Vous ne pouvez pas sélectionner plus de 25 sujets
Les noms de sujets doivent commencer par une lettre ou un nombre, peuvent contenir des tirets ('-') et peuvent comporter jusqu'à 35 caractères.
107 lignes
3.5 KiB
107 lignes
3.5 KiB
// 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 <algorithm>
|
|
#include <cassert>
|
|
|
|
#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_);
|
|
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_++;
|
|
}
|
|
|
|
} // namespace leveldb
|