At Sannsyn, we have been working for a while on this Solr Refiner project, mostly implemented in Kotlin. As it is now publicly available, I'd like to tip you about it here. Some of the stuff that we write about it, will concern Kotlin in particular, and I will make sure to mention such posts here when they come into existence. We have a short introduction here: https://hallvard-ystad.medium.com/introducing-tellusr-for-solr-search-enhancements-1c498c9e7252