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+/**
+ *    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). If the input value is not an
+ * array then we execute the inner side just once and return the result.
+ *
+ * If an optional 'fold' expression is provided then instead of the output array we combine
+ * individual results into a single output value. Another expression 'final' controls optional
+ * short-circuiting (a.k.a. early out) logic.
+ */
+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,
+                  boost::optional<size_t> nestedArraysDepth = boost::none);
+
+    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() const final;
+    const SpecificStats* getSpecificStats() const final;
+    std::vector<DebugPrinter::Block> debugPrint() const final;
+
+private:
+    void openInner(value::TypeTags tag, value::Value val);
+    bool traverse(value::SlotAccessor* inFieldAccessor,
+                  value::OwnedValueAccessor* outFieldOutputAccessor,
+                  size_t level);
+
+    // 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;
+
+    vm::ByteCode _bytecode;
+
+    bool _compiled{false};
+    bool _reOpenInner{false};
+};
+}  // namespace mongo::sbe