An apparatus and method are disclosed for converting an input data character stream into a variable length encoded data stream in a data compression system. The data compression system includes a history array. The history array has a plurality of entries and each entry of the history array is for storing a portion of the input data stream. The method for converting the input data character stream includes the following steps. Performing a search in a history array for the longest data string which matches the input data string. If the matching data string is found within the history buffer, the next step includes encoding the longest matching data string found by appending to the encoded data stream a tag indicating the longest matching data string was found and a string substitution code. If the matching data string is not found within the history array, the next step includes encoding the first character of the input data string by appending to the encoded data stream a raw data tag indicating that no matching data string was found and the first character of the input data string.

Data compression apparatus and method

An apparatus and method for converting an input data character stream into a variable length encoded data stream in a data compression system. A sliding window data compression algorithm is combined with Huffman encoding on the strings and raw bytes. The Huffman table, in a compressed form, is prepended to the encoded output data. The Huffman codes representing the shortest strings encode both the string length and part of the string offset. Assigning Huffman codes to represent the combined length and offset allows the use of a smaller sliding window size without sacrificing compression ratio. The smaller window size allows implementations in software and hardware to minimize memory usage, thus reducing cost.

Data compression apparatus and method using matching string searching and Huffman encoding

i ACKNOWLEDGMENTS ii TABLE OF CONTENTS iii LIST OF TABLES vi LIST OF FIGURES vii LIST OF DEFINITIONS viii

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Specifying and simulating the externally observable behavior of modules

A new pattern matcher has been implemented for the symbolic manipulation program SMP. The pattern matcher correctly treats associative and commutative functions, functions with arbitrary symmetries under permutations of their arguments, and patterns subject to arbitrary logical predicates. A pattern is said to match a candidate instance if the set of expressions it represents is a superset of those represented by the instance (either or both may contain generic symbols which match possibly infinite sets of expressions). The matcher is primarily syntactic, but is able to find matches for syntactically different expressions which become literally equivalent upon replacing the generic symbols in the pattern by their bindings to subexpressions of the instance and simplifying the result. The bindings must be determined by literal comparison with the instance, not by solving equations.

The SMP Pattern Matcher

An expression analysis package for REDUCE 2 is presented. This package, completely written in Standard LISP, can be considered as an extension of the algebraic mode. It allows to interactively dismantled and/or modify the last output expression as it is desplayed or printed. An interface with the Code Optimizer makes on line construction of optimized FORTRAN programs possible.

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Richard M. Cowan

Papers

Hashing, the key to rapid pattern matching