AdventOfCode23/Day1/CalibrationExtractor.cs

using System.Text.RegularExpressions;

namespace Day1;

public class CalibrationExtractor
{
    private readonly string _fileName;
    private readonly DigitTransformer _transformer;
    private readonly Regex _nonDigit = new Regex(@"[^\d]", RegexOptions.Compiled);

    public CalibrationExtractor(string fileName, DigitTransformer transformer = null)
    {
        _fileName = fileName;
        _transformer = transformer;
    }

    public IEnumerable<int> Extract()
    {
        using var reader = File.OpenText(_fileName);
        while (!reader.EndOfStream)
        {
            var line = reader.ReadLine();
            if (_transformer != null)
            {
                line = _transformer.Replace(line);
            }
            var calibrationValueStr = _nonDigit.Replace(line, "");
            if (calibrationValueStr.Length < 1)
            {
                throw new InvalidOperationException($"Not enough digits: '{line}'");
            }
            yield return int.Parse(new string(new char[] { calibrationValueStr.First(), calibrationValueStr.Last() }));
        }
    }
}