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diff --git a/src/mongo/db/exec/sort_key_generator.h b/src/mongo/db/exec/sort_key_generator.h
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+/**
+ *    Copyright (C) 2015 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 <memory>
+
+#include "mongo/bson/bsonobj.h"
+#include "mongo/db/index/btree_key_generator.h"
+#include "mongo/db/query/index_bounds.h"
+
+namespace mongo {
+
+class Collection;
+class WorkingSetMember;
+
+/**
+ * Maps a WSM value to a BSONObj key that can then be sorted via BSONObjCmp.
+ */
+class SortKeyGenerator {
+public:
+    /**
+     * 'sortSpec' is the BSONObj in the .sort(...) clause.
+     *
+     * 'queryObj' is the BSONObj in the .find(...) clause.  For multikey arrays we have to
+     * ensure that the value we select to sort by is within bounds generated by
+     * executing 'queryObj' using the virtual index with key pattern 'sortSpec'.
+     */
+    SortKeyGenerator(const Collection* collection,
+                     const BSONObj& sortSpec,
+                     const BSONObj& queryObj);
+
+    /**
+     * Returns the key used to sort 'member'.
+     */
+    Status getSortKey(const WorkingSetMember& member, BSONObj* objOut) const;
+
+    /**
+     * Passed to std::sort and used to sort the keys that are returned from getSortKey.
+     *
+     * Returned reference lives as long as 'this'.
+     */
+    const BSONObj& getSortComparator() const {
+        return _comparatorObj;
+    }
+
+private:
+    Status getBtreeKey(const BSONObj& memberObj, BSONObj* objOut) const;
+
+    /**
+     * In order to emulate the existing sort behavior we must make unindexed sort behavior as
+     * consistent as possible with indexed sort behavior.  As such, we must only consider index
+     * keys that we would encounter if we were answering the query using the sort-providing
+     * index.
+     *
+     * Populates _hasBounds and _bounds.
+     */
+    void getBoundsForSort(const BSONObj& queryObj, const BSONObj& sortObj);
+
+    // Not owned by us
+    const Collection* _collection;
+
+    // The object that we use to call woCompare on our resulting key.  Is equal to _rawSortSpec
+    // unless we have some $meta expressions.  Each $meta expression has a default sort order.
+    BSONObj _comparatorObj;
+
+    // The raw object in .sort()
+    BSONObj _rawSortSpec;
+
+    // The sort pattern with any non-Btree sort pulled out.
+    BSONObj _btreeObj;
+
+    // If we're not sorting with a $meta value we can short-cut some work.
+    bool _sortHasMeta;
+
+    // True if the bounds are valid.
+    bool _hasBounds;
+
+    // The bounds generated from the query we're sorting.
+    IndexBounds _bounds;
+
+    // Helper to extract sorting keys from documents.
+    std::unique_ptr<BtreeKeyGenerator> _keyGen;
+
+    // Helper to filter keys, ensuring keys generated with _keyGen are within _bounds.
+    std::unique_ptr<IndexBoundsChecker> _boundsChecker;
+};
+
+}  // namespace mongo