/**
 *    Copyright (C) 2019-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 "mongo/db/exec/sbe/expressions/expression.h"
#include "mongo/db/exec/sbe/stages/stages.h"
#include "mongo/db/exec/sbe/vm/vm.h"

namespace mongo::sbe {
/**
 * This is an array traversal operator. If the input value coming from the 'outer' side is an array
 * then we execute the 'inner' side exactly once for every element of the array. The results from
 * the 'inner' side are then collected into the output array value. The traversal is recursive and
 * the structure of nested arrays is preserved (up to optional depth 'nestedArraysDepth'). If the
 * input value is not an array then we execute the inner side just once and return the result.
 *
 * The 'inField' slot contains the value from the outer side which ie being traversed. When the
 * traversal is complete, the resulting value is placed in the slot given by 'outField'. Note that
 * it is legal for 'outField' and 'inField' to be the same slot when there are no fold or final
 * expressions. The 'outFieldInner' slot is the slot that the traverse stage should read in order to
 * get the resulting value whenever it executes the inner side. In other words, when the inner side
 * is re-opened and executed once for a particular array element, the 'outFieldInner' slot holds the
 * result of this operation (which may subsequently be passed to the fold/final expressions).
 *
 * The 'outerCorrelated' slots are slots from the outer side that are made visible to the inner
 * side.
 *
 * If an optional 'foldExpr' expression is provided, then instead of the output array we combine
 * individual results into a single output value. Another expression 'finalExpr' controls optional
 * short-circuiting (a.k.a. early out) logic.
 *
 * Debug string representation:
 *
 *  traverse outputSlot outputFromInnerBranchSlot inputSlot [<correlated slots>]
 *          { foldExpr }? { finalExpr }?
 *          from childStage
 *          in childStage
 */
class TraverseStage final : public PlanStage {
public:
    TraverseStage(std::unique_ptr<PlanStage> outer,
                  std::unique_ptr<PlanStage> inner,
                  value::SlotId inField,
                  value::SlotId outField,
                  value::SlotId outFieldInner,
                  value::SlotVector outerCorrelated,
                  std::unique_ptr<EExpression> foldExpr,
                  std::unique_ptr<EExpression> finalExpr,
                  PlanNodeId planNodeId,
                  boost::optional<size_t> nestedArraysDepth,
                  bool participateInTrialRunTracking = true);

    std::unique_ptr<PlanStage> clone() const final;

    void prepare(CompileCtx& ctx) final;
    value::SlotAccessor* getAccessor(CompileCtx& ctx, value::SlotId slot) final;
    void open(bool reOpen) final;
    PlanState getNext() final;
    void close() final;

    std::unique_ptr<PlanStageStats> getStats(bool includeDebugInfo) const final;
    const SpecificStats* getSpecificStats() const final;
    std::vector<DebugPrinter::Block> debugPrint() const final;
    size_t estimateCompileTimeSize() const final;

protected:
    void doSaveState(bool relinquishCursor) final;
    void doRestoreState(bool relinquishCursor) final;

private:
    void openInner(value::TypeTags tag, value::Value val);
    bool traverse(value::SlotAccessor* inFieldAccessor,
                  value::OwnedValueAccessor* outFieldOutputAccessor,
                  size_t level);
    PlanState getNextOuterSide() {
        _isReadingLeftSide = true;
        auto ret = _children[0]->getNext();
        _isReadingLeftSide = false;
        return ret;
    }

    // The input slot holding value being traversed.
    const value::SlotId _inField;

    // The output slot holding result of the traversal.
    const value::SlotId _outField;

    // The result of a single iteration of the traversal.
    const value::SlotId _outFieldInner;

    // Slots from the 'outer' side that are explicitly accessible on the 'inner' side.
    const value::SlotVector _correlatedSlots;

    // Optional folding expression for combining array elements.
    const std::unique_ptr<EExpression> _fold;

    // Optional boolean expression controlling short-circuiting of the fold.
    const std::unique_ptr<EExpression> _final;

    // Optional nested arrays recursion depth.
    const boost::optional<size_t> _nestedArraysDepth;

    value::SlotAccessor* _inFieldAccessor{nullptr};
    value::ViewOfValueAccessor _correlatedAccessor;
    value::OwnedValueAccessor _outFieldOutputAccessor;
    value::SlotAccessor* _outFieldInputAccessor{nullptr};

    std::unique_ptr<vm::CodeFragment> _foldCode;
    std::unique_ptr<vm::CodeFragment> _finalCode;

    std::list<value::ArrayAccessor> _inArrayAccessors;

    vm::ByteCode _bytecode;

    bool _compiled{false};
    bool _reOpenInner{false};
    TraverseStats _specificStats;

    // Tracks whether or not we're reading from the left child or the right child.
    // This is necessary for yielding.
    bool _isReadingLeftSide = false;
};
}  // namespace mongo::sbe