What Sensor Fusion algo is google AOSP using? See the source code here

Question

I'm looking into Googles code here:

https://cs.android.com/android/platform/superproject/+/master:frameworks/native/services/sensorservice/Fusion.cpp;bpv=0;bpt=0

For the experts: what algorithm is used here? It looks like a Kalman filter to me. Or is it a Madgwick implementation?

I'm currently trying to decide / choose a sensor fusion algo for my application. Basially looking for the best one around. I compiled and tested Googles one, gives decent results but far from perfect.