What would be a Vanilla replacement for [filter~]'s core ?

It doesn't have a help-file, I could only find the source with comments:

https://git.iem.at/pd/iemlib/-/blob/master/src/filter~.c?ref_type=heads

/* ---------- filter~ - slow dynamic filter-kernel 1. and 2. order ----------- */

It seems to have some interpolation algorithm to deal with the limited update rate of control-rate-only-inputs.
I don't need this, we can omit that, - just interested in what kind of filter is this?

it has several modes:

lp1, lp2, hp1, hp2,
lp1c, lp2c, hp1c, hp2c,
ap1, ap2, ap1c, ap2c,
bpq2, rbpq2, bsq2,
bpw2, rbpw2, bsw2,
dlp1, dlp2, dhp1, dhp2,
dlp1c, dlp2c, dhp1c, dhp2c,
dap1, dap2, dap1c, dap2c,
dbpq2, drbpq2, dbsq2,
dbpw2, drbpw2, dbsw2

And is used in many of IEMLIB's filters ...

Is this a biquad?

@lacuna Just because anyone else has replied yet ...

I don't really understand the code of IEMlib's [filter~] but I'm pretty sure that it is indeed a biquad filter, that is: if the filter (defined by the first argument) is second order. First order filters use a similar but cheaper algorithm. Higher order filters are implemented as a cascade of first and second order sections depending on whether the filter order is even or uneven.

To get the biquad coefficients right, it's important to notice the differences in implementation. (There has been confusion in the past because Max and Pd use different implementations.) Both Pd's [biquad~] and IEMlib's [filter~] seem to use Direct-Form II, but I'm not sure about that.