#nullable enable

using System;

using System.Text;

using System.Diagnostics.CodeAnalysis;

​

Console.Write(new Lexer("56 ").MakeToken());

​

public struct Lexer

{

    int? pos;

    string text;

    char? currChar;

    public Lexer(string txt)

    {

        pos = -1;

        text = txt;

        currChar = null;

    }

    public string MakeToken()

    {

        StringBuilder str = new("[");

        if (currChar is null) Advance();

        StringBuilder num = new("");

        int numc = 0;

        while (currChar is not null)

        {

            if(int.TryParse($"{currChar}", out _))

            {

                num.Append(currChar);

                numc++;

                Advance();

                continue;

            }

            else if (numc is not 0)

            {

                Token integer = Token.Int.With(value: int.

                    Parse(num.ToString()).ToString());

                str.Append(integer.ToString() + ", ");

                numc = 0;

            }

            Advance();

        }

        str.Remove(str.Length - 2, str.Length);

        str.Append("]");

        return str.ToString();

    }

    public char? Advance(ushort count = 1)

    {

        pos = pos + count < text.Length? pos + count: null;

        currChar = int.TryParse($"{pos}", out var p)? text[p]: null;

        return currChar;

    }

}

​

public struct Token

{

    public static readonly Token Int = new("INT");

    public string TokenType { get; init; }

    public string? Value { get; init; }

    public Token([NotNull] string tokenType, string? value = null)

    {

        TokenType = tokenType;

        Value = value;

    }

    public Token With(string? tt = null, string? value = null) => new()

    {

        TokenType = tt ?? this.TokenType, Value = value

    };

    public override string ToString() => Value is null?

        TokenType: $"{TokenType}: {Value}";

}