AdventOfCode/AdventOfCode2023/Day08.cs

namespace AdventOfCode2023;

/*
--- Day 8: Haunted Wasteland ---

You're still riding a camel across Desert Island when you spot a sandstorm quickly approaching. When you turn to warn the Elf, she disappears before your eyes! To be fair, she had just finished warning you about ghosts a few minutes ago.

One of the camel's pouches is labeled "maps" - sure enough, it's full of documents (your puzzle input) about how to navigate the desert. At least, you're pretty sure that's what they are; one of the documents contains a list of left/right instructions, and the rest of the documents seem to describe some kind of network of labeled nodes.

It seems like you're meant to use the left/right instructions to navigate the network. Perhaps if you have the camel follow the same instructions, you can escape the haunted wasteland!

After examining the maps for a bit, two nodes stick out: AAA and ZZZ. You feel like AAA is where you are now, and you have to follow the left/right instructions until you reach ZZZ.

This format defines each node of the network individually. For example:

RL

AAA = (BBB, CCC)
BBB = (DDD, EEE)
CCC = (ZZZ, GGG)
DDD = (DDD, DDD)
EEE = (EEE, EEE)
GGG = (GGG, GGG)
ZZZ = (ZZZ, ZZZ)

Starting with AAA, you need to look up the next element based on the next left/right instruction in your input. In this example, start with AAA and go right (R) by choosing the right element of AAA, CCC. Then, L means to choose the left element of CCC, ZZZ. By following the left/right instructions, you reach ZZZ in 2 steps.

Of course, you might not find ZZZ right away. If you run out of left/right instructions, repeat the whole sequence of instructions as necessary: RL really means RLRLRLRLRLRLRLRL... and so on. For example, here is a situation that takes 6 steps to reach ZZZ:

LLR

AAA = (BBB, BBB)
BBB = (AAA, ZZZ)
ZZZ = (ZZZ, ZZZ)

Starting at AAA, follow the left/right instructions. How many steps are required to reach ZZZ?


*/

public class Day08 : IDay
{
    public string ResolvePart1(string[] inputs)
    {
        const string startNodeKey = "AAA";
        const string endNodeKey = "ZZZ";
        Map map = new(inputs);

        MapWalker walker = new(map, startNodeKey);
        while (walker.CurrentNode.Key != endNodeKey)
        {
            walker.Step();
        }
        return walker.Steps.ToString();
    }

    public string ResolvePart2(string[] inputs)
    {
        throw new NotImplementedException();
    }

    private class MapNode
    {
        public string Key { get; set; }
        public string LeftKey { get; set; }
        public string RightKey { get; set; }

        public MapNode(string strMapNode)
        {
            Key = strMapNode.Substring(0, 3);
            LeftKey = strMapNode.Substring(7, 3);
            RightKey = strMapNode.Substring(12, 3);
        }
    }

    private class Map
    {
        private readonly List<MapNode> _mapNodes;
        private readonly Dictionary<string, MapNode> _dictMapNodes;
        private readonly string _strLeftRightInstructions;

        public Map(string[] inputs)
        {
            _mapNodes = new List<MapNode>();
            _dictMapNodes = new Dictionary<string, MapNode>();
            _strLeftRightInstructions = inputs[0];
            for (int i = 2; i < inputs.Length; i++)
            {
                MapNode node = new(inputs[i]);
                _mapNodes.Add(node);
                _dictMapNodes.Add(node.Key, node);
            }
        }

        public List<MapNode> Nodes
        {
            get { return _mapNodes; }
        }

        public MapNode? GetByKey(string key)
        {
            return _dictMapNodes.GetValueOrDefault(key);
        }

        public char GetInstruction(int step)
        {
            char leftRightInstruction = _strLeftRightInstructions[step % _strLeftRightInstructions.Length];
            return leftRightInstruction;
        }
    }

    private class MapWalker
    {
        private readonly Map _map;

        private MapNode _currentNode;
        private int _steps;

        public MapWalker(Map map, string startKey)
        {
            _map = map;
            _currentNode = map.GetByKey(startKey) ?? map.Nodes.First();
            _steps = 0;
        }

        public void Step()
        {
            char leftRightInstruction = _map.GetInstruction(_steps);
            if (leftRightInstruction == 'L')
            {
                _currentNode = _map.GetByKey(_currentNode.LeftKey) ?? _map.Nodes.First();
            }
            if (leftRightInstruction == 'R')
            {
                _currentNode = _map.GetByKey(_currentNode.RightKey) ?? _map.Nodes.First();
            }
            _steps++;
        }

        public MapNode CurrentNode
        {
            get { return _currentNode; }
        }

        public int Steps
        {
            get { return _steps; }
        }
    }
}