An approximation to the greedy algorithm for differential compression
IBM Journal of Research and Development - Spintronics
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We present algorithms for in-place differential file compression, where a target file of size n is compressed with respect to a source file of size m using no additional space; that is, the space for the source file is overwritten by the decompressed target file so that at no time is more than a total of MAX(m,n) space used. From a theoretical point of view, an optimal solution (best possible compression) to this problem is known to be NP-hard. We begin by presenting a constant-factor approximation algorithm. We then present fast practical algorithms based on sliding window data compression. Although NP-hardness implies that these algorithms cannot always be optimal, the asymptotic optimality for an information source of sliding window methods along with their ability for constant-factor approximation is evidence that they should work well for this problem in practice. We present experiments that indicate that these algorithms compare favorably with traditional practical approaches, even those that are not in-place, while at the same time having low encoding complexity and extremely low decoding complexity.