Day 10 (Part 2)

Day10_2/Day10_2.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>net8.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <None Remove="input.txt" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <Content Include="input.txt">
+      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
+    </Content>
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\Tools\Tools.csproj" />
+  </ItemGroup>
+
+</Project>

Day10_2/Program.cs

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+using Tools;
+
+var input = File.ReadAllLines("input.txt");
+
+var width = input[0].Length;
+var height = input.Length;
+
+var map = new char[width, height];
+var distance = new int[width, height];
+var mainLoop = new bool[width, height];
+
+var startingPosition = new Point();
+
+for (int x = 0; x < width; x++)
+{
+    for (int y = 0; y < height; y++)
+    {
+        distance[x, y] = -1;
+        map[x, y] = input[y][x];
+        if (map[x, y] == 'S')
+        {
+            startingPosition = new Point(x, y);
+        }
+    }
+}
+
+var loopLength = 0;
+
+var stack = new Stack<(Point previous, Point current, int steps)>();
+stack.Push((startingPosition, startingPosition, 0));
+while (stack.Count > 0)
+{
+    var (previousPosition, currentPosition, steps) = stack.Pop();
+    if (map[currentPosition.X, currentPosition.Y] == 'S' &&
+        distance[currentPosition.X, currentPosition.Y] != -1)
+    {
+        loopLength = steps;
+        break;
+    }
+
+    distance[currentPosition.X, currentPosition.Y] = steps;
+    steps++;
+
+    foreach (var direction in Directions.WithoutDiagonals)
+    {
+        var nextPosition = currentPosition + direction;
+        if (nextPosition.X < 0 || nextPosition.X >= width || nextPosition.Y < 0 || nextPosition.Y >= height)
+        {
+            continue;
+        }
+
+        if (nextPosition == previousPosition)
+        {
+            continue;
+        }
+
+        if (map[nextPosition.X, nextPosition.Y] == '.')
+        {
+            continue;
+        }
+
+        if (map[nextPosition.X, nextPosition.Y] != 'S' && distance[nextPosition.X, nextPosition.Y] != -1)
+        {
+            continue;
+        }
+
+        var currentPipe = map[currentPosition.X, currentPosition.Y];
+        var nextPipe = map[nextPosition.X, nextPosition.Y];
+
+        bool isValid = false;
+        if (direction == new Point(1, 0))
+        {
+            var nextPipeIsValid = (nextPipe == '7' || nextPipe == 'J' || nextPipe == '-' || nextPipe == 'S');
+            var currentPipeIsValid = (currentPipe == 'L' || currentPipe == '-' || currentPipe == 'S' || currentPipe == 'F');
+            isValid = nextPipeIsValid && currentPipeIsValid;
+        }
+        else if (direction == new Point(-1, 0))
+        {
+            var nextPipeIsValid = (nextPipe == 'F' || nextPipe == 'L' || nextPipe == '-' || nextPipe == 'S');
+            var currentPipeIsValid = (currentPipe == 'J' || currentPipe == '7' || currentPipe == 'S' || currentPipe == '-');
+            isValid = nextPipeIsValid && currentPipeIsValid;
+        }
+        else if (direction == new Point(0, 1))
+        {
+            var nextPipeIsValid = (nextPipe == 'L' || nextPipe == 'J' || nextPipe == '|' || nextPipe == 'S');
+            var currentPipeIsValid = (currentPipe == 'F' || currentPipe == '|' || currentPipe == 'S' || currentPipe == '7');
+            isValid = nextPipeIsValid && currentPipeIsValid;
+        }
+        else if (direction == new Point(0, -1))
+        {
+            var nextPipeIsValid = (nextPipe == 'F' || nextPipe == '7' || nextPipe == '|' || nextPipe == 'S');
+            var currentPipeIsValid = (currentPipe == 'L' || currentPipe == 'J' || currentPipe == 'S' || currentPipe == '|');
+            isValid = nextPipeIsValid && currentPipeIsValid;
+        }
+
+        if (isValid)
+        {
+            stack.Push((currentPosition, nextPosition, steps));
+        }
+    }
+}
+
+var originalLoopLength = loopLength;
+mainLoop[startingPosition.X, startingPosition.Y] = true;
+var loopPosition = startingPosition;
+while (loopLength > 0)
+{
+    loopLength--;
+    foreach (var direction in Directions.WithoutDiagonals)
+    {
+        var nextPosition = loopPosition + direction;
+        if (nextPosition.X < 0 || nextPosition.X >= width || nextPosition.Y < 0 || nextPosition.Y >= height)
+        {
+            continue;
+        }
+
+        if (distance[nextPosition.X, nextPosition.Y] == loopLength)
+        {
+            mainLoop[nextPosition.X, nextPosition.Y] = true;
+            loopPosition = nextPosition;
+            break;
+        }
+    }
+}
+
+long counter = 0;
+for (var y = 0; y < height; y++)
+{
+    for (var x = 0; x < width; x++)
+    {
+        if (!mainLoop[x, y])
+        {
+            if (InMainLoop(x, y))
+            {
+                counter++;
+                Console.Write('I');
+            }
+            else
+            {
+                Console.Write('O');
+            }
+        }
+        else
+        {
+            Console.Write('X');
+        }
+    }
+    Console.WriteLine();
+}
+
+Console.WriteLine(counter);
+
+bool InMainLoop(int x, int y)
+{
+    var crossings = 0;
+    x--;
+    for (; x >= 0; x--)
+    {
+        if (mainLoop[x, y] && (map[x, y] == '|' || map[x, y] == 'J' || map[x, y] == 'L' ))
+        {
+            crossings++;
+        }
+    }
+
+    return crossings % 2 != 0;
+}

Day10_2/Properties/launchSettings.json

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+{
+  "profiles": {
+    "Run": {
+      "commandName": "Project"
+    }
+  }
+}