[Leveldb源码剖析疑问]-block_builder.cc之Add函数

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?

 void BlockBuilder::Add(const Slice& key, const Slice& value) {
   Slice last_key_piece(last_key_);
   assert(!finished_);
   assert(counter_ <= options_->block_restart_interval);
   assert(buffer_.empty() // No values yet?
          || options_->comparator->Compare(key, last_key_piece) > );
   size_t shared = ;
   if (counter_ < options_->block_restart_interval) {
     // See how much sharing to do with previous string
     const size_t min_length = std::min(last_key_piece.size(), key.size());
     while ((shared < min_length) && (last_key_piece[shared] == key[shared])) {
       shared++;
     }
   } else {
     // Restart compression
     restarts_.push_back(buffer_.size());
     counter_ = ;
   }
   const size_t non_shared = key.size() - shared;

   // Add "<shared><non_shared><value_size>" to buffer_
   PutVarint32(&buffer_, shared);
   PutVarint32(&buffer_, non_shared);
   PutVarint32(&buffer_, value.size());

   // Add string delta to buffer_ followed by value
   buffer_.append(key.data() + shared, non_shared);
   buffer_.append(value.data(), value.size());

   // Update state
   last_key_.resize(shared);
   last_key_.append(key.data() + shared, non_shared);
   assert(Slice(last_key_) == key);
   counter_++;
 }