added some collection extensions, namely Combinations

MLEM/Extensions/CollectionExtensions.cs

@@ -0,0 +1,55 @@
+using System.Collections.Generic;
+using System.Linq;
+
+namespace MLEM.Extensions {
+    /// <summary>
+    /// A set of extensions for dealing with collections of various kinds
+    /// </summary>
+    public static class CollectionExtensions {
+
+        /// <summary>
+        /// This method returns a set of possible combinations of n items from n different sets, where the order of the items in each combination is based on the order of the input sets.
+        /// For a version of this method that returns indices rather than entries, see <see cref="IndexCombinations{T}"/>.
+        /// <example>
+        /// Given the input set <c>{{1, 2, 3}, {A, B}, {+, -}}</c>, the returned set would contain the following sets:
+        /// <code>
+        /// {1, A, +}, {1, A, -}, {1, B, +}, {1, B, -},
+        /// {2, A, +}, {2, A, -}, {2, B, +}, {2, B, -},
+        /// {3, A, +}, {3, A, -}, {3, B, +}, {3, B, -}
+        /// </code>
+        /// </example>
+        /// </summary>
+        /// <param name="things">The different sets to be combined</param>
+        /// <typeparam name="T">The type of the items in the sets</typeparam>
+        /// <returns>All combinations of set items as described</returns>
+        public static IEnumerable<IEnumerable<T>> Combinations<T>(this IEnumerable<IEnumerable<T>> things) {
+            var combos = Enumerable.Repeat(Enumerable.Empty<T>(), 1);
+            foreach (var t in things)
+                combos = combos.SelectMany(c => t.Select(c.Append));
+            return combos;
+        }
+
+        /// <summary>
+        /// This method returns a set of possible combinations of n indices of items from n different sets, where the order of the items' indices in each combination is based on the order of the input sets.
+        /// For a version of this method that returns entries rather than indices, see <see cref="Combinations{T}"/>.
+        /// <example>
+        /// Given the input set <c>{{1, 2, 3}, {A, B}, {+, -}}</c>, the returned set would contain the following sets:
+        /// <code>
+        /// {0, 0, 0}, {0, 0, 1}, {0, 1, 0}, {0, 1, 1},
+        /// {1, 0, 0}, {1, 0, 1}, {1, 1, 0}, {1, 1, 1},
+        /// {2, 0, 0}, {2, 0, 1}, {2, 1, 0}, {2, 1, 1}
+        /// </code>
+        /// </example>
+        /// </summary>
+        /// <param name="things">The different sets to be combined</param>
+        /// <typeparam name="T">The type of the items in the sets</typeparam>
+        /// <returns>All combinations of set items as described</returns>
+        public static IEnumerable<IEnumerable<int>> IndexCombinations<T>(this IEnumerable<IEnumerable<T>> things) {
+            var combos = Enumerable.Repeat(Enumerable.Empty<int>(), 1);
+            foreach (var t in things)
+                combos = combos.SelectMany(c => t.Select((o, i) => c.Append(i)));
+            return combos;
+        }
+
+    }
+}