mirror of https://github.com/facebook/rocksdb.git
134 lines
4.7 KiB
C++
134 lines
4.7 KiB
C++
// Copyright (c) 2011-present, Facebook, Inc. All rights reserved.
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// This source code is licensed under the BSD-style license found in the
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// LICENSE file in the root directory of this source tree. An additional grant
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// of patent rights can be found in the PATENTS file in the same directory.
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// This source code is also licensed under the GPLv2 license found in the
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// COPYING file in the root directory of this source tree.
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//
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// Copyright (c) 2011 The LevelDB Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style license that can be
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// found in the LICENSE file. See the AUTHORS file for names of contributors.
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//
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// BlockBuilder generates blocks where keys are prefix-compressed:
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//
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// When we store a key, we drop the prefix shared with the previous
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// string. This helps reduce the space requirement significantly.
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// Furthermore, once every K keys, we do not apply the prefix
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// compression and store the entire key. We call this a "restart
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// point". The tail end of the block stores the offsets of all of the
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// restart points, and can be used to do a binary search when looking
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// for a particular key. Values are stored as-is (without compression)
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// immediately following the corresponding key.
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//
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// An entry for a particular key-value pair has the form:
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// shared_bytes: varint32
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// unshared_bytes: varint32
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// value_length: varint32
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// key_delta: char[unshared_bytes]
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// value: char[value_length]
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// shared_bytes == 0 for restart points.
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//
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// The trailer of the block has the form:
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// restarts: uint32[num_restarts]
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// num_restarts: uint32
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// restarts[i] contains the offset within the block of the ith restart point.
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#include "table/block_builder.h"
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#include <algorithm>
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#include <assert.h>
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#include "rocksdb/comparator.h"
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#include "db/dbformat.h"
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#include "util/coding.h"
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namespace rocksdb {
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BlockBuilder::BlockBuilder(int block_restart_interval, bool use_delta_encoding)
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: block_restart_interval_(block_restart_interval),
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use_delta_encoding_(use_delta_encoding),
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restarts_(),
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counter_(0),
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finished_(false) {
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assert(block_restart_interval_ >= 1);
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restarts_.push_back(0); // First restart point is at offset 0
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estimate_ = sizeof(uint32_t) + sizeof(uint32_t);
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}
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void BlockBuilder::Reset() {
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buffer_.clear();
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restarts_.clear();
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restarts_.push_back(0); // First restart point is at offset 0
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estimate_ = sizeof(uint32_t) + sizeof(uint32_t);
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counter_ = 0;
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finished_ = false;
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last_key_.clear();
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}
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size_t BlockBuilder::EstimateSizeAfterKV(const Slice& key, const Slice& value)
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const {
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size_t estimate = CurrentSizeEstimate();
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estimate += key.size() + value.size();
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if (counter_ >= block_restart_interval_) {
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estimate += sizeof(uint32_t); // a new restart entry.
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}
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estimate += sizeof(int32_t); // varint for shared prefix length.
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estimate += VarintLength(key.size()); // varint for key length.
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estimate += VarintLength(value.size()); // varint for value length.
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return estimate;
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}
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Slice BlockBuilder::Finish() {
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// Append restart array
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for (size_t i = 0; i < restarts_.size(); i++) {
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PutFixed32(&buffer_, restarts_[i]);
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}
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PutFixed32(&buffer_, static_cast<uint32_t>(restarts_.size()));
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finished_ = true;
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return Slice(buffer_);
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}
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void BlockBuilder::Add(const Slice& key, const Slice& value) {
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assert(!finished_);
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assert(counter_ <= block_restart_interval_);
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size_t shared = 0; // number of bytes shared with prev key
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if (counter_ >= block_restart_interval_) {
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// Restart compression
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restarts_.push_back(static_cast<uint32_t>(buffer_.size()));
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estimate_ += sizeof(uint32_t);
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counter_ = 0;
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if (use_delta_encoding_) {
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// Update state
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last_key_.assign(key.data(), key.size());
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}
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} else if (use_delta_encoding_) {
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Slice last_key_piece(last_key_);
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// See how much sharing to do with previous string
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shared = key.difference_offset(last_key_piece);
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// Update state
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// We used to just copy the changed data here, but it appears to be
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// faster to just copy the whole thing.
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last_key_.assign(key.data(), key.size());
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}
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const size_t non_shared = key.size() - shared;
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const size_t curr_size = buffer_.size();
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// Add "<shared><non_shared><value_size>" to buffer_
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PutVarint32Varint32Varint32(&buffer_, static_cast<uint32_t>(shared),
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static_cast<uint32_t>(non_shared),
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static_cast<uint32_t>(value.size()));
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// Add string delta to buffer_ followed by value
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buffer_.append(key.data() + shared, non_shared);
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buffer_.append(value.data(), value.size());
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counter_++;
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estimate_ += buffer_.size() - curr_size;
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}
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} // namespace rocksdb
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