A real-time biquad in pure Kotlin

A biquad is five coefficients and four state values per channel. No library is needed and no allocation happens per sample:

val y = b0 * x + b1 * x1 + b2 * x2 - a1 * y1 - a2 * y2
x2 = x1; x1 = x
y2 = y1; y1 = y

The coefficients come from the standard audio-EQ-cookbook formulas (Robert Bristow-Johnson). For a low-pass, with ω = 2π·fc/fs, α = sin(ω)/(2Q):