];
  const playerNames = taken.map((s) => (s.kind === 'human' ? s.displayName : 'Bot'));
  const botSeats = taken.flatMap((s, i) => (s.kind === 'bot' ? [i as PlayerIndex] : []));
  return { ok: true, playerNames, botSeats };
}