Day 12

This one is sloppier than I'd like, but these visual problems always throw me for a loop anyway. I was lucky I was able to use the ugly "num corners" trick quickly here.

src/12.cs

@@ -0,0 +1,210 @@
+using aoc2024.Util;
+
+namespace aoc2024;
+
+internal class Day12 : Day
+{
+    private char[][] grid = [];
+
+    internal override void Parse()
+    {
+        var lines = Util.Parsing.ReadAllLines($"{GetDay()}").ToList();
+        grid = new char[lines.Count][];
+        for (int y = 0; y < lines.Count; y++)
+        {
+            var row = new char[lines[y].Length];
+            for (int x = 0; x < lines[y].Length; x++)
+            {
+                row[x] = lines[y][x];
+            }
+            grid[y] = row;
+        }
+    }
+
+    internal override string Part1()
+    {
+        HashSet<ivec2> used = [];
+        HashSet<(char, HashSet<ivec2>)> plots = [];
+        for (int y = 0; y < grid.Length; y++)
+        {
+            for (int x = 0; x < grid[y].Length; x++)
+            {
+                var v = new ivec2(x, y);
+                if (used.Contains(v))
+                {
+                    continue;
+                }
+
+                var plot = (grid[y][x], new HashSet<ivec2>());
+                plots.Add(plot);
+                var q = new Queue<ivec2>();
+                q.Enqueue(v);
+                plot.Item2.Add(v);
+                while (q.Count > 0)
+                {
+                    var check = q.Dequeue();
+                    var neighbors = check.GetBoundedOrthogonalNeighbors(0, 0, grid[0].Length - 1, grid.Length - 1);
+                    foreach (var n in neighbors)
+                    {
+                        if (grid[n.y][n.x] == grid[y][x] && !plot.Item2.Contains(n))
+                        {
+                            q.Enqueue(n);
+                            plot.Item2.Add(n);
+                        }
+                    }
+                }
+
+                used.UnionWith(plot.Item2);
+            }
+        }
+
+        long total = 0;
+        foreach (var plot in plots)
+        {
+            var area = plot.Item2.Count;
+            var perimeter = 0;
+            foreach (var plotPt in plot.Item2)
+            {
+                perimeter += plotPt.GetOrthogonalNeighbors().Count(pt => !plot.Item2.Contains(pt));
+            }
+
+            var plotTotal = area * perimeter;
+            total += plotTotal;
+        }
+
+        return $"Area * perimeter cost: <+white>{total}";
+    }
+
+    private bool IsSame(char c, ivec2 pt)
+    {
+        if (!pt.IsWithinRange(0, 0, grid[0].Length - 1, grid.Length - 1))
+        {
+            return false;
+        }
+
+        return grid[pt.y][pt.x] == c;
+    }
+
+    private int NumCorners(IEnumerable<ivec2> points)
+    {
+        int numCorners = 0;
+        foreach (var pt in points)
+        {
+            var ch = grid[pt.y][pt.x];
+            // convex
+            {
+                // top-left
+                if (!IsSame(ch, pt - new ivec2(1, 0)) &&
+                    !IsSame(ch, pt - new ivec2(0, 1)))
+                {
+                    numCorners++;
+                }
+
+                // top-right
+                if (!IsSame(ch, pt + new ivec2(1, 0)) &&
+                    !IsSame(ch, pt - new ivec2(0, 1)))
+                {
+                    numCorners++;
+                }
+
+                // bottom-left
+                if (!IsSame(ch, pt - new ivec2(1, 0)) &&
+                    !IsSame(ch, pt + new ivec2(0, 1)))
+                {
+                    numCorners++;
+                }
+
+                // bottom-right
+                if (!IsSame(ch, pt + new ivec2(1, 0)) &&
+                    !IsSame(ch, pt + new ivec2(0, 1)))
+                {
+                    numCorners++;
+                }
+            }
+            // concave
+            {
+                // top-left
+                if (IsSame(ch, pt - new ivec2(1, 0)) &&
+                    IsSame(ch, pt - new ivec2(0, 1)) &&
+                    !IsSame(ch, pt - ivec2.ONE))
+                {
+                    numCorners++;
+                }
+                // top-right
+                if (IsSame(ch, pt + new ivec2(1, 0)) &&
+                    IsSame(ch, pt - new ivec2(0, 1)) &&
+                    !IsSame(ch, pt + new ivec2(1, -1)))
+                {
+                    numCorners++;
+                }
+                // bottom-left
+                if (IsSame(ch, pt - new ivec2(1, 0)) &&
+                    IsSame(ch, pt + new ivec2(0, 1)) &&
+                    !IsSame(ch, pt + new ivec2(-1, 1)))
+                {
+                    numCorners++;
+                }
+                // bottom-right
+                if (IsSame(ch, pt + new ivec2(1, 0)) &&
+                    IsSame(ch, pt + new ivec2(0, 1)) &&
+                    !IsSame(ch, pt + new ivec2(1, 1)))
+                {
+                    numCorners++;
+                }
+            }
+        }
+
+        return numCorners;
+    }
+
+    internal override string Part2()
+    {
+        HashSet<ivec2> used = [];
+        List<(char, HashSet<ivec2>)> plots = [];
+        for (int y = 0; y < grid.Length; y++)
+        {
+            for (int x = 0; x < grid[y].Length; x++)
+            {
+                var v = new ivec2(x, y);
+                if (used.Contains(v))
+                {
+                    continue;
+                }
+
+                var plot = (grid[y][x], new HashSet<ivec2>());
+                plots.Add(plot);
+                var q = new Queue<ivec2>();
+                q.Enqueue(v);
+                plot.Item2.Add(v);
+                while (q.Count > 0)
+                {
+                    var check = q.Dequeue();
+                    var neighbors = check.GetBoundedOrthogonalNeighbors(0, 0, grid[0].Length - 1, grid.Length - 1);
+                    foreach (var n in neighbors)
+                    {
+                        if (grid[n.y][n.x] != grid[y][x] || plot.Item2.Contains(n))
+                        {
+                            continue;
+                        }
+
+                        q.Enqueue(n);
+                        plot.Item2.Add(n);
+                    }
+                }
+
+                used.UnionWith(plot.Item2);
+            }
+        }
+
+        long total = 0;
+        foreach (var plot in plots)
+        {
+            var area = plot.Item2.Count;
+            var numSides = NumCorners(plot.Item2);
+
+            total += area * numSides;
+        }
+
+        return $"Area * sides cost: <+white>{total}";
+    }
+}