Parameterised compression for sparse bitmaps

Full-text retrieval systems often use either a bitmap or an inverted file to identify which documents contain which terms, so that the documents containing any combination of query terms can be quickly located. Bitmaps of term occurrences are large, but are usually sparse, and thus are amenable to a variety of compression techniques. Here we consider techniques in which the encoding of each bitvector within the bitmap is parameterised, so that a different code can be used for each bitvector. Our experimental results show that the new methods yield better compression than previous techniques.