DRC enhancements:

- Shapes are added to existing categories rather than creating
  a new category with the same name
- The category name in "output" can be an array creating a
  hierarchy of categories.

src/drc/drc/built-in-macros/_drc_engine.rb

@@ -3226,7 +3226,23 @@ def _output(data, *args)
         output_rdb = channel.rdb
         output_cell = channel.cell
 
-        cat = output_rdb.create_category(args[0].to_s)
+        categories = args[0]
+        if !categories.is_a?(Array)
+          categories = [ categories.to_s ]
+        end
+
+        cat = nil
+        categories.each do |c|
+          ccat = nil
+          if cat
+            ccat = cat.each_sub_category.find { |i| i.name == c }
+          else
+            ccat = output_rdb.each_category.find { |i| i.name == c }
+          end
+          cat = ccat ? ccat : output_rdb.create_category(cat, c)
+        end
+        cat ||= output_rdb.create_category("default")
+
         args[1] && cat.description = args[1]
 
         cat.scan_collection(output_cell, RBA::CplxTrans::new(self.dbu), data)

src/drc/drc/built-in-macros/_drc_layer.rb

@@ -5111,7 +5111,11 @@ def nets(*args)
     # This method will copy the content of the layer to the specified output.
     #
     # If a report database is selected for the output, the specification has to include a 
-    # category name and optionally a category description.
+    # category name and optionally a category description. The category name can be an 
+    # array of strings - in that case, a hierarchy of categories is created
+    # with the first array item being the top level category name.
+    # Shapes are added to an existing category, if a category with the given
+    # name already exists.
     #
     # If the layout is selected for the output, the specification can consist of
     # one to three parameters: a layer number, a data type (optional, default is 0)

src/rdb/rdb/gsiDeclRdb.cc

@@ -1342,6 +1342,7 @@ Class<rdb::Database> decl_ReportDatabase ("rdb", "ReportDatabase",
     "@brief Creates a new sub-category\n"
     "@param parent The category under which the category should be created\n"
     "@param name The name of the category\n"
+    "Since version 0.29.1, 'parent' can be nil. In that case, a top-level category is created."
   ) +
   gsi::method ("category_by_path", &rdb::Database::category_by_name, gsi::arg ("path"),
     "@brief Gets a category by path\n"

src/rdb/rdb/rdb.cc

@@ -1275,6 +1275,10 @@ Database::import_cells (const Cells &cells)
 Category *
 Database::create_category (Category *parent, const std::string &name)
 {
+  if (! parent) {
+    return create_category (name);
+  }
+
   set_modified ();
 
   Category *cat = create_category (&parent->sub_categories (), name);