Use private constant to test broken libxml version

[ashmaroli]

What problem is this PR intended to solve?

Unnecessary memory usage.

The issue

While profiling a project for optimizing memory usage, I was pointed to the following line:

allocated memory by location
-----------------------------------
  14.61 MB  nokogiri-1.10.3/lib/nokogiri/xml/node.rb:852

which is:

nokogiri/lib/nokogiri/xml/node.rb

Line 852 in c45658a

return dump_html if Nokogiri.uses_libxml? && %w[2 6] === LIBXML_VERSION.split('.')[0..1]

If the above code is frequently called, then for every call:

• A new array is allocated via %w[2 6]
• An interim array is generated via LIBXML_VERSION.split('.')
• And yet another array is generated via Array#[]<range>

The Solution

The solution is to avoid generating objects and directly check the version string against a substring.
But since its wasteful to repeatedly test a CONSTANT string, stash the entire evaluation in a private constant and then read its value as required.

Have you included adequate test coverage?

This doesn't change behavior nor introduce a new behavior.

Does this change affect the C or the Java implementations?

No, it doesn't.

[codeclimate]

Code Climate has analyzed commit 5558d13 and detected 0 issues on this pull request.

The test coverage on the diff in this pull request is 100.0% (80% is the threshold).

This pull request will bring the total coverage in the repository to 93.8% (0.0% change).

View more on Code Climate.

[flavorjones]

Thanks for pointing out that we could be more efficient with memory here. It looks like this PR is failing on JRuby, I'll take a look this weekend and may end up editing this patchset to make it all work.

[ashmaroli]

@flavorjones Thank you for informing me about the failed check.
I've pushed a commit to check if LIBXML_VERSION is defined beforehand.
Note: I didn't use Nokogiri.uses_libxml? to avoid invoking a VersionInfo instance.

[flavorjones]

Merged manually! Thank you again for this contribution.