2024 Day 6 (brute force)

Author: premun

src/2024/06/Program.cs

@@ -1,6 +1,76 @@
 using AdventOfCode.Common;
+using Coor = AdventOfCode.Common.Coor<int>;
 
-var lines = Resources.GetInputFileLines();
+char[,] map = Resources.GetInputFileLines().ParseAsArray();
+Coor start = map.AllCoordinates().First(c => map.Get(c) == '^');
 
-Console.WriteLine($"Part 1: {""}");
-Console.WriteLine($"Part 2: {""}");
+var directions = new Dictionary<char, Coor>
+{
+    { '^', Coor.Up },
+    { '>', Coor.Right },
+    { 'V', Coor.Down },
+    { '<', Coor.Left },
+};
+
+Simulate(map);
+var traversedPath = map.AllCoordinates()
+    .Where(c => map.Get(c) != '.' && map.Get(c) != '#')
+    .ToList();
+
+var possibleObstaclePositions = 0;
+foreach (var path in traversedPath.Where(c => c != start))
+{
+    try
+    {
+        var newMap = Resources.GetInputFileLines().ParseAsArray();
+        newMap.Set(path, '#');
+        Simulate(newMap);
+    }
+    catch (LoopException)
+    {
+        possibleObstaclePositions++;
+    }
+}
+
+Console.WriteLine($"Part 1: {traversedPath.Count}");
+Console.WriteLine($"Part 2: {possibleObstaclePositions}");
+
+static void Simulate(char[,] map)
+{
+    var position = map.AllCoordinates().First(c => map.Get(c) == '^');
+    var direction = Coor.Up;
+    map.Set(position, (char)0);
+
+    while (true)
+    {
+        var current = map.Get(position);
+        map.Set(position, (char)(current switch
+        {
+            '.' => 1,
+            _ => current + 1,
+        }));
+
+        var next = position + direction;
+        if (!next.InBoundsOf(map))
+        {
+            break;
+        }
+
+        if (map.Get(next) == '#')
+        {
+            direction = direction.TurnRight();
+            continue;
+        }
+
+        // We can only come to a field twice (from side and from top/down)
+        // If we come a third time, it's a loop
+        if (map.Get(next) != '.' && map.Get(next) > 3)
+        {
+            throw new LoopException();
+        }
+
+        position = next;
+    }
+}
+
+file class LoopException : Exception;

src/Common/Coor.cs

@@ -101,4 +101,13 @@ public static void Visualize(this ICollection<Coor<int>> coors, Coor<int>? min,
             Console.WriteLine();
         }
     }
+
+    public static Coor<T> TurnRight<T>(this Coor<T> direction) where T : INumber<T> => direction switch
+    {
+        _ when direction == Coor<T>.Up => Coor<T>.Right,
+        _ when direction == Coor<T>.Right => Coor<T>.Down,
+        _ when direction == Coor<T>.Down => Coor<T>.Left,
+        _ when direction == Coor<T>.Left => Coor<T>.Up,
+        _ => throw new NotImplementedException()
+    };
 }