Time scheduler in C#

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

namespace Idaligo.Time
{
	public class ShiftedAnchor : IAnchor
	{
		private Scale scale;
		private Anchor anchor;
		private int number;

		public ShiftedAnchor(Scale baseScale, Scale scale, int number)
			: this(new Anchor(baseScale), scale, number)
		{
		}

		protected ShiftedAnchor(Anchor @base, Scale scale, int number)
		{
			if (@base == null) throw new ArgumentNullException("@base");
			if (number < 0) throw new ArgumentOutOfRangeException("number", number, "Must be 0 or greater.");
			Validate(@base, scale, number);
			this.anchor = @base;
			this.scale = scale;
			this.number = number;
		}

		private void Validate(Anchor @base, Scale scale, int number)
		{
#warning Left undone: ShiftedAnchor.ValidateScaleAndNumber
		}

		public bool AtPoint(DateTime at)
		{
			bool result;
			if (this.scale == Scale.Week)
			{
				DateTime starts = (this.anchor.AtPoint(at)) ? at : this.anchor.Previous(at);
				if (starts.DayOfWeek != DayOfWeek.Sunday)
				{
					starts = starts.AddDays(-(int) starts.DayOfWeek).AddDays(7);
				}

				if (this.number == 0)
				{
					result = starts == at;
				}
				else
				{
					result = at == this.Next(starts);
				}
			}
			else
			{
				if (this.number == 0)
				{
					result = this.anchor.AtPoint(at);
				}
				else
				{
					result = at == this.Next(this.anchor.Previous(at));
				}
			}
			return result;
		}



		public IEnumerable<DateTime> Walk(DateTime from)
		{
			if (this.AtPoint(from))
			{
				yield return from;
			}
			while (true)
			{
				from = this.Next(from);
				yield return from;
			}
		}

		public DateTime Next(DateTime from)
		{
			return this.Go((this.anchor.AtPoint(from)) ? from : this.anchor.Previous(from), from, true);
		}

		private DateTime Go(DateTime starts, DateTime from, bool forward)
		{
			DateTime result;

			if (this.scale == Scale.Year)
			{
				throw new InvalidProgramException();
			}
			else if (this.scale == Scale.Month)
			{
				if (this.anchor.Scale == Scale.Year)
				{
					if (this.number > 11)
					{
						throw new InvalidOperationException();
					}
					else
					{
						result = starts.AddMonths(this.number);
					}
				}
				else
				{
					throw new InvalidOperationException();
				}
			}
			else if (this.scale == Scale.Week)
			{
				#region Week
				if (starts.DayOfWeek != DayOfWeek.Sunday)
				{
					starts = starts.AddDays(-(int) starts.DayOfWeek).AddDays(7);
				}

				if (this.anchor.Scale == Scale.Year)
				{
					if (this.number > 51)
					{
						throw new InvalidOperationException();
					}
					else
					{
						result = starts.AddDays(this.number * 7);
						while ((result.DayOfYear - 1) / 7 != this.number)
						{
							starts = (forward) ? this.anchor.Next(starts) : this.anchor.Previous(starts);
							if (starts.DayOfWeek != DayOfWeek.Sunday)
							{
								starts = starts.AddDays(-(int) starts.DayOfWeek).AddDays(7);
							}
							result = starts.AddDays(this.number * 7);
						}
					}
				}
				else if (this.anchor.Scale == Scale.Month)
				{
					if (this.number > 4)
					{
						throw new InvalidOperationException();
					}
					else
					{
						result = starts.AddDays(this.number * 7);
						while (result.Day / 7 != this.number)
						{
							starts = (forward) ? this.anchor.Next(starts) : this.anchor.Previous(starts);
							if (starts.DayOfWeek != DayOfWeek.Sunday)
							{
								starts = starts.AddDays(-(int) starts.DayOfWeek).AddDays(7);
							}
							result = starts.AddDays(this.number * 7);
						}
					}
				}
				else
				{
					throw new InvalidOperationException();
				}
				#endregion
			}
			else if (this.scale == Scale.Day)
			{
				#region Day
				if (this.anchor.Scale == Scale.Year)
				{
					if (this.number > 365)
					{
						throw new InvalidOperationException();
					}
					else if (this.number > 364)
					{
						// 365 (366th - leap year)
						result = starts.AddDays(this.number);
						while (result.DayOfYear != 366)
						{
							starts = (forward) ? this.anchor.Next(starts) : this.anchor.Previous(starts);
							result = starts.AddDays(this.number);
						}
					}
					else
					{
						// 364 (365th - usual year)
						result = starts.AddDays(this.number);
					}
				}
				else if (this.anchor.Scale == Scale.Month)
				{
					if (this.number > 30)
					{
						throw new InvalidOperationException();
					}
					else if (this.number > 27)
					{
						// 30 (31th)
						// 29 (30th)
						// 28 (29th)
						result = starts.AddDays(this.number);
						while (result.Date.Day != this.number + 1)
						{
							starts = (forward) ? this.anchor.Next(starts) : this.anchor.Previous(starts);
							result = starts.AddDays(this.number);
						}
					}
					else
					{
						result = starts.AddDays(this.number);
					}
				}
				else if (this.anchor.Scale == Scale.Week)
				{
					if (this.number > 6)
					{
						throw new InvalidOperationException();
					}
					else
					{
						result = starts.AddDays(this.number);
					}
				}
				else
				{
					throw new InvalidOperationException();
				}
				#endregion
			}
			else if (this.scale == Scale.Hour)
			{
				#region Hour
				if (this.anchor.Scale == Scale.Year)
				{
					if (this.number > 8783 /* 366x24 - 1 */)
					{
						throw new InvalidOperationException();
					}
					else if (this.number > 8759 /* 365x24 - 1 */)
					{
						result = starts.AddHours(this.number);
						while ((result.DayOfYear - 1) * 24 + result.Hour != this.number)
						{
							starts = (forward) ? this.anchor.Next(starts) : this.anchor.Previous(starts);
							result = starts.AddHours(this.number);
						}
					}
					else
					{
						result = starts.AddHours(this.number);
					}
				}
				else if (this.anchor.Scale == Scale.Month)
				{
					if (this.number > 773 /* 31x24 - 1*/)
					{
						throw new InvalidOperationException();
					}
					else if (this.number > 671 /* 28x24 - 1 */)
					{
						result = starts.AddHours(this.number);
						while ((result.Day - 1) * 24 + result.Hour != this.number)
						{
							starts = (forward) ? this.anchor.Next(starts) : this.anchor.Previous(starts);
							result = starts.AddHours(this.number);
						}
					}
					else
					{
						result = starts.AddHours(this.number);
					}
				}
				else if (this.anchor.Scale == Scale.Week)
				{
					if (this.number > 167)
					{
						throw new InvalidOperationException();
					}
					else
					{
						result = starts.AddHours(this.number);
					}
				}
				else if (this.anchor.Scale == Scale.Day)
				{
					if (this.number > 23)
					{
						throw new InvalidOperationException();
					}
					else
					{
						result = starts.AddHours(this.number);
					}
				}
				else
				{
					throw new InvalidOperationException();
				}
				#endregion
			}
			else if (this.scale == Scale.Minute)
			{
				#region Minute
				if (this.anchor.Scale == Scale.Year)
				{
					if (this.number > 527039 /* 366x24x60 - 1*/)
					{
						throw new InvalidOperationException();
					}
					else if (this.number > 525599 /* 365x24x60 - 1*/)
					{
						result = starts.AddMinutes(this.number);
						while ((result.DayOfYear - 1) * 24 * 60 + result.Hour * 60 + result.Minute != this.number)
						{
							starts = (forward) ? this.anchor.Next(starts) : this.anchor.Previous(starts);
							result = starts.AddMinutes(this.number);
						}
					}
					else
					{
						result = starts.AddMinutes(this.number);
					}
				}
				else if (this.anchor.Scale == Scale.Month)
				{
					if (this.number > 44639 /* 31x24x60 - 1 */)
					{
						throw new InvalidOperationException();
					}
					else if (this.number > 40319 /* 28x24x60 - 1 */)
					{
						result = starts.AddMinutes(this.number);
						while ((result.Day - 1) * 24 * 60 + result.Hour * 60 + result.Minute != this.number)
						{
							starts = (forward) ? this.anchor.Next(starts) : this.anchor.Previous(starts);
							result = starts.AddMinutes(this.number);
						}
					}
					else
					{
						result = starts.AddMinutes(this.number);
					}
				}
				else if (this.anchor.Scale == Scale.Week)
				{
					if (this.number > 8399 /* 24x7x60 -1 */)
					{
						throw new InvalidOperationException();
					}
					else
					{
						result = starts.AddMinutes(this.number);
					}
				}
				else if (this.anchor.Scale == Scale.Day)
				{
					if (this.number > 1439 /* 24x60 - 1*/)
					{
						throw new InvalidOperationException();
					}
					else
					{
						result = starts.AddMinutes(this.number);
					}
				}
				else if (this.anchor.Scale == Scale.Hour)
				{
					if (this.number > 59)
					{
						throw new NotSupportedException();
					}
					else
					{
						result = starts.AddMinutes(this.number);
					}
				}
				else
				{
					throw new InvalidOperationException();
				}
				#endregion
			}
			else if (this.scale == Scale.Second)
			{
				result = starts.AddSeconds(this.number);
			}
			else
			{
				throw new NotSupportedException(this.scale.ToString());
			}

			if (forward)
			{
				if (result <= from)
				{
					if (this.scale == Scale.Week)
					{
						starts = this.anchor.Next(starts);
						if (starts.DayOfWeek != DayOfWeek.Sunday)
						{
							starts = starts.AddDays(-(int) starts.DayOfWeek).AddDays(7);
						}
					}
					else
					{
						starts = this.anchor.Next(starts);
					}
					result = this.Go(starts, from, forward);
				}
			}
			else
			{
				if (result >= from)
				{
					if (this.scale == Scale.Week)
					{
						starts = this.anchor.Previous(starts);
						if (starts.DayOfWeek != DayOfWeek.Sunday)
						{
							starts = starts.AddDays(-(int) starts.DayOfWeek).AddDays(7);
						}
					}
					else
					{
						starts = this.anchor.Previous(starts);
					}
					result = this.Go(starts, from, forward);
				}
			}

			return result;
		}


		public DateTime Previous(DateTime from)
		{
			return this.Go((this.anchor.AtPoint(from)) ? from : this.anchor.Previous(from), from, false);
		}

	}
}