Abstract
Lossless data compression has been widely studied in computer science. One of the most widely used lossless data compressions is Lempel-Ziv (LZ) 77 parsing, which achieves a high compression ratio. Bidirectional (a.k.a. macro) parsing is a lossless data compression technique that computes a sequence of phrases copied from another substring (target phrase) on either the left or the right position in an input string. Navarro et al. recently showed that a large gap may exist between the size of the smallest bidirectional parse of a given string and that of LZ77 parsing. In addition, finding the smallest bidirectional parse of a given text is NP-complete. Several variants of bidirectional parsing algorithm have been proposed thus far, but no prior work for bidirectional parsing algorithm has achieved compression that is at least as good as LZ77 parsing for any string. In this paper, we present the first practical bidirectional parsing algorithm named LZ77 parsing with right reference (LZRR), in which the number of LZRR phrases is theoretically guaranteed to be not larger than the number of LZ77 phrases. Experimental results using benchmark strings show that the number of LZRR phrases is approximately six percent smaller than that of LZ77 phrases.
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