Optimization of Element#namespace and Element#namespaces with @parent_namespaces cache

## Why?
for fix get_namespace performance
> # FIXME: This DOUBLES the time XPath searches take

Co-authored-by: tomoya ishida <tomoyapenguin@gmail.com>

Author: naitoh

lib/rexml/element.rb

@@ -589,10 +589,8 @@ def prefixes
     #   d.elements['//c'].namespaces # => {"x"=>"1", "y"=>"2", "z"=>"3"}
     #
     def namespaces
-      namespaces = {}
-      namespaces = parent.namespaces if parent
-      namespaces = namespaces.merge( attributes.namespaces )
-      return namespaces
+      @parent_namespaces ||= parent ? parent.namespaces : {}
+      @parent_namespaces.merge(attributes.namespaces)
     end
 
     # :call-seq:
@@ -619,17 +617,9 @@ def namespace(prefix=nil)
       if prefix.nil?
         prefix = prefix()
       end
-      if prefix == ''
-        prefix = "xmlns"
-      else
-        prefix = "xmlns:#{prefix}" unless prefix[0,5] == 'xmlns'
-      end
-      ns = nil
-      target = self
-      while ns.nil? and target
-        ns = target.attributes[prefix]
-        target = target.parent
-      end
+      prefix = (prefix == '') ? 'xmlns' : prefix.delete_prefix("xmlns:")
+      ns = namespaces[prefix]
+
       ns = '' if ns.nil? and prefix == 'xmlns'
       return ns
     end
@@ -653,6 +643,7 @@ def namespace(prefix=nil)
     #   e.namespaces # => {"xmlns"=>"bar", "baz"=>"bat"}
     #
     def add_namespace( prefix, uri=nil )
+      self.each{|e| e.reset_parent_namespaces }
       unless uri
         @attributes["xmlns"] = prefix
       else
@@ -685,6 +676,7 @@ def add_namespace( prefix, uri=nil )
     #   d.to_s # => "<a/>"
     #
     def delete_namespace namespace="xmlns"
+      self.each{|e| e.reset_parent_namespaces }
       namespace = "xmlns:#{namespace}" unless namespace == 'xmlns'
       attribute = attributes.get_attribute(namespace)
       attribute.remove unless attribute.nil?
@@ -1516,6 +1508,11 @@ def write(output=$stdout, indent=-1, transitive=false, ie_hack=false)
       formatter.write( self, output )
     end
 
+    protected
+    def reset_parent_namespaces
+      @parent_namespaces = nil
+      self.each{|e| e.reset_parent_namespaces }
+    end
 
     private
     def __to_xpath_helper node

lib/rexml/xpath_parser.rb

@@ -492,12 +492,10 @@ def node_test(path_stack, nodesets, any_type: :element)
                 if strict?
                   raw_node.name == name and raw_node.namespace == ""
                 else
-                  # FIXME: This DOUBLES the time XPath searches take
                   ns = get_namespace(raw_node, prefix)
                   raw_node.name == name and raw_node.namespace == ns
                 end
               else
-                # FIXME: This DOUBLES the time XPath searches take
                 ns = get_namespace(raw_node, prefix)
                 raw_node.name == name and raw_node.namespace == ns
               end
@@ -507,7 +505,6 @@ def node_test(path_stack, nodesets, any_type: :element)
               elsif prefix.empty?
                 raw_node.name == name and raw_node.namespace == ""
               else
-                # FIXME: This DOUBLES the time XPath searches take
                 ns = get_namespace(raw_node.element, prefix)
                 raw_node.name == name and raw_node.namespace == ns
               end