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/**
* Copyright (C) 2018-present MongoDB, Inc.
*
* This program is free software: you can redistribute it and/or modify
* it under the terms of the Server Side Public License, version 1,
* as published by MongoDB, Inc.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* Server Side Public License for more details.
*
* You should have received a copy of the Server Side Public License
* along with this program. If not, see
* <http://www.mongodb.com/licensing/server-side-public-license>.
*
* As a special exception, the copyright holders give permission to link the
* code of portions of this program with the OpenSSL library under certain
* conditions as described in each individual source file and distribute
* linked combinations including the program with the OpenSSL library. You
* must comply with the Server Side Public License in all respects for
* all of the code used other than as permitted herein. If you modify file(s)
* with this exception, you may extend this exception to your version of the
* file(s), but you are not obligated to do so. If you do not wish to do so,
* delete this exception statement from your version. If you delete this
* exception statement from all source files in the program, then also delete
* it in the license file.
*/
#pragma once
#include <boost/optional/optional.hpp>
#include <cstddef>
#include <vector>
#include "mongo/db/exec/document_value/document.h"
#include "mongo/base/status.h"
namespace mongo {
/**
* Implements a sequential cache of Documents, up to an optional maximum size. Can be in one of
* three states: building, serving, or abandoned. See SequentialDocumentCache::CacheStatus.
*/
class SequentialDocumentCache {
SequentialDocumentCache(const SequentialDocumentCache&) = delete;
SequentialDocumentCache& operator=(const SequentialDocumentCache&) = delete;
public:
explicit SequentialDocumentCache(size_t maxCacheSizeBytes) : _maxSizeBytes(maxCacheSizeBytes) {}
SequentialDocumentCache(SequentialDocumentCache&& moveFrom)
: _status(moveFrom._status),
_maxSizeBytes(moveFrom._maxSizeBytes),
_sizeBytes(moveFrom._sizeBytes),
_cacheIter(std::move(moveFrom._cacheIter)),
_cache(std::move(moveFrom._cache)) {}
SequentialDocumentCache& operator=(SequentialDocumentCache&& moveFrom) {
_cacheIter = std::move(moveFrom._cacheIter);
_maxSizeBytes = moveFrom._maxSizeBytes;
_cache = std::move(moveFrom._cache);
_sizeBytes = moveFrom._sizeBytes;
_status = moveFrom._status;
return *this;
}
/**
* Defines the states that the cache may be in at any given time.
*/
enum class CacheStatus {
// The cache is being filled. More documents may be added. A newly instantiated cache is in
// this state by default.
kBuilding,
// The caller has invoked freeze() to indicate that no more Documents need to be added. The
// cache is read-only at this point.
kServing,
// The maximum permitted cache size has been exceeded, or the caller has explicitly
// abandoned the cache. Cannot add more documents or call getNext.
kAbandoned,
};
/**
* Adds a document to the back of the cache. May only be called while the cache is in
* 'kBuilding' mode.
*/
void add(Document doc);
/**
* Moves the cache into 'kServing' (read-only) mode, and attempts to release any excess
* allocated memory. May only be called while the cache is in 'kBuilding' mode.
*/
void freeze();
/**
* Abandons the cache, marking it as 'kAbandoned' and freeing any memory allocated while
* building.
*/
void abandon();
/**
* Returns the next Document in sequence from the cache, or boost::none if the end of the cache
* has been reached. May only be called while in 'kServing' mode.
*/
boost::optional<Document> getNext();
/**
* Resets the cache iterator to the beginning of the cache. May only be called while the cache
* is in 'kServing' mode.
*/
void restartIteration();
CacheStatus status() const {
return _status;
}
size_t sizeBytes() const {
return _sizeBytes;
}
size_t maxSizeBytes() const {
return _maxSizeBytes;
}
size_t count() const {
return _cache.size();
}
bool isBuilding() const {
return _status == CacheStatus::kBuilding;
}
bool isServing() const {
return _status == CacheStatus::kServing;
}
bool isAbandoned() const {
return _status == CacheStatus::kAbandoned;
}
private:
CacheStatus checkCacheSize(const Document& doc);
CacheStatus _status = CacheStatus::kBuilding;
size_t _maxSizeBytes = 0;
size_t _sizeBytes = 0;
std::vector<Document>::iterator _cacheIter;
std::vector<Document> _cache;
};
} // namespace mongo
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