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

@@ -188,30 +188,32 @@ module Oga
       #
       def on_predicate(ast_node, context)
         test, predicate = *ast_node
+        final_nodes     = XML::NodeSet.new
 
-        initial_nodes = process(test, context)
-        final_nodes   = XML::NodeSet.new
-        xpath_index   = 1
+        context.each do |context_node|
+          initial_nodes = process(test, XML::NodeSet.new([context_node]))
+          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
+          initial_nodes.each do |xml_node|
+            retval = with_node_set(initial_nodes) do
+              process(predicate, XML::NodeSet.new([xml_node]))
             end
 
-            final_nodes << 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
 
-          xpath_index += 1
+            # Node sets, strings, booleans, etc
+            elsif retval
+              if retval.respond_to?(:empty?) and retval.empty?
+                next
+              end
+
+              final_nodes << xml_node
+            end
+
+            xpath_index += 1
+          end
         end
 
         return final_nodes

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]
-      @b2 = @document.children[0].children[1]
+      @a1 = @document.at_xpath('root/a[1]')
+      @a2 = @document.at_xpath('root/b/a[1]')
     end
 
-    example 'evaluate a predicate that returns the first <b> node' do
-      evaluate_xpath(@document, 'root/b[1]').should == node_set(@b1)
-    end
-
-    example 'evaluate a predicate that returns the second <b> node' do
-      evaluate_xpath(@document, 'root/b[2]').should == node_set(@b2)
+    example 'return a node set containing all first <a> nodes' do
+      evaluate_xpath(@document, 'descendant-or-self::node()/a[1]')
+        .should == node_set(@a1, @a2)
     end
   end
 end