[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) > 0);
size_t shared = 0;
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_ = 0;
}
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_++;
}