Showdown — room protocol

Contract between the frontend and the backend (Rust, in backend/). The design goal is no database: each room lives entirely in server memory and disappears when its last player disconnects.

Transport

One WebSocket per player: GET /ws/room/{roomId} (upgrade).
All frames are JSON text messages.
The server serves the built frontend (frontend/dist) for every other route, so deep links like /room/lucky-coyote-07 load the SPA.

Room lifecycle

A room is created in memory the first time someone connects to its id.
Presence is the connection: when a socket closes, that player is removed and a new state snapshot is broadcast (this is why friends vanish when they close the tab).
When the last socket closes, the room is deleted. Watchers count as connections (a TV keeps the room alive) but never as players.
playerId is generated by the browser and persisted in localStorage. A reconnect with the same playerId replaces the old seat (refresh keeps your identity and your vote).

Client → server messages

{ "type": "join", "playerId": "uuid", "name": "Billy", "emoji": "🤠" } // first message on every (re)connect
{ "type": "watch" }                   // alternative first message: spectator (TV display)
{ "type": "vote", "value": "5" }      // value: deck card, or null to retract
{ "type": "reveal" }                  // anyone may reveal; ignored if no votes yet
{ "type": "reset" }                   // anyone may start a new round
{ "type": "deck", "cards": ["1","2","3"] } // anyone may swap the deck; implies reset

A socket that opens with watch instead of join is a watcher: it receives every state snapshot but never appears in players, and any other message it sends is ignored. The frontend's TV view (/room/{id}/tv) connects this way.

Server → client messages

The server replies to every change (join, leave, vote, reveal, reset, deck) by broadcasting one full-state snapshot to every socket in the room. Snapshots are small (≤ ~16 players), so no deltas are needed.

{
  "type": "state",
  "state": {
    "roomId": "lucky-coyote-07",
    "deck": ["0","1","2","3","5","8","13","21","34","?","☕"],
    "revealed": false,
    "round": 3,
    "players": [
      { "id": "uuid", "name": "Billy", "emoji": "🤠", "voted": true, "vote": null }
    ]
  }
}

Rules the server must enforce:

vote is hidden (null) in snapshots until revealed is true — only voted leaks before the showdown.
Votes are rejected while revealed is true.
reset and deck increment round, clear all votes and set revealed = false. Clients use the round counter to clear local UI state.
reveal with zero votes is a no-op.

Server shape (implemented in `backend/`)

All rooms in one Mutex<HashMap<roomId, Room>>. Each socket is a single async task: it registers (join/watch), subscribes to the room's broadcast channel, then loops — relaying snapshots out and applying client messages in. Disconnect cleanup removes the seat (and the room, when its socket count hits zero). No persistence anywhere.

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