HR algorithm: how does it work?

Doing some work on the HRM would be amazing - @Mi's explanation sounds pretty much spot on.

Hopefully the code itself is in its own file so should be reasonably easy to compile without Espruino with a few simple stubs.

So to find a robust peak detector seems to be a valuable task.

Yes - it's worth looking in GitHub history as I did previously use a more robust detector, but at least in my testing I had found the current one performed better.

One thing to add is the hardware itself auto-adjusts exposure, and it's actually possible that during high accelerations it actually needs modifying to stop doing that.