You're already almost 1.5km (almost a mile) below the surface of the ocean, already so deep that you can't see any sunlight. What you can see, however, is a giant squid that has attached itself to the outside of your submarine.

Maybe it wants to play bingo?

Read the full puzzle.

using System;
using System.Collections.Generic;
using System.Linq;

namespace AdventOfCode.Y2021.Day04;

[ProblemName("Giant Squid")] 
class Solution : Solver {

    public object PartOne(string input) => BoardsInOrderOfCompletion(input).First().score;
    public object PartTwo(string input) => BoardsInOrderOfCompletion(input).Last().score;

    IEnumerable<BingoBoard> BoardsInOrderOfCompletion(string input) {

        var blocks = input.Split("\n\n");

        // first block contains the numbers to be drawn, rest describe bingo boards:
        var numbers = blocks[0].Split(",");
        var boards = (from block in blocks.Skip(1) select new BingoBoard(block)).ToHashSet();

        // let's play the game
        foreach (var number in numbers) {
            foreach (var board in boards.ToArray()) {
                board.AddNumber(number);
                if (board.score > 0) {
                    yield return board;
                    boards.Remove(board);
                }
            }
        }
    }
}

record Cell(string number, bool marked = false);

// Let's be ho-ho-hoOOP this time.
class BingoBoard {

    public int score { get; private set; }
    private List<Cell> cells;

    IEnumerable<Cell> CellsInRow(int irow) =>
        from icol in Enumerable.Range(0, 5) select cells[irow * 5 + icol];

    IEnumerable<Cell> CellsInCol(int icol) =>
        from irow in Enumerable.Range(0, 5) select cells[irow * 5 + icol];

    public BingoBoard(string st) {

        // split the input into words & read them numbers into cells
        cells = (
            from word in st.Split(" \n".ToArray(), StringSplitOptions.RemoveEmptyEntries)
            select new Cell(word)
        ).ToList();
    }

    public void AddNumber(string number) {

        var icell = cells.FindIndex(cell => cell.number == number);

        if (icell >= 0) {

            // mark the cell
            cells[icell] = cells[icell] with { marked = true };

            // if the board is completed, compute score
            for (var i = 0; i < 5; i++) {
                if (
                    CellsInRow(i).All(cell => cell.marked) ||
                    CellsInCol(i).All(cell => cell.marked)
                ) {

                    var unmarkedNumbers =
                        from cell in cells where !cell.marked select int.Parse(cell.number);

                    score = int.Parse(number) * unmarkedNumbers.Sum();
                }
            }
        }
    }
}

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