YouTube audio stream url extractor

YtAudio.cs

// requires Jint
//
// this can be used for video streams as well, with few modifications

public async Task<string> GetYtAudioUrl(string id)
{
	await Task.Yield();

	var utf8 = new UTF8Encoding(false);
	var wc = new WebClient();
	wc.Encoding = utf8;

	var dats = await wc.DownloadStringTaskAsync(new Uri(string.Format("https://youtube.com/get_video_info?video_id={0}&el=detailpage", id)));
	var dat = dats.Split('&')
		.Where(xs => !string.IsNullOrWhiteSpace(xs))
		.Select(xs => xs.Split('='))
		.ToDictionary(xsa => xsa[0], xsa => HttpUtility.UrlDecode(xsa[1]));

	if (dat.ContainsKey("reason"))
		throw new Exception(dat["reason"]);

	// You can use those to get some video info
	//var thumb = dat["thumbnail_url"];
	//var title = dat["title"];
	//var authr = dat["author"];

	var fmtss = dat["adaptive_fmts"];
	var fmts = fmtss.Split(',')
		.Where(xs => !string.IsNullOrWhiteSpace(xs))
		.Select(xs => xs.Split('&')
			.Where(xxs => !string.IsNullOrWhiteSpace(xxs))
			.Select(xxs => xxs.Split('='))
			.ToDictionary(xxsa => xxsa[0], xxsa => HttpUtility.UrlDecode(xxsa[1])))
		.Select(xd => new { url = xd["url"], type = xd["type"], bitrate = int.Parse(xd["bitrate"]), sig = xd.ContainsKey("s") ? xd["s"] : null })
		.OrderByDescending(xa => xa.bitrate);
	
	var fmt = fmts.FirstOrDefault(xa => xa.type.StartsWith("audio/mp4"));
	if (fmt == null)
		throw new InvalidOperationException("The audio stream did not contain suitable formats.");
		
	var url = fmt.url;

	if (fmt.sig != null)
	{
		var sig = fmt.sig;

		// decode
		var dpage = await wc.DownloadStringTaskAsync(string.Concat("https://www.youtube.com/watch?v=", id));

		var dreg = new Regex(@"""js"":""(.+?)""(\}|,)");
		var dm = dreg.Match(dpage);
		var djloc = string.Concat("https:", Regex.Unescape(dm.Groups[1].Value));

		var djs = await wc.DownloadStringTaskAsync(djloc);

		dreg = new Regex(@"\.set\(""signature"",(([a-zA-Z]+?))\(.+?\)\);");
		dm = dreg.Match(djs);
		var dfunc = dm.Groups[1].Value;

		dreg = new Regex(string.Concat(dfunc, @"=function\(([a-zA-Z]+?)\)\{(.+?)\};"));
		dm = dreg.Match(djs);

		var dargn = dm.Groups[1].Value;
		var dalg = dm.Groups[2].Value;
		var djsfunc = string.Concat("var unscramble = function(", dargn, ") { ", dalg, " };");

		var dalgps = dalg.Split(';');
		var dalgrs = new HashSet<string>();
		foreach (var dalgp in dalgps)
			if (!dalgp.StartsWith(dargn) && !dalgp.StartsWith(string.Concat(dargn, "=")) && !dalgp.StartsWith("return "))
				dalgrs.Add(dalgp.Split('.')[0]);

		dreg = new Regex(string.Concat("var ", dalgrs.First(), @"=\{(.+?)\};"), RegexOptions.Singleline);
		dm = dreg.Match(djs);

		dalg = dm.Groups[0].Value;

		djsfunc = string.Concat(dalg, "\n", djsfunc, "\nunscramble(sig);");

		var jseng = new Engine();
		sig = jseng.SetValue("sig", sig)
			.Execute(djsfunc)
			.GetCompletionValue()
			.ToObject()
			.ToString();

		url = string.Concat(url, "&signature=", sig);
	}
	
	return url;
}