heat map - Where is the GeoServer heatmap rendering transformation source code?

I'm trying to find the source code for the GeoServer Heatmap rendering transformation. I can't find anything relevant in the GeoServer GitHub repo.

This page in the developer guide includes a code snippet from the transformation but when I search for "public class HeatmapProcess implements GeoServerProcess" that page in the developer guide is the only result.

I'd like to learn how the transformation produces grid cell values as there's no documentation on the internal logic.

Answer

GeoServer uses the heatmap process from GeoTools. Here is the link.