Advent of Code - 2025 - Day 4

var input = await File.ReadAllLinesAsync("input.txt");
var grid = new List<PaperRoll>();

for (var row = 0; row < input.Length; row++)
{
    for (var column = 0; column < input[row].Length; column++)
    {
        if (input[row][column] == '@')
        {
            grid.Add(new PaperRoll(row, column, []));
        }
    }
}

var part2 = 0;

while (true)
{
    for (int i = 0; i < grid.Count; i++)
    {
        var roll = grid[i];
        var adjacent = grid.Where(r =>
            Math.Abs(r.Row - roll.Row) <= 1 &&
            Math.Abs(r.Column - roll.Column) <= 1 &&
            r != roll).ToList();

        grid[i] = roll with { Adjacent = adjacent };
    }

    var pickableRolls = grid.Where(r => r.Adjacent.Count < 4).ToList();

    if (part2 == 0)
    {
        Console.WriteLine($"Part 1: {pickableRolls.Count}");
    }

    if (pickableRolls.Count == 0)
    {
        Console.WriteLine($"Part 2: {part2}");
        break;
    }

    part2 += pickableRolls.Count;
    grid = [.. grid.Except(pickableRolls)];
}

record PaperRoll(int Row, int Column, List<PaperRoll> Adjacent);