HeartRateVariability
Measure pulse, parasympathetic vs sympathetic, measure HRV, i.e. variability between heart beats
This was done as an exercise for me to learn more about signal processing, statistics and the VDSP library.
The numbers seem reasonable, but I haven't validated them, not tested them on anybody but me, so I'm not putting this on the app store, nor include it in existing apps. Use the data as you like, don't bet your life on it.
I had hoped to write this more as functional Swift rather than its present state of C style but hey, speed is everything here and C style is faster. May undertake making more functional in the future, or it may just sit here neglected and unloved.