Program a farming drone in real C++. Watch it work.

🚀 Play now → cropbot.fly.dev

A browser-based farming game where you write C++ code to automate a drone. Your code is compiled to WebAssembly via Emscripten and runs live in the browser — no plugins, no downloads, just a text editor and a field to farm.

You write C++ in the browser. Click Run. The server compiles your code with emcc, sends back a .wasm binary, and the drone starts executing your main() — tick by tick, on a real grid, with real crops.

int main() {
    while (true) {
        if (get_cell_state() == CellState::EMPTY)   till();
        if (get_cell_state() == CellState::TILLED)  plant(CropType::PUMPKIN);
        if (get_cell_state() == CellState::READY)   harvest();

        if (get_water_level() < 30 && get_tank() >= 15)
            water();

        move_east();
    }
}

npm install
node server.js
# open http://localhost:3000

Requirements: Node.js, Emscripten (emcc in PATH)

All functions are available without any #include. The header is automatically prepended to your code before compilation.

void move(Direction dir);
void move_north();
void move_south();
void move_east();
void move_west();

void till();                  // EMPTY → TILLED
void plant(CropType crop);    // TILLED → PLANTED
void harvest();               // READY  → EMPTY, earn gold
void water();                 // costs 15 tank units, gives cell +50 water
void wait(int ticks);

CellState get_cell_state();
CellState get_cell_state(int x, int y);
CropType  get_crop();
CropType  get_crop(int x, int y);
int get_water_level();
int get_water_level(int x, int y);
int get_tank();
int get_max_tank();
int get_energy();
int get_max_energy();
int is_at_base();             // 1 if drone is at (0,0)
int get_x();
int get_y();
int get_ticks();
int get_score();

int  get_gold();
int  get_seeds(CropType crop);
void buy_seeds(CropType crop, int count);
void buy_water(int packs);    // 50 tank units per pack, 10 gold each
void print(int val);

enum class Direction  { NORTH, EAST, SOUTH, WEST };
enum class CellState  { EMPTY, TILLED, PLANTED, GROWING, READY, BASE };
enum class CropType   { WHEAT, POTATO, PUMPKIN, CORN, MUSHROOM, COFFEE };

Mushroom: grows only if water_level > 40 — requires preventive watering strategy. Water: every planted/growing cell loses 2 water/tick. Growth pauses at 0. Tank: holds water for water() calls. Refills at base (+8/tick). Battery: every action costs 1 energy. Recharges at base (+10/tick).

• Farming actions (till, plant, water, harvest) are no-ops at base
• Auto-refills tank and battery each tick while the drone is here
• is_at_base() returns 1 when drone is at (0, 0)

Purchased from the Extra panel in-game. Permanent until reset.

Cost multiplier: ×1.65 per level.

Browser
├── index.html          UI, styles, API reference panel
├── game.js             Game engine, renderer, WASM orchestration
│   ├── growTick()      Crop growth + water drain per tick
│   ├── droneActions{}  Bridge between WASM calls and game state
│   ├── execWasmAction()Dispatches action codes from Worker
│   └── wasmActionLoop()Listens to SharedArrayBuffer via Atomics.waitAsync
└── wasm-worker.js      Web Worker — runs compiled WASM
    └── callMain()      Blocks Worker via Atomics.wait, wakes main thread

Server (Node.js)
├── POST /compile       Runs emcc, returns .wasm bytes
└── COOP/COEP headers   Required for SharedArrayBuffer to work

The Worker runs main() which blocks on Atomics.wait() for every drone API call. The main thread uses Atomics.waitAsync() to react, executes the action, writes the result into a SharedArrayBuffer, then wakes the Worker with Atomics.notify().

Worker              SharedArrayBuffer           Main thread
  │                 [ACTION][ARG0][ARG1]           │
  │── write action ──────────────────────────────> │
  │── Atomics.wait(RESPONSE) ─────────────────>    │
  │                                   execWasmAction()
  │                 [RESULT][RESPONSE=1]            │
  │<─────────────────────────────── Atomics.notify ─│
  │── read result ──────────────────────────────    │

farm.cpp/
├── index.html        Markup, styles, in-game API panel
├── game.js           All game logic (~1700 lines)
├── wasm-worker.js    Web Worker for WASM execution
├── drone_api.h       Drone API header (auto-prepended on compile)
├── server.js         Express server: /compile endpoint + static files
├── package.json      Dependencies (express)
├── ideas/            Design notes and feature ideas
│   ├── crops.md
│   └── field_expansion.md
└── README.md

Three files must be updated in sync:

1. drone_api.h

extern "C" { int get_something(); }

2. game.js

// in const ACT:
GET_SOMETHING: 31,

// in QUERY_ACTIONS set (if it costs 0 ticks):
ACT.GET_SOMETHING,

// in execWasmAction() switch:
case ACT.GET_SOMETHING: result = droneActions.get_something(); break;

// in droneActions{}:
get_something() { return /* value */; },

3. wasm-worker.js

// in const ACT:
GET_SOMETHING: 31,

// in makeDroneEnv():
get_something: () => callMain(ACT.GET_SOMETHING),

SharedArrayBuffer is blocked by browsers unless the page is served with:

Cross-Origin-Opener-Policy: same-origin
Cross-Origin-Embedder-Policy: require-corp

Opening index.html directly via file:// will not work. Any static file server that can set these headers works — the Node.js server is just the simplest setup that also handles compilation.