Thanks! The flood fill is an implementation of the “span filling” algorithm described in the wikipedia link below. I made a naive flood fill first that used more memory and was noticeably slower, but this one runs pretty well.
It’s kind of fun just to make spirals/mazes and then watch it flood fill them. :)
https://en.wikipedia.org/wiki/Flood_fill#Span_filling
Most of the drawing tools are built on top of the GBDK drawing.h api, which saved a lot of time vs writing them all entirely from scratch.