static int d2charsave_checksum(unsigned char const *data, unsigned int len, unsigned int offset){
  int checksum;
  unsigned int i;
  unsigned int ch;

  if (!data) return 0;
  checksum=0;
  for (i=0; i<len; i++) {
    ch=data[i];
    if (i>=offset && i<offset+sizeof(int)) ch=0;
    ch+=(checksum<0);
    checksum=2*checksum+ch;
  }
  return checksum;
}