Screeps WebAssembly module demo

binding.js

let i32;
let u8;
let creeps;

export function initialize(wasm) {
	const { memory } = wasm.exports;
	const reset = () => {
		i32 = new Int32Array(memory.buffer);
		u8 = new Uint8Array(memory.buffer);
	};
	reset();
	wasm.exports.memory.addGrowCallback?.(reset);
}

export function evaluate(string) {
	return eval(String.fromCharCode(...u8.subarray(string, u8.indexOf(0, string))));
}

export function getTime() {
	return Game.time;
}

export function findCreeps(addr) {
	creeps = Object.values(Game.creeps);
	for (let ii = 0; ii < creeps.length; ++ii) {
		i32[(addr >>> 2) + ii * 2] = creeps[ii].pos.x;
		i32[(addr >>> 2) + ii * 2 + 1] = creeps[ii].pos.y;
	}
	return creeps.length;
}

export function dispatchMoveIntent(creep, direction) {
	return creeps[creep].move(direction);
}

loop.cc

#include <ctime>
#include <iostream>
#include <random>

#define IMPORT(module, symbol) __attribute__((import_module(module), import_name(symbol)))
#define EXPORT extern "C" __attribute__((used))

struct Creep {
	int x, y;
};

IMPORT("./binding.js", "evaluate") int evaluate(const char* source);
IMPORT("./binding.js", "getTime") int get_time();
IMPORT("./binding.js", "findCreeps") int find_creeps(Creep creeps[]);
IMPORT("./binding.js", "dispatchMoveIntent") int dispatch_move_intent(int creep, int direction);

EXPORT void loop() {
	// Initialize random
	std::random_device rd;
	std::mt19937 gen{rd()};
	std::uniform_int_distribution distrib(1, 8);

	// Log current time, takes TZ environment into account
	const auto time = std::time(nullptr);
	char time_str[100];
	if (std::strftime(time_str, sizeof(time_str), "%c", std::localtime(&time))) {
		std::cout << "Starting tick[" << get_time() << "] @ " << time_str << '\n';
	}

	// Spawn creeps
	evaluate("Game.spawns.Spawn1.createCreep([ MOVE ])");

	// Move creeps in a random direction
	Creep creeps[16];
	const auto length = find_creeps(creeps);
	for (int ii = 0; ii < length; ++ii) {
		const auto move = distrib(gen);
		std::cout << "Moving creep[" << creeps[ii].x << ", " << creeps[ii].y << "] -> " << move;
		std::cout << " = " << dispatch_move_intent(ii, distrib(gen)) << '\n';
	}
}

main.js

export { loop } from './loop.wasm?TZ=CST+6';

Makefile

# emcc: `-fwasm-exceptions` and node: `--experimental-wasm-eh` are available but not super well
# supported right now, so C++ exceptions will exit the wasm runtime via a JS exception.

CXX = em++
LD = emcc
CXXFLAGS = -O3 -std=c++20
EMFLAGS = -s ERROR_ON_UNDEFINED_SYMBOLS=0 -s ALLOW_MEMORY_GROWTH=1 --no-entry

loop.wasm: loop.o

%.wasm: %.o
	$(LD) -o $@ $^ $(EMFLAGS)

.PHONY: clean
clean:
	$(RM) *.o *.wasm