2024 Day 16 Part 1

Author: premun

src/2024/16/Program.cs

@@ -1,6 +1,93 @@
 using AdventOfCode.Common;
+using Coor = AdventOfCode.Common.Coor<int>;
 
-var lines = Resources.GetInputFileLines();
+var map = Resources.GetInputFileLines()
+    .ParseAsArray();
 
-Console.WriteLine($"Part 1: {""}");
+var start = map.AllCoordinates().First(c => map.Get(c) == 'S');
+var corner = new[]
+{
+    (Coor.Up, Coor.Left),
+    (Coor.Up, Coor.Right),
+    (Coor.Down, Coor.Left),
+    (Coor.Down, Coor.Right),
+};
+var end = map.AllCoordinates().First(c => map.Get(c) == 'E');
+var direction = Coor.Right;
+
+// Places where we can change direction
+var crossroads = map.AllCoordinates()
+    .Where(c => map.Get(c) == '.' && corner.Any(d => map.Get(c + d.Item1) == '.' && map.Get(c + d.Item2) == '.'))
+    .Append(start)
+    .Append(end)
+    .ToHashSet();
+
+var neighbours = new Dictionary<Coor, List<(Coor Neighbour, int Distance)>>();
+
+foreach (var c in crossroads)
+{
+    foreach (var dir in Coor.FourWayNeighbours)
+    {
+        var current = c;
+        var dist = 0;
+        do
+        {
+            current += dir;
+            dist++;
+
+            if (crossroads.Contains(current))
+            {
+                if (!neighbours.TryGetValue(c, out var list))
+                {
+                    list = [];
+                    neighbours[c] = list;
+                }
+                list.Add((current, dist));
+                break;
+            }
+        } while (map.Get(current) == '.');
+    }
+}
+
+var scores = Resources.GetInputFileLines()
+    .ParseAsArray(c => c switch
+    {
+        'S' => 0,
+        '#' => int.MinValue,
+        _ => int.MaxValue,
+    });
+
+var shortestPaths = new Dictionary<Coor, List<Coor>>();
+var queue = new PriorityQueue<State, int>();
+queue.Enqueue(new(start, Coor.Right, []), 0);
+
+while (queue.TryDequeue(out var current, out var score))
+{
+    if (scores.Get(current.Position) < score) continue;
+
+    scores.Set(current.Position, score);
+    shortestPaths[current.Position] = current.Path;
+
+    if (!neighbours.TryGetValue(current.Position, out var currentNeighbours))
+    {
+        continue;
+    }
+
+    foreach (var nc in currentNeighbours.Where(n => n.Neighbour != current.Position))
+    {
+        var newDirection = (current.Position - nc.Neighbour).Normalize();
+        var newScore = score + nc.Distance + (newDirection == current.Direction ? 0 : 1000);
+        if (scores.Get(nc.Neighbour) > newScore)
+        {
+            queue.Enqueue(new(nc.Neighbour, newDirection, [..current.Path, nc.Neighbour]), newScore);
+        }
+    }
+}
+
+var shortestPath = shortestPaths[end];
+map.Print(c => shortestPath.Contains(c) ? ('*', ConsoleColor.Cyan) : (map.Get(c), ConsoleColor.White));
+
+Console.WriteLine($"Part 1: {scores.Get(end)}");
 Console.WriteLine($"Part 2: {""}");
+
+file record struct State(Coor Position, Coor Direction, List<Coor> Path);

src/Common/Coor.cs

@@ -60,6 +60,17 @@ public IEnumerable<Coor<T>> GetFourWayNeighbours()
         { 'v', Down },
     };
 
+    public Coor<T> Normalize()
+    {
+        T x = T.Zero;
+        if (X > T.Zero) x = T.One;
+        if (X < T.Zero) x = -T.One;
+        T y = T.Zero;
+        if (Y > T.Zero) y = T.One;
+        if (Y < T.Zero) y = -T.One;
+        return new Coor<T>(y, x);
+    }
+
     public static bool operator ==(Coor<T> me, (T, T) other) => new Coor<T>(other.Item1, other.Item2) == me;
     public static bool operator !=(Coor<T> me, (T, T) other) => !(me == other);
     public static Coor<T> operator +(Coor<T> me, (T, T) other) => new(Y: me.Y + other.Item1, X: me.X + other.Item2);

src/Common/MultiDimensionalArrayExtensions.cs

@@ -88,6 +88,23 @@ public static void Print<T>(this T[,] map, Func<Coor<int>, char> printOverride)
         }
     }
 
+    public static void Print<T>(this T[,] map, Func<Coor<int>, (char, ConsoleColor)> printOverride)
+    {
+        for (var y = 0; y < map.Height(); y++)
+        {
+            for (var x = 0; x < map.Width(); x++)
+            {
+                var tmp = Console.ForegroundColor;
+                var c = printOverride.Invoke(new(y, x));
+                Console.ForegroundColor = c.Item2;
+                Console.Write(c.Item1);
+                Console.ForegroundColor = tmp;
+            }
+
+            Console.WriteLine();
+        }
+    }
+
     // Returns a set of vectors of possible ways to traverse a rectangle:
     // - Rows
     // - Columns