Initial check in lib

src.lib/dpglib.TreeBlockContext.pas

@@ -0,0 +1,63 @@
+// The context needed to add root,child elements to a Tree. There is only one alternative
+// (i.e., a list of children). We subclass to specialize. MakeGrammar.addElementToCurrentAlt
+// will work correctly now for either a block of alts or a Tree child list.
+//
+// The first time addAlternativeElement is called, it sets the root element
+// rather than adding it to one of the alternative lists.  Rather than have
+// the grammar duplicate the rules for grammar atoms etc... we use the same
+// grammar and same refToken behavior etc...  We have to special case somewhere
+// and here is where we do it.
+
+unit dpglib.TreeBlockContext;
+
+interface
+uses
+   dpglib.types,
+   dpglib.BlockContext;
+
+type
+   TTreeBlockContext = class( TBlockContext)
+      protected
+         fNextElementIsRoot: boolean;
+
+      public
+         procedure   AddAlternativeElem( pElem: IAlternativeElem); override;
+
+      public
+         procedure   AfterConstruction; override;
+   end;
+
+implementation
+
+{ TTreeBlockContext }
+
+// ================================================================================================
+// After Construction
+// ================================================================================================
+procedure TTreeBlockContext.AfterConstruction;
+begin
+   inherited;
+   fNextElementIsRoot := true;
+end;
+
+// ================================================================================================
+// Add Alternative Elem
+// ================================================================================================
+procedure TTreeBlockContext.AddAlternativeElem(pElem: IAlternativeElem);
+var
+   tree: ITreeElem;
+
+begin
+   Block.QueryInterface( ITreeElem, tree);
+
+   if fNextElementIsRoot then
+   begin
+      tree.Root := pElem as IGrammarAtom;
+      fNextElementIsRoot := false
+   end
+
+   else
+      inherited
+end;
+
+end.