static long raf_findBeginningOfLine(RandomAccessFile raf, long pos, int bufSize) ctex {
  byte[] buf = new byte[bufSize];
  while (pos > 0) {
    long start = Math.max(pos-bufSize, 0);
    raf.seek(start);
    raf.readFully(buf, 0, (int) Math.min(pos-start, bufSize));
    int idx = lastIndexOf_byteArray(buf, (byte) '\n');
    if (idx >= 0) ret start+idx+1;
    pos = start;
  }
  ret 0;
}