// Copyright (c) 2011 The LevelDB Authors. All rights reserved. // Use of this source code is governed by a BSD-style license that can be // found in the LICENSE file. See the AUTHORS file for names of contributors. // // BlockBuilder generates blocks where keys are prefix-compressed: // // When we store a key, we drop the prefix shared with the previous // string. This helps reduce the space requirement significantly. // Furthermore, once every K keys, we do not apply the prefix // compression and store the entire key. We call this a "restart // point". The tail end of the block stores the offsets of all of the // restart points, and can be used to do a binary search when looking // for a particular key. Values are stored as-is (without compression) // immediately following the corresponding key. // // An entry for a particular key-value pair has the form: // shared_bytes: varint32 // unshared_bytes: varint32 // value_length: varint32 // key_delta: char[unshared_bytes] // value: char[value_length] // shared_bytes == 0 for restart points. // // The trailer of the block has the form: // restarts: uint32[num_restarts] // num_restarts: uint32 // restarts[i] contains the offset within the block of the ith restart point. #include "table/block_builder.h" #include #include #include "leveldb/comparator.h" #include "leveldb/options.h" #include "util/coding.h" namespace leveldb { BlockBuilder::BlockBuilder(const Options* options) : options_(options), restarts_(), counter_(0), finished_(false) { assert(options->block_restart_interval >= 1); restarts_.push_back(0); // First restart point is at offset 0 } void BlockBuilder::Reset() { buffer_.clear(); restarts_.clear(); restarts_.push_back(0); // First restart point is at offset 0 counter_ = 0; finished_ = false; last_key_.clear(); } size_t BlockBuilder::CurrentSizeEstimate() const { return (buffer_.size() + // Raw data buffer restarts_.size() * sizeof(uint32_t) + // Restart array sizeof(uint32_t)); // Restart array length } Slice BlockBuilder::Finish() { // Append restart array for (size_t i = 0; i < restarts_.size(); i++) { PutFixed32(&buffer_, restarts_[i]); } PutFixed32(&buffer_, restarts_.size()); finished_ = true; return Slice(buffer_); } void BlockBuilder::Add(const Slice& key, const Slice& value) { Slice last_key_piece(last_key_); // 创建一个Slice对象last_key_piece,它是last_key_的引用,表示上一个添加的键 assert(!finished_); // 断言检查是否已经开始构建块(finished_为false)。如果已经开始构建(即finished_为true),则不应该再添加新的键值对 assert(counter_ <= options_->block_restart_interval); // 断言检查当前键的计数是否超过重启间隔,这是为了控制压缩的频率 assert(buffer_.empty() // 断言检查buffer_是否为空(即还没有添加任何键值对),或者当前添加的键大于(在排序顺序上)上一个添加的键 || options_->comparator->Compare(key, last_key_piece) > 0); // 检查是否需要重启压缩。如果当前键的计数小于重启间隔,则尝试与上一个键共享前缀;如果达到或超过重启间隔,则重启压缩(在restarts_中记录当前buffer_的大小,并将counter_重置为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; } // 计算当前键与上一个键非共享的部分的大小 const size_t non_shared = key.size() - shared; // Add "" to buffer_ // 将共享长度、非共享长度和值的大小(都以Varint32格式)添加到buffer_中。这允许在读取时解码这些值并重建原始键和值 PutVarint32(&buffer_, shared); PutVarint32(&buffer_, non_shared); PutVarint32(&buffer_, value.size()); // Add string delta to buffer_ followed by value // 将键的非共享部分和值追加到buffer_中 buffer_.append(key.data() + shared, non_shared); buffer_.append(value.data(), value.size()); // Update state // 更新last_key_为当前键(仅保留共享部分和非共享部分),然后增加键的计数 last_key_.resize(shared); last_key_.append(key.data() + shared, non_shared); assert(Slice(last_key_) == key); counter_++; } } // namespace leveldb