System.Text.RegularExpressions.RegexNode.ReduceConcatenation C# (CSharp) Method

        internal RegexNode ReduceConcatenation()
        {
            // Eliminate empties and concat adjacent strings/chars

            bool wasLastString;
            RegexOptions optionsLast;
            RegexOptions optionsAt;
            int i;
            int j;

            if (_children == null)
                return new RegexNode(Empty, _options);

            wasLastString = false;
            optionsLast = 0;

            for (i = 0, j = 0; i < _children.Count; i++, j++)
            {
                RegexNode at;
                RegexNode prev;

                at = _children[i];

                if (j < i)
                    _children[j] = at;

                if (at._type == Concatenate &&
                    ((at._options & RegexOptions.RightToLeft) == (_options & RegexOptions.RightToLeft)))
                {
                    for (int k = 0; k < at._children.Count; k++)
                        at._children[k]._next = this;

                    _children.InsertRange(i + 1, at._children);
                    j--;
                }
                else if (at._type == Multi ||
                         at._type == One)
                {
                    // Cannot merge strings if L or I options differ
                    optionsAt = at._options & (RegexOptions.RightToLeft | RegexOptions.IgnoreCase);

                    if (!wasLastString || optionsLast != optionsAt)
                    {
                        wasLastString = true;
                        optionsLast = optionsAt;
                        continue;
                    }

                    prev = _children[--j];

                    if (prev._type == One)
                    {
                        prev._type = Multi;
                        prev._str = Convert.ToString(prev._ch, CultureInfo.InvariantCulture);
                    }

                    if ((optionsAt & RegexOptions.RightToLeft) == 0)
                    {
                        if (at._type == One)
                            prev._str += at._ch.ToString();
                        else
                            prev._str += at._str;
                    }
                    else
                    {
                        if (at._type == One)
                            prev._str = at._ch.ToString() + prev._str;
                        else
                            prev._str = at._str + prev._str;
                    }
                }
                else if (at._type == Empty)
                {
                    j--;
                }
                else
                {
                    wasLastString = false;
                }
            }

            if (j < i)
                _children.RemoveRange(j, i - j);

            return StripEnation(Empty);
        }