writing a custom Filter plugin?

[Jonathan Rochkind]

Does anyone know of any tutorials, basic examples, and/or documentation 
on writing your own Filter plugin for Solr? For Solr 4.x/4.3?

I would like a Solr 4.3 version of the normalization filters found here 
for Solr 1.4: https://github.com/billdueber/lib.umich.edu-solr-stuff

But those are old, for Solr 1.4.

Does anyone have any hints for writing a simple substitution Filter for 
Solr 4.x?  Or, does a simple sourcecode example exist anywhere?