transport.js

function createTransport({fps, reset, update, draw}) {
	const [UNKNOWN, READY, RUNNING] = enumeration('UNKNOWN', 'READY', 'RUNNING');

	const frameLength = Math.floor(1000 / fps);
	const clock = {dt: null};
	let state = UNKNOWN, timer = null, lastFrame = null, nextFrame = null;

	function _start() {
		if (state === RUNNING) return;
		if (state === UNKNOWN) _reset();
		
		state = RUNNING;
		lastFrame = Date.now() - frameLength;
		tick();

		function tick() {
			const frameStart = Date.now();
			const nextFrame = frameStart + frameLength;
			clock.dt = frameStart - lastFrame;
			lastFrame = frameStart;
			update(clock);
			draw();
			timer = setTimeout(tick, nextFrame - Date.now());
		}
	}

	function _stop() {
		if (state !== RUNNING) return;
		clearTimeout(timer);
		timer = null;
		state = READY;
	}

	function _reset() {
		_stop();
		reset();
		state = READY;
	}

	return {
		start: _start,
		stop: _stop,
		reset: _reset
	};
}