Optimum String Match Choices in LZSS

Abstract

The LZ77 and LZ78 compression algorithms perform a greedy choice when looking for the next string of input symbols to match. That is, the longest string of symbols which is found in the current dictionary is chosen as the next match. Many variations of LZ77 and LZ78 have been proposed; some of these attempt to improve compression by sometimes choosing a non-maximal string, if it appears that such a choice might improve the overall compression ratio. These approaches make this decision based upon local criteria in an attempt to minimise the number of strings matched. In this paper we present an algorithm which computes a set of matches designed to minimize the number of bits output, not necessarily the number of strings matched.