[itecz26]

_xpath_match_condition() turns each combinator of a complex :not() argument into a reverse axis nested in the previous step's predicate (:not(x * * *) becomes not(ancestor::*[ancestor::*[ancestor::*[self::x]]])), and libxml2 evaluates a nested predicate again for every node the outer axis reaches, so the generated XPath takes time that grows like C(depth, k) for k descendant combinators, or C(siblings, k) with ~. that's an algorithmic-complexity DoS for anything that runs user-supplied selectors against a page: on 3.13 with lxml 6.1.3, :not(x * * * * * * * *) (23 chars) takes 4.35 s on a 32-element document nested 30 deep, and tr:not(x ~ * ~ * ~ *) takes 5.25 s on a plain 200-row table. emitting the chain as one location path (parent::*[self::b]/parent::*[self::a]) means the same thing in a boolean context but lets libxml2 merge the node-set at each step, which brings those two down to 0.5 ms and 146 ms. single-combinator output doesn't change; the e:not(a > b > c) expectation in test_translation now has the path form, the two longer chains added there fail without the change, two multi-combinator selects go into test_select, and old vs new translations select the same elements on ~40k random complex :not() selectors over random documents.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (0a0c197) to head (1c389d3).

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #219   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            3         3           
  Lines         1019      1023    +4     
  Branches       170       170           
=========================================
+ Hits          1019      1023    +4     

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[AdrianAtZyte]

Looks good to me. Please, drop the code comment and the test comment about nested predicates though, that reasoning is already in the commit message, and the code reads fine without them.

[itecz26]

makes sense, dropped both comments in 1c389d3.