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[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?
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 "<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 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 }
[Leveldb源码剖析疑问]-block_builder.cc之Add函数
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