AdventOfCode23/Day1/WordToDigitTransform.cs

using System.Text.RegularExpressions;

namespace Day1;

public partial class WordToDigitTransform : IStringTransform
{
    
    [GeneratedRegex("one|two|three|four|five|six|seven|eight|nine", RegexOptions.IgnoreCase, "en-US")]
    private static partial Regex NumberWords();
    
    private static readonly Dictionary<string, char> Digits = new()
    {
        ["one"] = '1',
        ["two"] = '2',
        ["three"] = '3',
        ["four"] = '4',
        ["five"] = '5',
        ["six"] = '6',
        ["seven"] = '7',
        ["eight"] = '8',
        ["nine"] = '9',
    };

    public string Transform(string input)
    {
        var result = new List<char>();
        for (var i = 0; i < input.Length; i++)
        {
            var current = input.Substring(i);
            var next = false;
            foreach (var kvp in Digits)
            {
                if (current.StartsWith(kvp.Key))
                {
                    result.Add(kvp.Value);
                    next = true;
                    break;
                }
            }

            if (!next)
            {
                result.Add(current[0]);
            }
        }

        return string.Concat(result);
    }
}