Support for the "following-sibling" axis.

This also comes with some small cleanups regarding
XPath::Evaluator#node_matches?. This change removes the need to, every time,
also use can_match_node?() to prevent NoMethodError errors from popping up.

lib/oga/xpath/evaluator.rb

@@ -101,9 +101,7 @@ module Oga
         nodes = XML::NodeSet.new
 
         context.each do |xml_node|
-          if can_match_node?(xml_node) and node_matches?(xml_node, ast_node)
-            nodes << xml_node
-          end
+          nodes << xml_node if node_matches?(xml_node, ast_node)
         end
 
         return nodes
@@ -144,7 +142,7 @@ module Oga
           while has_parent?(xml_node)
             xml_node = xml_node.parent
 
-            if can_match_node?(xml_node) and node_matches?(xml_node, ast_node)
+            if node_matches?(xml_node, ast_node)
               nodes << xml_node
               break
             end
@@ -265,8 +263,43 @@ module Oga
 
             next unless check
 
-            if can_match_node?(doc_node) and node_matches?(doc_node, ast_node)
+            nodes << doc_node if node_matches?(doc_node, ast_node)
+          end
+        end
+
+        return nodes
+      end
+
+      ##
+      # Evaluates the `following-sibling` axis.
+      #
+      # @param [Oga::XPath::Node] ast_node
+      # @param [Oga::XML::NodeSet] context
+      # @return [Oga::XML::NodeSet]
+      #
+      def on_axis_following_sibling(ast_node, context)
+        nodes = XML::NodeSet.new
+
+        context.each do |context_node|
+          check  = false
+          parent = context_node.respond_to?(:parent) ? context_node.parent : nil
+
+          @document.each_node do |doc_node|
+            # Skip child nodes of the current context node, compare all
+            # following nodes.
+            if doc_node == context_node
+              check = true
+              throw :skip_children
+            end
+
+            if !check or parent != doc_node.parent
+              next
+            end
+
+            if node_matches?(doc_node, ast_node)
               nodes << doc_node
+
+              throw :skip_children
             end
           end
         end
@@ -307,6 +340,8 @@ module Oga
       # @return [Oga::XML::NodeSet]
       #
       def node_matches?(xml_node, ast_node)
+        return false unless can_match_node?(xml_node)
+
         ns, name = *ast_node
 
         name_matches = xml_node.name == name || name == '*'

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

@@ -0,0 +1,59 @@
+require 'spec_helper'
+
+describe Oga::XPath::Evaluator do
+  context 'following-sibling axis' do
+    before do
+      # Strip whitespace so it's easier to retrieve/compare elements.
+      @document = parse(<<-EOF.strip.gsub(/\s+/m, ''))
+<root>
+  <foo>
+    <bar></bar>
+    <baz>
+      <baz></baz>
+    </baz>
+  </foo>
+  <baz></baz>
+</root>
+      EOF
+
+      @first_baz  = @document.children[0].children[0].children[1]
+      @second_baz = @first_baz.children[0]
+      @third_baz  = @document.children[0].children[1]
+      @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-sibling::foo')
+      end
+
+      it_behaves_like :empty_node_set
+    end
+
+    context 'matching nodes in the current context' do
+      before do
+        @set = @evaluator.evaluate('root/foo/following-sibling::baz')
+      end
+
+      it_behaves_like :node_set, :length => 1
+
+      example 'return the third <baz> node' do
+        @set[0].should == @third_baz
+      end
+    end
+
+    context 'matching nodes in other contexts' do
+      before do
+        @set = @evaluator.evaluate('root/foo/bar/following-sibling::baz')
+      end
+
+      it_behaves_like :node_set, :length => 1
+
+      example 'return the first <baz> node' do
+        @set[0].should == @first_baz
+      end
+    end
+  end
+end

spec/oga/xpath/evaluator/general_spec.rb

@@ -61,6 +61,12 @@ describe Oga::XPath::Evaluator do
     example 'return true if a node is matched without having a namespace' do
       @evaluator.node_matches?(@name_node, s(:test, '*', 'a')).should == true
     end
+
+    example 'return false when trying to match an XML::Text instance' do
+      text = Oga::XML::Text.new(:text => 'Foobar')
+
+      @evaluator.node_matches?(text, s(:test, nil, 'a')).should == false
+    end
   end
 
   context '#can_match_node?' do