LPF destroys data on the source buffer

[j1elo]

I have a Buffer with 48kHz audio as PCM signed 16-bit little-endian (FFmpeg format s16le). Then I'm doing this simple test to find out that the resample function is affecting the source data:

while (...) {
  const audioBuffer = Buffer.from(...);
  const resampled = WaveResampler.resample(audioBuffer, 48000, 16000, {
    LPF: true,
  });
  // `resampled` is not used. `audioBuffer` has been modified but it shouldn't.
  fs.appendFileSync("/tmp/test.raw", audioBuffer);
}

Now playing this audio with FFmpeg, the audio contents have been destroyed and everything is just noise, except that the original audio can be kind of noticed very subtle in the background:

ffplay -f s16le -ar 48k -ac 1 /tmp/test.raw

Setting LPF: false avoids the issue. The mistake seems to be in the downsample_() function, which saves data directly into the source buffer, when I believe it shouldn't:

samples[i]  = filter.filter(samples[i]);

As it is, no LPF downsampling can be done with this library with default settings, because the resulting audio is garbled.