AdventOfCode/Days/Day4.cs

using Spectre.Console;

namespace AdventOfCode.Days;

public class Day4 : Day
{
    public override int Number => 4;
    public override string Name => "Printing Department";

    public const int Length = 136;

    public override void RunPart1(bool display = true)
    {
        var accessibleRollsOfPaper = 0;

        var map = ParseMap(Input);

        Span<(int i, int j)> adjacentPositions =
        [
            (-1, -1), (-1, 0), (-1, 1),
            ( 0, -1),              ( 0, 1),
            ( 1, -1), ( 1, 0), ( 1, 1)
        ];

        for (var i = 0; i < Length; i++)
        {
            for (var j = 0; j < Length; j++)
            {
                // There is no paper roll at position (i, j)
                if (!map[i, j])
                {
                    continue;
                }

                // Check if a paper roll is accessible
                var adjacentRolls = 0;
                foreach (var adjacentPosition in adjacentPositions)
                {
                    var targetI = i + adjacentPosition.i;
                    var targetJ = j + adjacentPosition.j;

                    // Check that the adjacent position is not out of bounds
                    if (targetI is < 0 or >= Length || targetJ is < 0 or >= Length)
                    {
                        continue;
                    }

                    if (map[targetI, targetJ])
                    {
                        adjacentRolls++;
                    }
                }

                if (adjacentRolls < 4)
                {
                    accessibleRollsOfPaper++;
                }
            }
        }

        if (display)
        {
            AnsiConsole.MarkupLine($"[green]Accessible rolls of paper: [yellow]{accessibleRollsOfPaper}[/][/]");
        }
    }

    public override void RunPart2(bool display = true)
    {
        var accessibleRollsOfPaper = 0;

        var map = ParseMap(Input);

        Span<(int i, int j)> adjacentPositions =
        [
            (-1, -1), (-1, 0), (-1, 1),
            ( 0, -1),              ( 0, 1),
            ( 1, -1), ( 1, 0), ( 1, 1)
        ];

        int removedRollsOfPaper;
        // Loop until we cannot remove any roll anymore
        do
        {
            removedRollsOfPaper = 0;

            for (var i = 0; i < Length; i++)
            {
                for (var j = 0; j < Length; j++)
                {
                    // There is no paper roll at position (i, j)
                    if (!map[i, j])
                    {
                        continue;
                    }

                    // Check if a paper roll is accessible
                    var adjacentRolls = 0;
                    foreach (var adjacentPosition in adjacentPositions)
                    {
                        var targetI = i + adjacentPosition.i;
                        var targetJ = j + adjacentPosition.j;

                        // Check that the adjacent position is not out of bounds
                        if (targetI is < 0 or >= Length || targetJ is < 0 or >= Length)
                        {
                            continue;
                        }

                        if (map[targetI, targetJ])
                        {
                            adjacentRolls++;
                        }
                    }

                    if (adjacentRolls < 4)
                    {
                        removedRollsOfPaper++;

                        // Remove roll of paper
                        map[i, j] = false;
                    }
                }
            }

            accessibleRollsOfPaper += removedRollsOfPaper;
        } while (removedRollsOfPaper > 0);

        if (display)
        {
            AnsiConsole.MarkupLine($"[green]Accessible rolls of paper: [yellow]{accessibleRollsOfPaper}[/][/]");
        }
    }

    private static bool[,] ParseMap(string input)
    {
        // Map is a  square
        var map = new bool[Length, Length];

        var i = 0;
        foreach (var line in input.EnumerateLines())
        {
            var j = 0;

            foreach (var value in line)
            {
                map[i, j] = value is '@';

                j++;
            }

            i++;
        }

        return map;
    }
}