AdventOfCode/Days/Day2.cs

using Spectre.Console;

namespace AdventOfCode.Days;

public class Day2 : Day
{
    public override int Number => 2;
    public override string Name => "Red-Nosed Reports";

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

        foreach (var line in Input.AsSpan().EnumerateLines())
        {
            var isSafe = true;
            var levels = line.Split(' ');

            var previousDirection = 0;
            var previousLevel = int.MinValue;
            foreach (var levelRange in levels)
            {
                var level = int.Parse(line[levelRange]);

                // First level, skip to next
                if (previousLevel == int.MinValue)
                {
                    previousLevel = level;

                    continue;
                }

                // Check that direction is preserved
                var direction = level - previousLevel;

                if (previousDirection != 0 && Math.Sign(direction) != Math.Sign(previousDirection))
                {
                    isSafe = false;
                    break;
                }

                // Check that distance >= 1 and <= 3
                if (Math.Abs(direction) is < 1 or > 3)
                {
                    isSafe = false;
                    break;
                }

                previousLevel = level;
                previousDirection = direction;
            }

            if (isSafe)
            {
                safeReports++;
            }
        }

        if (display)
        {
            AnsiConsole.MarkupLine($"[green]Number of safe reports: [yellow]{safeReports}[/][/]");
        }
    }

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

        foreach (var line in Input.AsSpan().EnumerateLines())
        {
            if (IsSafe(line))
            {
                safeReports++;
            }
        }

        if (display)
        {
            AnsiConsole.MarkupLine($"[green]Number of safe reports: [yellow]{safeReports}[/][/]");
        }

        return;

        bool IsSafe(ReadOnlySpan<char> line, Range? toSkip = null)
        {
            var levels = line.Split(' ');

            var previousDirection = 0;
            var previousLevel = int.MinValue;
            Range previousLevelRange = default;
            Range firstLevelRange = default;

            foreach (var levelRange in levels)
            {
                if (levelRange.Equals(toSkip))
                {
                    // This level can be skipped using the problem dampener
                    continue;
                }

                var level = int.Parse(line[levelRange]);

                // First level, skip to next
                if (previousLevel == int.MinValue)
                {
                    previousLevel = level;
                    previousLevelRange = levelRange;
                    firstLevelRange = levelRange;

                    continue;
                }

                // Check that direction is preserved
                var direction = level - previousLevel;

                if (previousDirection != 0 && Math.Sign(direction) != Math.Sign(previousDirection))
                {
                    // Also try by removing current or previous level if it's first try (problem dampener)
                    return toSkip is null
                           && (IsSafe(line, levelRange) || IsSafe(line, previousLevelRange) || IsSafe(line, firstLevelRange));
                }

                // Check that distance >= 1 and <= 3
                if (Math.Abs(direction) is < 1 or > 3)
                {
                    // Also try by removing current or previous level if it's first try (problem dampener)
                    return toSkip is null
                           && (IsSafe(line, levelRange) || IsSafe(line, previousLevelRange) || IsSafe(line, firstLevelRange));
                }

                previousLevel = level;
                previousDirection = direction;
                previousLevelRange = levelRange;
            }

            return true;
        }
    }
}