Extract Combinations method to AdventOfCode.Extensions and refactor usages to improve modularity and reduce duplication

2025-12-12 12:35:01 +01:00
parent d92293f631
commit d0b612154a
7 changed files with 75 additions and 69 deletions
@@ -0,0 +1,52 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+
+namespace AdventOfCode.Extensions;
+
+public static class EnumerableExtensions
+{
+    public static IEnumerable<TValue[]> Combinations<TValue>(this IEnumerable<TValue> values, int count)
+    {
+        var pool = values.ToArray();
+        var poolLength = pool.Length;
+        if (count > poolLength)
+            yield break;
+        var indices = Range(0, count).ToArray();
+        yield return GetCombination(indices, pool);
+        while (true)
+        {
+            var validIndex = false;
+            var currentIndex = 0;
+            foreach (var i in Range(0, count).Reverse())
+            {
+                currentIndex = i;
+                if (indices[i] != i + poolLength - count)
+                {
+                    validIndex = true;
+                    break;
+                }
+            }
+            if(!validIndex)
+                yield break;
+            
+            indices[currentIndex] += 1;
+            foreach (var j in Range(currentIndex + 1, count))
+            {
+                Index ix = j - 1;
+                indices[j] = indices[ix] + 1;
+            }
+
+            yield return GetCombination(indices, pool);
+        }
+
+        static TValue[] GetCombination(int[] innerIndices, TValue[] innerPool) => 
+            innerIndices.Select(i => innerPool[i]).ToArray();
+
+        static IEnumerable<int> Range(int start, int end)
+        {
+            for (var i = start; i < end; i++)
+                yield return i;
+        }
+    }
+}