XPath optimize preparation for positional predicates that uses last()

[tompng]

Expand optimizable simple positional queries to support [last()] [last()-N] and [position() <=> last()-N].
This will open the door for future optimization.

Simple positional predicates that may be optimized are represented as:

Before: [:==, 1], [:<, 2], [:>, 3]
↓
After: [:index_eq, 0], [:index_lt, 1], [:index_gt, 2]
Added: [:reverse_index_eq, 0], [:reverse_index_lt, 2], [:reverse_index_gt, 4]

Example

Example optimization of preceding-sibling/following-sibling (not included in this PR)

def preceding_following_sibling(nodeset, tester, selector, reverse:)
  ...
  when :reverse_index_eq, :reverse_index_lt, :reverse_index_gt
    nodeset.group_by(&:parent).flat_map do |parent, sibling_nodes|
      anchors = Set.new.compare_by_identity
      sibling_nodes.each {|sibling| anchors << sibling }
      children = parent.children
      children = children.reverse if reverse

      # Different anchor node gives the same reverse-index. We only need to check with the first anchor
      followings = children.drop_while {|child| !anchors.include?(child) }.drop(1)
      candidates = followings.select(&tester).reverse
      case operator
      when :reverse_index_eq
        matched = candidates[value] if value >= 0
        matched ? [matched] : []
      when :reverse_index_lt
        value >= 0 ? candidates[0...value] : []
      when :reverse_index_gt
        value >= 0 ? candidates.drop(value + 1) : candidates
      end
    end
  when :nodesets
    ...
  end
end

doc = REXML::Document.new('<root>'+('<a>'+'<b/><c/>'*500+'</a>')*2+'</root>')
REXML::XPath.match(doc, '//c/preceding-sibling::*[position()<last()-500]').size
# processing time:  9.740037s → 0.059448s
# => 996

[Copilot]

Pull request overview

This PR expands the internal representation of “simple positional predicates” in the XPath parser to support last()-based forms (e.g., [last()], [last()-N], and comparisons against last()-N), enabling future axis-scan optimizations while preserving correct XPath behavior.

Changes:

• Extends position_operation to recognize last()/last()-INTEGER and to normalize positional predicates into 0-based :index_* and :reverse_index_* selector forms.
• Updates sibling-axis scanning and the generic slow-path selector application to operate on the new selector operators/0-based indexing.
• Adds test coverage for inverted position() comparisons and last()-based predicates on preceding-/following-sibling axes.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
lib/rexml/xpath_parser.rb	Adds last()-aware predicate detection and migrates selector operators to 0-based :index_* / :reverse_index_*, updating scan/slow-path handling accordingly.
test/xpath/test_axis_preceding_sibling.rb	Adds assertions covering last()/last()-N and inverted position() comparisons for sibling axes.

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