I went through the list and the comment about Apache Lucene really piqued my interest. I found the Lucene code in GitHub but some of the sources have more boilerplate Java comments than code! Anyways, I remembered Java Docs are often excellent so here's the Lucene Java Docs:

https://lucene.apache.org/core/7_5_0/core/index.html

From my experience, studying the API Docs for Java and using it as a guide to the code makes understanding the code much easier.

The code is here:

https://github.com/apache/lucene-solr