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diff --git a/src/mongo/db/pipeline/accumulator_covariance.cpp b/src/mongo/db/pipeline/accumulator_covariance.cpp
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+++ b/src/mongo/db/pipeline/accumulator_covariance.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 <cmath>
+
+#include "mongo/db/pipeline/accumulator.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_NON_REMOVABLE_WINDOW_FUNCTION(
+    covarianceSamp, window_function::ExpressionFromAccumulator<AccumulatorCovarianceSamp>::parse);
+
+REGISTER_NON_REMOVABLE_WINDOW_FUNCTION(
+    covariancePop, window_function::ExpressionFromAccumulator<AccumulatorCovariancePop>::parse);
+
+void AccumulatorCovariance::processInternal(const Value& input, bool merging) {
+    tassert(5424000, "$covariance can't be merged", !merging);
+
+    // Currently we only support array with 2 numeric values as input value. Other types of input or
+    // non-numeric arrays have no impact on covariance.
+    if (!input.isArray())
+        return;
+    const auto& arr = input.getArray();
+    if (arr.size() != 2)
+        return;
+    if (!arr[0].numeric() || !arr[1].numeric())
+        return;
+
+    // This is an implementation of the following algorithm:
+    // https://en.wikipedia.org/wiki/Algorithms_for_calculating_variance#Online
+    double x = arr[0].coerceToDouble();
+    double y = arr[1].coerceToDouble();
+    double dx = x - _meanX;
+
+    _count++;
+    _meanX += (dx / _count);
+    _meanY += (y - _meanY) / _count;
+    _cXY += dx * (y - _meanY);
+}
+
+AccumulatorCovariance::AccumulatorCovariance(ExpressionContext* const expCtx, bool isSamp)
+    : AccumulatorState(expCtx), _isSamp(isSamp) {
+    _memUsageBytes = sizeof(*this);
+}
+
+void AccumulatorCovariance::reset() {
+    _count = 0;
+    _meanX = 0;
+    _meanY = 0;
+    _cXY = 0;  // 0 only makes sense if "_count" > 0.
+}
+
+Value AccumulatorCovariance::getValue(bool toBeMerged) {
+    const double adjustedCount = (_isSamp ? _count - 1 : _count);
+
+    if (adjustedCount <= 0)
+        return Value(BSONNULL);  // Covariance not well defined in this case.
+
+    return Value(_cXY / adjustedCount);
+}
+
+boost::intrusive_ptr<AccumulatorState> AccumulatorCovarianceSamp::create(
+    ExpressionContext* const expCtx) {
+    return new AccumulatorCovarianceSamp(expCtx);
+}
+
+boost::intrusive_ptr<AccumulatorState> AccumulatorCovariancePop::create(
+    ExpressionContext* const expCtx) {
+    return new AccumulatorCovariancePop(expCtx);
+}
+
+}  // namespace mongo