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

浏览数：27 / 时间：2015年06月12日

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 string
10     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 compression
16     restarts_.push_back(buffer_.size());
17     counter_ = 0;
18   }
19   const size_t non_shared = key.size() - shared;
20 
21   // Add "<shared><non_shared><value_size>" 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 value
27   buffer_.append(key.data() + shared, non_shared);
28   buffer_.append(value.data(), value.size());
29 
30   // Update state
31   last_key_.resize(shared);
32   last_key_.append(key.data() + shared, non_shared);
33   assert(Slice(last_key_) == key);
34   counter_++;
35 }