presto-main/src/main/java/io/prestosql/operator/window/AggregateWindowFunction.java/AggregateWindowFunction/resetAccumulator
class AggregateWindowFunction:    private void resetAccumulator()
    {
        if (currentStart >= 0) {
            accumulator = accumulatorFactory.createAccumulator();
            currentStart = -1;
            currentEnd = -1;
        }
    }