Basic boilerplate for converting CSS to XPath.

lib/oga.rb

@@ -50,3 +50,4 @@ require_relative 'oga/xpath/evaluator'
 
 require_relative 'oga/css/lexer'
 require_relative 'oga/css/parser'
+require_relative 'oga/css/transformer'

lib/oga/css/transformer.rb

@@ -0,0 +1,30 @@
+module Oga
+  module CSS
+    ##
+    # Transforms an CSS AST into a corresponding XPath AST.
+    #
+    class Transformer < AST::Processor
+      def on_class(node)
+        name, test = node.to_a
+
+        unless test
+          test = s(:test, nil, '*')
+        end
+
+        predicate = s(
+          :eq,
+          s(:axis, 'attribute', s(:test, nil, 'class')),
+          s(:string, name)
+        )
+
+        return s(:axis, 'child', test.updated(nil, test.children + [predicate]))
+      end
+
+      private
+
+      def s(type, *children)
+        return AST::Node.new(type, children)
+      end
+    end # Transformer
+  end # CSS
+end # Oga