HR algorithm: how does it work?

Hi,

I have some students developing an algorithm to detect HR from raw PPG. We are trying to make the algorithm somewhat hardware indipendent, so the outcome could be applicable to Bangle as well (if good enough!). But first I wanted to understand how the current implementation works.

I know you used to compute autocorrelation at some point, but the current algorithm seems more like a peak detector? As the code is not greatly commented, can anybody (Gordon?) explain a little what is the idea behind?

Thanks a lot,
Dario