Academic code copied by library author
In fact something great happened: Your research did have impact in the real world and seems to be useful - not everyone can say that!

When it comes to legal issues, things are usually complicated and will involve copyright issues (of yours and of the journal / publisher) as well as authorship issues. Those vary between countries and it will be difficult to give an advise without proper juristical background and line by line comparison of your publications and the library code.

But as I understood your question, you are more interested in the academic credit, and therefore I would suggest to contact the authors, tell them you are happy that your work is used by them, and offer to be part of the development team. You might add a phrase like it would be a pleasure to be listed as author of the library - an in most cases they will understand...