Day 11 solution

Pretty happy with this one overall. I tripped myself up on part 2 and caused a huge headache by not realizing that I was passing the already-modified array from part 1 into part 2. I left some code to copy the 2d array because that was my original solution (and it's not straightforward to copy jagged arrays), but then realized that the synchronized flash doesn't happen until after step 100 (at least for my input and the demo input) so we can just account for that in part 2 and pick up where part 1 left off.

advent-of-code-2021.csproj

@@ -48,6 +48,9 @@
 	  <None Update="inputs\10.txt">
 	    <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
 	  </None>
+	  <None Update="inputs\11.txt">
+	    <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
+	  </None>
 	</ItemGroup>
 
 </Project>

inputs/11.txt

@@ -0,0 +1,10 @@
+3265255276
+1537412665
+7335746422
+6426325658
+3854434364
+8717377486
+4522286326
+6337772845
+8824387665
+6351586484

src/11.cs

@@ -0,0 +1,142 @@
+namespace aoc2021;
+
+internal class Day11
+{
+    internal static void Go()
+    {
+        Logger.Log("Day 11");
+        Logger.Log("-----");
+
+        var lines = File.ReadAllLines("inputs/11.txt");
+        var grid = new byte[lines.Length, lines[0].Length];
+        for (int i = 0; i < lines.Length; i++)
+        {
+            var line = lines[i];
+            for (int j = 0; j < line.Length; j++)
+            {
+                var num = line[j];
+                grid[i, j] = (byte)char.GetNumericValue(num);
+            }
+        }
+
+        //var gridCopy = new byte[lines.Length, lines[0].Length];
+        //Buffer.BlockCopy(grid, 0, gridCopy, 0, grid.Length);
+
+        Part1(grid);
+        Part2(grid);
+
+        Logger.Log("");
+    }
+
+    private static IEnumerable<(int, int)> Adjacent(byte[,] grid, int i, int j)
+    {
+        if (i > 0)
+        {
+            if (j > 0)
+            {
+                yield return (i - 1, j - 1);
+            }
+            yield return (i - 1, j);
+            if (j < grid.GetLength(1) - 1)
+            {
+                yield return (i - 1, j + 1);
+            }
+        }
+        if (j > 0)
+        {
+            yield return (i, j - 1);
+        }
+        if (j < grid.GetLength(1) - 1)
+        {
+            yield return (i, j + 1);
+        }
+        if (i < grid.GetLength(1) - 1)
+        {
+            if (j > 0)
+            {
+                yield return (i + 1, j - 1);
+            }
+            yield return (i + 1, j);
+            if (j < grid.GetLength(1) - 1)
+            {
+                yield return (i + 1, j + 1);
+            }
+        }
+    }
+
+    private static void Flash(byte[,] grid, int i, int j, List<(int, int)> flashed)
+    {
+        flashed.Add((i, j));
+        foreach (var pt in Adjacent(grid, i, j))
+        {
+            grid[pt.Item1, pt.Item2]++;
+            if (grid[pt.Item1, pt.Item2] == 10)
+            {
+                Flash(grid, pt.Item1, pt.Item2, flashed);
+            }
+        }
+    }
+
+    private static void Part1(byte[,] grid)
+    {
+        using var t = new Timer();
+
+        long numFlashes = 0;
+        for (int step = 0; step < 100; step++)
+        {
+            var flashed = new List<(int, int)>();
+            for (int i = 0; i < grid.GetLength(0); i++)
+            {
+                for (int j = 0; j < grid.GetLength(1); j++)
+                {
+                    grid[i, j]++;
+                    if (grid[i, j] == 10)
+                    {
+                        Flash(grid, i, j, flashed);
+                    }
+                }
+            }
+
+            foreach (var pt in flashed)
+            {
+                grid[pt.Item1, pt.Item2] = 0;
+                numFlashes++;
+            }
+        }
+
+        Logger.Log($"part1: {numFlashes}");
+    }
+
+    private static void Part2(byte[,] grid)
+    {
+        using var t = new Timer();
+
+        int step = 101;
+        for (; ; step++)
+        {
+            var flashed = new List<(int, int)>();
+            for (int i = 0; i < grid.GetLength(0); i++)
+            {
+                for (int j = 0; j < grid.GetLength(1); j++)
+                {
+                    grid[i, j]++;
+                    if (grid[i, j] == 10)
+                    {
+                        Flash(grid, i, j, flashed);
+                    }
+                }
+            }
+
+            if (flashed.Count == grid.GetLength(0) * grid.GetLength(1))
+            {
+                break;
+            }
+            foreach (var pt in flashed)
+            {
+                grid[pt.Item1, pt.Item2] = 0;
+            }
+        }
+
+        Logger.Log($"part2: {step}");
+    }
+}

src/main.cs

@@ -33,7 +33,11 @@ switch (arg)
         aoc2021.Day09.Go();
         break;
 
-    default:
+    case "10":
         aoc2021.Day10.Go();
         break;
+
+    default:
+        aoc2021.Day11.Go();
+        break;
 }