C#

namespace Day04;

static class Program
{
    public record struct Point(int Row, int Col);

    static void Main(string[] args)
    {
        var sample = File.ReadAllLines("sample.txt");
        var data = File.ReadAllLines("data.txt");

        Console.WriteLine($"Part 1 (sample): {SolvePart1(sample)}");
        Console.WriteLine($"Part 1 (data): {SolvePart1(data)}");

        Console.WriteLine($"Part 2 (sample): {SolvePart2(sample)}");
        Console.WriteLine($"Part 2 (data): {SolvePart2(data)}");
    }

    private static readonly string Search = "XMAS";

    private static readonly Func<Point, Point>[] DirectionalMoves =
    {
        p => new Point(p.Row + 1, p.Col),
        p => new Point(p.Row + 1, p.Col + 1),
        p => new Point(p.Row, p.Col + 1),
        p => new Point(p.Row - 1, p.Col + 1),
        p => new Point(p.Row - 1, p.Col),
        p => new Point(p.Row - 1, p.Col - 1),
        p => new Point(p.Row, p.Col - 1),
        p => new Point(p.Row + 1, p.Col - 1),
    };

    private static readonly Func<Point, Point>[] ForwardSlashMoves =
    {
        p => new Point(p.Row - 1, p.Col - 1),
        p => new Point(p.Row + 1, p.Col + 1),
    };
    
    private static readonly Func<Point, Point>[] BackSlashMoves =
    {
        p => new Point(p.Row + 1, p.Col - 1),
        p => new Point(p.Row - 1, p.Col + 1),
    };

    static long SolvePart1(string[] data)
    {
        return Enumerable
            .Range(0, data.Length)
            .SelectMany(row => Enumerable.Range(0, data[row].Length)
                .Select(col => new Point(row, col)))
            .Where(p => IsMatch(data, p, Search[0]))
            .Sum(p => DirectionalMoves
                .Count(move => DeepMatch(data, move(p), move, Search, 1)));
    }

    static long SolvePart2(string[] data)
    {
        return Enumerable
            .Range(0, data.Length)
            .SelectMany(row => Enumerable.Range(0, data[row].Length)
                .Select(col => new Point(row, col)))
            .Where(p => IsMatch(data, p, 'A'))
            .Count(p => CheckDiagonalMoves(data, p, ForwardSlashMoves)
                        && CheckDiagonalMoves(data, p, BackSlashMoves));
    }

    static bool CheckDiagonalMoves(string[] data, Point p, Func<Point, Point>[] moves)
        => (IsMatch(data, moves[0](p), 'S') && IsMatch(data, moves[1](p), 'M'))
           || (IsMatch(data, moves[0](p), 'M') && IsMatch(data, moves[1](p), 'S'));

    static bool DeepMatch(string[] data, Point p, Func<Point, Point> move, string search, int searchIndex) =>
        (searchIndex >= search.Length) ? true :
        (!IsMatch(data, p, search[searchIndex])) ? false :
        DeepMatch(data, move(p), move, search, searchIndex + 1);

    static bool IsMatch(string[] data, Point p, char searchChar)
        => IsInBounds(data, p) && (data[p.Row][p.Col] == searchChar);

    static bool IsInBounds(string[] data, Point p) =>
        (p.Row >= 0) && (p.Col >= 0) && (p.Row < data.Length) && (p.Col < data[0].Length);
}