AdventOfCode2025/AdventOfCode2025/Solutions/Day4.cs

using AdventOfCode2025.Inputs;

namespace AdventOfCode2025.Solutions;

public class Day4() : Solution(4)
{
    private const char NoneChar = '.';
    private const char PaperChar = '@';

    public override string SolvePart1()
    {
        var grid = PaddingField(Input.Lines)
            .Select(line => line.ToCharArray())
            .ToArray();

        return GetRemovablePaper(grid).Count().ToString();
    }

    public override string SolvePart2()
    {
        var grid = PaddingField(Input.Lines)
            .Select(line => line.ToCharArray())
            .ToArray();
        List<(int, int)> paperCanBeRemoved;
        var paperCount = 0;
        do
        {
            paperCanBeRemoved = GetRemovablePaper(grid).ToList();
            paperCount += paperCanBeRemoved.Count;
            foreach (var (row, col) in paperCanBeRemoved)
            {
                grid[row][col] = NoneChar;
            }
        } while (paperCanBeRemoved.Count > 0);

        return paperCount.ToString();
    }

    private static IEnumerable<(int row, int col)> GetRemovablePaper(char[][] grid)
    {
        const int threshold = 3;
        for (var row = 1; row < grid.Length - 1; row++)
        {
            for (var col = 1; col < grid[0].Length - 1; col++)
            {
                if (grid[row][col] != PaperChar)
                {
                    continue;
                }

                if (!HasMorePaperNeighborsAs(col, row, threshold, grid))
                {
                    yield return (row, col);
                }
            }
        }
    }

    private static bool HasMorePaperNeighborsAs(int x, int y, int threshold, char[][] grid)
    {
        var directions = new (int dx, int dy)[] { (-1, -1), (0, -1), (1, -1), (-1, 0), (1, 0), (0, 1), (-1, 1), (1, 1) };
        var paperNeighbors = 0;
        foreach (var (dx, dy) in directions)
        {
            var nx = x + dx;
            var ny = y + dy;
            if (nx < 0 || nx >= grid[0].Length || ny < 0 || ny >= grid.Length)
            {
                continue;
            }

            if (grid[ny][nx] != PaperChar)
            {
                continue;
            }

            paperNeighbors++;
            if (paperNeighbors > threshold)
            {
                return true;
            }
        }

        return false;
    }

    private IEnumerable<string> PaddingField(IEnumerable<string> field)
    {
        List<string> lines = field.ToList();
        var width = lines.First().Length;
        lines.Insert(0, new string(NoneChar, width));
        lines.Add(new string(NoneChar, width));
        return lines.Select(l => NoneChar + l + NoneChar);
    }
}