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107 linhas
3.5 KiB
107 linhas
3.5 KiB
// 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 <cassert>
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#include "leveldb/comparator.h"
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#include "leveldb/options.h"
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#include "util/coding.h"
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namespace leveldb {
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BlockBuilder::BlockBuilder(const Options* options)
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: options_(options), restarts_(), counter_(0), finished_(false) {
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assert(options->block_restart_interval >= 1);
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restarts_.push_back(0); // First restart point is at offset 0
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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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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::CurrentSizeEstimate() const {
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return (buffer_.size() + // Raw data buffer
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restarts_.size() * sizeof(uint32_t) + // Restart array
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sizeof(uint32_t)); // Restart array length
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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_, 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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Slice last_key_piece(last_key_);
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assert(!finished_);
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assert(counter_ <= options_->block_restart_interval);
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assert(buffer_.empty() // No values yet?
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|| options_->comparator->Compare(key, last_key_piece) > 0);
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size_t shared = 0;
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if (counter_ < options_->block_restart_interval) {
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// See how much sharing to do with previous string
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const size_t min_length = std::min(last_key_piece.size(), key.size());
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while ((shared < min_length) && (last_key_piece[shared] == key[shared])) {
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shared++;
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}
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} else {
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// Restart compression
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restarts_.push_back(buffer_.size());
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counter_ = 0;
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}
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const size_t non_shared = key.size() - shared;
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// Add "<shared><non_shared><value_size>" to buffer_
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PutVarint32(&buffer_, shared);
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PutVarint32(&buffer_, non_shared);
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PutVarint32(&buffer_, 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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// Update state
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last_key_.resize(shared);
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last_key_.append(key.data() + shared, non_shared);
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assert(Slice(last_key_) == key);
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counter_++;
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}
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} // namespace leveldb
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