Basic support for the XPath "parent" axis.

The usage of `parent::node()` is not yet supported.

lib/oga/xpath/evaluator.rb

@@ -307,6 +307,27 @@ module Oga
         return nodes
       end
 
+      ##
+      # Evaluates the `parent` axis.
+      #
+      # @param [Oga::XPath::Node] ast_node
+      # @param [Oga::XML::NodeSet] context
+      # @return [Oga::XML::NodeSet]
+      #
+      def on_axis_parent(ast_node, context)
+        nodes = XML::NodeSet.new
+
+        context.each do |context_node|
+          next unless has_parent?(context_node)
+
+          parent = context_node.parent
+
+          nodes << parent if node_matches?(parent, ast_node)
+        end
+
+        return nodes
+      end
+
       ##
       # Returns a node set containing all the child nodes of the given set of
       # nodes.

spec/oga/xpath/evaluator/axes/parent_spec.rb

@@ -0,0 +1,42 @@
+require 'spec_helper'
+
+describe Oga::XPath::Evaluator do
+  context 'parent axis' do
+    before do
+      @document  = parse('<a><b></b></a>')
+      @evaluator = described_class.new(@document)
+    end
+
+    context 'matching nodes without parents' do
+      before do
+        @set = @evaluator.evaluate('parent::a')
+      end
+
+      it_behaves_like :empty_node_set
+    end
+
+    context 'matching nodes with parents' do
+      before do
+        @set = @evaluator.evaluate('a/b/parent::a')
+      end
+
+      it_behaves_like :node_set, :length => 1
+
+      example 'return the <a> node' do
+        @set[0].should == @document.children[0]
+      end
+    end
+
+    context 'matching nodes with parents using the short axis form' do
+      before do
+        @set = @evaluator.evaluate('a/b/parent::node()')
+      end
+
+      it_behaves_like :node_set, :length => 1
+
+      example 'return the <a> node' do
+        @set[0].should == @document.children[0]
+      end
+    end
+  end
+end