Add `Permutations` method to `EnumerableExtensions` with corresponding tests

AdventOfCode.Extensions.Tests/EnumerableExtensionsTest.cs

@@ -34,4 +34,33 @@ public class EnumerableExtensionsTest
 
         Assert.Contains([1, 2, 3], actual);
     }
+    
+    [Fact]
+    public void Permutations_12_2_equals_11_12_21_22()
+    {
+        int[] data = [1, 2];
+
+        var actual = data.Permutations(2).ToArray();
+
+        Assert.Contains([1, 1], actual);
+        Assert.Contains([1, 2], actual);
+        Assert.Contains([2, 1], actual);
+        Assert.Contains([2, 2], actual);
+    }
+    
+    [Fact]
+    public void Permutations_123_2_equals_11_12_13_21_22_23_31_32_33()
+    {
+        int[] data = [1, 2, 3];
+
+        var actual = data.Permutations(2).ToArray();
+
+        Assert.Contains([1, 1], actual);
+        Assert.Contains([1, 2], actual);
+        Assert.Contains([2, 1], actual);
+        Assert.Contains([2, 2], actual);
+        Assert.Contains([3, 1], actual);
+        Assert.Contains([3, 2], actual);
+        Assert.Contains([3, 3], actual);
+    }
 }

AdventOfCode.Extensions/EnumerableExtensions.cs

@@ -16,8 +16,8 @@ public static class EnumerableExtensions
         yield return GetCombination(indices, pool);
         while (true)
         {
-            int idx;
+            var idx = count - 1;
-            for(idx = count - 1; idx >= 0; idx--)
+            for(;idx >= 0; --idx)
             {
                 var isIndexBelowMax = indices[idx] != idx + poolLength - count;
                 if (isIndexBelowMax) 
@@ -34,8 +34,47 @@ public static class EnumerableExtensions
 
             yield return GetCombination(indices, pool);
         }
-
-        static TValue[] GetCombination(int[] innerIndices, TValue[] innerPool) => 
-            innerIndices.Select(i => innerPool[i]).ToArray();
     }
+    
+    public static IEnumerable<TValue[]> Permutations<TValue>(this IEnumerable<TValue> values, int count)
+    {
+        var pool = values.ToArray();
+        if (count > pool.Length)
+            yield break;
+        foreach (var p in InnerPermutations(pool, count)) 
+            yield return p;
+        
+        foreach (var p in InnerPermutations([..pool.Reverse()], count)) 
+            yield return p;
+        yield break;
+
+        static IEnumerable<TValue[]> InnerPermutations(TValue[] pool, int count)
+        {
+            var indices = Enumerable.Repeat(0, count).ToArray();
+            yield return GetCombination(indices, pool);
+            while (true)
+            {
+                var idx = count - 1;
+                for(;idx >= 0; --idx)
+                {
+                    var isIndexBelowMax = indices[idx] != idx + pool.Length - count;
+                    if (isIndexBelowMax) 
+                        break;
+                }
+                if(idx < 0)
+                    break;
+            
+                indices[idx] += 1;
+                for (var j = idx + 1; j < count; j++)
+                {
+                    indices[j] = indices[j - 1];
+                }
+
+                yield return GetCombination(indices, pool);
+            }
+        }
+    }
+    
+    private static TValue[] GetCombination<TValue>(int[] innerIndices, TValue[] innerPool) => 
+        innerIndices.Select(i => innerPool[i]).ToArray();
 }