Streaming windowed sinc interpolation/resampling: trying to understand a Rust implementation
This looks partially similar to the BASIC code in my Q&amp;D resampling cookbook here:  http://www.nicholson.com/rhn/dsp.html#3  which computes both the Sinc and the window (von Hann) on the fly (per sample).
But it appears that the .rs code is not determining the lowpass cutoff frequency (phi upper and lower) correctly.  It needs to know whether the resampling is up or down in frequency, so that the Sinc computation can use something below half the lesser of the two frequencies.
Computation of one new resampled sample at a time works just fine, as long as the rate change is fixed (or increases slightly), and the initial and ending transients (related to the Sinc window width (or depth?)) are ignored (or accepted as transients).