[Libreoffice] How to write an import filter?

Hello Libre-office developers,

I'm interested in learning how to write an import filter for some legacy
document formats. I tried to figure out by myself but failed. Could you
point some documents / wiki pages, or even some files in the source code
repository?

Especially I need to know how to register an import filter to the
libreoffice installation and how the core invokes a filter, i.e. the
entry points of the filters, to start some hacking.

Thank you for reading.

Best regards,
mete0r