lfilter static method

Float32List lfilter(Float32List input, { required Float32List b, required Float32List a, Float32List? out, })

Applies a causal linear digital filter (IIR/FIR) to input.

The implemented difference equation is: a[0] * y[n] = sum_{k=0..nb-1} b[k] * x[n-k] - sum_{k=1..na-1} a[k] * y[n-k]

Conventions:

• b are feed-forward (numerator) coefficients.
• a are feedback (denominator) coefficients.
• a[0] must be non-zero.

Z-domain form:

                    -1              -M
        b[0] + b[1]z  + ... + b[M] z
Y(z) = -------------------------------- X(z)
                    -1              -N
        a[0] + a[1]z  + ... + a[N] z

Behavior:

• Uses a direct-form II transposed stateful core in native code.
• Returns one output sample per input sample (out.length == input.length).
• If out is provided, it is filled in-place and returned.

Implementation

static Float32List lfilter(
  Float32List input, {
  required Float32List b,
  required Float32List a,
  Float32List? out,
}) {
  final L = input.length;
  final nb = b.length;
  final na = a.length;

  ffi.Pointer<ffi.Float> bPtr = ffi.nullptr;
  ffi.Pointer<ffi.Float> aPtr = ffi.nullptr;
  try {
    aPtr = calloc<ffi.Float>(na);
    bPtr = calloc<ffi.Float>(nb);
    aPtr.asTypedList(L).setAll(0, a);
    bPtr.asTypedList(L).setAll(0, b);
  } catch (_) {
    if (bPtr != ffi.nullptr) calloc.free(bPtr);
    if (aPtr != ffi.nullptr) calloc.free(aPtr);
    throw StateError("Failed to create lfilter context");
  }

  final ctx = yl_iir_ctx_create_df2t_f32(bPtr, nb, aPtr, na);
  if (ctx == ffi.nullptr) {
    throw StateError("Failed to create lfilter context");
  }
  ffi.Pointer<ffi.Float> inPtr = ffi.nullptr;
  ffi.Pointer<ffi.Float> outPtr = ffi.nullptr;
  try {
    inPtr = calloc<ffi.Float>(L);
    outPtr = calloc<ffi.Float>(L);
    inPtr.asTypedList(L).setAll(0, input);
    // double errorSqrt = yl_lpc_ctx_process_frame(ctx, inPtr, L, 0, outPtr);
    yl_iir_ctx_process_f32(ctx, inPtr, outPtr, L);

    out = out ?? Float32List(L);
    out.setAll(0, outPtr.asTypedList(L));
    return out;
  } finally {
    if (inPtr != ffi.nullptr) calloc.free(inPtr);
    if (outPtr != ffi.nullptr) calloc.free(outPtr);
    if (bPtr != ffi.nullptr) calloc.free(bPtr);
    if (aPtr != ffi.nullptr) calloc.free(aPtr);
    yl_iir_ctx_destroy(ctx);
  }
}