static int offset(long index) {
    return (int) (index & SEGMENT_MASK);
  }
Computes the offset associated with a given index.

@param index an index into a big array.
@return the associated offset (in the associated {@linkplain #segment(long) segment}).
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/execution/operator/source/relational/aggregation/grouped/array/BigArrays.java