using System;

using System.Net.Http;

using System.Collections.Generic;

using System.Linq;

​

// Get data

var response = await new HttpClient().GetStringAsync("https://tompafireadventofcode.free.beeceptor.com/day4"); 

​

const string sectionDelimeter = "\n\n";

var sections = response.Split(sectionDelimeter);

​

var inputSection = sections.First();

var boardSections = sections.Skip(1);

​

var input = inputSection.Split(',', StringSplitOptions.TrimEntries | StringSplitOptions.RemoveEmptyEntries).Select(int.Parse);

var boards = GetBoardsFromText(boardSections);

​

​

​

// Step 1

​

var boardsStep1 = boards.Select(b => b with { }).ToList();

​

var finalNumber = 0;

Board boardWithFullLine = null;

​

foreach (var calledNumber in input)

{

    var matchingValues = boardsStep1.SelectMany(b => b.Rows.SelectMany(r => r.Values)).Where(x => x.Value == calledNumber).ToList();

    matchingValues.ForEach(x => x.IsSelected = true);

​

    boardWithFullLine = boardsStep1.FirstOrDefault(b => 

        b.Rows.Any(r => r.Values.All(x => x.IsSelected)) ||

        b.Columns.Any(c => c.Values.All(x => x.IsSelected)));

​

    if (boardWithFullLine != null)

    {

        finalNumber = calledNumber;

        break;

    }

}

​

if (boardWithFullLine != null)

{

    var sumOfUnmarkedValues = boardWithFullLine.Rows.SelectMany(r => r.Values).Where(x => !x.IsSelected).Sum(x => x.Value);

​

    Console.WriteLine(finalNumber);

    Console.WriteLine(sumOfUnmarkedValues);

​

    var result = finalNumber * sumOfUnmarkedValues;

​

    Console.WriteLine(result);

}

​

​

// Step 2

​

var boardsStep2 = boards.Select(b => b with { }).ToList();

​

var finalNumber2 = 0;

Board lastBoardWithFullLine = null;

​

var remainingBoards = boardsStep2;

​

foreach (var calledNumber in input)

{

    var matchingValues = remainingBoards.SelectMany(b => b.Rows.SelectMany(r => r.Values)).Where(x => x.Value == calledNumber).ToList();

    matchingValues.ForEach(x => x.IsSelected = true);

​

    var boardsdWithFullLine = remainingBoards.Where(b =>

        b.Rows.Any(r => r.Values.All(x => x.IsSelected)) ||

        b.Columns.Any(c => c.Values.All(x => x.IsSelected)));

​

    if (boardsdWithFullLine.Any())

    {

        finalNumber2 = calledNumber;

        lastBoardWithFullLine = boardsdWithFullLine.Last();

        remainingBoards = remainingBoards.Except(boardsdWithFullLine).ToList();

    }

}

​

if (lastBoardWithFullLine != null)

{

    var sumOfUnmarkedValues = lastBoardWithFullLine.Rows.SelectMany(r => r.Values).Where(x => !x.IsSelected).Sum(x => x.Value);

​

    Console.WriteLine(finalNumber2);

    Console.WriteLine(sumOfUnmarkedValues);

​

    var result = finalNumber2 * sumOfUnmarkedValues;

​

    Console.WriteLine(result);

}

​

IEnumerable<Board> GetBoardsFromText(IEnumerable<string> sections)

{

    foreach (var section in sections)

    {

        var rows = section.Split("\n").Select(line =>

        {

            var values = line.Split(' ', StringSplitOptions.TrimEntries | StringSplitOptions.RemoveEmptyEntries).Select((val, index) => new CellValue(index, int.Parse(val)))

            .ToList();

            return new Row(values);

        }).ToList();

​

        yield return new Board(rows);

    }

}

​

public record Row(List<CellValue> Values);

​

public record Column(List<CellValue> Values);

​

public record CellValue(int Index, int Value)

{

    public bool IsSelected { get; set; }

}

​

public record Board(IEnumerable<Row> Rows)

{

    public List<Column> Columns => Rows.SelectMany(row => row.Values).GroupBy(x => x.Index).Select(x => new Column(x.ToList())).ToList();

}

​