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diff --git a/src/mongo/db/pipeline/accumulator_locf.cpp b/src/mongo/db/pipeline/accumulator_locf.cpp
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+++ b/src/mongo/db/pipeline/accumulator_locf.cpp
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+/**
+ *    Copyright (C) 2021-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.
+ */
+
+#include "mongo/platform/basic.h"
+
+#include "mongo/db/pipeline/accumulator_for_window_functions.h"
+
+#include "mongo/db/exec/document_value/value.h"
+#include "mongo/db/pipeline/accumulation_statement.h"
+#include "mongo/db/pipeline/expression.h"
+#include "mongo/db/pipeline/window_function/window_function_expression.h"
+
+namespace mongo {
+
+REGISTER_WINDOW_FUNCTION_CONDITIONALLY(
+    locf,
+    mongo::window_function::ExpressionFromWindowlessAccumulator<AccumulatorLocf>::parse,
+    multiversion::FeatureCompatibilityVersion::kVersion_5_1,
+    feature_flags::gFlagFill.isEnabledAndIgnoreFCV());
+
+AccumulatorLocf::AccumulatorLocf(ExpressionContext* const expCtx)
+    : AccumulatorForWindowFunctions(expCtx) {
+    _memUsageBytes = sizeof(*this) + _lastNonNull.getApproximateSize();
+}
+
+void AccumulatorLocf::processInternal(const Value& input, bool merging) {
+    tassert(6050100, "$locf can't be merged", !merging);
+
+    if (!input.nullish()) {
+        _lastNonNull = input;
+        _memUsageBytes = sizeof(*this) + _lastNonNull.getApproximateSize();
+    }
+}
+
+Value AccumulatorLocf::getValue(bool toBeMerged) {
+    tassert(6050102, "$locf can't be merged", !toBeMerged);
+    return _lastNonNull;
+}
+
+void AccumulatorLocf::reset() {
+    _lastNonNull = Value();
+    _memUsageBytes = sizeof(*this) + _lastNonNull.getApproximateSize();
+}
+
+boost::intrusive_ptr<AccumulatorState> AccumulatorLocf::create(ExpressionContext* const expCtx) {
+    return new AccumulatorLocf(expCtx);
+}
+}  // namespace mongo