Improve Node#each_recursive performance (#139)

Fix #134

## Summary

This PR does:

- Add `benchmark/each_recursive.yaml`
- Rewrite `Node#each_recursive` implementation for performance
- Add a test for `Node#each_recursive`

The performance of `Node#each_recursive` is improved 60~80x faster.

## Details

`each_recursive` is too much slow as I described in #134.
I improved this performance by rewriting its implementation in this PR.

Also, I added a benchmark in `benchmark/each_recursive.yaml` and the
following is a result on my laptop:

```
RUBYLIB= BUNDLER_ORIG_RUBYLIB= /Users/makenowjust/Projects/github.com/makenowjust/simple-dotfiles/.asdf/installs/ruby/3.3.2/bin/ruby -v -S benchmark-driver /Users/makenowjust/Projects/github.com/ruby/rexml/benchmark/each_recursive.yaml
ruby 3.3.2 (2024-05-30 revision e5a195edf6) [arm64-darwin23]
Calculating -------------------------------------
                     rexml 3.2.6      master  3.2.6(YJIT)  master(YJIT)
      each_recursive      11.279     686.502       17.926        1.470k i/s -     100.000 times in 8.866303s 0.145666s 5.578360s 0.068018s

Comparison:
                   each_recursive
        master(YJIT):      1470.2 i/s
              master:       686.5 i/s - 2.14x  slower
         3.2.6(YJIT):        17.9 i/s - 82.01x  slower
         rexml 3.2.6:        11.3 i/s - 130.35x  slower

```

We can see that the performance is improved 60~80x faster.

Additionally, I added a new test for `Node#each_recursive`.
It was missing, but we need it to confirm not to break the previous
behavior.

Thank you.

---------

Co-authored-by: Sutou Kouhei <kou@cozmixng.org>

benchmark/each_recursive.yaml

@@ -0,0 +1,40 @@
+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'
+
+  xml_source = +"<root>"
+  100.times do
+    x_node_source = ""
+    100.times do
+      x_node_source = "<x>#{x_node_source}</x>"
+    end
+    xml_source << x_node_source
+  end
+  xml_source << "</root>"
+
+  document = REXML::Document.new(xml_source)
+
+benchmark:
+  each_recursive: document.each_recursive { |_| }

lib/rexml/node.rb

@@ -52,10 +52,14 @@ def parent?
 
     # Visit all subnodes of +self+ recursively
     def each_recursive(&block) # :yields: node
-      self.elements.each {|node|
-        block.call(node)
-        node.each_recursive(&block)
-      }
+      stack = []
+      each { |child| stack.unshift child if child.node_type == :element }
+      until stack.empty?
+        child = stack.pop
+        yield child
+        n = stack.size
+        child.each { |grandchild| stack.insert n, grandchild if grandchild.node_type == :element }
+      end
     end
 
     # Find (and return) first subnode (recursively) for which the block

test/test_document.rb

@@ -209,6 +209,42 @@ def test_gt_linear_performance_attribute_value
       end
     end
 
+    def test_each_recursive
+      xml_source = <<~XML
+        <?xml version="1.0" encoding="UTF-8" standalone="yes"?>
+        <root name="root">
+          <x name="1_1">
+            <x name="1_2">
+              <x name="1_3" />
+            </x>
+          </x>
+          <x name="2_1">
+            <x name="2_2">
+              <x name="2_3" />
+            </x>
+          </x>
+          <!-- comment -->
+          <![CDATA[ cdata ]]>
+        </root>
+      XML
+
+      expected_names = %w[
+        root
+        1_1 1_2 1_3
+        2_1 2_2 2_3
+      ]
+
+      document = REXML::Document.new(xml_source)
+
+      # Node#each_recursive iterates elements only.
+      # This does not iterate XML declerations, comments, attributes, CDATA sections, etc.
+      actual_names = []
+      document.each_recursive do |element|
+        actual_names << element.attributes["name"]
+      end
+      assert_equal(expected_names, actual_names)
+    end
+
     class WriteTest < Test::Unit::TestCase
       def setup
         @document = REXML::Document.new(<<-EOX)