day 12

src/aoc/Year2024/Day12.cs

@@ -1,20 +1,180 @@
+using aoc.Common;
+using aoc.Common.BfsImpl;
+using MoreLinq;
+
 namespace aoc.Year2024;
 
 internal partial class Day12
 {
-    internal partial class Part1
-    {
-        public string Solve(string input)
-        {
-            throw new NotImplementedException();
-        }
-    }
-
-    internal partial class Part2
-    {
-        public string Solve(string input)
-        {
-            throw new NotImplementedException();
-        }
-    }
+	internal partial class Part1
+	{
+		private readonly Example example = new(
+			"""
+			RRRRIICCFF
+			RRRRIICCCF
+			VVRRRCCFFF
+			VVRCCCJFFF
+			VVVVCJJCFE
+			VVIVCCJJEE
+			VVIIICJJEE
+			MIIIIIJJEE
+			MIIISIJEEE
+			MMMISSJEEE
+			""", 1930);
+
+		public int Solve(char[,] input)
+		{
+			var remainingPlots = input.AsEnumerable().Select(x => x.point).ToHashSet();
+
+			var price = 0;
+			while (remainingPlots.Count != 0)
+			{
+				var start = remainingPlots.First(); 
+				// While traversing the graph, for each visited node, we not only determine its adjacent nodes 
+				// but also count how many of them lie outside this region.
+				var g = Bfs.Common.StartWith((plot: start, plant: input.At(start), sides: (int?)null))
+					.WithAdjacency(p =>
+					{
+						var dirs = Directions.All4();
+
+						var allAdj = dirs.Select(d => d + p.plot).ToList();
+
+						var (inside, outside) =
+							allAdj.Partition(adj => input.TryAt(adj, out var adjValue) && adjValue == p.plant);
+
+						return inside.Select(x => (x, input.At(x), (int?)null))
+							.Append((p.plot, p.plant, outside.Count()));
+
+					}).WithFolder(L.Empty<(V<int> plot, int sides)>(), (acc, path) =>
+					{
+						if (path.HeadItem.sides is null)
+							return (acc, TraversalResult.Continue);
+						return (acc.Prepend((path.HeadItem.plot, path.HeadItem.sides.Value)), TraversalResult.Continue);
+					}).Run().ToList();
+
+				var area = g.Select(x => x.plot).Count();
+				var perimeter = g.Sum(x => x.sides);
+
+				price += area * perimeter;
+
+				remainingPlots.ExceptWith(g.Select(x => x.plot));
+			}
+
+			return price;
+		}
+
+		public char[,] Parse(string input)
+		{
+			return Character.Letter.Map().Parse(input);
+		}
+	}
+
+	internal partial class Part2
+	{
+		private readonly Example example = new(
+			"""
+			RRRRIICCFF
+			RRRRIICCCF
+			VVRRRCCFFF
+			VVRCCCJFFF
+			VVVVCJJCFE
+			VVIVCCJJEE
+			VVIIICJJEE
+			MIIIIIJJEE
+			MIIISIJEEE
+			MMMISSJEEE
+			""", 1206);
+
+		private readonly Example example1 = new(
+			"""
+			EEEEE
+			EXXXX
+			EEEEE
+			EXXXX
+			EEEEE
+			""", 236);
+
+		private readonly Example example2 = new(
+			"""
+			AAAAAA
+			AAABBA
+			AAABBA
+			ABBAAA
+			ABBAAA
+			AAAAAA
+			""", 368);
+		private readonly Example example3 = new(
+			"""
+			AAAA
+			BBCD
+			BBCC
+			EEEC
+			""", 80);
+
+		public int Solve(char[,] input)
+		{
+			var remainingPoints = input.AsEnumerable().ToHashSet();
+
+			var price = 0;
+			while (remainingPoints.Count != 0)
+			{
+				var start = (remainingPoints.First().point, el: remainingPoints.First().element);
+				var region = Bfs.Common.StartWith(start)
+					.WithAdjacency(p =>
+					{
+						var dirs = Directions.All4();
+
+						return dirs.Select(d => d + p.point)
+							.Where(input.Inside)
+							.Select(adj => (adj, input.At(adj)))
+							.Where(adj => adj.Item2 == p.el);
+					})
+					.WithVisitedKey(x => x.point)
+					.WithFolder(L.Empty<V<int>>(),
+						(acc, path) => (acc.Prepend(path.HeadItem.point), TraversalResult.Continue)
+					).Run().ToList();
+
+				var area = region.Count;
+				var extendedRegion = 
+					region.SelectMany(x => Directions.All8().Append(V<int>.Zero).Select(d => d + x))
+						.Distinct().ToList();
+				// Counting corners instead of sides
+				// Scanning the region with a 2x2 window to determine how many corners are in it by counting the plots.
+				var corners = extendedRegion
+					.Sum(x =>
+					{
+						// 
+						//  X .
+						//  . .
+						// 
+						var blockToFindCorner = new [] { x, Directions.W + x, Directions.S + x, Directions.SW + x };
+						var pointsFoundInBlock = blockToFindCorner.Where(l => region.Contains(l)).ToList();
+						return pointsFoundInBlock switch
+						{
+							//   A .   . .   . .   . A   A A   A .   . A   A A     
+							//   . .   A .   . A   . .   A .   A A   A A   . A     
+							{Count: 1 or 3} => 1,
+							//  A .    . A 
+							//  . A    A . 
+							[var found1, var found2] when found1.MLen(found2) == 2 => 2,
+							//  A A   A A   . A   . .   A .   . . 
+							//  A A   . .   . A   A A   A .   . . 
+							_ => 0
+						};
+					});
+
+				var sides = corners;
+				price += area * sides;
+
+				remainingPoints = remainingPoints.ExceptBy(region, x => x.point).ToHashSet();
+			}
+
+			return price;
+		}
+
+		public char[,] Parse(string input)
+		{
+			return Character.Letter.Map().Parse(input);
+		}
+	}
 }

tests/AoC.Tests/InputTests.cs

@@ -30,5 +30,7 @@ private static IEnumerable<PartInputCaseData> GetCases()
 		yield return new PartInputCaseData(10, 2, "1116");
 		yield return new PartInputCaseData(11, 1, "235850");
 		yield return new PartInputCaseData(11, 2, "279903140844645");
+		yield return new PartInputCaseData(12, 1, "1486324");
+		yield return new PartInputCaseData(12, 2, "898684");
 	}
 }