LZW data compression and decompression apparatus and method using grouped data characters to reduce dictionary accesses

In a data compressor, the input stream of data characters is formed into one or more data character groupings where each grouping comprises a predetermined number of the data characters (grouped character) followed by one or more of the input data characters. The formed input is compared to similarly configured stored strings until a longest match is determined. Each stored string has a code associated therewith and the code of the longest match is output by the compressor. An extended string is stored comprising the longest match extended by the data character that caused the mismatch. A next compression cycle begins with a grouped character comprising the data character that caused the mismatch concatenated by one less than the predetermined number of the next following data characters. In one embodiment, data character strings comprise an initial grouped character followed by as many data characters as can be matched. In another embodiment, a string is comprised of consecutive grouped characters followed by one or more data characters up to a maximum of one less than the predetermined number. In this embodiment, when extension of a string for storage would result in the predetermined number of data characters following the consecutive grouped characters, the predetermined number of data characters is appended to the consecutive grouped characters as a further grouped character. In this embodiment two dictionaries are utilized, one for storing strings of consecutive grouped characters and the other for storing data character extensions of strings stored in the first dictionary.