RtfToTLParser

public class RtfToTLParser : RtfSarParser
{
    private bool _bold;
    private bool _italic;
    private bool _firstPard;

    private string _groupText;
    private string _lastKey;
    private int? _field;

    private int _level = -1;
    private int _length;

    private Stack<TLMessageEntityBase> _entities;

    public IReadOnlyList<TLMessageEntityBase> Entities { get; private set; }

    public override void StartRtfDocument()
    {
        _entities = new Stack<TLMessageEntityBase>();
        Entities = null;

        _bold = false;
        _italic = false;
        _firstPard = false;

        _groupText = null;
        _lastKey = null;
        _field = null;

        _level = -1;
        _length = 0;
    }

    public override void StartRtfGroup()
    {
        if (_level >= 0 && !_firstPard) return;

        if (_field.HasValue)
            _field++;

        WriteLine($"StartRtfGroup:\tlevel: {++_level}", _level);
    }

    public override void RtfControl(string key, bool hasParameter, int parameter)
    {
        if (_firstPard && key == "'" && hasParameter)
        {
            if (_field.HasValue && _lastKey == "fldinst")
            {
                _groupText += (char)parameter;
            }
            else if (_field.HasValue && _lastKey.Equals("fldrslt"))
            {
                _groupText += (char)parameter;
            }
            else if (_bold || _italic)
            {
                _groupText += (char)parameter;
            }
            else
            {
                _groupText += (char)parameter;
                HandleBasicText();
                _groupText = string.Empty;
            }
        }
    }

    public override void RtfKeyword(string key, bool hasParameter, int parameter)
    {
        if (key.Equals("pard"))
        {
            _firstPard = true;
        }
        else if (key.Equals("field"))
        {
            _field = !hasParameter || (hasParameter && parameter == 1) ? new int?(0) : null;
        }
        else if (key.Equals("b"))
        {
            if (!hasParameter || (hasParameter && parameter == 1))
            {
                _groupText = string.Empty;
                _bold = true;
            }
            else
            {
                HandleBoldText();
                _groupText = string.Empty;
                _bold = false;
            }
        }
        else if (key.Equals("i"))
        {
            if (!hasParameter || (hasParameter && parameter == 1))
            {
                _groupText = string.Empty;
                _italic = true;
            }
            else
            {
                HandleItalicText();
                _groupText = string.Empty;
                _italic = false;
            }
        }
        else if (key.Equals("fldinst") || key.Equals("fldrslt"))
        {
            _lastKey = key;
        }
    }

    public override void RtfText(string text)
    {
        if (_firstPard)
        {
            if (_field.HasValue && _lastKey == "fldinst")
            {
                if (text.IndexOf("HYPERLINK") == 0)
                    _groupText += text.Substring("HYPERLINK ".Length);
                else
                    _groupText += text;
            }
            else if (_field.HasValue && _lastKey.Equals("fldrslt"))
            {
                _groupText += text;
            }
            else if (_bold || _italic)
            {
                _groupText += text;
            }
            else
            {
                _groupText += text;
                HandleBasicText();
                _groupText = string.Empty;
            }
        }
    }

    public override void EndRtfGroup()
    {
        if (_firstPard)
        {
            if (_bold) HandleBoldText();
            else if (_italic) HandleItalicText();
            else if (_field.HasValue && _field == 2 && _lastKey.Equals("fldinst")) HandleHyperlinkUrl();
            else if (_field.HasValue && _field == 2 && _lastKey.Equals("fldrslt")) HandleHyperlinkText();
            else if (string.IsNullOrEmpty(_groupText) == false) HandleBasicText();

            _bold = false;
            _italic = false;
            _lastKey = string.Empty;
            _groupText = string.Empty;

            if (_field.HasValue)
                _field--;

            if (_field.HasValue && _field < 0)
                _field = null;

            WriteLine($"EndRtfGroup:\tlevel: {_level--}", _level + 1);
        }
    }

    private void HandleBoldText()
    {
        _entities.Push(new TLMessageEntityBold { Offset = _length, Length = _groupText.Length });
        _length += _groupText.Length;
        WriteLine("BOLD   TEXT:\t\"" + _groupText + "\"", _level + 1);
    }

    private void HandleItalicText()
    {
        _entities.Push(new TLMessageEntityItalic { Offset = _length, Length = _groupText.Length });
        _length += _groupText.Length;
        WriteLine("ITALIC TEXT:\t\"" + _groupText + "\"", _level + 1);
    }

    private void HandleHyperlinkUrl()
    {
        var userId = int.Parse(_groupText.Trim().Trim('"'));
        //var user = InMemoryCacheService.Current.GetUser(userId) as TLUser;
        //if (user != null)
        //{
        _entities.Push(new TLInputMessageEntityMentionName { UserId = userId });
        //}
        WriteLine("HYPERLINK URL:\t" + _groupText, _level + 1);
    }

    private void HandleHyperlinkText()
    {
        var mention = _entities.Peek() as TLInputMessageEntityMentionName;
        if (mention != null)
        {
            mention.Offset = _length;
            mention.Length = _groupText.Length;
        }

        _length += _groupText.Length;
        WriteLine("HYPERLINK TEXT:\t\"" + _groupText + "\"", _level + 1);
    }

    private void HandleBasicText()
    {
        _length += _groupText.Length;
        WriteLine("BASIC  TEXT:\t\"" + _groupText + "\"", _level + 1);
    }

    public override void EndRtfDocument()
    {
        if (Entities == null)
        {
            Entities = new List<TLMessageEntityBase>(_entities);
        }

        Debugger.Break();
    }

    private void WriteLine(string str, int level)
    {
        for (int i = 0; i < level; i++)
        {
            str = '\t' + str;
        }

        Debug.WriteLine(str);
    }
}