/**
 *    Copyright (C) 2020-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,
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 */

#include "mongo/platform/basic.h"

#include "mongo/db/query/sbe_plan_ranker.h"

namespace mongo::sbe::plan_ranker {
namespace {
/**
 * A plan ranker for the SBE plan stage tree. Defines productivity as a cumulative number of
 * physical reads from the storage performed by all stages in the plan which can read from the
 * storage, divided by the total number of advances of the root stage, which corresponds to the
 * number of returned documents.
 */
class DefaultPlanScorer final : public mongo::plan_ranker::PlanScorer<PlanStageStats> {
public:
    DefaultPlanScorer(const QuerySolution* solution) : _solution{solution} {
        invariant(_solution);
    }

protected:
    double calculateProductivity(const mongo::sbe::PlanStageStats* root) const final {
        auto numReads{calculateNumberOfReads(root)};

        // We add one to the number of advances so that plans which returned zero documents still
        // have a productivity of non-zero. This allows us to compare productivity scores between
        // plans with zero advances. For example, a plan which did zero advances but examined ten
        // documents would have a score of (0 + 1)/10, while a plan which did zero advances but
        // examined a hundred documents would have a score of (0 + 1)/100.
        //
        // Similarly, we add one to the number of reads in case 0 reads were performed. This could
        // happen if a plan encounters EOF right away, for example.
        return static_cast<double>(root->common.advances + 1) / static_cast<double>(numReads + 1);
    }

    std::string getProductivityFormula(const mongo::sbe::PlanStageStats* root) const final {
        auto numReads{calculateNumberOfReads(root)};
        StringBuilder sb;

        sb << "(" << (root->common.advances + 1) << " advances)/(" << numReads << " numReads)";

        return sb.str();
    }

    double getNumberOfAdvances(const mongo::sbe::PlanStageStats* stats) const final {
        return stats->common.advances;
    }

    bool hasStage(StageType type, const mongo::sbe::PlanStageStats* stats) const final {
        // In SBE a plan stage doesn't map 1-to-1 to a solution node, and can expand into a subtree
        // of plan stages, each having its own plan stage stats. So, to answer whether an SBE plan
        // stage stats tree contains a stage of the given 'type', we need to look into the solution
        // tree instead.
        return _solution->hasNode(type);
    }

private:
    const QuerySolution* _solution;
};
}  // namespace

std::unique_ptr<mongo::plan_ranker::PlanScorer<PlanStageStats>> makePlanScorer(
    const QuerySolution* solution) {
    return std::make_unique<DefaultPlanScorer>(solution);
}
}  // namespace mongo::sbe::plan_ranker