var boards = new List<Board>();
using var sr = new StreamReader("../inputs/day04.txt");
var lines = sr.ReadToEnd().Split("\n").Select(x => x.Trim()).ToList();
var numbers = lines.First().Split(",").Select(x => Convert.ToByte(x)).ToList();
lines.RemoveAt(0); //Drop numbers
lines.RemoveAt(0); //Drop whitespace

var currentBoard = new List<string>();
foreach (var line in lines)
{
    if (!string.IsNullOrEmpty(line))
    {
        currentBoard.Add(line);
    }
    else
    {
        boards.Add(new Board(currentBoard));
        currentBoard = new List<string>();
    }
}

var part1Done = false;
foreach (var number in numbers)
{
    foreach (var board in boards)
        board.Mark(number);

    var winningBoards =
        boards.Where(board => board.Win).ToArray(); // Need to enumerate this before the loop or it will break
    foreach (var board in winningBoards)
    {
        if (!part1Done)
        {
            Console.WriteLine($"2021 Day 04 Part 1 result: {board.Score * number}");
            part1Done = true;
        }

        if (boards.Count == 1)
            Console.WriteLine($"2021 Day 04 Part 2 result: {board.Score * number}");

        boards.Remove(board);
    }

    if (!boards.Any())
        break;
}

private class Board
{
    private readonly byte?[,] _rows;
    internal int Score => _rows.Cast<byte?>().Aggregate(0, (score, number) => score + (number ?? 0));
    private bool HorizontalWin => DidRowWin(_rows);
    private bool VerticalWin => DidRowWin(Transpose());
    internal bool Win => HorizontalWin || VerticalWin;

    internal Board(IEnumerable<string> inputRows)
    {
        _rows = new byte?[5, 5];

        for (var r = 0; r < inputRows.Count(); r++)
        {
            var nums = inputRows
                .ElementAt(r)
                .Split(" ")
                .Where(x => !string.IsNullOrEmpty(x))
                .Select(x => Convert.ToByte(x));

            for (var c = 0; c < nums.Count(); c++)
            {
                _rows[c, r] = nums.ElementAt(c);
            }
        }
    }

    internal void Mark(byte number)
    {
        for (var r = 0; r < _rows.GetLength(0); r++)
        {
            for (var c = 0; c < _rows.GetLength(1); c++)
            {
                if (_rows[c, r].HasValue && _rows[c, r].Value == number)
                    _rows[c, r] = null;
            }
        }
    }

    private byte?[,] Transpose()
    {
        var cols = new byte?[5, 5];

        for (var c = 0; c < 5; c++)
        {
            for (var r = 0; r < 5; r++)
            {
                cols[c, r] = _rows[r, c];
            }
        }

        return cols;
    }

    private static bool DidRowWin(byte?[,] rows)
    {
        var win = true;

        for (var r = 0; r < rows.GetLength(0); r++)
        {
            win = true;

            for (var c = 0; c < rows.GetLength(1); c++)
            {
                if (rows[c, r].HasValue)
                    win = false;
            }

            if (win)
                break;
        }

        return win;
    }
}