Compressed String Dictionaries

Nieves Brisaboa, Rodrigo Cánovas, Francisco Claude, Miguel Mart&iacaute;nez-Prieto, and Gonzalo Navarro

The problem of storing a set of strings -- a string dictionary -- in compact form appears naturally in many cases. While classically it has represented a small part of the whole data to be processed (e.g., for Natural Language processing or for indexing text collections), recent applications in Web engines, RDF graphs, Bioinformatics, and many others, handle very large string dictionaries, whose size is a significant fraction of the whole data. Thus efficient approaches to compress them are necessary. In this paper we empirically compare time and space performance of some existing alternatives, as well as new ones we propose. We show that space reductions of up to 20% of the original size of the strings is possible while supporting dictionary searches within a few microseconds, and up to 10% within a few tens or hundreds of microseconds.