Day 4

Day4/Program.cs

@@ -0,0 +1,98 @@
+var lines = File.ReadAllLines("input.txt");
+var charArray = new char[lines.Length, lines[0].Length];
+
+for (var i = 0; i < charArray.GetLength(0); i++)
+for (var j = 0; j < charArray.GetLength(1); j++)
+{
+    charArray[i, j] = lines[i][j];
+}
+
+Console.WriteLine($"Part1: {FindXmas(charArray)}\nPart2: {FindMas(charArray)}");
+
+int FindMas(char[,] grid)
+{
+    var rows = grid.GetLength(0);
+    var cols = grid.GetLength(1);
+
+    var directions = new[,]
+    {
+        { -1, 1 }, // diagonal up right
+        { 1, 1 }, // diagonal down right
+        { -1, -1 }, // diagonal up left
+        { 1, -1 } // diagonal down left
+    };
+
+    List<string> masCenterCoordinates = new();
+
+    for (var r = 0; r < rows; r++)
+    {
+        for (var c = 0; c < cols; c++)
+        {
+            for (var d = 0; d < directions.GetLength(0); d++)
+            {
+                var masCoordinates = CheckWord(r, c, directions[d, 0], directions[d, 1], grid, "MAS");
+                if (masCoordinates != null)
+                {
+                    masCenterCoordinates.Add($"{masCoordinates[1][0]},{masCoordinates[1][1]}");
+                }
+            }
+        }
+    }
+
+    var masCountDict = masCenterCoordinates.GroupBy(item => item).ToDictionary(group => group.Key, group => group.Count()).Where(x => x.Value > 1);
+
+    return masCountDict.Count();
+}
+
+int FindXmas(char[,] grid)
+{
+    var rows = grid.GetLength(0);
+    var cols = grid.GetLength(1);
+    var xmasCount = 0;
+
+    var directions = new[,]
+    {
+        { -1, 0 }, // up
+        { 0, 1 }, // right
+        { 0, -1 }, // left
+        { 1, 0 }, // down
+        { -1, 1 }, // diagonal up right
+        { 1, 1 }, // diagonal down right
+        { -1, -1 }, // diagonal up left
+        { 1, -1 } // diagonal down left
+    };
+
+    for (var r = 0; r < rows; r++)
+    {
+        for (var c = 0; c < cols; c++)
+        {
+            for (var d = 0; d < directions.GetLength(0); d++)
+            {
+                if (CheckWord(r, c, directions[d, 0], directions[d, 1], grid, "XMAS") != null)
+                {
+                    xmasCount++;
+                }
+            }
+        }
+    }
+
+    return xmasCount;
+}
+
+List<int[]>? CheckWord(int r, int c, int dr, int dc, char[,] grid, string word)
+{
+    var characterCoordinates = new List<int[]>();
+
+    for (var i = 0; i < word.Length; i++)
+    {
+        var newR = r + i * dr;
+        var newC = c + i * dc;
+        if (!IsValid(newR, newC, grid) || grid[newR, newC] != word[i]) return null;
+
+        characterCoordinates.Add([newR, newC]);
+    }
+
+    return characterCoordinates;
+}
+
+bool IsValid(int r, int c, char[,] grid) => r >= 0 && r < grid.GetLength(0) && c >= 0 && c < grid.GetLength(1);