Day 24. Overall pleased with this solution.

Definitely could use some optimization in not creating so many new maps,
but runtime is acceptable for now.

2020.csproj

@@ -74,6 +74,9 @@
     <None Update="23input.txt">
       <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
     </None>
+    <None Update="24input.txt">
+      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
+    </None>
   </ItemGroup>
 
 </Project>

24input.txt

@@ -0,0 +1,292 @@
+nwswswnenwswsweswseswswnwswsweswsw
+swseswsenwswnenwsesesenwseseesesenwenw
+seswseswseswswseswwswneseswsese
+nwwwsewenwwwwswwwe
+seswwwseeseseseseseseswseswneneesew
+eenewewweseneneneenene
+nenenenenwnwwswseenwwneneenwneswnwnw
+swneswwswwnewwwswwswwww
+seseweewwnwwnwwsewwnwnenewnwne
+nwwewewnwnwwwwnwswnwwsenwneww
+sewnewswswwsenwsenenwneswswsesweenw
+nwwnwnenwnwwnwsewnwseewsenwsesenwnwnw
+enwnenwnwswnwnenesenwneswnenwnenwwnenese
+neneneneneswneneenwenenenenenenewnwse
+nwenwswsenwwnwwnwwnwwnwneww
+wnwnwnwnwnenenwnesenwnwnwneswnwnenwsewne
+wnwnwnwsesenwwwnenewnwnwnwnww
+wnewwwswwwwwsw
+nenwnenenewnenwnesenesenewnenenw
+eeseeseeesenwesesese
+seneseneneeeneewwswenweeneeneenene
+wswsesenwseeeeseseseseseswnewseswnwnwse
+swnenewwneneneneneneneseenenenenenee
+swenwwwwewweswswwswswwswwnww
+neswnenenenwswnwnwnwneneneneewnwneenw
+eeesewswseenweewnwweeseene
+swnwseeswneenwsesenwseeswnw
+enwwnwwwswnwnwnenenenenwnenwnesesesenw
+swwwwwwnewnewewwnewwsesw
+eneseswwneseewnwnweswswwwnenwnenwne
+sewsesesesenwnesewnweeswswswneseswwsw
+nwneswnwnenenwnenweneswnesenwswnewnw
+seneneseseseewsenenwswsewseswswsenesw
+nwneseneenesewwnesenene
+swwnwwsenwewswneewnweweswewnewse
+nenenwseneswnwnwnwsenwwnwnenwnenenenw
+neneneswneneenweesweneeeewenenw
+swswswsenewseseseewswnesewswswseseswnw
+eswwnwnwwswnwwwnwnwewwnw
+swneseswenwseswsesesenwneseseseseswsese
+nesenesenewnenenenwsenewnewsenwwnwne
+swswseswseseswswnwseswsenesweswsw
+swnwnwwswwswswswswwswsweneseswswnewne
+nenenenenenwswneneswsenwnesenenenwnenenwnee
+newnwneneeswneswnenwwsenenwneneenwnenw
+swneeesesenesesewseseeesewse
+nenwnwswseeenwseswneenewwene
+enenenenenenenwnenesw
+nwwswwwseneewwwwnwwwswnenweww
+sewseneeesenwesesesesewneswseeew
+swnwnweneswneneseswnwnenenenenenenwnese
+nwsenwneseswwewnweswneewnwnwnwenw
+wsenwwnwwnwenwwwnweswnw
+swswwewnwnenesesewsesewneesewnene
+neswnwwseswenenenwneseneeneenewsesww
+nwwwsesenesesesesesenwseneeswnwsenewse
+wnwswneswswswsweswne
+enesenweneswsewneenenenwswnewnwnwsw
+enenwnwswsenenwnwswnwwnwnwnwnwwnwwnw
+wswswswwswwsesenwnwswnewsewswswnese
+swwswneswswswneswswswsw
+swswseswsesesenwseeeswseseswnwseseswnewsw
+nwseeseseseswwesesesesesesesenesesenwe
+swnwewewwwnwnwwnwnenwwwnwsewnw
+swswswswnwswsenwswwweswwswsw
+swenwswnesenwswesweseswwnewswenwswswnw
+enewswnwwwwswswneewneewsw
+nwsewnenwnwwnwnwnwenwnwnwnwnwswsesenwnw
+wneeseseswseseesenweeseenwsenesewwne
+senenwnwnenwnwnwnewnwnwnwnwenwnwswnesw
+seseseseeseseseneseewse
+neneeeneewweeneeneesesw
+swswnwseswswwneseswswswswnwneswseswwswe
+wsenenwwwseseeseswsewneneneneseswse
+seeseeneeeswswseseenwneewneeseese
+eeseeeeswsenweweenwneeewseene
+wnenewnenenenesesenewnwnwnenenenwsenwne
+neneneeswswnenenenenwenwnenwneneneswne
+nwnwnwnwswwwnwnwwneenwwwswnwnwsenw
+eneweneeneneenesweneneneesweswnww
+neseseseseswsesesenwseseswseseseseswnew
+seseseswswseseswsewne
+swnwnewsweswswweswswswswwswswsesenwswsw
+neewnwswnwswswewswnenwseswnwswswswsesw
+esewnewwwwwswwwwwnwseewwwne
+swswswnwswesweswswsweswswswswsewswnwsw
+swneswswwnwswswswwswneswsweewswswsw
+swwwesesewwswseneseneseseseeswsesesw
+nwswnwnwnweswwnwnwenwnwnwnwe
+nwwwnenwseseswnewsenesesewneswswwwe
+nwnenenenwsenwwnenwnewseneewse
+wwnwswnewwwwnw
+swneneswnesenenenewnesenwewnewnenene
+nenwneeneneesenwnesenewsw
+neseseeewswweesesese
+seenwsesesenwseeseseseseesesese
+wwwwwswnewswswwwwnwswnewesw
+swswwswswseswsenwwwseneeneswseswnwneese
+nesenweseswesewsenwwsesesesesesee
+neswseswsenwsweseneswswswseswswswswswwwnw
+sesesewswseneswnwswswswswnwsesenwswesesw
+nwwswesenewnwswswnwswwnwenenwne
+eeeeneeweeeneneseeeenw
+seseneseswwsewnwneseeeseneseswsenesw
+nwnwneenwwnewewenwwnwnenwsesenenenw
+neneneneswnenenwneneneeneneswneswswnene
+wswseewneneseenewsesewwwswnwnwnwnwnw
+nwnwnwnwnwnenwnwswnwnenenwsenwe
+swnwnweswnwseeeeenwsesewswwnenesw
+ewswseswnwwnwwnwnwsweweeenwnenw
+wnewwnwneswsenwsewwenewnwnwwwww
+nwnenenenewnwsenwnenwnwnesewnenwnwnwnwse
+swsenewneswneneneenwnw
+sweseswneeeneesenwnwneneenwnesenww
+seswswneswswseswwseswnenwseseneswswswswse
+nenwsenenwneneewneneneneneneswswene
+wsewwwwwnewwwwwwwnw
+swswseseeswswswwswsesesesewneneseswnw
+nenweswnwnenenwnwnenwnewsewwnenenwe
+neeewnenwnwnwnwnwsenwnwnwswwnwnwnwnw
+swsenwswswneseswenewewsenwwnesewsenwne
+wswwswwweeswswwswswnwnwnwene
+nwneneneneseswnenwnesweswnenenwnenesene
+wseseswswsewseneeswneewswseseswsene
+swsenweeneswswsesesene
+weswswsenewnwwewwswswenwsewnwnesw
+neswnewsenwewnesenwsweseeseene
+seswswseseswwneswseenweswswswswswnwnw
+eseswswsweeswseswswsenwswswswswsewsenw
+enewnwswsesweneswseswseenwsewnwsee
+neseneswseweseenweeweesweeenew
+wnwenesenwswnwnewsenwnwswnwnesewnwwese
+swnwswswseeseseeneswswswswswswswswsww
+wsesewwneseswswswseswnwswseeswneswewsw
+nwnenenwnwenwswnwnenewnwnwswnwsesenwnww
+eswseseswsewseseswsesesesenese
+nwswswseeswswseswswsenweseewnwsesese
+seseeeeenewswnwwnwswsenwnenwnwnwwsesw
+neeweewnenwneenwnwsenwsenwsesesene
+nenwseeseenwenwseeswswnweeeneeswse
+swswseswswswswswenwwwswnwswneswswesww
+swswneswnwswswswswswseewswewswwnene
+nwneeneenwnewswnenwnenwswnenwnwnwenwne
+nenenenenwnenesweneseneneneswnwnenenenw
+ewseesweenweneseenwesweewwew
+eseseeseseseeweesewseesesene
+wneeneneenwweswenwwseneeenesesese
+nwnwnenesenenenwwnwwnwsenwnwnwnenenwse
+senewnwseesenwwseseswesewesesenenesw
+seneeewnwwwsewnenwewnwswswse
+neswwewwwwwwwswsweewwwww
+wwsweswwswewwsenewwswwswswswnw
+nwnwnwnwneneswnwsenwenwnwnwneenewnwnw
+senwwnwneneeswneneeeweeenenenesw
+nweneswneneswneenwnewnwnwnwnwnenenwnesenw
+sewneswnweweswnesewnwwwwnewswsww
+wneswswswwswsewswwswswswewswsw
+senwneseswseeseswneseswneswswswswswsenw
+ewwsweneenwsene
+seeneeseeenewnwewswnwswswsesenww
+wsesewwsewwswwnewswwnewwwwwnew
+wnwsewwwnwwnewsenwnwnewnwnesenwnw
+swswseseswsenewsesesesesesesesenwsenwse
+nwwwwneswwwwwseswewwwwnenew
+seewneeeeseenwsewenwsesw
+nwnenwwneneseswnwew
+neeenwenwesweeeeeeeseesw
+nwnwwswewswnwnenwnwnwnwsenene
+ewnwsesesesenwswesesesewswseesesese
+neenwwsewsenwseeeeewwswneeneswne
+swsenwseeswsewnwweneseneseeswsesesee
+swsenwnweseswseenwseswseseseenwwsesene
+eseenenenwneswnwnwneeeeneseneenew
+swnenesweeeswneswenenwswneeeeseswse
+sewsesesweeeeweeneeneseswesee
+swwwwwwewnwnwwwewwwwnw
+seswswenenwnweeneneneneswwenenenenenwne
+seseeeseswwneeesenww
+nwnwnwswneenwnenwneneswnwswnwsw
+swswswswseswneswswswswnwseswsww
+wnwwnwwsenwnwwneenwnwnwsesenwnwenww
+eseeseeesewseseswesesenwnenwesese
+swswnenwnenenwnenwenenenwenenwnwswnwnwswne
+newwsewwwnwwnewsew
+swswswseseswwseswswneswneneswnwnwswsesw
+eeeesenwswseeeeewwseeneeeee
+neswswseswneeswswneswswseswswww
+neseeesesesewnwseswsesesewseseswwnwsee
+nwnwnesenenwnwsesweswswenwnwnwewnwwse
+eswswswswswseeeswwseewsenenwwsesww
+newwnesesesweeneewwenee
+swnwswwnwswseeneeseeneswswenwswwnesw
+sesesweeseswseeseseenweseeswnesenenw
+wwwwwwseewwwwwwwwswwnesene
+swnwnwnenwesweswswsenwwneneenwnwnww
+swnwnwewenwenweenwnwswwwnwnwnwww
+wsweswswswswnwswswsweswswswswswswswne
+neneneswwnwnenwnwneswneenwsesenene
+swewswseneswnwswseswswseseenwswswsee
+enweeesweseseweeeeeenwswee
+sewseseswnenesewenwneeneneneewnewnew
+esenwswewneswneswnwnenwnwnwenwenwnw
+swneswwswwswswswwwwenwswswsweswsw
+newseswseswnwseseneeswsenwwswnw
+wwesweewswswnwwewewwwenwnesenw
+seswenwseswenwwseewnenenwnesenwsese
+wwwwnewwwswswww
+nwwnwnwswnesenenenwwenwenwwnwnwsenwwnw
+nwsewesesewswsesweseseneseneseswnese
+eseeeeenwseeseswe
+swseenwseswwneneswsweseswseswnwwnene
+eenwewesweseneweswnenwswsewenee
+wewwwwwwwwwnewsewwwwsenw
+wwwsewwwwneww
+senwneneenenwsenwnesenenenenewnewsesw
+eweeeeeeee
+nwnewneneneseneneenewseneneswsenenwnenw
+swsewswnwneeenwseweseenwwwenesww
+swswnwswneneswsesewswseswsewswnesese
+newwnwnwwwwnwswswnenwnwneenwnwswnw
+eenwsenwnewwneneenenwnenwsewwwsee
+eseesenweeseseswseenesenweeewswnwe
+nwenwenewnwwwnwswwsweswseswnwwe
+nwseneeswswseseseswswswseswseeewswnw
+seeeenweeweeeeesweeenee
+eneweswnesenenewnenewneswnenesesww
+nwwnewswnwwwnwe
+swswswswswseswswswswwswswnesw
+wwwsewnenwwwwnwsewnw
+neeeseseesewseseesesewsesesene
+nwnwswswneswwswseswswswsweswswseneswswsw
+sesesenewsewseseswseseseseswnesesese
+seneseswswseswseeeseenwnwseenesewnwse
+nwwnwwwwwnwwewe
+swseswwsweswswswnwswsweswswswswneswse
+swseeswswswswswswswwnwnwswwnesesenwswsw
+senwseseswsesenesenenwsesesewsesesesese
+ewswwswnwseneswseswnenwswsenwseseswswne
+sweswseswseswwwseneneenesenwseswnwsenw
+nwwweswnwswnwswswseeseswsenenwsesww
+sesewseesenwesweseeneseeneseswnewese
+esewseeeesewenwseeweenesewew
+eneseneswnesewwneenwneneswewsewnesw
+newneweeseenesweneeeseenwneeswe
+seswseseneswswswswswswsw
+wsesesesesenesesenwseesenwsesesesesese
+nenenenesenenewnenenene
+seswwnwswwsesesweswsweswenesenwene
+neswesenenesenenweeswseeenwnewnwnene
+eeeeeeeenweeneswenenwnweswsw
+seneenenesweneswneewnwswnene
+swswwswswswwswswnwwwnweenwwseswww
+swswwswneswseneswswswsenesw
+swwswswswswswsweswswswswsw
+seneseseseenwnwsweswnwseseswseswwneswswsw
+nenwnwswnenwewneswnwnewnenenesenenee
+nwseseesenwwswseeenesesesesenwnwew
+swnwswseenewswwswswswwwwwnwswwe
+nwneenesenwneewnewneseeneneesw
+neswnwwnenweswneneneneswnenwwneenwneene
+neeneneswwsenwnwneneneseenenenewsene
+swswnewseseseswswseswneswnwseeseswsese
+swnwwswseswwnwsweeswwnweenewnwsw
+nenweeeeswneweseswneswne
+weneenwnenenenenwnenwswnewneeneneenesw
+swnenwnwwwswwwnenewwnwsewnesewnw
+neswswwswswseesweneswswsewswswwswse
+nenenenwwsenwnwnenwnwnenwsenenwnwnwsenw
+nwseseswseesenwseswswsenwese
+wswneswnwneneswseseswwnwseswswnesenwsesew
+weeeewsweeeneneneswesenwswee
+senwnwwwnwsenwnwwwnwsesenesesenewsene
+wwwesewwsenwwwwswweswwswnenww
+swwneneseswswwswsewwwsw
+enenwneneeneeseneswnwneeneswnesenene
+eseesenenenewwneneseenenewnenenewnene
+wenwswewnwewnweswseeeneeneenesw
+neneneneswnenewsenwneseneenenwneswnenewne
+eneswneneeseeswenewneeeeeenwnee
+eswswsenwneseeseeeeeswnweeeewne
+nwewwwwenwwwwewnwwnwwewsw
+nwwwwnwnesenwwseenwswnwewnwswwnewnw
+seweeweeenwseeeeenwseeeene
+wswsenenewwwwwwwswwswwwneseww
+nenenenweeseneneneeeeeweewswse
+neneneneenweseeneswswnenenweeneswee
+sesweeeswenwnese
+enweseneneneneenwneenwneswneswneswnew
+senenweseneneswsesenwseswswnwwswnwsewse
+nwnwnwenwnwnwnwnwnwnwswnwnwnwsesenwnwnw
+wwwnwnwwenwnwnwnwsenwswww
+nwnwnweswnenenwnesenwwswnenenenwnesenwe

Program.cs

@@ -26,6 +26,7 @@
          Q21.Go();
          Q22.Go();
          Q23.Go();
+         Q24.Go();
          Util.Log($"Total time={(System.DateTime.Now - start).TotalMilliseconds}ms");
       }
    }

Q24.cs

@@ -0,0 +1,174 @@
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Linq;
+
+namespace _2020
+{
+   class Q24
+   {
+      static readonly List<List<string>> list = new List<List<string>>();
+      static Dictionary<Tuple<int, int>, int> tiles = new Dictionary<Tuple<int, int>, int>();
+
+      public static void Go()
+      {
+         var start = DateTime.Now;
+         MakeList();
+         Util.Log($"Q24 MakeList took {(DateTime.Now - start).TotalMilliseconds:N0}ms");
+         var p1start = DateTime.Now;
+         Part1();
+         Util.Log($"Q24 part1 took {(DateTime.Now - p1start).TotalMilliseconds:N0}ms");
+         var p2start = DateTime.Now;
+         Part2();
+         Util.Log($"Q24 part2 took {(DateTime.Now - p2start).TotalMilliseconds:N0}ms");
+
+         Util.Log($"Q24 took {(DateTime.Now - start).TotalMilliseconds:N0}ms");
+      }
+
+      static void MakeList()
+      {
+         foreach (var line in File.ReadAllLines("24input.txt"))
+         {
+            var sublist = new List<string>();
+            for (int i = 0; i < line.Length; i++)
+            {
+               if (line[i] == 'e' || line[i] == 'w')
+               {
+                  sublist.Add(line[i].ToString());
+               }
+               else
+               {
+                  i++;
+                  sublist.Add($"{line[i - 1]}{line[i]}");
+               }
+            }
+
+            list.Add(sublist);
+         }
+      }
+
+      static void Part1()
+      {
+         foreach (var inst in list)
+         {
+            var row = 0;
+            var col = 0;
+            foreach (var step in inst)
+            {
+               if (step.StartsWith('n'))
+               {
+                  row--;
+               }
+               else if (step.StartsWith('s'))
+               {
+                  row++;
+               }
+               
+               if (step.EndsWith('e'))
+               {
+                  col++;
+                  if (step.Length == 1)
+                  {
+                     col++;
+                  }
+               }
+               else if (step.EndsWith('w'))
+               {
+                  col--;
+                  if (step.Length == 1)
+                  {
+                     col--;
+                  }
+               }
+            }
+
+            var tilePos = Tuple.Create(row, col);
+            if (!tiles.ContainsKey(tilePos))
+            {
+               tiles.Add(tilePos, 0);
+            }
+            tiles[tilePos]++;
+         }
+
+         var flipped = tiles.Values.Aggregate(0, (sum, curr) => IsFlipped(curr) ? (sum + 1) : sum);
+
+         Util.Log($"Q24Part1: flipped={flipped}");
+      }
+      
+      static List<Tuple<int, int>> GetNeighbors(Tuple<int, int> coords)
+      {
+         var result = new List<Tuple<int, int>>();
+         result.Add(Tuple.Create(coords.Item1 - 1, coords.Item2 - 1));
+         result.Add(Tuple.Create(coords.Item1    , coords.Item2 - 2));
+         result.Add(Tuple.Create(coords.Item1 + 1, coords.Item2 - 1));
+         result.Add(Tuple.Create(coords.Item1 - 1, coords.Item2 + 1));
+         result.Add(Tuple.Create(coords.Item1    , coords.Item2 + 2));
+         result.Add(Tuple.Create(coords.Item1 + 1, coords.Item2 + 1));
+         return result;
+      }
+
+      static bool IsFlipped(int val)
+      {
+         return (val & 1) == 1;
+      }
+
+      static int NumNeighborsFlipped(Tuple<int, int> coords)
+      {
+         var result = 0;
+         foreach (var neighbor in GetNeighbors(coords))
+         {
+            if (tiles.ContainsKey(neighbor) && IsFlipped(tiles[neighbor]))
+            {
+               result++;
+            }
+         }
+         return result;
+      }
+
+      static Dictionary<Tuple<int, int>, int> PopulateNeighbors(Dictionary<Tuple<int, int>, int> inList)
+      {
+         var tempList = new Dictionary<Tuple<int, int>, int>(inList);
+         foreach (var pair in inList)
+         {
+            if (IsFlipped(pair.Value))
+            {
+               foreach (var neighbor in GetNeighbors(pair.Key))
+               {
+                  if (!tempList.ContainsKey(neighbor))
+                  {
+                     tempList.Add(neighbor, 0);
+                  }
+               }
+            }
+         }
+
+         return tempList;
+      }
+
+      static void Part2()
+      {
+         for (int day = 0; day < 100; day++)
+         {
+            var tileState = PopulateNeighbors(tiles);
+            foreach (var pair in tileState)
+            {
+               var numNeighborsFlipped = NumNeighborsFlipped(pair.Key);
+               var isFlipped = IsFlipped(pair.Value);
+               if (isFlipped && (numNeighborsFlipped == 0 || numNeighborsFlipped > 2))
+               {
+                  tileState[pair.Key]++;
+               }
+               else if (!isFlipped && numNeighborsFlipped == 2)
+               {
+                  tileState[pair.Key]++;
+               }
+            }
+
+            tiles = new Dictionary<Tuple<int, int>, int>(tileState);
+         }
+
+         var flipped = tiles.Values.Aggregate(0, (sum, curr) => IsFlipped(curr) ? (sum + 1) : sum);
+         Util.Log($"Q24Part2: flipped={flipped}");
+      }
+   }
+}