Variable-Length codes for space-efficient grammar-based compression

  • Authors:

  • Yoshimasa Takabatake;Yasuo Tabei;Hiroshi Sakamoto

  • Affiliations:

  • Kyushu Institute of Technology, Iizuka-shi, Fukuoka, Japan;ERATO Minato Project, Japan Science and Technology Agency, Sapporo, Japan;Kyushu Institute of Technology, Iizuka-shi, Fukuoka, Japan,PRESTO JST, Kawaguchi, Saitama, Japan

  • Venue:
  • SPIRE'12 Proceedings of the 19th international conference on String Processing and Information Retrieval
  • Year:
  • 2012

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Abstract

Dictionary is a crucial data structure to implement grammar-based compression algorithms. Such a dictionary should access any codes in O(1) time for an efficient compression. A standard dictionary consisting of fixed-length codes consumes a large amount of memory of 2n logn bits for n variables. We present novel dictionaries consisting of variable-length codes for offline and online grammar-based compression algorithms. In an offline setting, we present a dictionary of at most min {nlogn+2n+o(n), 3nlogσ(1+o(1))} bits of space where σ n. In an online setting, we present a dictionary of at most $\frac{7}{4}n\log n + 4n + o(n)$ bits of space for a constant alphabet and unknown n. Experiments revealed that memory usage in our dictionary was much smaller than that of state-of-the-art dictionaries.