LogReader.cs

using System;
using System.IO;

public class LogSearcher
{
	public static int DateLength = "2013-12-29 17:37:56".Length;

	public void Search(string path, DateTime start, DateTime end, TextWriter results)
	{
		using (var file = File.Open(path, FileMode.Open))
		using (var reader = new StreamReader(file))
		{
			var startOfSearchRegion = 0L;
			var endOfSearchRegion = file.Length - 1;
			var offset = endOfSearchRegion/2;
			var lastOffset = offset;
			while (true)
			{
				string line = ReadLineAt(reader, ref offset); // offset is moved to start of line
				while (line[0] == '#' && offset > 0)
				{
					--offset;
					line = ReadLineAt(reader, ref offset); // offset is moved to start of line
				}
				if(offset == 0)
				{
					break;
				}
				var date = DateTime.Parse(line.Substring(0, DateLength));
				if (date < start)
				{
					startOfSearchRegion = offset;
				}
				else
				{
					endOfSearchRegion = offset;
				}
				if (lastOffset == offset)
				{
					break;
				}
				lastOffset = offset;
				offset = (startOfSearchRegion + endOfSearchRegion)/2;
			}
			// Now just read until we reach the end date or EOF
			reader.BaseStream.Seek(offset, SeekOrigin.Begin);
			reader.DiscardBufferedData();
			while (!reader.EndOfStream)
			{
				var line = reader.ReadLine();
				if (line[0] == '#' || line.Length < DateLength)
				{
					results.WriteLine(line);
				}
				else
				{
					var date = DateTime.Parse(line.Substring(0, DateLength));
					if (date < start) continue;
					if (date > end) break;
					results.WriteLine(line);
				}
			}
		}
	}

	// This function will move the offset backwards until we get to the begininning of the current line.
	// I'm seeking backwards because otherwise we'll never get the first line!
	// ex: The computed offset could initially be at the ^ in this snippet of data:
	// "\n2013-12-29 17:37:56 127.0.0.1 GET /test - 8^0 - 127.0.0.1\n"
	private static string ReadLineAt(StreamReader reader, ref long offset)
	{
		reader.BaseStream.Seek(offset, SeekOrigin.Begin);
		reader.DiscardBufferedData();
		if (offset == 0)
		{
			return reader.ReadLine();
		}
		do
		{
			// can't rely on reader.BaseStream.Position or SeekOrigin.Current due to buffering.
			reader.BaseStream.Seek(--offset, SeekOrigin.Begin);
			reader.DiscardBufferedData();
		} while (reader.Peek() != '\n' && offset != 0);
		if (offset > 0)
		{
			reader.BaseStream.Seek(++offset, SeekOrigin.Begin);
			reader.DiscardBufferedData();
		}
		return reader.ReadLine();
	}
}