Support for the XPath "following" axis.

lib/oga/xpath/evaluator.rb

@@ -241,6 +241,35 @@ module Oga
         return on_test(node, context) + on_axis_descendant(node, context)
       end
 
+      ##
+      # Evaluates the `following` axis.
+      #
+      # @param [Oga::XPath::Node] node
+      # @param [Oga::XML::NodeSet] context
+      # @return [Oga::XML::NodeSet]
+      #
+      def on_axis_following(node, context)
+        nodes = XML::NodeSet.new
+
+        context.each do |context_node|
+          current = context_node
+
+          while current.respond_to?(:next)
+            next_node = current.next
+
+            if can_match_node?(next_node) and node_matches?(next_node, node)
+              nodes << next_node
+            end
+
+            # When this returns nil the loop automaticall breaks since `nil`
+            # doesn't respond to `next`.
+            current = next_node
+          end
+        end
+
+        return nodes
+      end
+
       ##
       # Returns a node set containing all the child nodes of the given set of
       # nodes.

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

@@ -0,0 +1,47 @@
+require 'spec_helper'
+
+describe Oga::XPath::Evaluator do
+  context 'following axis' do
+    before do
+      document   = parse('<a><b></b><c></c><c></c></a>')
+
+      @first_c   = document.children[0].children[1]
+      @second_c  = document.children[0].children[2]
+      @evaluator = described_class.new(document)
+    end
+
+    # This should return an empty set since the document doesn't have any
+    # following nodes.
+    context 'using a document as the root' do
+      before do
+        @set = @evaluator.evaluate('following::a')
+      end
+
+      it_behaves_like :empty_node_set
+    end
+
+    context 'matching following elements using a name' do
+      before do
+        @set = @evaluator.evaluate('a/b/following::c')
+      end
+
+      it_behaves_like :node_set, :length => 2
+
+      example 'return the first <c> node' do
+        @set[0].should == @first_c
+      end
+
+      example 'return the second <c> node' do
+        @set[1].should == @second_c
+      end
+    end
+
+    context 'matching following elements using a wildcard' do
+      before do
+        @set = @evaluator.evaluate('a/b/following::*')
+      end
+
+      it_behaves_like :node_set, :length => 2
+    end
+  end
+end