In-place differential file compression

Algorithms for in-place differential file compression were presented, 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 is used. From a theoretical point of view, an optimal solution (best possible compression) to this problem is known to be NP-hard. The constant-factor approximation algorithm is presented. 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. The experiments indicate that these algorithms compare favorably with traditional approaches, even those that are not in-place, while at the same time having low encoding complexity and extremely low decoding complexity.