day 24

aoc2020.test/DayTests.cs

@@ -30,7 +30,7 @@ public class DayTests
     [DataRow(typeof(Day21), "2436", "dhfng,pgblcd,xhkdc,ghlzj,dstct,nqbnmzx,ntggc,znrzgs")]
     [DataRow(typeof(Day22), "32856", "33805")]
     [DataRow(typeof(Day23), "36542897", "562136730660")]
-    //[DataRow(typeof(Day24), "", "")]
+    [DataRow(typeof(Day24), "282", "3445")]
     //[DataRow(typeof(Day25), "", "")]
     public void CheckAllDays(Type dayType, string part1, string part2)
     {

aoc2020/Day24.cs

@@ -5,11 +5,80 @@ namespace aoc2020;
 /// </summary>
 public sealed class Day24 : Day
 {
+    private static readonly Dictionary<string, (int q, int r, int s)> Directions = new()
+    {
+        { "e", (1, 0, -1) },
+        { "w", (-1, 0, 1) },
+        { "se", (0, 1, -1) },
+        { "sw", (-1, 1, 0) },
+        { "nw", (0, -1, 1) },
+        { "ne", (1, -1, 0) }
+    };
+
+    private Dictionary<(int q, int r, int s), Tile> _tiles;
+
     public Day24() : base(24, "Lobby Layout")
     {
+        _tiles = Input
+            .Select(Tile.FromLine)
+            .GroupBy(t => t.Location)
+            .Where(g => g.Count() % 2 == 1)
+            .Select(t => t.First())
+            .ToDictionary(t => t.Location);
     }
 
-    public override string Part1() => "";
+    public override string Part1() => $"{_tiles.Count}";
 
-    public override string Part2() => "";
-}
+    public override string Part2()
+    {
+        foreach (var _ in Enumerable.Range(0, 100))
+        {
+            _tiles = _tiles
+                .SelectMany(t => Directions.Select(d => t.Value + d.Value))
+                .Distinct()
+                .Where(t =>
+                {
+                    var neighborCount = Directions
+                        .Select(d => t + d.Value)
+                        .Count(neighbor => _tiles.ContainsKey(neighbor.Location));
+
+                    return neighborCount == 2 || _tiles.ContainsKey(t.Location) && neighborCount == 1;
+                })
+                .ToDictionary(t => t.Location);
+        }
+
+        return $"{_tiles.Count}";
+    }
+
+    private record Tile
+    {
+        public (int q, int r, int s) Location { get; private init; }
+
+        public static Tile FromLine(string route)
+        {
+            (int q, int r, int s) location = (0, 0, 0);
+            var direction = "";
+            foreach (var c in route)
+            {
+                if (c is 'n' or 's')
+                {
+                    direction += c;
+                    continue;
+                }
+
+                direction += c;
+                var (q, r, s) = Directions[direction];
+                location = (location.q + q, location.r + r, location.s + s);
+                direction = "";
+            }
+
+            return new Tile { Location = location };
+        }
+
+        public static Tile operator +(Tile t, (int q, int r, int s) direction) =>
+            t with
+            {
+                Location = (t.Location.q + direction.q, t.Location.r + direction.r, t.Location.s + direction.s)
+            };
+    }
+}

aoc2020/aoc2020.csproj

@@ -14,10 +14,10 @@
   </ItemGroup>
 
 	<ItemGroup>
-		<Using Include="System.Collections.Generic"/>
-		<Using Include="System.Collections.Immutable"/>
-		<Using Include="System.Text"/>
-		<Using Include="System.Text.RegularExpressions"/>
+		<Using Include="System.Collections.Generic" />
+		<Using Include="System.Collections.Immutable" />
+		<Using Include="System.Text" />
+		<Using Include="System.Text.RegularExpressions" />
 	</ItemGroup>
 
 </Project>