void add(byte[] buf, int offset, int len) {
    /*
     * For faster hashing, use combinatorial generation
     * http://www.eecs.harvard.edu/~kirsch/pubs/bbbf/esa06.pdf
     */
    HashKey<byte[]> hashKey = new ByteArrayHashKey(buf, offset, len);
    int hash1 = this.hash.hash(hashKey, 0);
    int hash2 = this.hash.hash(hashKey, hash1);
    setHashLoc(hash1, hash2);
  }
Creates another similar Bloom filter. Does not copy the actual bits, and sets the new filter's
key count to zero.
@return a Bloom filter with the same configuration as this
hbase-server/src/main/java/org/apache/hadoop/hbase/util/BloomFilterChunk.java