sclass JustCountingOutputStream extends OutputStream {
  long counter;
  
  @Override
  public void write(int b) throws IOException {
    ++counter;
  }

  @Override
  public void write(byte[] b) throws IOException {
    counter += b.length;
  }

  @Override
  public void write(byte[] b, int off, int len) throws IOException {
    counter += len;
  }
  
  long getFilePointer aka get() { ret counter; }
}

Began life as a copy of #1029366