Day 6

Part 2 gave me more trouble than it should have, mostly because I was initially turning and stepping at the same time which ended up being a big mistake.

This runs in a few seconds in Debug, but only a half a second in Release. I feel like it can be much faster, but I don't have any ideas on doing that just yet.

src/06.cs

@@ -0,0 +1,102 @@
+using aoc2024.Util;
+
+namespace aoc2024;
+
+internal class Day06 : Day
+{
+    private bool[][] grid = [];
+    private ivec2 startPos;
+    private ivec2 startDir;
+    internal override void Parse()
+    {
+        var lines = Util.Parsing.ReadAllLines($"{GetDay()}").ToList();
+        grid = new bool[lines.Count][];
+        for (int i = 0; i < lines.Count; i++)
+        {
+            var line = lines[i];
+            var row = new bool[line.Length];
+            for (int j = 0; j < line.Length; j++)
+            {
+                if (line[j] == '#')
+                {
+                    row[j] = true;
+                }
+                else
+                {
+                    row[j] = false;
+                    if (line[j] != '.')
+                    {
+                        startPos = new ivec2(j, i);
+                        startDir = ivec2.DirFromChar(line[j]);
+                    }
+                }
+            }
+
+            grid[i] = row;
+        }
+    }
+
+    private (bool, HashSet<ivec2>) HasLoop()
+    {
+        var currPos = startPos;
+        var currDir = startDir;
+        var loopSet = new HashSet<(ivec2, ivec2)>(10000);
+        var visited = new HashSet<ivec2>(10000);
+        while (true)
+        {
+            visited.Add(currPos);
+            if (!loopSet.Add((currPos, currDir)))
+            {
+                return (true, visited);
+            }
+
+            var nextPos = currPos + currDir;
+            if (!nextPos.IsWithinRange(0, 0, grid[0].Length - 1, grid.Length - 1))
+            {
+                return (false, visited);
+            }
+
+            if (grid[nextPos.y][nextPos.x])
+            {
+                currDir = currDir.GetRotatedRight();
+            }
+            else
+            {
+                currPos += currDir;
+            }
+        }
+    }
+
+    internal override string Part1()
+    {
+        var (_, visited) = HasLoop();
+        return $"Locations visited: <+white>{visited.Count}";
+    }
+
+    internal override string Part2()
+    {
+        var (_, allVisited) = HasLoop();
+
+        int numBlockedLoops = 0;
+        foreach (var pos in allVisited)
+        {
+            var (x, y) = (pos.x, pos.y);
+            if (startPos == pos)
+            {
+                continue;
+            }
+
+            grid[y][x] = true;
+
+            var (hasLoop, _) = HasLoop();
+            if (hasLoop)
+            {
+                numBlockedLoops++;
+            }
+
+            grid[y][x] = false;
+        }
+
+        return $"# obstructions that cause a loop: <+white>{numBlockedLoops}";
+    }
+}

src/Util/Vec2.cs

@@ -38,6 +38,15 @@ public readonly struct ivec2 : IEquatable<ivec2>, IComparable<ivec2>, IComparabl
     public long ManhattanDistance => Abs(this).Sum;
     public long ManhattanDistanceTo(ivec2 other) => System.Math.Abs(x - other.x) + System.Math.Abs(y - other.y);
 
+    public static ivec2 DirFromChar(char ch) => ch switch
+    {
+        '^' => UP,
+        '>' => RIGHT,
+        '<' => LEFT,
+        'v' => DOWN,
+        _ => throw new FormatException($"Invalid direction {ch}"),
+    };
+
     public ivec2 GetBestDirectionTo(ivec2 p)
     {
         ivec2 diff = p - this;