Achieving the Capability of a Dictionary Based Data Compression Technique OLZW and its Variants

In this paper, the capability of a lossless dictionary based data compression technique OLZW and its variants MOLZW, MOLZWV1 and MOLZWV2 have been discussed with respect to LZW technique. In OLZW technique, initially the dictionary is empty. If the phrase to be encoded is already in dictionary, it encoded with the code in the dictionary. Length of code is not fixed and is calculated by the highest index in the dictionary. Otherwise, ASCII code is used to encode the same. The technique eliminates some limitations of LZW technique and works very well for small size files particularly. MOLZW is the modified form of OLZW where additionally a checking is performed to determine the dictionary is full or not before insertion of new entry into the dictionary. When the dictionary gets full, the least recently used dictionary phrase is deleted. In MOLZWV1, no phrases of dictionary are deleted. But, phrases added to the dictionary are all the string formed by concatenation of previous match and prefix of current match including current match itself. MOLZWV2 combines both the above MOLZW and MOLZWV1 techniques. The experimental results show that OLZW technique offers better compression rates for small size files particularly than its counter parts and MOLZW techniques work well not only for small size files but also for large size files.