Restructuring Compressed Texts without Explicit Decompression

Abstract

We consider the problem of {\em restructuring} compressed texts without explicit decompression. We present algorithms which allow conversions from compressed representations of a string $T$ produced by any grammar-based compression algorithm, to representations produced by several specific compression algorithms including LZ77, LZ78, run length encoding, and some grammar based compression algorithms. These are the first algorithms that achieve running times polynomial in the size of the compressed input and output representations of $T$. Since most of the representations we consider can achieve exponential compression, our algorithms are theoretically faster in the worst case, than any algorithm which first decompresses the string for the conversion.