completed day 4

AdventOfCode2025/AdventOfCode2025.csproj

@@ -24,6 +24,10 @@
       <Content Include="Input\DayThree.txt">
         <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
       </Content>
+      <None Remove="Input\DayFour.txt" />
+      <Content Include="Input\DayFour.txt">
+        <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
+      </Content>
     </ItemGroup>
 
 </Project>

AdventOfCode2025/Lobby.cs

@@ -8,10 +8,13 @@ public class Lobby : IAdventSolution
     public AdventSolution Solve(string input)
     {
         var lines = input.SplitLines();
-        var total = 0;
+        var total = 0UL;
+        var total2 = 0UL;
         foreach (var line in lines)
         {
-            var first = '0';
+            total += GreatestXDigits(line, 2);
+            total2 += GreatestXDigits(line, 12);
+            /*var first = '0';
             var second = '0';
             for (var i = 0; i < line.Length; i++)
             {
@@ -25,11 +28,41 @@ public class Lobby : IAdventSolution
                     second = line[i];
                 }
             }
-            
+            total += int.Parse($"{first}{second}");*/
-            
-            total += int.Parse($"{first}{second}");
         }
         
-        return new AdventSolution(total.ToString(), null);
+        return new AdventSolution(total.ToString(), total2.ToString());
+    }
+
+    private readonly List<char> _chars = new();
+
+    private ulong GreatestXDigits(string input, int digits)
+    {
+        _chars.Clear();
+        _chars.Add(input[0]);
+        for (var i = 1; i < input.Length; i++)
+        {
+            var remainingChars = input.Length - i - 1;
+            var c = input[i];
+            if(!IsReplace(c, remainingChars, digits) && _chars.Count < digits)
+            {
+                _chars.Add(c);
+            }
+        }
+        
+        return ulong.Parse(string.Join("", _chars));
+    }
+
+    private bool IsReplace(char c, int remaining, int digits)
+    {
+        for (var i = 0; i < _chars.Count; i++)
+        {
+            if (c <= _chars[i] || (remaining < digits - (i+1))) continue;
+            _chars.RemoveRange(i, _chars.Count - i);
+            _chars.Add(c);
+            return true;
+        }
+
+        return false;
     }
 }

AdventOfCode2025/PrintingDepartment.cs

@@ -0,0 +1,57 @@
+using AdventOfCode2025.Utils;
+
+namespace AdventOfCode2025;
+
+public class PrintingDepartment : IAdventSolution
+{
+    private readonly HashSet<Coordinate> _existingRolls = new();
+    public AdventSolution Solve(string input)
+    {
+        var map = input.SplitLines();
+        for (var i = 0; i < map.Length; i++)
+        {
+            for (var j = 0; j < map[i].Length; j++)
+            {
+                if(map[i][j] != '.')
+                    _existingRolls.Add(new Coordinate(j, i));
+            }
+        }
+        var initialCount = _existingRolls.Count;
+        var accessible = -1;
+        var firstAccessible = -1;
+        var toRemove = new HashSet<Coordinate>();
+        while(accessible != 0 && _existingRolls.Count > 0)
+        {
+            accessible = 0;
+            foreach (var c in _existingRolls)
+            {
+                if (!IsAccessible(c)) continue;
+                accessible++;
+                toRemove.Add(c);
+            }
+            
+            if (firstAccessible == -1) firstAccessible = accessible;
+            
+            _existingRolls.RemoveWhere(x => toRemove.Contains(x));
+            toRemove.Clear();
+        } 
+
+        return new AdventSolution(firstAccessible.ToString(), (initialCount - _existingRolls.Count).ToString());
+    }
+
+    private readonly Coordinate[] _directions =
+    [
+        new(-1, -1), new( 0, -1), new( 1, -1),
+        new(-1,  0),              new( 1,  0),
+        new(-1,  1), new( 0,  1), new( 1,  1)
+    ];
+    private bool IsAccessible(Coordinate position)
+    {
+        var neighbors = 0;
+        foreach (var dir in _directions)
+        {
+            if(_existingRolls.Contains(position + dir)) neighbors++;
+        }
+        return neighbors < 4;
+    }
+}

AdventOfCode2025/Program.cs

@@ -10,3 +10,6 @@ AdventSolver<GiftShop>.Solve(dayTwoInput, 2, "Gift Shop");
 
 var dayThreeInput = await File.ReadAllTextAsync("./Input/DayThree.txt");
 AdventSolver<Lobby>.Solve(dayThreeInput, 3, "Lobby");
+
+var dayFourInput = await File.ReadAllTextAsync("./Input/DayFour.txt");
+AdventSolver<PrintingDepartment>.Solve(dayFourInput, 4, "Printing Department");

AdventOfCode2025/Utils/Coordinate.cs

@@ -0,0 +1,16 @@
+using System.Numerics;
+
+namespace AdventOfCode2025.Utils;
+
+public record struct Coordinate(int X, int Y) : IAdditionOperators<Coordinate,Coordinate,Coordinate>, ISubtractionOperators<Coordinate,Coordinate,Coordinate>
+{
+    public static Coordinate operator +(Coordinate left, Coordinate right)
+    {
+        return new Coordinate(left.X + right.X, left.Y + right.Y);
+    }
+
+    public static Coordinate operator -(Coordinate left, Coordinate right)
+    {
+        return new Coordinate(left.X - right.X, left.Y - right.Y);
+    }
+}