edit: C# 3.0: SelfJoinByOffset

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Uses "Tuple<T1, T2>"[1].

[1]C#: Tuple[T1, T2]/219

{{
\[Flags]
public enum IncludeFringe
{
	None = 0,
	Begin = 1,
	End = 2,
	BeginAndEnd = Begin | End
}

/// <summary>
/// Joins a sequence to itself, offset by some number of elements.
/// </summary>
/// <typeparam name="T"></typeparam>
/// <param name="list"></param>
/// <param name="offset"></param>
/// <returns>
/// A Tuple with the second element being <code>offset</code> elements past the first.
/// The second element will be null for the last <code>offset</code> element(s) of the list.
/// </returns>
public static IEnumerable<Tuple<T, T>> SelfJoinByOffset<T>(this IEnumerable<T> list, int offset, IncludeFringe fringes)
{
	if (offset <= 0)
		throw new ArgumentOutOfRangeException("offset", "Must be greater than zero.");

/* Let's say the offset is 3, and we have 5 elements total
	 * 
	 * A B C D E
	 * 
	 * Then we will always have 3 elements in the Queue.  Here's the Q before we start yield returning:
	 * 
	 * A B C D E
	 * ^ ^ ^       enqueued
	 * 
	 * Now we yield return [(A, D), (B, E), (C, null), (D, null), (E, null)]
	 * 
	 * What we're really doing is:
	 *   yield return (Q.Dequeue, e.Current);
	 */

bool includeBegin = (fringes & IncludeFringe.Begin) != 0;
	bool includeEnd = (fringes & IncludeFringe.End) != 0;

using (IEnumerator<T> e = list.GetEnumerator())
	{
		var q = new Queue<T>(offset);
		bool hasMore = true;

while (offset-- > 0 && hasMore)
		{
			if (hasMore = e.MoveNext())
			{
				q.Enqueue(e.Current);

if (includeBegin)
					yield return new Tuple<T, T>(default(T), e.Current);
			}
		}

while (q.Count > 0)
		{
			if (hasMore = e.MoveNext())
				q.Enqueue(e.Current);

if (!hasMore && !includeEnd)
				yield break;

yield return new Tuple<T, T>(q.Dequeue(), hasMore ? e.Current : default(T));
		}
	}
}
}}
The input (with `offset` 1 and `IncludeFringe.End`)
{{
0, 1, 2, 3
}}
would get returned as 4 `Tuple<int, int>`s:
{{
Tuple<int, int>.First:  0, 1, 2, 3
Tuple<int, int>.Second: 1, 2, 3, 0 <
                                 ^ this is default(int)
}}

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