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XPath nodeset always in document-order #320

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23 changes: 10 additions & 13 deletions lib/rexml/xpath_parser.rb
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Original file line number Diff line number Diff line change
Expand Up @@ -262,7 +262,7 @@ def expr( path_stack, nodeset, context=nil )
nodesets
end
when :ancestor
nodeset = step(path_stack) do
nodeset = step(path_stack, order: :reversed) do
nodesets = []
# new_nodes = {}
nodeset.each do |node|
Expand All @@ -280,7 +280,7 @@ def expr( path_stack, nodeset, context=nil )
nodesets
end
when :ancestor_or_self
nodeset = step(path_stack) do
nodeset = step(path_stack, order: :reversed) do
nodesets = []
# new_nodes = {}
nodeset.each do |node|
Expand Down Expand Up @@ -325,7 +325,7 @@ def expr( path_stack, nodeset, context=nil )
nodesets
end
when :preceding_sibling
nodeset = step(path_stack, order: :reverse) do
nodeset = step(path_stack, order: :reversed) do
nodesets = []
nodeset.each do |node|
raw_node = node.raw_node
Expand All @@ -342,7 +342,7 @@ def expr( path_stack, nodeset, context=nil )
nodesets
end
when :preceding
nodeset = step(path_stack, order: :reverse) do
nodeset = step(path_stack, order: :reversed) do
unnode(nodeset) do |node|
preceding(node)
end
Expand Down Expand Up @@ -460,7 +460,7 @@ def expr( path_stack, nodeset, context=nil )
leave(:expr, path_stack, nodeset) if @debug
end

def step(path_stack, any_type: :element, order: :forward)
def step(path_stack, any_type: :element, order: :ordered)
nodesets = yield
begin
enter(:step, path_stack, nodesets) if @debug
Expand All @@ -471,7 +471,7 @@ def step(path_stack, any_type: :element, order: :forward)
nodesets = evaluate_predicate(predicate_expression, nodesets)
end
if nodesets.size == 1
ordered_nodeset = nodesets[0]
ordered_nodeset = order == :ordered ? nodesets.first : nodesets.first.reverse
else
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@naitoh naitoh Jun 4, 2026

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@tompng
Thank you for this PR.

But, performance has decreased because the optimization for nodesets.size == 1 has been removed.
Is there any way to improve this?

$ git checkout master
$ rake build
rexml 3.4.5 built to pkg/rexml-3.4.5.gem.
$ gem install pkg/rexml-3.4.5.gem
$ git checkout no_reverse_order
$ benchmark-driver benchmark/xpath_no_reverse_order.yaml
Calculating -------------------------------------
                     master(a98066c)       after  master(a98066c,YJIT)  after(YJIT) 
               child         280.227      19.016               544.496       31.084 i/s -     100.000 times in 0.356853s 5.258603s 0.183656s 3.217105s
          descendant          18.445       9.682                28.523       15.258 i/s -     100.000 times in 5.421532s 10.327928s 3.505883s 6.554030s

Comparison:
                            child
master(a98066c,YJIT):       544.5 i/s 
     master(a98066c):       280.2 i/s - 1.94x  slower
         after(YJIT):        31.1 i/s - 17.52x  slower
               after:        19.0 i/s - 28.63x  slower

                       descendant
master(a98066c,YJIT):        28.5 i/s 
     master(a98066c):        18.4 i/s - 1.55x  slower
         after(YJIT):        15.3 i/s - 1.87x  slower
               after:         9.7 i/s - 2.95x  slower
  • benchmark/xpath_no_reverse_order.yaml
loop_count: 100
contexts:
  - name: master(a98066c)
    gems:
      rexml: 3.4.5
    require: false
    prelude: require 'rexml'
  - name: after
    prelude: |
      $LOAD_PATH.unshift(File.expand_path("lib"))
      require 'rexml'
  - name: master(a98066c,YJIT)
    gems:
      rexml: 3.4.5
    require: false
    prelude: |
      require 'rexml'
      RubyVM::YJIT.enable
  - name: after(YJIT)
    prelude: |
      $LOAD_PATH.unshift(File.expand_path("lib"))
      require 'rexml'
      RubyVM::YJIT.enable

prelude: |
  require "rexml"
  xml = "<root>" + (1..2000).map { |i| "<item id='#{i}'/>" }.join + "</root>"
  doc = REXML::Document.new(xml)

benchmark:
  child: REXML::XPath.match(doc, "/root/item")
  descendant: REXML::XPath.match(doc, "//item")

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@tompng tompng Jun 5, 2026

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Is there any way to improve this?

Yes, there are several ways.

  • A. Improve performance of sort

    • Should be done in a separate PR

  • B. Mark nodesets ordering

    • ordered_nodeset = marked_as_ordered? ? nodesets.first : nodesets.first.reverse
    • Ordering is currently :ordered | :reversed but in Optimize XPath step #315 we need to add :unordered
    • Conflicts with C

  • C. Delay nodeset ordering on demand

    • Sort the final result and in some functions such number(xpath)
    • Requires predicate position-independency analytics, requires sorted nodeset or not
    • After doing this, optimization of B needs to be extended. We need to re-implement B in a different way again.

I implemented B with changing the meaning of order: keyword parameter

  • Before: key to specify the output order: :forward | :reverse
  • After: key to specify the state of nodesets ordering: :ordered | :reversed | :unordered(future)

Perhaps this approach will be a blocker implementing C(delayed nodeset ordering), and optimizing XPathParser#step (#315) but we can consider it later.

seen = {}.compare_by_identity
raw_nodes = []
Expand All @@ -483,8 +483,9 @@ def step(path_stack, any_type: :element, order: :forward)
raw_nodes << raw_node
end
end
ordered_nodeset = sort(raw_nodes, order)
ordered_nodeset = sort(raw_nodes)
end

new_nodeset = []
ordered_nodeset.each do |node|
new_nodeset << XPathNode.new(node, position: new_nodeset.size + 1)
Expand Down Expand Up @@ -667,7 +668,7 @@ def leave(tag, *args)
# in and out of function calls. If I knew what the index of the nodes was,
# I wouldn't have to do this. Maybe add a document IDX for each node?
# Problems with mutable documents. Or, rewrite everything.
def sort(array_of_nodes, order)
def sort(array_of_nodes)
new_arry = []
array_of_nodes.each { |node|
node_idx = []
Comment on lines 672 to 674
Expand All @@ -679,11 +680,7 @@ def sort(array_of_nodes, order)
new_arry << [ node_idx.reverse, node ]
}
ordered = new_arry.sort_by do |index, node|
if order == :forward
index
else
index.map(&:-@)
end
index
end
ordered.collect do |_index, node|
node
Expand Down
2 changes: 1 addition & 1 deletion test/xpath/test_axis_preceding_sibling.rb
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Original file line number Diff line number Diff line change
Expand Up @@ -23,7 +23,7 @@ def test_preceding_sibling_axis
assert_equal "6", context.attributes["id"]

prev = XPath.first(context, "preceding-sibling::f")
assert_equal "5", prev.attributes["id"]
assert_equal "3", prev.attributes["id"]

prev = XPath.first(context, "preceding-sibling::f[1]")
assert_equal "5", prev.attributes["id"]
Expand Down
65 changes: 48 additions & 17 deletions test/xpath/test_base.rb
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Original file line number Diff line number Diff line change
Expand Up @@ -382,32 +382,39 @@ def test_preceding
start = XPath.first( d, "/a/b[@id='1']" )
assert_equal 'b', start.name
c = XPath.first( start, "preceding::c" )
assert_equal '2', c.attributes['id']
assert_equal '0', c.attributes['id']

c1, c0 = XPath.match( d, "/a/b/c[@id='2']/preceding::node()" )
assert_equal '1', c1.attributes['id']
b0, b2, c0, c1 = XPath.match( d, "/a/b/c[@id='2']/preceding::node()" )
assert_equal 'b', b0.name
assert_equal 'b', b2.name
assert_equal 'c', c0.name
assert_equal 'c', c1.name

assert_equal '0', b0.attributes['id']
assert_equal '2', b2.attributes['id']
assert_equal '0', c0.attributes['id']
assert_equal '1', c1.attributes['id']

c2, c1, c0, b, b2, b0 = XPath.match( start, "preceding::node()" )
b0, b2, b, c0, c1, c2 = XPath.match( start, "preceding::node()" )

assert_equal 'c', c2.name
assert_equal 'c', c1.name
assert_equal 'c', c0.name
assert_equal 'b', b.name
assert_equal 'b', b2.name
assert_equal 'c', c1.name
assert_equal 'c', c2.name
assert_equal 'b', b0.name
assert_equal 'b', b2.name
assert_equal 'b', b.name

assert_equal '2', c2.attributes['id']
assert_equal '1', c1.attributes['id']
assert_equal '0', c0.attributes['id']
assert b.attributes.empty?
assert_equal '2', b2.attributes['id']
assert_equal '1', c1.attributes['id']
assert_equal '2', c2.attributes['id']
assert_equal '0', b0.attributes['id']
assert_equal '2', b2.attributes['id']
assert b.attributes.empty?

d = REXML::Document.new("<a><b/><c/><d/></a>")
matches = REXML::XPath.match(d, "/a/d/preceding::node()")
assert_equal("c", matches[0].name)
assert_equal("b", matches[1].name)
assert_equal("b", matches[0].name)
assert_equal("c", matches[1].name)

s = "<a><b><c id='1'/></b><b><b><c id='2'/><c id='3'/></b><c id='4'/></b><c id='NOMATCH'><c id='5'/></c></a>"
d = REXML::Document.new(s)
Expand All @@ -425,7 +432,7 @@ def test_preceding_multiple
XML
doc = REXML::Document.new(source)
matches = REXML::XPath.match(doc, "a/d/preceding::*")
assert_equal(["d", "c", "b"], matches.map(&:name))
assert_equal(["b", "c", "d"], matches.map(&:name))
end

def test_following_multiple
Expand Down Expand Up @@ -498,7 +505,7 @@ def test_preceding_sibling_across_multiple_nodes
XML
doc = REXML::Document.new(source)
matches = REXML::XPath.match(doc, "a/b/x/preceding-sibling::*")
assert_equal(["e", "d", "c"], matches.map(&:name))
assert_equal(["c", "d", "e"], matches.map(&:name))
end

def test_preceding_sibling_within_single_node
Expand All @@ -511,7 +518,7 @@ def test_preceding_sibling_within_single_node
XML
doc = REXML::Document.new(source)
matches = REXML::XPath.match(doc, "a/b/x/preceding-sibling::*")
assert_equal(["e", "x", "d", "c"], matches.map(&:name))
assert_equal(["c", "d", "x", "e"], matches.map(&:name))
end

def test_following
Expand Down Expand Up @@ -901,6 +908,30 @@ def test_ordering
r.collect {|element| element.attribute("id").value})
end

def test_order_consistency
doc = REXML::Document.new('<a><b><c><d id="1"><d id="2"/></d></c></b></a>')
assert_equal('a', REXML::XPath.first(doc, '//d/ancestor::*').name)
assert_equal('a', REXML::XPath.first(doc, '//d[@id="2"]/ancestor::*').name)
assert_equal(%w[a b c d], REXML::XPath.match(doc, '//d/ancestor::*').map(&:name))
assert_equal(%w[a b c d], REXML::XPath.match(doc, '//d[@id="2"]/ancestor::*').map(&:name))

assert_equal('a', REXML::XPath.first(doc, '//d/ancestor-or-self::*').name)
assert_equal('a', REXML::XPath.first(doc, '//d[@id="2"]/ancestor-or-self::*').name)
assert_equal(%w[a b c d d], REXML::XPath.match(doc, '//d/ancestor-or-self::*').map(&:name))
assert_equal(%w[a b c d d], REXML::XPath.match(doc, '//d[@id="2"]/ancestor-or-self::*').map(&:name))

doc = REXML::Document.new('<a><b/><c/><d id="1"/><d id="2"/></a>')
assert_equal('b', REXML::XPath.first(doc, '//d/preceding::*').name)
assert_equal('b', REXML::XPath.first(doc, '//d[@id="2"]/preceding::*').name)
assert_equal(%w[b c d], REXML::XPath.match(doc, '//d/preceding::*').map(&:name))
assert_equal(%w[b c d], REXML::XPath.match(doc, '//d[@id="2"]/preceding::*').map(&:name))

assert_equal('b', REXML::XPath.first(doc, '//d/preceding-sibling::*').name)
assert_equal('b', REXML::XPath.first(doc, '//d[@id="2"]/preceding-sibling::*').name)
assert_equal(%w[b c d], REXML::XPath.match(doc, '//d/preceding-sibling::*').map(&:name))
assert_equal(%w[b c d], REXML::XPath.match(doc, '//d[@id="2"]/preceding-sibling::*').map(&:name))
end

def test_descendant_or_self_ordering
source = "<a>
<b>
Expand Down
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