# Smack a Brother

![1](/screenshots/1.png)

![2](/screenshots/2.png)

![3](/screenshots/3.png)

![4](/screenshots/4.png)

Online platform fighter. Lobbies of up to 16 players, games of up to 8. Play it in the browser [here](https://shweetz.net/smack-a-brother/).

## Clarification
This application was developed with the assistance of a large language model.

## Build

After cloning and entering the repository's directory, make sure to initialize the submodules:

```git submodule update --init --recursive```

Everything (SDL, libdatachannel, simdjson) is built statically from the submodules. The game's resources are in `resources/` and get linked next to the executable.

### Linux

Needs OpenSSL, plus the X11/Wayland and audio headers SDL builds against.

```
mkdir build
cd build
cmake ..
make
```

### Web

Needs the [Emscripten SDK](https://emscripten.org/docs/getting_started/downloads.html).

```
emcmake cmake -B build-web -DCMAKE_BUILD_TYPE=Release
cmake --build build-web
```

This makes `smack-a-brother.html`, `.js`, `.wasm` and `.data`. Rename the `.html` to `index.html` and serve all four.

### Server

The server builds on its own, without SDL. It needs OpenSSL (`libssl-dev` on Debian).

```
cmake -S server -B build-server -DCMAKE_BUILD_TYPE=Release
cmake --build build-server
```

Run it with:

```smack-a-brother-server [port] [first UDP port] [last UDP port]```

Defaults are `47800`, `47810` and `47899`. It only listens on `127.0.0.1`, so put it behind a reverse proxy that handles TLS. For nginx:

```
location /smack-a-brother/ws {
    proxy_pass http://127.0.0.1:47800;
    proxy_http_version 1.1;
    proxy_set_header Upgrade $http_upgrade;
    proxy_set_header Connection "upgrade";
    proxy_set_header Host $host;
    proxy_read_timeout 120s;
}
```

Open the UDP range in the firewall (e.g. `ufw allow 47810:47899/udp`).

## Netcode

- The server runs every lobby: a sandbox everyone in the lobby shares, plus the games queued in it. Each is its own simulation. All of them run on one thread, at 60 ticks a second.
- Clients send their inputs ahead of time. Each tick the server takes what has arrived (repeating a player's last inputs if they're late), steps the simulation, and sends everyone the inputs it used. Every 60 ticks it also sends a checksum.
- Clients run the same simulation about 3 ticks ahead of the server, guessing other players' inputs. When the server's inputs differ from the guess, the client rolls back and replays.
- The simulation is deterministic on every platform (strict floating point, own math functions), so desktop and web play together.
- Clients connect over a WebSocket first (lobbies, chat, WebRTC signaling). Then they open a WebRTC connection to the server with two data channels: a reliable one, and an unreliable one for inputs and frames. If WebRTC can't connect, everything stays on the WebSocket.
- STUN: `stun.l.google.com:19302`.
- The server address is set in Settings. Default: `wss://shweetz.net/smack-a-brother/ws`.

### Joining a lobby directly
- Desktop: `smack-a-brother --join CODE`
- Web: `https://shweetz.net/smack-a-brother/?lobby=CODE`

### Environment variables
- `SAB_SERVER_URL`: use a different server, e.g. `ws://127.0.0.1:47800`
- `SAB_NET_LAG`: add this many milliseconds of lag
- `SAB_NET_LOSS`: drop this percent of input packets
- `SAB_NET_STATS`: log how the client is keeping up with the server

## License

The game's code and art are released into the public domain under [CC0 1.0](LICENSE). Not covered by that: the Ithaca font (SIL Open Font License, see `resources/font/LICENSE.txt`), the sound effects cut from the Sound Ideas libraries (see `resources/sfx/SOURCES.md`), and the libraries in `external/`, each under its own license.