[WIP] feat(cache): bring up lfu

[SukkaW]

What does it do?

This PR is a part of proposal #3897, bring up LFU cache for fragment_cache().

How to test

git clone -b fragment_cache_lru https://github.com/sukkaw/hexo.git
cd hexo
npm install
npm test

Screenshots

Pull request tasks

• Add test cases for the changes.
• Passed the CI test.

[coveralls]

Coverage increased (+0.0008%) to 97.136% when pulling dd14ff2 on SukkaW:fragment_cache_lru into e423773 on hexojs:master.

[coveralls]

Coverage increased (+0.0008%) to 97.136% when pulling c7c34cd on SukkaW:fragment_cache_lru into e423773 on hexojs:master.

[SukkaW]

I have run some test by using Object.keys(cache).length to know the size of cache for some popular themes:

Theme	Current cache size
hexo-theme-light	1
hexo-theme-tranquilpeak	3
hexo-theme-landscape	4
hexo-theme-cake	17
hexo-theme-next	18
hexo-theme-suka	18
hexo-theme-icarus	98

Those data are from one-language site. Multi-language site could have larger size of cache.

Maybe 50 is still to large. We could use 15 or 20 (I prefer 20).

[jiangtj]

It is best to be exposed to the outside for configuration, If some theme has larger cache length, it can adjust it.

[SukkaW]

Most of users have no idea about the cache size of their themes.

[jiangtj]

Most of users have no idea about the cache size of their themes.

But as a theme developer. If he knew clearly that he needed 50+, how to achieve it?

[SukkaW]

@jiangtj In fact he/she doesn't. It is better to cache as less as possible because larger size of cache means higher memory usage. It is not a wise idea to cache everything in the fragment_cache. That's how LFU works, by only caching the most usefuls.