const TXBUF = 0;
const RXBUF = 32;
const TXHEAD = 64;
const TXTAIL = 65;
const RXHEAD = 66;
const RXTAIL = 67;

const TXBUF_SIZE = 32;
const RXBUF_SIZE = 32;
const PERIPHS_SIZE = 68;

const POLL_INTERVAL_MS = 20;

const COLS = 80;
const ROWS = 36;
const TAB_WIDTH = 8;

const CURSOR_IDLE_TIME_MS = 1000;

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.encoder = new TextEncoder('utf-8');

		this.output = document.getElementById('output');
		this.rx_queue = [];
		this.timer = setInterval(() => this.poll(), POLL_INTERVAL_MS);

		this.grid = Array.from(
			{ length: ROWS },
			() => new Array(COLS).fill(' '));
		this.cursor = { x: 0, y: 0 };
		this.range = {
			start: { x: 0, y: 0 },
			end: { x: 0, y: 0 }
		};
		this.idle_timer = null;
		document.addEventListener('keydown', (e) => this.handle_keydown(e));

		this.flush_output();
		document.getElementById('cursor').classList.add('blinking');

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

	poll() {
		const txhead = Atomics.load(this.mem_u8, TXHEAD);
		const txtail = Atomics.load(this.mem_u8, TXTAIL);
		if (txhead != txtail)
			this.handle_tx_data(txhead, txtail);

		if (this.rx_queue.length != 0) {
			const rxhead = Atomics.load(this.mem_u8, RXHEAD);
			const rxtail = Atomics.load(this.mem_u8, RXTAIL);
			if (this.fifo_next(rxtail) != rxhead)
				this.handle_rx_data(rxhead, rxtail);
		}
	}

	fifo_next(idx) {
		return (idx + 1) & 0x1f;
	}

	handle_tx_data(head, tail) {
		const data = [];
		do {
			data.push(this.mem_u8[TXBUF + head]);
			head = this.fifo_next(head);
		} while (head != tail);
		Atomics.store(this.mem_u8, TXHEAD, head);
		this.print(this.decoder.decode(new Uint8Array(data)));
	}

	handle_rx_data(head, tail) {
		do {
			this.mem_u8[RXBUF + tail] = this.rx_queue.shift();
			tail = this.fifo_next(tail);
		} while (this.fifo_next(tail) != head && this.rx_queue.length != 0);
		Atomics.store(this.mem_u8, RXTAIL, tail);
	}

	print(str) {
		for (const cp of str) {
			switch (cp) {
			case '\n':
				do
					this.print_character(' ');
				while (this.range.start.x != 0);	
				break;
			case '\t':
				do
					this.print_character(' ');
				while (this.range.start.x % TAB_WIDTH != 0);
				break;
			default:
				this.print_character(cp);
				break;
			}
		}
		this.flush_output();
	}

	print_character(c) {
		this.shift_up(this.range.start);
		this.grid[this.range.start.y][this.range.start.x] = c;
		this.cursor = this.next_cell(this.cursor);
		this.range.start = this.next_cell(this.range.start);
	}

	handle_keydown(e) {
		if (e.key.length == 1 && !e.ctrlKey && !e.altKey && !e.metaKey) {
			this.shift_up(this.cursor);
			this.grid[this.cursor.y][this.cursor.x] = e.key;
			this.cursor_move(1, 0);
		} else if (e.key == 'Enter') {
			this.cursor.y = this.range.end.y + 1;
			this.cursor.x = 0;
			this.submit_line();
			Object.assign(this.range.start, this.cursor);
			Object.assign(this.range.end, this.cursor);
		} else if (e.key == 'ArrowLeft' || e.key == 'b' && e.ctrlKey) {
			this.cursor_move(-1, 0);
		} else if (e.key == 'ArrowRight' || e.key == 'f' && e.ctrlKey) {
			this.cursor_move(1, 0);
		} else if (e.key == 'ArrowUp' || e.key == 'p' && e.ctrlKey) {
			this.cursor_move(0, -1);
		} else if (e.key == 'ArrowDown' || e.key == 'n' && e.ctrlKey) {
			this.cursor_move(0, 1);
		} else if (e.key == 'Backspace') {
			if (this.cursor_move(-1, 0))
				this.shift_down(this.cursor);
		} else if (e.key == 'Delete' || e.key == 'd' && e.ctrlKey) {
			this.shift_down(this.cursor);
		} else if (e.key == 'Home' || e.key == 'a' && e.ctrlKey) {
			Object.assign(this.cursor, this.range.start);
		} else if (e.key == 'End' || e.key == 'e' && e.ctrlKey) {
			Object.assign(this.cursor, this.range.end);
		} else {
			return;
		}

		e.preventDefault();
		this.flush_output();

		document.getElementById('cursor').classList.remove('blinking');
		if (this.idle_timer)
			clearTimeout(this.idle_timer);
		this.idle_timer = setTimeout(() => {
			document.getElementById('cursor').classList.add('blinking');
		}, CURSOR_IDLE_TIME_MS);
	}

	submit_line() {
		let line = '';
		let cell = this.range.start;
		while (cell.x != this.range.end.x && cell.y != this.range.y) {
			line += this.grid[cell.y][cell.x];
			cell = this.next_cell(cell);
		}
		line += '\n';
		for (let cu of this.encoder.encode(line)) {
			this.rx_queue.push(cu);
		}
	}

	cursor_move(dx, dy) {
		let x = this.cursor.x + dx;
		let y = this.cursor.y + dy;
		if (x < 0) {
			--y;
			x += COLS;
		} else if (x >= COLS) {
			++y;
			x -= COLS;
		}

		if (this.range.start.y > y || this.range.end.y < y)
			return false;
		if (y == this.range.start.y && this.range.start.x > x)
			return false;
		if (y == this.range.end.y && this.range.end.x < x)
			return false;

		this.cursor.x = x;
		this.cursor.y = y;
		return true;
	}

	prev_cell(cell) {
		if (cell.x > 0)
			return { x: cell.x - 1, y: cell.y };
		else if (cell.y > 0)
			return { x: COLS - 1, y: cell.y - 1 };
		else
			return null;
	}

	next_cell(cell) {
		if (cell.x < COLS - 1)
			return { x: cell.x + 1, y: cell.y };
		else if (cell.y < ROWS - 1)
			return { x: 0, y: cell.y + 1 };
		else
			return null;
	}

	shift_up(start) {
		let cell = { x: this.range.end.x, y: this.range.end.y };
		while (cell.x != start.x || cell.y != start.y) {
			let prev = this.prev_cell(cell);
			this.grid[cell.y][cell.x] = this.grid[prev.y][prev.x];
			cell = prev;
		}
		this.range.end = this.next_cell(this.range.end);
	}

	shift_down(start) {
		let cell = { x: start.x, y: start.y };
		while (cell.x != this.range.end.x || cell.y != this.range.end.y) {
			let next = this.next_cell(cell);
			this.grid[cell.y][cell.x] = this.grid[next.y][next.x];
			cell = next;
		}
		this.grid[this.range.end.y][this.range.end.x] = ' ';
		this.range.end = this.prev_cell(this.range.end);
	}

	flush_output() {
		const html = this.grid.map((row, y) => {
			return row.map((c, x) => {
				const ec = this.html_escape(c);
				if (x == this.cursor.x && y == this.cursor.y)
					return '<span id="cursor">' + ec + '</span>';
				else
					return ec;
			}).join('');
		}).join('\n');
		this.output.innerHTML = html;
	}

	html_escape(c) {
		switch (c) {
		case '"':
			return '&quot;';
		case "'":
			return '&apos;';
		case '&':
			return '&amp;';
		case '<':
			return '&lt;';
		case '>':
			return '&gt;';
		default:
			return c;
		}
	}
}

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