void VersionEdit::EncodeTo(std::string* dst) const {  if (has_comparator_) {  // 记录Comparator    PutVarint32(dst, kComparator);    PutLengthPrefixedSlice(dst, comparator_);  }  if (has_log_number_) {  // 记录Log Numer     PutVarint32(dst, kLogNumber);    PutVarint64(dst, log_number_);  }  if (has_prev_log_number_) {  // 记录Prev Log Number，现在已废弃，一般为0    PutVarint32(dst, kPrevLogNumber);    PutVarint64(dst, prev_log_number_);  }  if (has_next_file_number_) {  // 记录下一个文件序号     PutVarint32(dst, kNextFileNumber);    PutVarint64(dst, next_file_number_);  }  if (has_last_sequence_) {  // 记录最大的sequence num     PutVarint32(dst, kLastSequence);    PutVarint64(dst, last_sequence_);  }  // 记录每一级Level下次compaction的起始Key    for (size_t i = 0; i < compact_pointers_.size(); i++) {    PutVarint32(dst, kCompactPointer);    PutVarint32(dst, compact_pointers_[i].first);  // level    PutLengthPrefixedSlice(dst, compact_pointers_[i].second.Encode());  }  // 记录每一级需要删除的文件    for (DeletedFileSet::const_iterator iter = deleted_files_.begin();       iter != deleted_files_.end();       ++iter) {    PutVarint32(dst, kDeletedFile);    PutVarint32(dst, iter->first);   // level    PutVarint64(dst, iter->second);  // file number  }  // 记录每一级需要有效的sst以及其smallest与largest的key    for (size_t i = 0; i < new_files_.size(); i++) {    const FileMetaData& f = new_files_[i].second;    PutVarint32(dst, kNewFile);    PutVarint32(dst, new_files_[i].first);  // level    PutVarint64(dst, f.number);    PutVarint64(dst, f.file_size);    PutLengthPrefixedSlice(dst, f.smallest.Encode());    PutLengthPrefixedSlice(dst, f.largest.Encode());  }}