using System;
using System.Web;
using System.IO;
using System.Configuration;

namespace Clarius.WebSite
{
	/// <summary>
	/// A simple <see cref="IHttpModule"/> that buffers the response in-memory via the 
	/// <see cref="HttpResponse.Filter"/> stream property and then appends 
	/// Google Analytics (https://www.google.com/analytics/) script 
	/// before the page body closing tag.
	/// </summary>
	/// <remarks>The app settings key (<see cref="AppSettingsKey"/> 
	/// must be present in the web.config file in order for the module to kick-in. If it's not, 
	/// the filter will not be added.
	/// </remarks>
	/// <example>
	/// <code>
	/// &lt;configuration&gt;
	///		&lt;appSettings&gt;
	///			&lt;add key="GoogleAnalyticsAccount" value="[Google Analytics Account #"/&gt;
	///		&lt;/appSettings&gt;
	/// 		&lt;httpModules&gt;
	/// 			&lt;add name="GoogleAnalytics" type="Clarius.WebSite.GoogleAnalyticsModule, Clarius.WebSite"/&gt;
	/// 		&lt;/httpModules&gt;
	/// &lt;/configuration&gt;
	/// </code>
	/// </example>
	public class GoogleAnalyticsModule : IHttpModule
	{
		public const string AppSettingsKey = "GoogleAnalyticsAccount";

		const string GoogleScript = @"<script src='http://www.google-analytics.com/urchin.js' type='text/javascript'>
</script>
<script type='text/javascript'>
_uacct = '{0}';
urchinTracker();
</script>";

		HttpApplication application;
		string accountNumber;

		public void Dispose()
		{
			application.BeginRequest -= new EventHandler(OnBeginRequest);
		}

		public void Init(HttpApplication context)
		{
			accountNumber = ConfigurationSettings.AppSettings[AppSettingsKey];
			if (accountNumber != null && accountNumber.Length > 0)
			{
				context.BeginRequest += new EventHandler(OnBeginRequest);
				application = context;
			}
		}

		void OnBeginRequest(object sender, EventArgs e)
		{
			application.Response.Filter = new AnalyticsStream(application.Response.Filter, accountNumber);
		}

		class AnalyticsStream : Stream
		{
			Stream innerStream;
			string accountNumber;
			MemoryStream memory = new MemoryStream();

			public AnalyticsStream(Stream innerStream, string accountNumber)
			{
				this.innerStream = innerStream;
				this.accountNumber = accountNumber;
			}

			public override void Close()
			{
				memory.Position = 0;
				using (StreamWriter writer = new StreamWriter(innerStream))
				{
					using (StreamReader reader = new StreamReader(memory))
					{
						bool headerDone = false;
						while (reader.Peek() != -1)
						{
							// Find </body>
							if (MatchesOrWrite(reader, writer, '<', null) &&
								MatchesOrWrite(reader, writer, '/', "<") &&
								MatchesOrWrite(reader, writer, 'b', "</") &&
								MatchesOrWrite(reader, writer, 'o', "</b") &&
								MatchesOrWrite(reader, writer, 'd', "</bo") &&
								MatchesOrWrite(reader, writer, 'y', "</bod") &&
								MatchesOrWrite(reader, writer, '>', "</body"))
							{
								string script = String.Format(GoogleScript, accountNumber) + "</body>";
								writer.Write(script);
								while (reader.Peek() != -1)
									writer.Write((char)reader.Read());
							}
						}
					}
				}

				base.Close();
			}

			private bool MatchesOrWrite(StreamReader reader, StreamWriter writer, char target, string buffered)
			{
				if (reader.Peek() != -1)
				{
					char current = (char)reader.Read();
					if (current == target)
					{
						return true;
					}
					else
					{
						writer.Write(buffered);
						writer.Write(current);
					}
				}
				else
				{
					writer.Write(buffered);
				}

				return false;
			}

			public override bool CanRead
			{
				get { return memory.CanRead; }
			}

			public override bool CanSeek
			{
				get { return memory.CanSeek; }
			}

			public override bool CanWrite
			{
				get { return memory.CanWrite; }
			}

			public override void Flush()
			{
				memory.Flush();
			}

			public override long Length
			{
				get { return memory.Length; }
			}

			public override long Position
			{
				get
				{
					return memory.Position;
				}
				set
				{
					memory.Position = value;
				}
			}

			public override int Read(byte[] buffer, int offset, int count)
			{
				return memory.Read(buffer, offset, count);
			}

			public override long Seek(long offset, SeekOrigin origin)
			{
				throw new NotImplementedException();
			}

			public override void SetLength(long value)
			{
				memory.SetLength(value);
			}

			public override void Write(byte[] buffer, int offset, int count)
			{
				memory.Write(buffer, offset, count);
			}
		}
	}
}