Faster Queries on BWT-runs Compressed Indexes
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Although a significant number of compressed indexes for highly repetitive strings have been proposed thus far, developing compressed indexes that support faster queries remains a challenge. Run-length Burrows-Wheeler transform (RLBWT) is a lossless data compression by a reversible permutation of an input string and run-length encoding, and it has become a popular research topic in string processing. Recently, Gagie et al. presented r-index, an efficient compressed index on RLBWT whose space usage does not depend on text length. In this paper, we present a new compressed index on RLBWT, which we call r-index-f, in which r-index is improved for faster locate queries. We introduce a novel division of RLBWT into blocks, which we call balanced BWT-sequence as follows: the RLBWT of a string is divided into several blocks, and a parent-child relationship between each pair of blocks is defined. In addition, we present a novel backward search algorithm on the balanced BWT-sequences, resulting in faster locate queries of r-index-f. We also present new algorithms for solving the queries of count query, extract query, decompression and prefix search on r-index-f.
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