feat(backend): add private rooms with create/join and match repeat

Implements PROTOCOL.md's private-room flow: room:create/room:join
with a collision-checked numeric code, up to a 5-match repeating
series per room via game/session.js, random-theme selection that
excludes the previous match's theme, and room:closed for matchLimit
or quit (rooms never touch matchmaking's ban store). themeMode:
'chosen' gets a narrow resolveChosenTheme() stand-in pending #7's
real theme-voting flow.

Also adds a minimal opponentName lookup to registry.js
(registerSocket's new optional name arg + getPlayerName) since
room:joined needs it, mirroring the same addition landing on #5's
branch.

backend/src/index.js

@@ -2,6 +2,7 @@ import http from 'node:http';
 import { WebSocketServer } from 'ws';
 import { handleConnection } from './ws/connectionHandler.js';
 import { registerMatchmakingHandlers } from './matchmaking/queue.js';
+import { registerRoomHandlers } from './rooms/room.js';
 
 const PORT = process.env.PORT || 8080;
 
@@ -22,6 +23,8 @@ const server = http.createServer((req, res) => {
 const wss = new WebSocketServer({ server });
 wss.on('connection', handleConnection);
 
+registerRoomHandlers();
+
 server.listen(PORT, () => {
   console.log(`Matchmaking server listening on :${PORT}`);
 });

backend/src/rooms/README.md

@@ -1,5 +1,6 @@
 # rooms/
 
-Reserved for game-session/room state: tracking which sockets belong to a
-match, broadcasting round state, and cleaning up on disconnect. Not
-implemented yet.
+Numeric-code private rooms: `room.js` tracks live rooms (code -> room
+state), handles `room:create`/`room:join`, and replays
+`game/session.js` matches up to a 5-match cap unless a player quits.
+Implemented; theme voting for `themeMode: 'chosen'` rooms is ticket #7.

backend/src/rooms/room.js

@@ -0,0 +1,228 @@
+// Private rooms: numeric-code matches for two players who agree on a
+// code out-of-band (see PROTOCOL.md "Private rooms"). A room replays
+// the same round-session engine (game/session.js) up to 5 times in a
+// row, picking a random theme per match (excluding the previous
+// match's theme) unless the creator asked for `themeMode: 'chosen'`,
+// in which case real theme voting is ticket #7 — this ticket only
+// wires a narrow, synchronous stand-in (see resolveChosenTheme below).
+//
+// Rooms never import or touch matchmaking's ban store: quitting a
+// private room carries no penalty, unlike quitting a matchmaking match.
+
+import { registerHandler, onDisconnect } from '../ws/connectionHandler.js';
+import { getPlayerId, getPlayerName, sendTo } from '../ws/registry.js';
+import { createSession } from '../game/session.js';
+import { pickRandomThemeId } from '../themeIds.js';
+
+const MAX_MATCHES = 5;
+const CODE_MIN = 100000;
+const CODE_MAX = 999999;
+
+// code (number) -> room. A room's code is freed (removed from this map)
+// as soon as the room closes, so a future room:create can reuse it.
+const rooms = new Map();
+// playerId -> code, so onDisconnect (which only knows a playerId) can
+// find the room a player belongs to without scanning every room.
+const roomCodeByPlayerId = new Map();
+
+function generateCode() {
+  let code;
+  do {
+    code = CODE_MIN + Math.floor(Math.random() * (CODE_MAX - CODE_MIN + 1));
+  } while (rooms.has(code));
+  return code;
+}
+
+function otherPlayerId(room, playerId) {
+  return room.players.find((id) => id !== playerId);
+}
+
+function teardownRoom(room) {
+  rooms.delete(room.code);
+  for (const id of room.players) {
+    roomCodeByPlayerId.delete(id);
+  }
+}
+
+/**
+ * Closes a room whose match just ended via an explicit quit or a
+ * mid-match disconnect. game/session.js's own quitSession() already
+ * sent `room:playerLeft` + `room:closed` (reason: "quit") to the
+ * remaining player before calling onEnd — this just forgets the room.
+ */
+function closeAfterSessionQuit(room) {
+  teardownRoom(room);
+}
+
+/**
+ * Closes a room the session engine never got a chance to react to:
+ * either the 5-match series cap was reached (`matchLimit`), or a
+ * player quit/disconnected while no match was in flight (e.g. the
+ * creator alone, still waiting for an opponent). Notifies whoever is
+ * still connected, then forgets the room.
+ */
+function closeRoom(room, reason, quitterId) {
+  if (reason === 'quit') {
+    const remainingId = quitterId === undefined ? undefined : otherPlayerId(room, quitterId);
+    if (remainingId) {
+      sendTo(remainingId, { type: 'room:playerLeft' });
+      sendTo(remainingId, { type: 'room:closed', reason: 'quit' });
+    }
+  } else {
+    for (const id of room.players) {
+      sendTo(id, { type: 'room:closed', reason });
+    }
+  }
+  teardownRoom(room);
+}
+
+/**
+ * `themeMode: 'chosen'` theme resolution is not implemented by this
+ * ticket (#6) — real theme voting (`vote:start`/`vote:cast`/
+ * `vote:settled`) is ticket #7. This is a narrow, synchronous stand-in
+ * so #7 has an obvious integration point without this ticket blocking
+ * on it: swap this function's body for the real voting flow.
+ */
+function resolveChosenTheme(room, cb) {
+  // TODO(#7): replace with real theme-voting flow (startVote)
+  cb(pickRandomThemeId());
+}
+
+function resolveTheme(room, cb) {
+  if (room.themeMode === 'chosen') {
+    resolveChosenTheme(room, cb);
+    return;
+  }
+  // Random mode: uniform pick, excluding the previous match's theme so
+  // the same theme never plays twice in a row. Match 1 has no previous
+  // theme yet, so nothing is excluded.
+  cb(pickRandomThemeId(room.previousThemeId ?? undefined));
+}
+
+function startMatch(room) {
+  resolveTheme(room, (themeId) => {
+    room.currentThemeId = themeId;
+    const [playerA, playerB] = room.players;
+    room.session = createSession({
+      playerA,
+      playerB,
+      timeMode: room.timeMode,
+      themeId,
+      onEnd: (reason, quitterId) => handleSessionEnd(room, reason, quitterId),
+    });
+  });
+}
+
+function handleSessionEnd(room, reason, quitterId) {
+  if (!rooms.has(room.code)) return; // already torn down
+
+  if (reason === 'quit') {
+    closeAfterSessionQuit(room);
+    return;
+  }
+
+  // Normal completion (round:reveal delivered, no quit).
+  room.matchesPlayed += 1;
+  room.previousThemeId = room.currentThemeId;
+
+  if (room.matchesPlayed >= MAX_MATCHES) {
+    closeRoom(room, 'matchLimit');
+    return;
+  }
+
+  const nextMatch = room.matchesPlayed + 1;
+  for (const id of room.players) {
+    sendTo(id, { type: 'round:nextMatch', match: nextMatch });
+  }
+  startMatch(room);
+}
+
+/**
+ * Registers this module's message handlers and disconnect hook. Called
+ * once from index.js at boot so private rooms are actually reachable.
+ */
+export function registerRoomHandlers() {
+  registerHandler('room:create', (socket, message) => {
+    const ownerId = getPlayerId(socket);
+    if (!ownerId) return;
+
+    const { timeMode, themeMode } = message;
+    if (timeMode !== 1 && timeMode !== 2) return;
+    if (themeMode !== 'random' && themeMode !== 'chosen') return;
+
+    const code = generateCode();
+    const room = {
+      code,
+      ownerId,
+      timeMode,
+      themeMode,
+      players: [ownerId],
+      matchesPlayed: 0,
+      previousThemeId: null,
+      currentThemeId: null,
+      session: null,
+    };
+    rooms.set(code, room);
+    roomCodeByPlayerId.set(ownerId, code);
+
+    sendTo(ownerId, { type: 'room:created', code });
+  });
+
+  registerHandler('room:join', (socket, message) => {
+    const joinerId = getPlayerId(socket);
+    if (!joinerId) return;
+
+    const { code } = message;
+    const room = typeof code === 'number' ? rooms.get(code) : undefined;
+
+    if (!room) {
+      sendTo(joinerId, { type: 'room:error', reason: 'not_found' });
+      return;
+    }
+    if (room.players.length >= 2 || room.players.includes(joinerId)) {
+      sendTo(joinerId, { type: 'room:error', reason: 'full' });
+      return;
+    }
+
+    room.players.push(joinerId);
+    roomCodeByPlayerId.set(joinerId, room.code);
+
+    const ownerName = getPlayerName(room.ownerId) ?? '';
+    const joinerName = getPlayerName(joinerId) ?? '';
+    sendTo(room.ownerId, {
+      type: 'room:joined',
+      code: room.code,
+      opponentName: joinerName,
+      timeMode: room.timeMode,
+      themeMode: room.themeMode,
+    });
+    sendTo(joinerId, {
+      type: 'room:joined',
+      code: room.code,
+      opponentName: ownerName,
+      timeMode: room.timeMode,
+      themeMode: room.themeMode,
+    });
+
+    startMatch(room);
+  });
+
+  // Handles a dropped socket identically to an explicit player:quit for
+  // room-closing purposes. game/session.js's own onDisconnect hook
+  // already runs first (room.js imports session.js, so session.js's
+  // top-level onDisconnect() subscription registers before this one)
+  // and, for a mid-match disconnect, already tore this room down via
+  // handleSessionEnd -> closeAfterSessionQuit, clearing
+  // roomCodeByPlayerId for both players. So by the time this fires, a
+  // mid-match disconnect is a no-op here; only a disconnect with no
+  // active session (e.g. the creator alone, still waiting for an
+  // opponent) is still live in `rooms`/`roomCodeByPlayerId` and needs
+  // handling here.
+  onDisconnect((playerId) => {
+    const code = roomCodeByPlayerId.get(playerId);
+    if (code === undefined) return;
+    const room = rooms.get(code);
+    if (!room) return;
+    closeRoom(room, 'quit', playerId);
+  });
+}