advent-2024-05.cs

var state = ParseState.OrderingRules;
Dictionary<int, HashSet<int>> rules = [];
List<List<int>> pageOrders = [];

var reader = new StringReader(input);
while (reader.ReadLine() is string line)
{
    if (state == ParseState.OrderingRules)
    {
        if (line == "")
        {
            state = ParseState.PagesInUpdate;
            continue;
        }

        var parts = line.Split('|');
        var from = int.Parse(parts[0]);
        var to = int.Parse(parts[1]);


        if (!rules.TryGetValue(from, out var tos))
        {
            rules[from] = [to];
        }
        else
        {
            tos.Add(to);
        }
    }
    else if (state == ParseState.PagesInUpdate)
    {
        pageOrders.Add(line.Split(',').Select(int.Parse).ToList());
    }
}

bool IsPageOrderValid(List<int> pageOrder)
{
    for (int i = 1; i < pageOrder.Count; i++)
    {
        if (rules.TryGetValue(pageOrder[i - 1], out var tos) && tos.Contains(pageOrder[i]))
        {
            continue;
        }
        else
        {
            return false;
        }
    }
    
    return true;
}

var sum1 = 0;

foreach (var pageOrder in pageOrders)
{
    if (IsPageOrderValid(pageOrder))
    {
        sum1 += pageOrder[pageOrder.Count / 2];
    }
}

Console.WriteLine(sum1);

int sum2 = 0;

foreach (var pageOrder in pageOrders)
{
    if (!IsPageOrderValid(pageOrder))
    {
        pageOrder.Sort((a, b) => rules.TryGetValue(a, out var ato) && ato.Contains(b) ? -1 : 1);
        sum2 += pageOrder[pageOrder.Count / 2];
    }
}

Console.WriteLine(sum2);

enum ParseState
{
    OrderingRules,
    PagesInUpdate,
}