Damaged BZip Files Are Difficult to Repair

Authors:
Christian Hundt;Ulf Ochsenfahrt
Affiliations:
Fakultät für Informatik und Elektrotechnik, Universität Rostock, Germany;Fakultät für Informatik und Elektrotechnik, Universität Rostock, Germany
Venue:
COCOON '08 Proceedings of the 14th annual international conference on Computing and Combinatorics
Year:
2008

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Abstract

bzipis a program written by Julian Seward that is often used under Unix to compress single files. It splits the file into blocks which are compressed individually using a combination of the Burrows-Wheeler-Transformation, the Move-To-Front algorithm, Huffman and Runlength encoding. The author himself stated that compressed blocks that are damaged, i.e., part of which are lost, are essentially non-recoverable. This paper gives a formal proof that this is indeed true: focusing on the Burrows-Wheeler-Transformation, the problem of completing a transformed string, such that the decoded string obeys certain file format restrictions, is NP-hard.