Advent of Code 2024 - Day 5

var input = await File.ReadAllLinesAsync("input.txt");

var rules = new List<Rule>();
var updates = new List<List<int>>();

var parsingRules = true;

foreach (var line in input)
{
    if (parsingRules)
    {
        if (string.IsNullOrWhiteSpace(line))
        {
            parsingRules = false;
        }
        else
        {
            var numbers = line.Split('|').Select(int.Parse).ToArray();
            rules.Add(new Rule(numbers[0], numbers[1]));
        }
    }
    else
    {
        updates.Add(line.Split(',').Select(int.Parse).ToList());
    }
}

var part1 = 0;
var part2 = 0;

foreach (var update in updates)
{
    var isValid = true;
    var matchingRules = new List<Rule>();

    foreach (var rule in rules)
    {
        var indexLeft = update.IndexOf(rule.Left);
        var indexRight = update.IndexOf(rule.Right);

        if (indexLeft == -1 || indexRight == -1)
        {
            continue;
        }

        matchingRules.Add(rule);

        if (indexLeft > indexRight)
        {
            isValid = false;
        }
    }

    if (isValid)
    {
        part1 += update[update.Count / 2];
    }
    else
    {
        update.Sort((a, b) =>
        {
            var rule = matchingRules.Find(r => r.Contains(a) && r.Contains(b));

            if (rule == default || rule.Left == a)
            {
                return -1;
            }

            return 1;
        });

        part2 += update[update.Count / 2];
    }
}

Console.WriteLine($"Part 1: {part1}");
Console.WriteLine($"Part 2: {part2}");

internal record Rule(int Left, int Right)
{
    public bool Contains(int i) => Left == i || Right == i;
}