System.Text.RegularExpressions.RegexFCD.RegexFCFromRegexTree C# (CSharp) Method

        private RegexFC RegexFCFromRegexTree(RegexTree tree)
        {
            RegexNode curNode;
            int curChild;

            curNode = tree._root;
            curChild = 0;

            for (; ;)
            {
                if (curNode._children == null)
                {
                    // This is a leaf node
                    CalculateFC(curNode._type, curNode, 0);
                }
                else if (curChild < curNode._children.Count && !_skipAllChildren)
                {
                    // This is an interior node, and we have more children to analyze
                    CalculateFC(curNode._type | BeforeChild, curNode, curChild);

                    if (!_skipchild)
                    {
                        curNode = curNode._children[curChild];
                        // this stack is how we get a depth first walk of the tree.
                        PushInt(curChild);
                        curChild = 0;
                    }
                    else
                    {
                        curChild++;
                        _skipchild = false;
                    }
                    continue;
                }

                // This is an interior node where we've finished analyzing all the children, or
                // the end of a leaf node.
                _skipAllChildren = false;

                if (IntIsEmpty())
                    break;

                curChild = PopInt();
                curNode = curNode._next;

                CalculateFC(curNode._type | AfterChild, curNode, curChild);
                if (_failed)
                    return null;

                curChild++;
            }

            if (FCIsEmpty())
                return null;

            return PopFC();
        }

        /*
         * This is the one of the only two functions that should be called from outside.
         * It takes a RegexTree and computes the set of chars that can start it.
         */
        internal static RegexPrefix FirstChars(RegexTree t) {
            RegexFCD s = new RegexFCD();
            RegexFC fc = s.RegexFCFromRegexTree(t);

            if (fc == null || fc._nullable)
                return null;
            
            CultureInfo culture = ((t._options & RegexOptions.CultureInvariant) != 0) ? CultureInfo.InvariantCulture : CultureInfo.CurrentCulture;
            return new RegexPrefix(fc.GetFirstChars(culture), fc.IsCaseInsensitive());
        }