Basic support for the preceding-sibling xpath axis

lib/oga/xpath/evaluator.rb

@@ -354,6 +354,33 @@ module Oga
         return nodes
       end
 
+      ##
+      # Evaluates the `preceding-sibling` axis.
+      #
+      # @param [Oga::XPath::Node] ast_node
+      # @param [Oga::XML::NodeSet] context
+      # @return [Oga::XML::NodeSet]
+      #
+      def on_axis_preceding_sibling(ast_node, context)
+        nodes = XML::NodeSet.new
+
+        context.each do |context_node|
+          check  = true
+          parent = has_parent?(context_node) ? context_node.parent : nil
+
+          @document.each_node do |doc_node|
+            # Test everything *until* we hit the current context node.
+            if doc_node == context_node
+              break
+            elsif doc_node.parent == parent and node_matches?(doc_node, ast_node)
+              nodes << doc_node
+            end
+          end
+        end
+
+        return nodes
+      end
+
       ##
       # Returns a node set containing all the child nodes of the given set of
       # nodes.