AdventOfCode23/Day1/DigitTransformer.cs

using System.Text;

namespace Day1;

public class DigitTransformer
{
    private static readonly Dictionary<string, string> _digits = new Dictionary<string, string>
    {
        ["one"] = "1",
        ["two"] = "2",
        ["three"] = "3",
        ["four"] = "4",
        ["five"] = "5",
        ["six"] = "6",
        ["seven"] = "7",
        ["eight"] = "8",
        ["nine"] = "9",
    };

    public string Replace(string input)
    {
        var result = new StringBuilder();
        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.Append(kvp.Value);
                    next = true;
                    break;
                }
            }

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

        return result.ToString();
    }
}