AdventOfCodeDay9-Extensions.cs

using System;
using System.Collections.Generic;
using System.Linq;

namespace AdventOfCodeDay9
{
	public static class Extensions
	{
		public static IEnumerable<IEnumerable<T>> Permute<T>(this IEnumerable<T> sequence)
		{
			if (sequence == null)
			{
				yield break;
			}

			var list = sequence.ToList();

			if (!list.Any())
			{
				yield return Enumerable.Empty<T>();
			}
			else
			{
				var startingElementIndex = 0;

				foreach (var startingElement in list)
				{
					var remainingItems = list.AllExcept(startingElementIndex);

					foreach (var permutationOfRemainder in remainingItems.Permute())
					{
						yield return startingElement.Concat(permutationOfRemainder);
					}

					startingElementIndex++;
				}
			}
		}

		private static IEnumerable<T> Concat<T>(this T firstElement, IEnumerable<T> secondSequence)
		{
			yield return firstElement;
			if (secondSequence == null)
			{
				yield break;
			}

			foreach (var item in secondSequence)
			{
				yield return item;
			}
		}

		private static IEnumerable<T> AllExcept<T>(this IEnumerable<T> sequence, int indexToSkip)
		{
			if (sequence == null)
			{
				yield break;
			}

			var index = 0;

			foreach (var item in sequence.Where(item => index++ != indexToSkip))
			{
				yield return item;
			}
		}
	}
}