Time scheduler in C#

using System;
using System.Collections.Generic;
using System.Text;
using System.Diagnostics;

namespace Idaligo.Time
{
	public class Area : IArea
	{
		protected class Errors
		{
			protected Errors()
			{
			}

			public static ApplicationException NoRanges()
			{
				return new ApplicationException("There are no ranges in the area.");
			}
		}

		private List<IPeriod> list;

		public Area()
		{
			this.list = new List<IPeriod>();
		}

		public Area(IPeriod period)
			: this()
		{
			if (period == null) throw new ArgumentNullException("period");
			this.Add(period);
		}

		public void Add(IPeriod period)
		{
			if (period == null) throw new ArgumentNullException("period");
			this.list.Add(period);
		}

		public bool Hit(DateTime at)
		{
			bool result = false;
			foreach (IPeriod range in this.list)
			{
				if (range.Hit(at))
				{
					result = true;
					break;
				}
			}
			return result;
		}

		//public IEnumerable<TimeRange> Walk1(DateTime from)
		//{
		//    IEnumerator<IPeriod> iterator = this.list.GetEnumerator();
		//    if (iterator.MoveNext())
		//    {
		//        DateTime starts = DateTime.MaxValue;
		//        DateTime ends = DateTime.MinValue;
		//        bool found = false;
		//        TimeRange current = iterator.Current.CurrentPeriodIfAny(from);
		//        if (current != null)
		//        {
		//            found = true;
		//            starts = current.From;
		//            ends = current.Till;
		//        }

		//        while (iterator.MoveNext())
		//        {
		//            current = iterator.Current.CurrentPeriodIfAny(from);
		//            if (current != null)
		//            {
		//                if (found)
		//                {
		//                    if (current.From < starts)
		//                    {
		//                        starts = current.From;
		//                    }
		//                    if (current.Till > ends)
		//                    {
		//                        starts = current.Till;
		//                    }
		//                }
		//                else
		//                {
		//                    starts = current.From;
		//                    ends = current.Till;
		//                    found = true;
		//                }
		//            }
		//        }

		//        if (found)
		//        {
		//            yield return new TimeRange(starts, ends);
		//            from = ends;
		//        }
		//        else
		//        {
		//            // do nothing
		//        }

		//        while (true)
		//        {
		//            starts = this.StartingBoundary(from);
		//            iterator = this.list.GetEnumerator();
		//            while (iterator.MoveNext())
		//            {
		//                current = iterator.Current.CurrentPeriodIfAny(starts);
		//                if (current != null)
		//                {
		//                    if (current.Till > ends)
		//                    {
		//                        ends = current.Till;
		//                    }
		//                }
		//            }
		//            yield return new TimeRange(starts, ends);
		//            from = ends;
		//        }
		//    }
		//    else
		//    {
		//        throw Errors.NoRanges();
		//    }
		//}

		public IEnumerable<TimeRange> Walk(DateTime from)
		{
			IEnumerator<IPeriod> iterator = this.list.GetEnumerator();
			if (iterator.MoveNext())
			{
				DateTime starts = DateTime.MaxValue;
				DateTime ends = DateTime.MinValue;
				bool found = false;
				TimeRange current = iterator.Current.CurrentPeriodIfAny(from);
				if (current != null)
				{
					found = true;
					starts = current.From;
					ends = current.Till;
				}

				while (iterator.MoveNext())
				{
					current = iterator.Current.CurrentPeriodIfAny(from);
					if (current != null)
					{
						if (found)
						{
							if (current.From < starts)
							{
								starts = current.From;
							}
							if (current.Till > ends)
							{
								starts = current.Till;
							}
						}
						else
						{
							starts = current.From;
							ends = current.Till;
							found = true;
						}
					}
				}
				
				TimeRange last;
				if (found)
				{
					last = new TimeRange(starts, ends);
				}
				else
				{
					last = null;
					ends = from;
				}

				// in: last, from
				while (true)
				{
					starts = this.StartingBoundary(ends);
					ends = this.EndingBoundary(starts);
					if (last != null)
					{
						if (last.Till == starts)
						{
							last = new TimeRange(last.From, ends);
						}
						else
						{
							yield return last;
							last = new TimeRange(starts, ends);
						}
					}
					else
					{
						last = new TimeRange(starts, ends);
					}
				}
			}
			else
			{
				throw Errors.NoRanges();
			}
		}

		public DateTime EndingBoundary(DateTime starts)
		{
			DateTime result = starts;

			bool found = false;
			IEnumerator<IPeriod> iterator = this.list.GetEnumerator();
			while (iterator.MoveNext())
			{
				TimeRange current = iterator.Current.CurrentPeriodIfAny(starts);
				if (current != null)
				{
					if (current.Till > result)
					{
						found = true;
						result = current.Till;
					}
				}
			}
			if (!found)
			{
				// must be found!!!
				throw new InvalidProgramException();
			}
			return result;
		}

		public DateTime StartingBoundary(DateTime from)
		{
			DateTime result;

			IEnumerator<IPeriod> iterator = this.list.GetEnumerator();
			if (iterator.MoveNext())
			{
				TimeRange current = iterator.Current.CurrentPeriodIfAny(from);
				if (current != null && current.From == from)
				{
					result = from;
				}
				else
				{
					result = iterator.Current.Next(from).From;
					while (iterator.MoveNext())
					{
						current = iterator.Current.CurrentPeriodIfAny(from);
						if (current != null)
						{
							result = from;
						}
						else
						{
							DateTime next = iterator.Current.Next(from).From;
							if (result > next)
							{
								result = next;
							}
						}
					}
				}
			}
			else
			{
				throw Errors.NoRanges();
			}

			return result;
		}

		public bool IsEmpty
		{
			[DebuggerStepThrough]
			get
			{
				return this.list.Count < 1;
			}
		}
	}
}