code: func (b *bufWriter) offset() uint32 {
  	off, _ := b.file.Seek(0, 1)
  	off += int64(len(b.buf))
  	if int64(uint32(off)) != off {
  		log.Fatalf("index is larger than 4GB")
  	}
  	return uint32(off)
  }
docs: // offset returns the current write offset.