Evaluate XPath predicates for every context node.

Instead of evaluating a predicate once for all context nodes, they should
instead be evaluated separately per context node.

lib/oga/xpath/evaluator.rb

@@ -188,9 +188,10 @@ module Oga
       #
       def on_predicate(ast_node, context)
         test, predicate = *ast_node
-
-        initial_nodes = process(test, context)
         final_nodes     = XML::NodeSet.new
+
+        context.each do |context_node|
+          initial_nodes = process(test, XML::NodeSet.new([context_node]))
           xpath_index   = 1
 
           initial_nodes.each do |xml_node|
@@ -213,6 +214,7 @@ module Oga
 
             xpath_index += 1
           end
+        end
 
         return final_nodes
       end

spec/oga/xpath/evaluator/predicates_spec.rb

@@ -3,18 +3,23 @@ require 'spec_helper'
 describe Oga::XPath::Evaluator do
   context 'predicates' do
     before do
-      @document = parse('<root><b>10</b><b>20</b></root>')
+      @document = parse(<<-EOF)
+<root>
+  <a>10</a>
+  <b>
+    <a>20</a>
+    <a>30</3>
+  </b>
+</root>
+      EOF
 
-      @b1 = @document.children[0].children[0]
+      @a1 = @document.at_xpath('root/a[1]')
-      @b2 = @document.children[0].children[1]
+      @a2 = @document.at_xpath('root/b/a[1]')
     end
 
-    example 'evaluate a predicate that returns the first <b> node' do
+    example 'return a node set containing all first <a> nodes' do
-      evaluate_xpath(@document, 'root/b[1]').should == node_set(@b1)
+      evaluate_xpath(@document, 'descendant-or-self::node()/a[1]')
-    end
+        .should == node_set(@a1, @a2)
-
-    example 'evaluate a predicate that returns the second <b> node' do
-      evaluate_xpath(@document, 'root/b[2]').should == node_set(@b2)
     end
   end
 end