:nth-of-type(n) produces incorrect XPath

[yorickpeterse]

This code:

Nokogiri::CSS.xpath_for(':nth-of-type(n)')

Produces the following XPath:

//*[position() = n]

This is incorrect and will result in an empty node set being returned when used to query a document. Instead the XPath should be //*[position() >= 1] as nth-of-type(n) simply returns all nodes matching the type.

[knu]

You are right. Looks like CSS3 allows just n but nokogiri's parser is not aware of that. I'll fix it later.

[yorickpeterse]

If it helps, the XPath predicate that can be used for this is simply ((count(preceding-sibling::*) + 1) mod 1) = 0, though technically leaving a predicate out all together would produce the same results.

[knu]

Nokogiri::CSS.xpath_for(':nth-of-type(n)') should now return ["//*[(position() mod 1) = 0]"].

[knu]

Oh wait, x:nth-of-type(n) and x:nth(n) produce the same expression. The current implementation looks broken.

[yorickpeterse]

Argh, my bad. I grabbed an XPath expression for nth-of-child, not nth-of-type. For the latter it can simply be (position() mod 1) =0.

[knu]

Ah, yes, Nokogiri::CSS.xpath_for('x:nth-child(n)') produces ["//x[((count(preceding-sibling::*) + 1) mod 1) = 0]"] which seems correct.

[knu]

Could you confirm the fix?

[yorickpeterse]

Looks good to me, thanks!

[knu]

Thanks for the catch!