AdventOfCode23/Day8/MapData.cs

namespace Day8;

public class MapData
{
    public string Instructions { get; }
    public IList<Node> Nodes { get; }

    public MapData(string instructions, IList<Node> nodes)
    {
        Instructions = instructions;
        Nodes = nodes;
    }

    public long Walk()
    {
        var current = Nodes.First(n => n.IsStart);
        var steps = 0L;

        while (!current.IsEnd)
        {
            current = Move(steps, current);
            steps++;
        }

        return steps;
    }

    public long[] MultiWalk()
    {
        var starts = Nodes.Where(n => n.Name.Last() == 'A').ToArray();
        var cycleLengths = new List<long>();

        foreach (var node in starts)
        {
            var current = node;
            var steps = 0;
            
            while (!current.IsEnd)
            {
                current = Move(steps, current);
                steps++;
            }
            
            cycleLengths.Add(steps);
            Console.WriteLine(string.Join(", ", cycleLengths));
        }

        return cycleLengths.ToArray();
    }

    private Node Move(long step, Node node)
    {
        if (step >= 10000000)
        {
            throw new Exception("Over TEN MILLION");
        }
        
        var instruction = Instructions[(int)(step % Instructions.Length)];
        if (instruction != 'L' && instruction != 'R')
        {
            throw new Exception($"Unknown instruction: {instruction} @ {step}");
        }
        
        return (instruction == 'L' ? node.Left : node.Right)
            ?? throw new Exception($"Ended in null after: {instruction} @ {step}");
    }
}