multiplayer added

README.md

@@ -8,10 +8,57 @@ A modern [Scorched Earth](https://en.wikipedia.org/wiki/Scorched_Earth_(video_ga
 
 ## How to play
 
-Tanks take turns to shoot at other tanks. The last surviving tank wins the round. Player 1 (red) is controlled via the keyboard, while other players are randomly-picked AIs.
+Tanks take turns to shoot at other tanks. The last surviving tank wins the round. Player 1 (red) is controlled via the keyboard, while other players are randomly-picked AIs. You can also play online against friends (see [Multiplayer](#multiplayer)).
 
 Use the start menu to configure players (3-10), rounds (1-10), and terrain type before each battle. Between rounds, visit the market to spend your score on weapons.
 
+## Multiplayer
+
+Play over the internet (or LAN) with up to 6 human players; the remaining slots are filled by AIs. The game uses a **host-authoritative** model: one player's browser runs the simulation, a tiny Node.js relay forwards messages between players, and the other players render from synchronized snapshots.
+
+### Running the relay
+
+```sh
+cd server
+npm install
+PORT=8090 npm start
+```
+
+### Playing
+
+1. Open the game and set **Mode → Multiplayer** in the start menu.
+2. Pick **HOST GAME** to create a room — you'll get a 6-character room code. The host configures total players, rounds, and terrain in the lobby, then presses START GAME.
+3. Other players pick **JOIN GAME** and enter the code (plus their name).
+4. Play as usual: aim, fire, and shop in the market. The host's browser simulates the game for everyone; other players can take their turns from anywhere.
+
+### Hosting options
+
+- **Same machine as the game** (default): the relay URL is derived from the page URL, so a reverse proxy can forward `/ws` to the relay port.
+- **Dev/LAN override**: point the game at any relay with `?relay=ws://HOST:PORT` (e.g. `http://localhost:8123/?relay=ws://localhost:8090`).
+
+Example nginx location for the WebSocket endpoint:
+
+```nginx
+location /ws {
+    proxy_pass http://127.0.0.1:8090;
+    proxy_http_version 1.1;
+    proxy_set_header Upgrade $http_upgrade;
+    proxy_set_header Connection "upgrade";
+}
+```
+
+### How it works (short)
+
+- The relay (`server/server.js`) only routes messages: joins, rosters, commands (client → host), and broadcasts (host → clients). It holds no game state.
+- The host broadcasts a world snapshot on every state change (and every 100–250 ms during projectile/explosion resolution), plus the terrain bitmap as a PNG whenever it changes (craters, collapse, napalm).
+- Clients render by applying snapshots to the same game code; they simulate nothing, and their aim/fire/market actions are sent as commands to the host.
+
+### Control flow
+
+- Turn-based rounds, market economy, and scoring work exactly as in single-player.
+- If a remote player doesn't fire within 45 seconds of their turn starting, the host auto-fires for them.
+- If the host leaves, the game ends for everyone.
+
 ### Controls
 
 | Key | Function | With Shift | With Alt |
@@ -46,8 +93,8 @@ Use the start menu to configure players (3-10), rounds (1-10), and terrain type
 - [ ] Many different tanks to pick from
 - [ ] Many AIs to battle against (only Moron and Chooser implemented)
 - [ ] Configuration
-- [ ] Local Multiplayer
-- [ ] Internet Multiplayer
+- [X] Local Multiplayer
+- [X] Internet Multiplayer (host-authoritative, via `server/` relay)
 
 ## What's new
 
@@ -58,6 +105,7 @@ This fork adds significant enhancements over the original game:
 - Multi-round battles with persistent score tracking and weapon purchasing
 - Improved visual effects: lava explosions, enhanced flame animation, and particle effects
 - Napalm and Hot napalm weapons
+- Online multiplayer (up to 6 humans + AI fill) with a Node.js relay server
 
 ## Weapons