Add函数是给一个Data block中添加对应的key和value,函数源码如下,其中有一处不理解:

L30~L34是更新last_key_的,不理解这里干嘛不直接last_key_ = key.ToString();

写成

// Update state

last_key_.resize(shared);

last_key_.append(key.data() + shared, non_shared);

assert(Slice(last_key_) == key);

是有什么其他原因吗?

1 void BlockBuilder::Add(const Slice& key, const Slice& value) { 2   Slice last_key_piece(last_key_); 3   assert(!finished_); 4   assert(counter_ <= options_->block_restart_interval); 5   assert(buffer_.empty() // No values yet? 6          || options_->comparator->Compare(key, last_key_piece) > 0); 7   size_t shared = 0; 8   if (counter_ < options_->block_restart_interval) { 9     // See how much sharing to do with previous string10     const size_t min_length = std::min(last_key_piece.size(), key.size());11     while ((shared < min_length) && (last_key_piece[shared] == key[shared])) {12       shared++;13     }14   } else {15     // Restart compression16     restarts_.push_back(buffer_.size());17     counter_ = 0;18   }19   const size_t non_shared = key.size() - shared;20 21   // Add "
     
      
       
        " to buffer_22   PutVarint32(&buffer_, shared);23   PutVarint32(&buffer_, non_shared);24   PutVarint32(&buffer_, value.size());25 26   // Add string delta to buffer_ followed by value27   buffer_.append(key.data() + shared, non_shared);28   buffer_.append(value.data(), value.size());29 30   // Update state31   last_key_.resize(shared);32   last_key_.append(key.data() + shared, non_shared);33   assert(Slice(last_key_) == key);34   counter_++;35 }