Advent of Code 2024 - Day 4

var input = await File.ReadAllLinesAsync("input.txt");

var gridSize = input.Length;
var coordinatesX = new List<Position>();
var coordinatesA = new List<Position>();

for (var row = 0; row < gridSize; row++)
{
    for (var col = 0; col < gridSize; col++)
    {
        switch (input[row][col])
        {
            case 'X':
                coordinatesX.Add(new Position(row, col));
                break;
            case 'A':
                coordinatesA.Add(new Position(row, col));
                break;
        }
    }
}

var part1 = coordinatesX.Sum(CheckPositionX);
var part2 = coordinatesA.Count(CheckPositionA);

Console.WriteLine($"Part 1: {part1}");
Console.WriteLine($"Part 2: {part2}");

return;

int CheckPositionX(Position x)
{
    var result = 0;
    for (var row = -1; row <= 1; row++)
    {
        for (var col = -1; col <= 1; col++)
        {
            if (row == 0 && col == 0)
            {
                continue;
            }

            var m = x.Move(row, col);
            var a = m.Move(row, col);
            var s = a.Move(row, col);

            if (m.OutOfBounds(gridSize) ||
                a.OutOfBounds(gridSize) ||
                s.OutOfBounds(gridSize))
            {
                continue;
            }

            if (input[m.Row][m.Col] == 'M' &&
                input[a.Row][a.Col] == 'A' &&
                input[s.Row][s.Col] == 'S')
            {
                result++;
            }
        }
    }

    return result;
}

bool CheckPositionA(Position a)
{
    if (a.OutOfBounds(1, gridSize - 1))
    {
        return false;
    }

    var topLeft = input[a.Row - 1][a.Col - 1];
    var topRight = input[a.Row - 1][a.Col + 1];
    var bottomLeft = input[a.Row + 1][a.Col - 1];
    var bottomRight = input[a.Row + 1][a.Col + 1];

    return topLeft == 'M' && topRight == 'M' && bottomLeft == 'S' && bottomRight == 'S'
        || topLeft == 'M' && topRight == 'S' && bottomLeft == 'M' && bottomRight == 'S'
        || topLeft == 'S' && topRight == 'S' && bottomLeft == 'M' && bottomRight == 'M'
        || topLeft == 'S' && topRight == 'M' && bottomLeft == 'S' && bottomRight == 'M';
}

internal class Position(int row, int col)
{
    public readonly int Row = row;
    public readonly int Col = col;

    public Position Move(int row, int col)
    {
        return new Position(Row + row, Col + col);
    }

    public bool OutOfBounds(int size)
    {
        return OutOfBounds(0, size);
    }

    public bool OutOfBounds(int start, int end)
    {
        return Row < start || Row >= end || Col < start || Col >= end;
    }
}