HtmlLexicalAnalyzer.GetNextCharacter C# (CSharp) Method

    private void GetNextCharacter()
    {
        if (_nextCharacterCode == -1)
        {
            throw new InvalidOperationException("GetNextCharacter method called at the end of a stream");
        }

        _previousCharacter = _nextCharacter;

        _nextCharacter = _lookAheadCharacter;
        _nextCharacterCode = _lookAheadCharacterCode;
        // next character not an entity as of now
        _isNextCharacterEntity = false;

        this.ReadLookAheadCharacter();

        if (_nextCharacter == '&')
        {
            if (_lookAheadCharacter == '#')
            {
                // numeric entity - parse digits - &#DDDDD;
                int entityCode;
                entityCode = 0;
                this.ReadLookAheadCharacter();

                // largest numeric entity is 7 characters
                for (int i = 0; i < 7 && Char.IsDigit(_lookAheadCharacter); i++)
                {
                    entityCode = 10 * entityCode + (_lookAheadCharacterCode - (int)'0');
                    this.ReadLookAheadCharacter();
                }
                if (_lookAheadCharacter == ';')
                {
                    // correct format - advance
                    this.ReadLookAheadCharacter();
                    _nextCharacterCode = entityCode;

                    // if this is out of range it will set the character to '?'
                    _nextCharacter = (char)_nextCharacterCode;

                    // as far as we are concerned, this is an entity
                    _isNextCharacterEntity = true;
                }
                else
                {
                    // not an entity, set next character to the current lookahread character
                    // we would have eaten up some digits
                    _nextCharacter = _lookAheadCharacter;
                    _nextCharacterCode = _lookAheadCharacterCode;
                    this.ReadLookAheadCharacter();
                    _isNextCharacterEntity = false;
                }
            }
            else if (Char.IsLetter(_lookAheadCharacter))
            {
                // entity is written as a string
                string entity = "";

                // maximum length of string entities is 10 characters
                for (int i = 0; i < 10 && (Char.IsLetter(_lookAheadCharacter) || Char.IsDigit(_lookAheadCharacter)); i++)
                {
                    entity += _lookAheadCharacter;
                    this.ReadLookAheadCharacter();
                }
                if (_lookAheadCharacter == ';')
                {
                    // advance
                    this.ReadLookAheadCharacter();

                    if (HtmlSchema.IsEntity(entity))
                    {
                        _nextCharacter = HtmlSchema.EntityCharacterValue(entity);
                        _nextCharacterCode = (int)_nextCharacter;
                        _isNextCharacterEntity = true;
                    }
                    else
                    {
                        // just skip the whole thing - invalid entity
                        // move on to the next character
                        _nextCharacter = _lookAheadCharacter;
                        _nextCharacterCode = _lookAheadCharacterCode;
                        this.ReadLookAheadCharacter();

                        // not an entity
                        _isNextCharacterEntity = false;
                    }
                }
                else
                {
                    // skip whatever we read after the ampersand
                    // set next character and move on
                    _nextCharacter = _lookAheadCharacter;
                    this.ReadLookAheadCharacter();
                    _isNextCharacterEntity = false;
                }
            }
        }
    }