AdventOfCode23/Day8/Parser.cs

using System.Text.RegularExpressions;

namespace Day8;

public partial class Parser
{
    [GeneratedRegex(@"(\w+)\s*=\s*\((\w+)\s*,\s*(\w+)\)")]
    private partial Regex LineMatch();
    
    public MapData Parse(string inputFile)
    {
        using var reader = File.OpenText(inputFile);

        var instructions = reader.ReadLine()!;
        reader.ReadLine(); // discard empty line

        var nodeMap = new Dictionary<string, Node>();
        
        while (!reader.EndOfStream)
        {
            var line = reader.ReadLine()!;
            var match = LineMatch().Match(line);
            if (!match.Success)
            {
                throw new Exception($"Expected node declaration, but got: {line}");
            }

            var node = GetNode(nodeMap, match.Groups[1].Value);
            node.Left = GetNode(nodeMap, match.Groups[2].Value);
            node.Right = GetNode(nodeMap, match.Groups[3].Value);
        }

        if (!nodeMap.TryGetValue("AAA", out var start))
        {
            throw new Exception($"Cannot find start node");
        }

        return new MapData(instructions, nodeMap.Values.ToList());
    }

    private Node GetNode(Dictionary<string, Node> nodeMap, string nodeName)
    {
        if (nodeMap.TryGetValue(nodeName, out Node? value))
        {
            return value;
        }

        value = new Node(nodeName);
        nodeMap[nodeName] = value;

        return value;
    }
}