Added subset extensions.

Author: electricessence

source/Extensions.Subsets.cs

@@ -0,0 +1,100 @@
+using System;
+using System.Buffers;
+using System.Collections.Generic;
+
+namespace Open.Collections
+{
+	public static partial class Extensions
+	{
+		/// <summary>
+		/// Iteratively updates the buffer with the possible subsets.
+		/// </summary>
+		/// <returns>An enumerable that yields the buffer.</returns>
+		public static IEnumerable<T[]> SubsetsBuffered<T>(this IReadOnlyList<T> source, int count, T[] buffer)
+		{
+			if (count < 1)
+				throw new ArgumentOutOfRangeException(nameof(count), count, "Must greater than zero.");
+			if (count > source.Count)
+				throw new ArgumentOutOfRangeException(nameof(count), count, "Must be less than or equal to the length of the source set.");
+			if (buffer is null)
+				throw new ArgumentNullException(nameof(buffer));
+
+
+			if (count == 1)
+			{
+				foreach (var e in source)
+				{
+					buffer[0] = e;
+					yield return buffer;
+				}
+				yield break;
+			}
+			var pool = ArrayPool<int>.Shared;
+			var indices = pool.Rent(count);
+			try
+			{
+				for (int pos = 0, index = 0; ;)
+				{
+					for (; pos < count; pos++, index++)
+					{
+						indices[pos] = index;
+						buffer[pos] = source[index];
+					}
+					yield return buffer;
+					do
+					{
+						if (pos == 0) yield break;
+						index = indices[--pos] + 1;
+					}
+					while (index > source.Count - count + pos);
+				}
+			}
+			finally
+			{
+				pool.Return(indices);
+			}
+		}
+
+		/// <summary>
+		/// Iteratively updates a buffer with the possible subsets.
+		/// </summary>
+		/// <returns>An enumerable that yields a buffer that is at least the length of the count provided.</returns>
+		public static IEnumerable<T[]> SubsetsBuffered<T>(this IReadOnlyList<T> source, int count)
+		{
+			var pool = ArrayPool<T>.Shared;
+			var buffer = pool.Rent(count);
+
+			try
+			{
+				return SubsetsBuffered(source, count, buffer);
+			}
+			finally
+			{
+				pool.Return(buffer);
+			}
+		}
+
+		/// <summary>
+		/// Iteratively updates the provided buffer with the possible subsets of the length of the buffer.
+		/// </summary>
+		/// <returns>An enumerable that yields a buffer that is at least the length of the count provided.</returns>
+		public static IEnumerable<T[]> SubsetsBuffered<T>(this IReadOnlyList<T> source, T[] buffer)
+			=> SubsetsBuffered(source, buffer.Length, buffer);
+
+
+		/// <summary>
+		/// Iteratively returns the possible subsets of the source.
+		/// </summary>
+		/// <returns>An enumerable that yields each subset.</returns>
+		public static IEnumerable<T[]> Subsets<T>(this IReadOnlyList<T> source, int count)
+		{
+			foreach (var s in SubsetsBuffered(source, count))
+			{
+				var a = new T[count];
+				s.CopyTo(a, 0);
+				yield return a;
+			}
+		}
+
+	}
+}