Bidirectional delta files

摘要

A bidirectional delta file is a novel concept, introduced in this paper, for a two way delta file. Previous work focuses on single way differential compression called forwards and backwards delta files. Here we suggest to efficiently combine them into a single file so that the combined file is smaller than the combination of the two individual ones. Given the bidirectional delta file of two files S and T and the original file S, one can decode it in order to produce T. The same bidirectional delta file is used together with the file T in order to reconstruct S. This paper presents two main strategies for producing an efficient bidirectional delta file in terms of the memory storage it requires; a quadratic time, optimal, dynamic programming algorithm, and a linear time, greedy algorithm. Although the dynamic programming algorithm often produces better results than the greedy algorithm, it is impractical for large files, and it is only used for theoretical comparisons. Experiments between the implemented algorithms and the traditional way of using both forwards and backwards delta files are presented, comparing their processing time and their compression performance. These experiments show memory storage savings of about 25% using this bidirectional delta approach as compared to the compressed delta file constructed using the traditional way, while preserving approximately the same processing time for decoding.