#!/usr/bin/env node

var { Client } = require('undici');
var { parseArgs } = require('node:util');

var { values, positionals } = parseArgs({
	options: {
		help: {
			type: 'boolean'
		}
	},
	allowPositionals: true,
});

var help =
`Usage: webtimediff <URL>
Calculate system clock difference between this host and a web server.

  --help    display this help and exit
`;

if (values.help || positionals.length != 1) {
	console.log(help);
	return;
}

function sleep(ms) {
	return new Promise((res, rej) => setTimeout(_ => res(), ms));
}

((async function(url) {
	var conn = new Client(url.origin, {
		pipelining: 1000,
		headersTimeout: 10e3,
		bodyTimeout: 10e3,
	});

	let step = 20;
	let end = 2000;
	let errors = [];
	let resps = [];
	let firstRemoteTime = undefined;
	let foundChange = undefined;
	let reqtempl = {
		method: "OPTIONS",
		path: url.pathname + url.search,
		headers: {
			"user-agent": "webtimediff"
		},
		blocking: false,
		idempotent: true,
	}

	await conn.request(reqtempl);

	for (let i = 0; (!foundChange) && i < end; i += step) {
		if (errors.length > 0)
			break;

		let date = new Date();
		let ii=i;
		let res = conn.request(reqtempl).then(e => (
			e.date = new Date(e.headers.date),
			firstRemoteTime = firstRemoteTime ?? e.date,
			e.madeAt = date,
			e.doneAt = new Date(),
			foundChange = foundChange ??
				((firstRemoteTime && (firstRemoteTime.getTime() != e.date.getTime()))
					? {
						remoteTime: e.date.getTime()/1000,
						beginTime: e.madeAt.getTime()/1000,
						endTime: e.doneAt.getTime()/1000,
					}
					: foundChange),
			e));
		res.catch(e => errors.push(res));
		resps.push(res);

		if (i + step < end)
			await sleep(step);
	}

	if (errors.length > 0)
		await Promise.all(errors);

	Promise.all(resps).catch(e => {});

	if (!foundChange) {
		throw new Error("The remote clock does not work.");
	}

	let firstReq = [await (resps[0])].map(res => ({
		remoteTime: res.date.getTime()/1000,
		beginTime: res.madeAt.getTime()/1000,
		endTime: res.doneAt.getTime()/1000,
	}))[0];

	let reqEnds = (foundChange.endTime - firstReq.endTime);
	let reqBegins = (foundChange.beginTime - firstReq.beginTime)

	let lag = reqEnds / reqBegins;

	if (!(lag < 1.08)) {
		console.error("WARNING: Big Lag ("
			+reqEnds+
			"s / "
			+reqBegins+
			"s) detected, reported result may be inaccurate.");
	}

	let diff = (foundChange.remoteTime - (foundChange.beginTime + foundChange.endTime) / 2);

	console.log("remote_time = local_time "+(diff < 0 ? "-" : "+")+" " + Math.abs(Math.round(diff*1000)/1000)+ " s");
	process.exit();
})(new URL(positionals[0])))