const TXBUF = 0;
const RXBUF = 32;
const TXDATA = 64;
const TXHEAD = 65;
const TXTAIL = 66;
const RXDATA = 67;
const RXHEAD = 68;
const RXTAIL = 69;

const TXBUF_SIZE = 32;
const RXBUF_SIZE = 32;
const PERIPHS_SIZE = 70;

const POLL_INTERVAL_MS = 20;

class Emulator {
	constructor() {
		this.mem = new WebAssembly.Memory({
			initial: 1,
			maximum: 1,
			shared: true
		});
		this.mem_u8 = new Uint8Array(this.mem.buffer);
		for (let i = 0; i < PERIPHS_SIZE; ++i)
			this.mem_u8[i] = 0;

		this.decoder = new TextDecoder('utf-8');

		this.output = document.getElementById('output');
		this.input = document.getElementById('input');

		this.timer = setInterval(() => this.poll(), POLL_INTERVAL_MS);

		this.worker = new Worker('boot.js');
		this.worker.postMessage(this.mem);
	}

	poll() {
		const txdata = Atomics.load(this.mem_u8, TXDATA);
		if (txdata !== 0)
			this.handle_txdata();
	}

	handle_txdata() {
		const head = Atomics.load(this.mem_u8, TXHEAD);
		const tail = Atomics.load(this.mem_u8, TXTAIL);

		const data = [];
		let i = head;
		do {
			data.push(this.mem_u8[TXBUF + i]);
			i = (i + 1) % TXBUF_SIZE;
		} while (i !== tail);

		Atomics.store(this.mem_u8, TXHEAD, tail);

		// More data could have been added -- only clear TXDATA if
		// tail is unchanged.
		if (tail === Atomics.load(this.mem_u8, TXTAIL))
			Atomics.store(this.mem_u8, TXDATA, 0);

		const str = this.decoder.decode(new Uint8Array(data));
		this.output.innerText += str;
	}
}

window.addEventListener('DOMContentLoaded', () => {
	window.emu = new Emulator();
});