dwd.js

const fs = require("fs");
const https = require("https");
const jsdom = require("jsdom");
const unzipper = require("unzipper");

main();

function main() {
	// To get an overview, see:
	// https://www.dwd.de/DE/leistungen/met_verfahren_mosmix/met_verfahren_mosmix.html

	// For a list of station names, see:
	// https://www.dwd.de/DE/leistungen/met_verfahren_mosmix/mosmix_stationskatalog.cfg?view=nasPublication&nn=16102
	let station_name = "TRITTAU";

	let today = new Date();
	let dates = [today, calc_tomorrow(today)];

	// For a list of possible values, see:
	// https://opendata.dwd.de/weather/lib/MetElementDefinition.xml
	let values = [
		"TTT", // Temperature 2m above surface (in K)
		"FF" // Wind speed (in m/s)
	];

	query(station_name, dates, values, (err, results) => {
		if (err) {
			console.error(err);
			process.exit(1);
		}
		console.log(`====== FORECASTS ======

[Today]
Date:         ${format_date(results[0][0])}
Time:         ${format_date_time(results[0][0])}
Temperature:  ${round_two_dec_places(kelvin_to_celsius(results[0][1]))} °C
Wind speed:   ${round_two_dec_places(mps_to_kmh(results[0][2]))} km/h

[Tomorrow]
Date:         ${format_date(results[1][0])}
Time:         ${format_date_time(results[1][0])}
Temperature:  ${round_two_dec_places(kelvin_to_celsius(results[1][1]))} °C
Wind speed:   ${round_two_dec_places(mps_to_kmh(results[1][2]))} km/h

=======================`);
	});
}

function query(station_name, dates, units, cb) {
	let simple_date = get_simple_date(dates[0]);
	fetch_station_id(station_name, (err, station_id) => {
		if (err) return cb(err);
		fetch_latest_measurement(station_id, simple_date, err => {
			if (err) return cb(err);
			unzip_latest_measurement(station_id, simple_date, err => {
				if (err) return cb(err);
				parse_latest_measurenment(station_id, simple_date, (err, doc) => {
					if (err) return cb(err);
					let time_steps = get_time_steps(doc);
					let results = dates.map(date => {
						let closest_index = closest_date(time_steps, date.getTime());
						let values = units.map(unit => get_forecast(doc, unit)[closest_index]);
						return [time_steps[closest_index], ...values];
					});
					cb(null, results);
				});
			});
		});
	});
}

function fetch_station_id(station_name, cb) {
	ensure_file_downloaded(
		"https://www.dwd.de/DE/leistungen/met_verfahren_mosmix/mosmix_stationskatalog.cfg?view=nasPublication&nn=16102",
		".cache/mosmix_stationskatalog.cfg",
		err => {
			if (err) return cb(err);
			fs.readFile(".cache/mosmix_stationskatalog.cfg", (err, buf) => {
				if (err) return cb(err);
				let lines = buf.toString("utf-8").split("\n");
				let station_line = lines.find(l => l.includes(station_name));
				if (!station_line) return cb(new Error(`Unable to find station '${station_name}'`));
				cb(null, station_line.substr(12, 5));
			});
		}
	);
}

function fetch_latest_measurement(station_id, { year, month, day }, cb) {
	ensure_file_downloaded(
		`https://opendata.dwd.de/weather/local_forecasts/mos/MOSMIX_L/single_stations/${station_id}/kml/MOSMIX_L_LATEST_${station_id}.kmz`,
		`.cache/${station_id}_${year}_${month}_${day}.kmz`,
		cb
	);
}

function unzip_latest_measurement(station_id, { year, month, day }, cb) {
	fs.exists(`.cache/${station_id}_${year}_${month}_${day}.kml`, is_unzipped => {
		if (!is_unzipped) {
			fs
				.createReadStream(`.cache/${station_id}_${year}_${month}_${day}.kmz`)
				.pipe(unzipper.ParseOne(null, {}))
				.pipe(fs.createWriteStream(`.cache/${station_id}_${year}_${month}_${day}.kml`))
				.on("error", cb)
				.on("close", cb);
		} else {
			cb();
		}
	});
}

function parse_latest_measurenment(station_id, { year, month, day }, cb) {
	fs.readFile(`.cache/${station_id}_${year}_${month}_${day}.kml`, (err, buf) => {
		if (err) return cb(err);
		let xml = buf.toString("utf-8");
		let dom = new jsdom.JSDOM(xml, { contentType: "application/xml" });
		let doc = dom.window.document;
		cb(null, doc);
	});
}

function get_time_steps(doc) {
	return Array.from(doc.querySelectorAll("dwd\\:TimeStep")).map(ts => new Date(ts.textContent));
}

function get_forecast(doc, unit) {
	return doc
		.querySelector(`dwd\\:Forecast[dwd\\:elementName=${unit}] dwd\\:value`)
		.textContent.trim()
		.replace(/\s+/g, " ")
		.split(" ")
		.map(s => (s === "-" ? NaN : parseFloat(s)));
}

/*** UTILS ***/

function ensure_file_downloaded(url, path, cb) {
	fs.exists(path, is_cached => {
		if (!is_cached) {
			https.get(url, res => {
				res
					.pipe(fs.createWriteStream(path))
					.on("error", cb)
					.on("close", cb);
			});
		} else {
			cb();
		}
	});
}

function calc_tomorrow(today) {
	let tomorrow = new Date(today);
	tomorrow.setDate(tomorrow.getDate() + 1);
	return tomorrow;
}

function get_simple_date(date) {
	let year = date.getFullYear();
	let month = pad_two_zeros(date.getMonth() + 1);
	let day = pad_two_zeros(date.getDate());
	return { year, month, day };
}

function pad_two_zeros(n) {
	return n.toString().padStart(2, "0");
}

function kelvin_to_celsius(n) {
	return n - 273.15;
}

function mps_to_kmh(n) {
	return n * 3.6;
}

function round_two_dec_places(n) {
	return Math.round(n * 100 + Number.EPSILON) / 100;
}

function closest_date(dates, target_time) {
	let closest_distance = Infinity;
	let winner_index = -1;
	for (let i = 0; i < dates.length; i++) {
		let distance = Math.abs(dates[i].getTime() - target_time);
		if (distance < closest_distance) {
			closest_distance = distance;
			winner_index = i;
		}
	}
	return winner_index;
}

function format_date(date) {
	let year = date.getFullYear();
	let month = pad_two_zeros(date.getMonth() + 1);
	let day = pad_two_zeros(date.getDate());
	return `${day}.${month}.${year}`;
}

function format_date_time(date) {
	let hours = pad_two_zeros(date.getHours());
	let minutes = pad_two_zeros(date.getMinutes());
	return `${hours}:${minutes}`;
}