First implementation of Combinations.

AioNet.Linq/ExtendedLinq.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Generic;
+using System.Linq;
 
 namespace AioNet.Linq
 {
@@ -10,7 +11,36 @@ namespace AioNet.Linq
             int subsequenceLength
         )
         {
-            throw new NotImplementedException();
+            return source.Combinations<T>(subsequenceLength, Enumerable.Empty<T>());
+        }
+
+        private static IEnumerable<IEnumerable<T>> Combinations<T>(
+            this IEnumerable<T> source,
+            int subsequenceLength,
+            IEnumerable<T> build
+        )
+        {
+            if (build.Count() == subsequenceLength)
+            {
+                return new IEnumerable<T>[] { build };
+            }
+
+            if (!source.Any())
+            {
+                return Enumerable.Empty<IEnumerable<T>>();
+            }
+
+            IEnumerable<T> rest = source.Skip(1);
+
+            IEnumerable<IEnumerable<T>> combinationsSkipping = rest.Combinations(
+                subsequenceLength,
+                build
+            );
+            IEnumerable<IEnumerable<T>> combinationsTaking = rest.Combinations(
+                subsequenceLength,
+                build.Append(source.First())
+            );
+            return combinationsTaking.Concat(combinationsSkipping);
         }
     }
 }