/**
 * RSS Discovery of Universal Subscription Mechanism
 * http://www.kbcafe.com/rss/usm.html
 * Author: Randy Charles Morin - http://www.kbcafe.com
 * No warranty. Public domain. Attribution appreciated.
 **/
using System;

namespace kb.rss
{
	/// <summary>
	/// Summary description for Discovery.
	/// </summary>
	public class Discovery
	{
		public static string AutoDiscoverRss(string url)
		{
			return AutoDiscover(url, "application/rss+xml");
		}

		public static string AutoDiscoverAtom(string url)
		{
			return AutoDiscover(url, "application/atom+xml");
		}

		public static string AutoDiscover(string url, string type)
		{
			System.Net.WebRequest request = System.Net.WebRequest.Create(url);
			request.Timeout = 10000;
			System.Net.WebResponse response = request.GetResponse();
			System.IO.StreamReader reader = new System.IO.StreamReader(response.GetResponseStream());
			string s = reader.ReadToEnd();
			while(true)
			{
				string t = s.ToLower();
				int i = t.IndexOf("<link");
				if (i == -1)
				{
					break;
				}
				int j = t.IndexOf(">", i);
				if (j == -1)
				{
					break;
				}
				string link = s.Substring(i, j-i);
				if (link.Substring(link.Length-1,1) == "/")
				{
					link += ">";
				}
				else
				{
					link += "/>";
				}

				System.Xml.XmlDocument doc = new System.Xml.XmlDocument();
				doc.LoadXml(link);
				if ( ( doc.DocumentElement.GetAttribute("rel").ToLower() == "alternate" ) &&
					( doc.DocumentElement.GetAttribute("type") == type ) )
				{
					return doc.DocumentElement.GetAttribute("href");
				}
				s = s.Substring(j);
			}
			return null;
		}
	}
}