Day 23 solution

Part 2 takes 7 seconds on my macbook, which I'd like to improve, but this gets the answer.

advent-of-code-2022.csproj

@@ -91,6 +91,8 @@
 	  <EmbeddedResource Include="inputs\22a.txt" />
 	  <EmbeddedResource Include="inputs\23.txt" />
 	  <None Remove="inputs\24.txt" />
+	  <EmbeddedResource Include="inputs\23a.txt" />
+	  <EmbeddedResource Include="inputs\23b.txt" />
 	  <EmbeddedResource Include="inputs\24.txt" />
 	  <None Remove="inputs\25.txt" />
 	  <EmbeddedResource Include="inputs\25.txt" />

inputs/23.txt

@@ -1,5 +1,74 @@
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inputs/23a.txt

@@ -0,0 +1,7 @@
+....#..
+..###.#
+#...#.#
+.#...##
+#.###..
+##.#.##
+.#..#..

inputs/23b.txt

@@ -0,0 +1,6 @@
+.....
+..##.
+..#..
+.....
+..##.
+.....

src/23.cs

@@ -0,0 +1,129 @@
+using System.Collections.Immutable;
+using aoc2022.Util;
+
+namespace aoc2022;
+
+internal class Day23 : Day
+{
+    private readonly HashSet<ivec2> elves = new();
+
+    private static readonly ivec2[][] dirOrder =
+    {
+        new[]{ivec2.UP, ivec2.UP + ivec2.RIGHT, ivec2.UP + ivec2.LEFT},
+        new[]{ivec2.DOWN, ivec2.DOWN + ivec2.RIGHT, ivec2.DOWN + ivec2.LEFT},
+        new[]{ivec2.LEFT, ivec2.UP + ivec2.LEFT, ivec2.DOWN + ivec2.LEFT},
+        new[]{ivec2.RIGHT, ivec2.UP + ivec2.RIGHT, ivec2.DOWN + ivec2.RIGHT},
+    };
+
+    internal override void Parse()
+    {
+        var lines = Parsing.ReadAllLines("23").ToImmutableList();
+        for (int row = 0; row < lines.Count; row++)
+        {
+            var line = lines[row];
+            for (int col = 0; col < line.Length; col++)
+            {
+                if (line[col] == '#')
+                {
+                    elves.Add(new ivec2(col, row));
+                }
+            }
+        }
+    }
+
+    private static List<(ivec2 curr, ivec2 proposed)> ConsiderMove(ICollection<ivec2> positions, int orderStart)
+    {
+        List<(ivec2 curr, ivec2 proposed)> considerations = new(positions.Count);
+
+        foreach (var elfPos in positions)
+        {
+            if (!elfPos.GetNeighbors().Any(positions.Contains))
+            {
+                continue;
+            }
+
+            for (int i = 0; i < dirOrder.Length; i++)
+            {
+                var dirs = dirOrder[(i + orderStart) % dirOrder.Length];
+                if (!dirs.Any(d => positions.Contains(d + elfPos)))
+                {
+                    considerations.Add((elfPos, elfPos + dirs[0]));
+                    break;
+                }
+            }
+        }
+
+        return considerations;
+    }
+
+    private static int PlayOut(ICollection<ivec2> positions, int? maxRounds = null)
+    {
+        int round;
+        for (round = 0; (maxRounds == null || round < maxRounds); round++)
+        {
+            var considerations = ConsiderMove(positions, round);
+            if (!considerations.Any())
+            {
+                break;
+            }
+
+            for (int i = 0; i < considerations.Count; i++)
+            {
+                bool bDuped = false;
+                for (int j = i + 1; j < considerations.Count; j++)
+                {
+                    if (considerations[i].proposed == considerations[j].proposed)
+                    {
+                        bDuped = true;
+                        considerations.RemoveAt(j);
+                        j--;
+                    }
+                }
+
+                if (bDuped)
+                {
+                    considerations.RemoveAt(i);
+                    i--;
+                }
+            }
+
+            foreach (var move in considerations)
+            {
+                positions.Remove(move.curr);
+                positions.Add(move.proposed);
+            }
+        }
+
+        return round + 1;
+    }
+
+    private static long GetEmptySpots(ICollection<ivec2> positions)
+    {
+        var minY = positions.MinBy(p => p.y).y;
+        var minX = positions.MinBy(p => p.x).x;
+        var maxY = positions.MaxBy(p => p.y).y;
+        var maxX = positions.MaxBy(p => p.x).x;
+
+        var height = (maxY - minY) + 1;
+        var width = (maxX - minX) + 1;
+        var area = height * width;
+        var emptyGround = area - positions.Count;
+
+        return emptyGround;
+    }
+
+    internal override string Part1()
+    {
+        HashSet<ivec2> positions = new(elves);
+        PlayOut(positions, 10);
+        var emptyGround = GetEmptySpots(positions);
+        return $"Empty ground spaces after 10 rounds: <+white>{emptyGround}";
+    }
+
+    internal override string Part2()
+    {
+        HashSet<ivec2> positions = new(elves);
+        var round = PlayOut(positions);
+        return $"Rounds to settle: <+white>{round}";
+    }
+}

src/Util/Vec2.cs

@@ -58,6 +58,22 @@ public struct ivec2 : IEquatable<ivec2>, IComparable<ivec2>, IComparable
         return this + dir;
     }
 
+    public IEnumerable<ivec2> GetOrthogonalNeighbors()
+    {
+        foreach (var dir in FOURWAY)
+        {
+            yield return this + dir;
+        }
+    }
+
+    public IEnumerable<ivec2> GetNeighbors()
+    {
+        foreach (var dir in EIGHTWAY)
+        {
+            yield return this + dir;
+        }
+    }
+
     public long this[long i] => (i == 0) ? x : y;
 
     public static ivec2 operator +(ivec2 v) => v;