Improve using // in XPath performance (#249)

When using `//` in XPath, the deeper the tag hierarchy, the slower it
becomes due to the namespace acquisition process.
Caching namespace information improves performance when using `//` with
XPath.

## Benchmark (Comparison with rexml 3.4.1)

```
$ benchmark-driver benchmark/xpath.yaml
Calculating -------------------------------------
                                                     rexml 3.4.1      master  3.4.1(YJIT)  master(YJIT)
REXML::XPath.match(REXML::Document.new(xml), 'a//a')      29.215     234.909      108.945       492.410 i/s -     100.000 times in 3.422925s 0.425697s 0.917898s 0.203083s

Comparison:
             REXML::XPath.match(REXML::Document.new(xml), 'a//a')
                                        master(YJIT):       492.4 i/s
                                              master:       234.9 i/s - 2.10x  slower
                                         3.4.1(YJIT):       108.9 i/s - 4.52x  slower
                                         rexml 3.4.1:        29.2 i/s - 16.85x  slower
```

- YJIT=ON : 4.52x faster
- YJIT=OFF : 8.04x faster

---------

Co-authored-by: tomoya ishida <tomoyapenguin@gmail.com>
Co-authored-by: Sutou Kouhei <kou@clear-code.com>

Author: 3 people

benchmark/xpath.yaml

@@ -0,0 +1,32 @@
+loop_count: 100
+contexts:
+  - gems:
+      rexml: 3.2.6
+    require: false
+    prelude: require 'rexml'
+  - name: master
+    prelude: |
+      $LOAD_PATH.unshift(File.expand_path("lib"))
+      require 'rexml'
+  - name: 3.2.6(YJIT)
+    gems:
+      rexml: 3.2.6
+    require: false
+    prelude: |
+      require 'rexml'
+      RubyVM::YJIT.enable
+  - name: master(YJIT)
+    prelude: |
+      $LOAD_PATH.unshift(File.expand_path("lib"))
+      require 'rexml'
+      RubyVM::YJIT.enable
+
+prelude: |
+  require 'rexml/document'
+
+  DEPTH = 100
+  xml   = '<a>' * DEPTH + '</a>' * DEPTH
+  doc   = REXML::Document.new(xml)
+
+benchmark:
+  "REXML::XPath.match(REXML::Document.new(xml), 'a//a')" : REXML::XPath.match(doc, "a//a")

lib/rexml/attribute.rb

@@ -206,6 +206,10 @@ def xpath
       path += "/@#{self.expanded_name}"
       return path
     end
+
+    def document
+      @element&.document
+    end
   end
 end
 #vim:ts=2 sw=2 noexpandtab:

lib/rexml/document.rb

@@ -448,6 +448,20 @@ def document
     end
 
     private
+
+    attr_accessor :namespaces_cache
+
+    # New document level cache is created and available in this block.
+    # This API is thread unsafe. Users can't change this document in this block.
+    def enable_cache
+      @namespaces_cache = {}
+      begin
+        yield
+      ensure
+        @namespaces_cache = nil
+      end
+    end
+
     def build( source )
       Parsers::TreeParser.new( source, self ).parse
     end

lib/rexml/element.rb

@@ -589,10 +589,12 @@ 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
+      namespaces_cache = document&.__send__(:namespaces_cache)
+      if namespaces_cache
+        namespaces_cache[self] ||= calculate_namespaces
+      else
+        calculate_namespaces
+      end
     end
 
     # :call-seq:
@@ -619,17 +621,9 @@ 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
@@ -1516,8 +1510,15 @@ def write(output=$stdout, indent=-1, transitive=false, ie_hack=false)
       formatter.write( self, output )
     end
 
-
     private
+    def calculate_namespaces
+      if parent
+        parent.namespaces.merge(attributes.namespaces)
+      else
+        attributes.namespaces
+      end
+    end
+
     def __to_xpath_helper node
       rv = node.expanded_name.clone
       if node.parent

lib/rexml/xpath_parser.rb

@@ -78,7 +78,15 @@ def variables=( vars={} )
 
     def parse path, node
       path_stack = @parser.parse( path )
-      match( path_stack, node )
+      if node.is_a?(Array)
+        Kernel.warn("REXML::XPath.each, REXML::XPath.first, REXML::XPath.match dropped support for nodeset...", uplevel: 1)
+        return [] if node.empty?
+        node = node.first
+      end
+
+      node.document.__send__(:enable_cache) do
+        match( path_stack, node )
+      end
     end
 
     def get_first path, node
@@ -137,11 +145,6 @@ def first( path_stack, node )
 
 
     def match(path_stack, node)
-      if node.is_a?(Array)
-        Kernel.warn("REXML::XPath.each, REXML::XPath.first, REXML::XPath.match dropped support for nodeset...", uplevel: 1)
-        return [] if node.empty?
-        node = node.first
-      end
       nodeset = [XPathNode.new(node, position: 1)]
       result = expr(path_stack, nodeset)
       case result
@@ -494,24 +497,18 @@ 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
+                  raw_node.name == name and raw_node.namespace == get_namespace(raw_node, prefix)
                 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
+                raw_node.name == name and raw_node.namespace == get_namespace(raw_node, prefix)
               end
             when :attribute
               if prefix.nil?
                 raw_node.name == name
               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
+                raw_node.name == name and raw_node.namespace == get_namespace(raw_node.element, prefix)
               end
             else
               false

test/test_core.rb

@@ -653,18 +653,23 @@ def test_namespace
       assert_equal "<sean:blah>Some text</sean:blah>", out
     end
 
-
     def test_add_namespace
       e = Element.new 'a'
+      assert_equal("", e.namespace)
+      assert_nil(e.namespace('foo'))
       e.add_namespace 'someuri'
       e.add_namespace 'foo', 'otheruri'
       e.add_namespace 'xmlns:bar', 'thirduri'
-      assert_equal 'someuri', e.attributes['xmlns']
-      assert_equal 'otheruri', e.attributes['xmlns:foo']
-      assert_equal 'thirduri', e.attributes['xmlns:bar']
+      assert_equal("someuri", e.namespace)
+      assert_equal("otheruri", e.namespace('foo'))
+      assert_equal("otheruri", e.namespace('xmlns:foo'))
+      assert_equal("thirduri", e.namespace('bar'))
+      assert_equal("thirduri", e.namespace('xmlns:bar'))
+      assert_equal('someuri', e.attributes['xmlns'])
+      assert_equal('otheruri', e.attributes['xmlns:foo'])
+      assert_equal('thirduri', e.attributes['xmlns:bar'])
     end
 
-
     def test_big_documentation
       d = File.open(fixture_path("documentation.xml")) {|f| Document.new f }
       assert_equal "Sean Russell", d.elements["documentation/head/author"].text.tr("\n\t", " ").squeeze(" ")
@@ -764,9 +769,15 @@ def test_attributes_each
 
     def test_delete_namespace
       doc = Document.new "<a xmlns='1' xmlns:x='2'/>"
+      assert_equal("1", doc.root.namespace)
+      assert_equal("2", doc.root.namespace('x'))
+      assert_equal("2", doc.root.namespace('xmlns:x'))
       doc.root.delete_namespace
       doc.root.delete_namespace 'x'
-      assert_equal "<a/>", doc.to_s
+      assert_equal("<a/>", doc.to_s)
+      assert_equal("", doc.root.namespace)
+      assert_nil(doc.root.namespace('x'))
+      assert_nil(doc.root.namespace('xmlns:x'))
     end
 
     def test_each_element_with_attribute

test/xpath/test_base.rb

@@ -1193,6 +1193,16 @@ def test_namespaces_0
       assert_equal( 1,  XPath.match( d, "//x:*" ).size )
     end
 
+    def test_namespaces_cache
+      doc = Document.new("<a xmlns='1'><b/></a>")
+      assert_equal("<b/>", XPath.first(doc, "//b[namespace-uri()='1']").to_s)
+      assert_nil(XPath.first(doc, "//b[namespace-uri()='']"))
+
+      doc.root.delete_namespace
+      assert_nil(XPath.first(doc, "//b[namespace-uri()='1']"))
+      assert_equal("<b/>", XPath.first(doc, "//b[namespace-uri()='']").to_s)
+    end
+
     def test_ticket_71
       doc = Document.new(%Q{<root xmlns:ns1="xyz" xmlns:ns2="123"><element ns1:attrname="foo" ns2:attrname="bar"/></root>})
       el = doc.root.elements[1]