Wrap predicate AST nodes _around_ other nodes.

This means that "foo[1]" uses this AST:

    (predicate (test nil "foo") (int 1))

Instead of this AST:

    (test nil "foo" (int 1))

This makes it easier for the XPath evaluator to process predicates correctly.

lib/oga/xpath/evaluator.rb

@@ -163,45 +163,58 @@ module Oga
       end
 
       ##
-      # Processes a node test and optionally a predicate.
+      # Processes a node test.
       #
       # @param [AST::Node] ast_node
       # @param [Oga::XML::NodeSet] context
       # @return [Oga::XML::NodeSet]
       #
       def on_test(ast_node, context)
-        nodes       = XML::NodeSet.new
-        predicate   = ast_node.children[2]
-        xpath_index = 1
+        nodes = XML::NodeSet.new
 
         context.each do |xml_node|
-          next unless node_matches?(xml_node, ast_node)
+          nodes << xml_node if node_matches?(xml_node, ast_node)
+        end
 
-          if predicate
-            retval = with_node_set(context) do
-              process(predicate, XML::NodeSet.new([xml_node]))
+        return nodes
+      end
+
+      ##
+      # Processes a predicate.
+      #
+      # @param [AST::Node] ast_node
+      # @param [Oga::XML::NodeSet] context
+      # @return [Oga::XML::NodeSet]
+      #
+      def on_predicate(ast_node, context)
+        test, predicate = *ast_node
+
+        initial_nodes = process(test, context)
+        final_nodes   = XML::NodeSet.new
+        xpath_index   = 1
+
+        initial_nodes.each do |xml_node|
+          retval = with_node_set(initial_nodes) do
+            process(predicate, XML::NodeSet.new([xml_node]))
+          end
+
+          # Numeric values are used as node set indexes.
+          if retval.is_a?(Numeric)
+            final_nodes << xml_node if retval.to_i == xpath_index
+
+          # Node sets, strings, booleans, etc
+          elsif retval
+            if retval.respond_to?(:empty?) and retval.empty?
+              next
             end
 
-            # Numeric values are used as node set indexes.
-            if retval.is_a?(Numeric)
-              nodes << xml_node if retval.to_i == xpath_index
-
-            # Node sets, strings, booleans, etc
-            elsif retval
-              if retval.respond_to?(:empty?) and retval.empty?
-                next
-              end
-
-              nodes << xml_node
-            end
-          else
-            nodes << xml_node
+            final_nodes << xml_node
           end
 
           xpath_index += 1
         end
 
-        return nodes
+        return final_nodes
       end
 
       ##

lib/oga/xpath/parser.y

@@ -40,9 +40,8 @@ rule
     ;
 
   expression_members
-    : node_test_as_axis
+    : expression_with_predicate
     | operator
-    | axis
     | string
     | number
     | call
@@ -51,9 +50,14 @@ rule
     | variable
     ;
 
-  path_member
+  expression_with_predicate
     : node_test_as_axis
+    | node_test_as_axis predicate { s(:predicate, val[0], val[1]) }
     | axis
+    | axis predicate { s(:predicate, val[0], val[1]) }
+
+  path_member
+    : expression_with_predicate
     | call
     ;
 
@@ -83,7 +87,7 @@ rule
 
   node_test
     : node_name           { s(:test, *val[0]) }
-    | node_name predicate { s(:test, *val[0], val[1]) }
+    #| node_name predicate { s(:predicate, s(:test, *val[0]), val[1]) }
     | type_test           { val[0] }
     ;