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

@@ -163,7 +163,7 @@ 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
@@ -171,20 +171,36 @@ module Oga
       #
       def on_test(ast_node, context)
         nodes = XML::NodeSet.new
-        predicate   = ast_node.children[2]
-        xpath_index = 1
 
         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
+        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)
-              nodes << xml_node if retval.to_i == xpath_index
+            final_nodes << xml_node if retval.to_i == xpath_index
 
           # Node sets, strings, booleans, etc
           elsif retval
@@ -192,16 +208,13 @@ module Oga
               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] }
     ;
 

spec/oga/xpath/parser/calls_spec.rb

@@ -43,18 +43,13 @@ describe Oga::XPath::Parser do
       parse_xpath('div[@class="foo"]/bar()').should == s(
         :path,
         s(
-          :axis,
-          'child',
-          s(
-            :test,
-            nil,
-            'div',
+          :predicate,
+          s(:axis, 'child', s(:test, nil, 'div')),
           s(
             :eq,
             s(:axis, 'attribute', s(:test, nil, 'class')),
             s(:string, 'foo')
           )
-          )
         ),
         s(:call, 'bar')
       )
@@ -64,14 +59,14 @@ describe Oga::XPath::Parser do
       parse_xpath('A[@x]/B[@x]/bar()').should == s(
         :path,
         s(
-          :axis,
-          'child',
-          s(:test, nil, 'A', s(:axis, 'attribute', s(:test, nil, 'x')))
+          :predicate,
+          s(:axis, 'child', s(:test, nil, 'A')),
+          s(:axis, 'attribute', s(:test, nil, 'x'))
         ),
         s(
-          :axis,
-          'child',
-          s(:test, nil, 'B', s(:axis, 'attribute', s(:test, nil, 'x')))
+          :predicate,
+          s(:axis, 'child', s(:test, nil, 'B')),
+          s(:axis, 'attribute', s(:test, nil, 'x'))
         ),
         s(:call, 'bar')
       )

spec/oga/xpath/parser/predicates_spec.rb

@@ -4,32 +4,26 @@ describe Oga::XPath::Parser do
   context 'predicates' do
     example 'parse a single predicate' do
       parse_xpath('foo[@class="bar"]').should == s(
-        :axis,
-        'child',
+        :predicate,
+        s(:axis, 'child', s(:test, nil, 'foo')),
         s(
-          :test,
-          nil,
-          'foo',
-          s(:eq, s(:axis, 'attribute', s(:test, nil, 'class')), s(:string, 'bar'))
+          :eq,
+          s(:axis, 'attribute', s(:test, nil, 'class')),
+          s(:string, 'bar')
         )
       )
     end
 
     example 'parse a predicate using the or operator' do
       parse_xpath('foo[@x="bar" or @x="baz"]').should == s(
-        :axis,
-        'child',
-        s(
-          :test,
-          nil,
-          'foo',
+        :predicate,
+        s(:axis, 'child', s(:test, nil, 'foo')),
         s(
           :or,
           s(:eq, s(:axis, 'attribute', s(:test, nil, 'x')), s(:string, 'bar')),
           s(:eq, s(:axis, 'attribute', s(:test, nil, 'x')), s(:string, 'baz')),
         )
       )
-      )
     end
   end
 end

spec/oga/xpath/parser/variable_spec.rb

@@ -8,9 +8,9 @@ describe Oga::XPath::Parser do
 
     example 'parse a variable reference in a predicate' do
       parse_xpath('foo[$bar]').should == s(
-        :axis,
-        'child',
-        s(:test, nil, 'foo', s(:var, 'bar'))
+        :predicate,
+        s(:axis, 'child', s(:test, nil, 'foo')),
+        s(:var, 'bar')
       )
     end
   end