Abstract: The invention provides a method and apparatus for finding a longest and closest matching string in a history buffer prior to a current string. A search algorithm in accordance with the invention first tries to find the longest matching old string (MOS) in the history buffer as its major task, and in a case where two MOS's are found to have the same longest matching length, the search algorithm tries to select the MOS closest to the current position as its minor task. Linked lists are constructed as searching progresses to speed the search process. The linked lists define a fast-path array which points to all locations within the history buffer containing a specified code sequence. Pointers to locations outside the history buffer are optionally removed and their space returned to memory free space.

Abstract: In a block compressed data file, desired given non-compressed data is identified in a subset of a compressed data file and only the subset of the compressed data file containing the desired data is decompressed. Three operating modes according to application access level permission are allowed. The direct-read mode is used for application read-only permission. The file shadowing mode is used for application read/write permission when it is likely that the application will only read from the file without writing to it. The file putback mode is used for application read/write permission when it is likely the application will write to the file. Data caching of decompressed data blocks from a file avoids repetitious decompressing of a data block requested at different times. Block access of the compressed data file occurs through a table of offsets, either stored in the compressed data file itself or constructed from scratch.

Abstract: The invention provides a method and apparatus for finding a longest and closest matching string in a history buffer prior to a current string. A search algorithm in accordance with the invention first tries to find the longest matching old string (MOS) in the history buffer as its major task, and in a case where two MOS's are found to have the same longest matching length, the search algorithm tries to select the MOS closest to the current position as its minor task. Linked lists are constructed as searching progresses to speed the search process. The linked lists define a fast-path array which points to all locations within the history buffer containing a specified code sequence. Pointers to locations outside the history buffer are optionally removed and their space returned to memory free space.

Abstract: An apparatus and method are disclosed for finding a target string in a history buffer, where the found target string matches a given current string to a maximum practical length. A presorted array of array entries (SP) is defined where each entry uniquely identifies a value and a location of a respective string-start byte pair in the history buffer. The array entries are sorted primarily upon their string-start byte-pair values and secondarily upon their pointed-to locations. A direct lookup table (DLT) is further provided, indexable by each possible string-start byte pair that may appear in the history buffer. The DLT is used to locate a first array entry for a given string-start byte pair. To find a longest matching target string, the first two bytes of the current string are used as an index into the direct lookup table, and the given table entry is then used as an index into the pre-sorted SP array. The corresponding array entry is used as an index to a first target string in the buffer.

Abstract: A cooperating data compressor, compressed data format, and data decompressor. The compressor compresses an input data block (HB) to a compressed data block having the format. The decompressor decompresses the compressed data block to restore the original data block. The compressor has a direct lookup table (DLT) of 2.sup.8.times.N entries, each indexable by N bytes at a current HB location and identifying a target HB location. The compressor determines whether a target string at the target HB location matches a current string at the current HB location. If they do not match, the compressor outputs a literal representing a datum at the current location. If they match, the compressor outputs a vector from the current location to the target string. Compression speed is maximized by the direct addressing of the DLT by the current N bytes in the HB.