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

AdventOfCode.Extensions/AdventOfCode.Extensions.csproj

@@ -0,0 +1,9 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+    <PropertyGroup>
+        <TargetFramework>net9.0</TargetFramework>
+        <ImplicitUsings>disable</ImplicitUsings>
+        <Nullable>enable</Nullable>
+    </PropertyGroup>
+
+</Project>

AdventOfCode.Extensions/EnumerableExtensions.cs

@@ -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;
+        }
+    }
+}

AdventOfCode.sln

@@ -10,6 +10,8 @@ Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "AoC_2025.Runner", "AoC_2025
 EndProject
 Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "AdvenOfCode.Contracts", "AdvenOfCode.Contracts\AdvenOfCode.Contracts.csproj", "{0249A329-D5C1-4699-88BE-A668B362432E}"
 EndProject
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "AdventOfCode.Extensions", "AdventOfCode.Extensions\AdventOfCode.Extensions.csproj", "{1466A0F6-7C88-47FF-B8F8-FF30368E26DC}"
+EndProject
 Global
 	GlobalSection(SolutionConfigurationPlatforms) = preSolution
 		Debug|Any CPU = Debug|Any CPU
@@ -32,6 +34,10 @@ Global
 		{0249A329-D5C1-4699-88BE-A668B362432E}.Debug|Any CPU.Build.0 = Debug|Any CPU
 		{0249A329-D5C1-4699-88BE-A668B362432E}.Release|Any CPU.ActiveCfg = Release|Any CPU
 		{0249A329-D5C1-4699-88BE-A668B362432E}.Release|Any CPU.Build.0 = Release|Any CPU
+		{1466A0F6-7C88-47FF-B8F8-FF30368E26DC}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{1466A0F6-7C88-47FF-B8F8-FF30368E26DC}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{1466A0F6-7C88-47FF-B8F8-FF30368E26DC}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{1466A0F6-7C88-47FF-B8F8-FF30368E26DC}.Release|Any CPU.Build.0 = Release|Any CPU
 	EndGlobalSection
 	GlobalSection(NestedProjects) = preSolution
 		{4F395AE3-DABB-4546-A4D0-5A62425FE168} = {05EBAA1C-B33B-4AFE-8635-A58D7CA509A7}

AoC_2025/AoC_2025.csproj

@@ -8,6 +8,7 @@
 
     <ItemGroup>
       <ProjectReference Include="..\AdvenOfCode.Contracts\AdvenOfCode.Contracts.csproj" />
+      <ProjectReference Include="..\AdventOfCode.Extensions\AdventOfCode.Extensions.csproj" />
     </ItemGroup>
 
     <ItemGroup>

AoC_2025/Day08.cs

@@ -4,6 +4,7 @@ using System.Data.Common;
 using System.IO;
 using System.Linq;
 using AdvenOfCode.Contracts;
+using AdventOfCode.Extensions;
 using Coordinate = (long x, long y, long z);
 
 namespace AoC_2025;
@@ -92,21 +93,11 @@ public class Day08 : IPuzzleSolver<long>
 
     private IEnumerable<(Coordinate boxA, Coordinate boxB)> GetAllCombinationsSortedByDistance(IEnumerable<Coordinate> jBoxes)
     {
-        var combinations = Combinations(jBoxes)
+        var combinations = jBoxes.Combinations(2)
+            .Select(x => (a: x[0], b: x[1]))
             .OrderBy(x => GetDistance(x.a, x.b));
         return combinations;
     }
-
-    private IEnumerable<(TValue a, TValue b)> Combinations<TValue>(IEnumerable<TValue> values)
-    {
-        var enumeratedValues = values.Index().ToArray();
-        var pairs = 
-            from a in enumeratedValues
-            from b in enumeratedValues
-            where a.Index < b.Index
-            select (a.Item, b.Item);
-        return pairs;
-    }
     
     private long MultiplyLargestCircuitLength(List<HashSet<Coordinate>> circuits, int amountMultiply)
     {

AoC_2025/Day09.cs

@@ -4,6 +4,7 @@ using System.Data.Common;
 using System.IO;
 using System.Linq;
 using AdvenOfCode.Contracts;
+using AdventOfCode.Extensions;
 using Coordinate = (long x, long y);
 using CoordinatePair = (long minX, long minY, long maxX, long maxY);
 
@@ -54,7 +55,7 @@ public class Day09 : IPuzzleSolver<long>
 
     private IEnumerable<CoordinatePair> GetAllPossibleRectanglesAsPairs(Coordinate[] redTiles)
     {
-        return Combinations(redTiles).Select(x => AsPair(x.a, x.b));
+        return redTiles.Combinations(2).Select(x => AsPair(x[0], x[1]));
     }
 
     private CoordinatePair[] GetBordersAsPairs(Coordinate[] redTiles)
@@ -84,17 +85,6 @@ public class Day09 : IPuzzleSolver<long>
         return (Math.Abs(tileA.x - tileB.x) + 1) * (Math.Abs(tileA.y - tileB.y) + 1);
     }
 
-    private IEnumerable<(TValue a, TValue b)> Combinations<TValue>(IEnumerable<TValue> values)
-    {
-        var enumeratedValues = values.Index().ToArray();
-        var pairs = 
-            from a in enumeratedValues
-            from b in enumeratedValues
-            where a.Index < b.Index
-            select (a.Item, b.Item);
-        return pairs;
-    }
-
     private IEnumerable<(Coordinate a, Coordinate b)> GetBorders(Coordinate[] tiles)
     {
         return tiles.Zip([..tiles[1..], tiles[0]], (a, b) => (a, b));

AoC_2025/Day10.cs

@@ -6,6 +6,7 @@ using System.Data.Common;
 using System.IO;
 using System.Linq;
 using AdvenOfCode.Contracts;
+using AdventOfCode.Extensions;
 using Microsoft.Z3;
 
 namespace AoC_2025;
@@ -214,7 +215,7 @@ public class Day10 : IPuzzleSolver<long>
         yield return ([..Enumerable.Repeat(0, patternLength)], 0);
         for (var i = 1; i <= buttons.Length; i++)
         {
-            foreach (var buttonPattern in Combinations(buttons, i))
+            foreach (var buttonPattern in buttons.Combinations(i))
             {
                 var joltageLevels = GetJoltageLevelsForButtonPattern(patternLength, buttonPattern);
                 yield return (joltageLevels, buttonPattern.Length);
@@ -271,50 +272,6 @@ public class Day10 : IPuzzleSolver<long>
         var answer = answerExpression is IntNum number ? number.Int64 : 0;
         return answer;
     }
-    
-    private IEnumerable<TValue[]> Combinations<TValue>(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;
-        }
-    }
 }
 
 public class IntArrayEqualityComparer : IEqualityComparer<int[]>