AdventOfCode23/Day5/MapParser.cs

using System.Text.RegularExpressions;

namespace Day5;

public partial class MapParser
{
    [GeneratedRegex(@"\d+")]
    private partial Regex NumberList();
    
    public Almanac Parse(string inputFile)
    {
        using var reader = File.OpenText(inputFile);
        var almanac = new Almanac();
        
        var line = reader.ReadLine()!;
        if (!line.StartsWith("seeds:"))
        {
            throw new Exception("Expecting the first line to list the seeds");
        }

        foreach (Match m in NumberList().Matches(line.Substring(6)))
        {
            almanac.Seeds.Add(long.Parse(m.Value));
        }

        string source = string.Empty;
        
        while (!reader.EndOfStream)
        {
            line = reader.ReadLine()!;
            if (String.IsNullOrWhiteSpace(line))
            {
                line = reader.ReadLine()!;
                var parts = line.Split(new char[] {'-', ' '},
                    StringSplitOptions.TrimEntries | StringSplitOptions.RemoveEmptyEntries);

                if (parts.Length < 4)
                {
                    throw new Exception($"Expected src-to-dst map line: {line}");
                }

                source = parts[0];
                var destination = parts[2];
                
                almanac.Mappings.Add((source, destination));
            }
            else
            {
                var numbers = NumberList().Matches(line).Cast<Match>().Select(x => long.Parse(x.Value)).ToList();
                if (numbers.Count != 3)
                {
                    throw new Exception($"Expecting lines to contain exactly 3 numbers, but got: {line}");
                }

                if (!almanac.MappingData.ContainsKey(source))
                {
                    almanac.MappingData[source] = new Dictionary<long, long>();
                }

                var dstIndex = numbers[0];
                var srcIndex = numbers[1];
                var lenght = numbers[2];

                for (var i = 0; i < lenght; i++)
                {
                    almanac.MappingData[source].Add(srcIndex + i, dstIndex + i);
                }
            }
        }
        
        return almanac;
    }
}