I needed to brush-up my C skills to work on adding some features to this:
https://github.com/martin-ger/esp_wifi_repeater
After a few attempts at using new materials, I found myself re-reading my ancient copy of K&R.
There's plenty of issues with that book, but working-through it with a modern computer feels akin to manipulating a fission reactor with your bare hands.
These machines have a lot of power when you don't waste it.