/**
 * Copyright (C) 2016 MongoDB Inc.
 *
 * This program is free software: you can redistribute it and/or  modify
 * it under the terms of the GNU Affero General Public License, version 3,
 * as published by the Free Software Foundation.
 *
 * 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
 * GNU Affero General Public License for more details.
 *
 * You should have received a copy of the GNU Affero General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 *
 * 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 GNU Affero General 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 "mongo/db/pipeline/document_source.h"
#include "mongo/db/pipeline/value_comparator.h"

namespace mongo {

/**
 * This class is not a registered stage, it is only used as an optimized replacement for $sample
 * when the storage engine allows us to use a random cursor.
 */
class DocumentSourceSampleFromRandomCursor final : public DocumentSource {
public:
    GetNextResult getNext() final;
    const char* getSourceName() const final;
    Value serialize(boost::optional<ExplainOptions::Verbosity> explain = boost::none) const final;
    GetDepsReturn getDependencies(DepsTracker* deps) const final;

    StageConstraints constraints(Pipeline::SplitState pipeState) const final {
        return {StreamType::kStreaming,
                PositionRequirement::kFirst,
                HostTypeRequirement::kAnyShard,
                DiskUseRequirement::kNoDiskUse,
                FacetRequirement::kNotAllowed,
                TransactionRequirement::kAllowed};
    }

    static boost::intrusive_ptr<DocumentSourceSampleFromRandomCursor> create(
        const boost::intrusive_ptr<ExpressionContext>& expCtx,
        long long size,
        std::string idField,
        long long collectionSize);

private:
    DocumentSourceSampleFromRandomCursor(const boost::intrusive_ptr<ExpressionContext>& expCtx,
                                         long long size,
                                         std::string idField,
                                         long long collectionSize);

    /**
     * Keep asking for documents from the random cursor until it yields a new document. Errors if a
     * a document is encountered without a value for '_idField', or if the random cursor keeps
     * returning duplicate elements.
     */
    GetNextResult getNextNonDuplicateDocument();

    long long _size;

    // The field to use as the id of a document. Usually '_id', but 'ts' for the oplog.
    std::string _idField;

    // Keeps track of the documents that have been returned, since a random cursor is allowed to
    // return duplicates.
    ValueUnorderedSet _seenDocs;

    // The approximate number of documents in the collection (includes orphans).
    const long long _nDocsInColl;

    // The value to be assigned to the randMetaField of outcoming documents. Each call to getNext()
    // will decrement this value by an amount scaled by _nDocsInColl as an attempt to appear as if
    // the documents were produced by a top-k random sort.
    double _randMetaFieldVal = 1.0;
};

}  // namespace mongo