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Description
Hi,
I’m the maintainer of cssselect, which does in Python pretty much the same as Nokogiri for CSS selectors: translate them to XPath. It looks like the SimonSapin/cssselect#12 bug also applies to Nokogiri. Namely, the XPath translation of :nth-child() and similar pseudo-classes is wrong when used after the + or ~ combinator. Here is a test case:
require 'nokogiri'
doc = Nokogiri::XML('<root><child1/><child2/><child3/></root>')
puts doc.css(':nth-child(2)').map { |e| e.name }
puts doc.css('child1 ~ :nth-child(2)').map { |e| e.name }
Expected output: child2 child2. Actual output child2 child3.
The problem is in the XPath translation of the later selector: //child1/following-sibling::*[position() = 2 and self::*] gives the element at position 2 when counting from child1, while we want the position among the parent’s children.
I am not sure it is even possible to correctly translate this selector to XPath: the = XPath operator on node-sets compares the text content of elements, not their identity.
The issue is similar for SimonSapin/cssselect#4 and Nokogiri’s #394.