Retrieving root nodes using Node#root_node.

This method uses a loop to traverse upwards the DOM tree in order to find the
root document/element. While this might have an impact on performance I don't
expect Oga itself to call this method very often. The benefit is that Node
instances don't require users to manually pass the top level document as an
argument.

spec/oga/xml/node_spec.rb

@@ -16,6 +16,29 @@ describe Oga::XML::Node do
     end
   end
 
+  context '#children' do
+    example 'return an empty set by default' do
+      described_class.new.children.empty?.should == true
+    end
+
+    example 'return a set that was created manually' do
+      set  = Oga::XML::NodeSet.new([described_class.new])
+      node = described_class.new(:children => set)
+
+      node.children.should == set
+    end
+  end
+
+  context '#parent' do
+    example 'return the parent of the node' do
+      owner = described_class.new
+      set   = Oga::XML::NodeSet.new([], owner)
+      node  = described_class.new(:node_set => set)
+
+      node.parent.should == owner
+    end
+  end
+
   context '#children=' do
     example 'set the child nodes using an Array' do
       child = described_class.new
@@ -35,4 +58,56 @@ describe Oga::XML::Node do
       node.children[0].should == child
     end
   end
+
+  context '#previous' do
+    before do
+      owner = described_class.new
+      @n1   = described_class.new
+      @n2   = described_class.new
+      @set  = Oga::XML::NodeSet.new([@n1, @n2], owner)
+    end
+
+    example 'return the previous node' do
+      @n2.previous.should == @n1
+    end
+
+    example 'return nil if there is no previous node' do
+      @n1.previous.nil?.should == true
+    end
+  end
+
+  context '#next' do
+    before do
+      owner = described_class.new
+      @n1   = described_class.new
+      @n2   = described_class.new
+      @set  = Oga::XML::NodeSet.new([@n1, @n2], owner)
+    end
+
+    example 'return the next node' do
+      @n1.next.should == @n2
+    end
+
+    example 'return nil if there is no previous node' do
+      @n2.next.nil?.should == true
+    end
+  end
+
+  context '#root_node' do
+    before do
+      @n4  = described_class.new
+      @n3  = described_class.new(:children => [@n4])
+      @n2  = described_class.new
+      @n1  = described_class.new(:children => [@n2])
+      @doc = Oga::XML::Document.new(:children => [@n1])
+    end
+
+    example 'return the root document of an element' do
+      @n2.root_node.should == @doc
+    end
+
+    example 'return the root element of another element' do
+      @n4.root_node.should == @n3
+    end
+  end
 end