private Sketch<?> mergeSparseWithSparse(SparseSketch dest, SparseSketch other) {
        dest.merge(other);
        if (dest.size() > HyperLogLog.SPARSE_SKETCH_CONVERSION_THRESHOLD) {
            NormalSketch newSketch = new NormalSketch();
            newSketch.aggregate(dest.data());
            return newSketch;
        }
        return dest;
    }
Merges one of the two sketches into the other. The merge operation is performed in-place is possible.

@param left Either a {@link NormalSketch} or {@link SparseSketch}.
@param right Either a {@link NormalSketch} or {@link SparseSketch}.
@return The merged sketch. Is either first parameter, the other parameter or a new instance.
searchlib/src/main/java/com/yahoo/searchlib/aggregation/hll/SketchMerger.java