Use a stack based XPath evaluator.

This evaluator is capable of processing very, very basic XPath expressions. It
does not yet know anything about axes, functions, operators, etc.

lib/oga/xpath/evaluator.rb

@@ -4,92 +4,75 @@ module Oga
     # The Evaluator class is used to evaluate an XPath expression in the
     # context of a given document.
     #
-    class Evaluator < AST::Processor
+    class Evaluator
       ##
       # @param [Oga::XML::Document|Oga::XML::Node] document
       #
       def initialize(document)
         @document = document
+
+        reset
       end
 
-      def process(node, cursor)
-        return if node.nil?
-
-        node = node.to_ast
-
-        on_handler = :"on_#{node.type}"
-
-        if respond_to?(on_handler)
-          new_node = send(on_handler, node, cursor)
-        else
-          new_node = handler_missing(node)
-        end
-
-        node = new_node if new_node
-
-        return node
-      end
-
-      def process_all(nodes, cursor)
-        return nodes.to_a.map do |node|
-          process(node, cursor)
-        end
+      def reset
+        @context = @document.children
+        @stack   = XML::NodeSet.new
       end
 
       def evaluate(string)
-        ast    = Parser.new(string).parse
-        cursor = move_cursor(@document)
+        ast = Parser.new(string).parse
 
-        return process(ast, cursor)
-      end
+        process(ast)
 
-      def on_absolute(node, cursor)
-        if @document.is_a?(XML::Node)
-          cursor = move_cursor(@document.root_node)
-        else
-          cursor = move_cursor(@document)
-        end
+        nodes = @stack
 
-        return process_all(node.children, cursor)
-      end
-
-      def on_path(node, cursor)
-        test, children = *node
-
-        nodes = process(test, cursor)
-
-        unless nodes.empty?
-          nodes = process_all(children, nodes)
-        end
+        reset
 
         return nodes
       end
 
-      def on_test(node, cursor)
-        ns, name = *node
-        nodes    = XML::NodeSet.new
+      def process(node)
+        handler = "on_#{node.type}"
 
-        cursor.each do |xml_node|
+        if respond_to?(handler)
+          send(handler, node)
+        end
+      end
+
+      def on_path(node)
+        test, children = *node
+
+        process(test)
+
+        if children and !@stack.empty?
+          swap_context
+
+          process(children)
+        end
+      end
+
+      def on_test(node)
+        ns, name = *node
+
+        @context.each do |xml_node|
           next unless xml_node.is_a?(XML::Element)
 
           if xml_node.name == name and xml_node.namespace == ns
-            nodes << xml_node
+            @stack << xml_node
+          end
+        end
+      end
+
+      def swap_context
+        @context = XML::NodeSet.new
+
+        @stack.each do |xml_node|
+          xml_node.children.each do |child|
+            @context << child
           end
         end
 
-        return nodes
-      end
-
-      def move_cursor(location)
-        if location.is_a?(XML::Document)
-          return location.children
-
-        elsif location.is_a?(XML::Node)
-          return XML::NodeSet.new([location])
-
-        else
-          return location
-        end
+        @stack = XML::NodeSet.new
       end
     end # Evaluator
   end # XPath