Affiliations: David R. Cheriton School of Computer Science, University of Waterloo, Canada. [email protected] | Department of Computer Science, University of Chile, Chile. [email protected]
Note: [] Address for correspondence: David R. Cheriton School of Computer Science, University of Waterloo. 200 University Avenue West, Waterloo, Ontario, Canada. N2L 3G1. Funded in part by NSERC Canada, Go-Bell Scholarships program and David R. Cheriton Graduate Scholarships program.
Note: [] Funded in part by Millennium Institute on Cell Dynamics and Biotechnology (ICDB), Grant ICM P05-001-F, Mideplan, Chile.
Abstract: Self-indexes aim at representing text collections in a compressed format that allows extracting arbitrary portions and also offers indexed searching on the collection. Current self-indexes are unable of fully exploiting the redundancy of highly repetitive text collections that arise in several applications. Grammar-based compression is well suited to exploit such repetitiveness. We introduce the first grammar-based self-index. It builds on Straight-Line Programs (SLPs), a rather general kind of context-free grammars. If an SLP of n rules represents a text T[1, u], then an SLP-compressed representation of T requires 2n log2 n bits. For that same SLP, our self-index takes O(n log n) + n log2 u bits. It extracts any text substring of length m in time O((m + h) log n), and finds occ occurrences of a pattern string of length m in time O((m(m + h) + h occ) log n), where h is the height of the parse tree of the SLP. No previous grammar representation had achieved o(n) search time. As byproducts we introduce (i) a representation of SLPs that takes 2n log2 n(1 + o(1)) bits and efficiently supports more operations than a plain array of rules; (ii) a representation for binary relations with labels supporting various extended queries; (iii) a generalization of our self-index to grammar compressors that reduce T to a sequence of terminals and nonterminals, such as Re-Pair and LZ78.
Keywords: Grammar-based Compression, Straight-Line Programs, Self-Indexes, Compressed Text Databases, Highly Repetitive Sequences, Pattern Matching, Data Structures
