2024 Day 4

src/2024/04/Program.cs

@@ -1,6 +1,57 @@
 using AdventOfCode.Common;
+using Coor = AdventOfCode.Common.Coor<int>;
 
-var lines = Resources.GetInputFileLines("input.txt");
+var map = Resources.GetInputFileLines().ParseAsArray();
 
-Console.WriteLine($"Part 1: {""}");
-Console.WriteLine($"Part 2: {""}");
+// Take all possible vectors across the map (rows, cols, diagonals)
+int part1 = map.GetVectors()
+    .SelectMany(vector => Enumerable.Range(0, vector.Count - 3).Select(i => vector.Skip(i).Take(4).ToList()))
+    .Count(vector => vector.VectorContainsWord(map, "XMAS") || vector.VectorContainsWord(map, "SAMX"));
+
+// Get all X constelations of coordinates across the map
+int part2 = map.GetXMasks()
+    .Count(MaskContainsMasMas);
+
+Console.WriteLine($"Part 1: {part1}");
+Console.WriteLine($"Part 2: {part2}");
+
+bool MaskContainsMasMas(IList<Coor> mask)
+{
+    if (map.Get(mask[2]) != 'A') return false;
+
+    var lt = map.Get(mask[0]);
+    var rt = map.Get(mask[1]);
+    var lb = map.Get(mask[3]);
+    var rb = map.Get(mask[4]);
+
+    return ((lt == 'M' && rb == 'S') || (lt == 'S' && rb == 'M'))
+        && ((rt == 'M' && lb == 'S') || (rt == 'S' && lb == 'M'));
+}
+
+static file class Extensions
+{
+    public static bool VectorContainsWord(this IList<Coor> vector, char[,] map, string word)
+    {
+        if (vector.Count < word.Length) return false;
+        if (vector.Where((c, i) => map.Get(vector[i]) != word[i]).Any()) return false;
+        return true;
+    }
+
+    // Returns sets of coordinates in the shape of an X
+    public static IEnumerable<List<Coor<int>>> GetXMasks(this char[,] map)
+    {
+        var width = map.Width();
+        var height = map.Height();
+
+        for (int row = 0; row < height - 2; ++row)
+            for (int col = 0; col < width - 2; ++col)
+                yield return
+                [
+                    new Coor<int>(row, col),
+                    new Coor<int>(row, col + 2),
+                    new Coor<int>(row + 1, col + 1),
+                    new Coor<int>(row + 2, col),
+                    new Coor<int>(row + 2, col + 2),
+                ];
+    }
+}

src/Common/MultiDimensionalArrayExtensions.cs

@@ -1,4 +1,6 @@
-using System.Text;
+using System.Drawing;
+using System.Linq;
+using System.Text;
 
 namespace AdventOfCode.Common;
 
@@ -88,6 +90,49 @@ public static void Print<T>(this T[,] map, Func<Coor<int>, char> printOverride)
         }
     }
 
+    // Returns a set of vectors of possible ways to traverse a rectangle:
+    // - Rows
+    // - Columns
+    // - Diagonals
+    public static IEnumerable<List<Coor<int>>> GetVectors<T>(this T[,] map)
+    {
+        var width = map.Width();
+        var height = map.Height();
+
+        // Horizontal rows
+        for (int row = 0; row < width; row++)
+            yield return [.. Enumerable.Range(0, width).Select(col => new Coor<int>(row, col))];
+
+        // Vertical cols
+        for (int col = 0; col < width; col++)
+            yield return [.. Enumerable.Range(0, height).Select(row => new Coor<int>(row, col))];
+
+        for (int i = 0; i < height + width - 1; i++)
+        {
+            var diagonal1 = new List<Coor<int>>();
+            var diagonal2 = new List<Coor<int>>();
+
+            int row = Math.Min(i, height - 1);
+            int col = Math.Max(0, i - height + 1);
+            var c1 = new Coor<int>(row, col);
+            var c2 = new Coor<int>(row, width - col - 1);
+
+            do
+            {
+                diagonal1.Add(c1);
+                diagonal2.Add(c2);
+                row--;
+                col++;
+                c1 = new Coor<int>(row, col);
+                c2 = new Coor<int>(row, width - col - 1);
+            }
+            while (c1.InBoundsOf(map));
+
+            yield return diagonal1;
+            yield return diagonal2;
+        }
+    }
+
     public static T Get<T>(this T[,] items, Coor<int> coor)
         => items[coor.Y, coor.X];