Word-based block-sorting text compression

Block sorting is an innovative compression mechanism introduced in by M. Burrows and D.J. Wheeler (1994). It involves three steps: permuting the input one block at a time through the use of the Burrows-Wheeler transform (BWT); applying a move-to-front (MTF) transform to each of the permuted blocks; and then entropy coding the output with a Huffman or arithmetic coder. Until now, block-sorting implementations have assumed that the input message is a sequence of characters. In this paper, we extend the block-sorting mechanism to word-based models. We also consider other transformations as an alternative to MTF, and are able to show improved compression results compared to MTF. For large text files, the combination of word-based modelling, BWT and MTF-like transformations allows excellent compression effectiveness to be attained within reasonable resource costs.