Manually increment XPath indexes.

This ensures that the index is only incremented for elements that match a node
test.

lib/oga/xpath/evaluator.rb

@@ -169,15 +169,14 @@ module Oga
       # @return [Oga::XML::NodeSet]
       #
       def on_test(ast_node, context)
-        nodes     = XML::NodeSet.new
+        nodes       = XML::NodeSet.new
-        predicate = ast_node.children[2]
+        predicate   = ast_node.children[2]
+        xpath_index = 1
 
-        context.each_with_index do |xml_node, index|
+        context.each do |xml_node|
           next unless node_matches?(xml_node, ast_node)
 
           if predicate
-            xpath_index = index + 1
-
             retval = with_node_set(context) do
               process(predicate, XML::NodeSet.new([xml_node]))
             end
@@ -197,6 +196,8 @@ module Oga
           else
             nodes << xml_node
           end
+
+          xpath_index += 1
         end
 
         return nodes

spec/oga/xpath/evaluator/predicates_spec.rb

@@ -0,0 +1,22 @@
+require 'spec_helper'
+
+describe Oga::XPath::Evaluator do
+  context 'predicates' do
+    before do
+      @document  = parse('<root><a></a><b>10</b><b>20</b></root>')
+      @evaluator = described_class.new(@document)
+    end
+
+    context 'using predicate indexes' do
+      before do
+        @set = @evaluator.evaluate('root/b[2]')
+      end
+
+      it_behaves_like :node_set, :length => 1
+
+      example 'return the second <b> node' do
+        @set[0].should == @document.children[0].children[-1]
+      end
+    end
+  end
+end