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Lempel-Ziv Algorithms. LZ77 (Sliding Window). Variants: LZSS (Lempel-Ziv- Storer-Szymanski); Applications: gzip, Squeeze, LHA, PKZIP, ZOO. LZ78 ( Dictionary. version of LZ77, called LZSS, and one improved version of LZ78, called LZW. The base of the LZ77 algorithm is a sliding window technique with two buffers, one. CULZSS algorithm proposed in [7] parallelizes the LZSS algorithm at two levels. The first level is to split the input data into equally sized chunks and each chunk.

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If I don’t have a match, I move to the lzsx character in the sliding window. Since the dictionary size is fixed at N the largest offset may be N – 1and the longest string matching a series of characters in the dictionary may be N characters. However KMP attempts to use some information about the string we’re attempting to find a match for and the comparisons already made in order skip some comparisons that must fail.

The source code implementing a binary tree search is contained in the file tree. I chose to implement my dictionary as a character cyclic buffer sliding window. The source code implementing a linked list search is contained in the version 0. Decoding input requires the following steps: While I was studying the algorithm, I came across some implementations that stored encoded flags in groups of eight followed by the characters or encoded strings that they represent.

If the first characters match, I check the characters that follow. In the examples I have seen, N is typically or and the maximum length allowed for matching strings is typically between 10 and 20 characters. After implementing string matches with linked listsit seemed like a wasn’t much effort to try matching using hash tables.

Don’t worry about it if I lost you on the EOF discussion, just relax, because there’s no need to handle it specially.

For example, encoding a string from a dictionary of symbols, and allowing for matches of up to 15 symbols, will require 16 bits to store an offset and a length.


Since the dictionary is a sliding window of the last characters encoded by the algorithm, the binary search tree must be updated as old characters are removed from the dictionary and new characters are added to the dictionary.

I have already experimented with some of these techniques and plan to experiment with others as time allows.

Wikipedia provides pzss description of the algorithm used to insert or remove entries from a binary search tree. Corrects an error that occurs when trying to use the default output file for decoding. Keeping the goal of a 16 bit encoded string in mind, and the above requirement for a 12 bit offset, I’m left with 4 bits to encode the string length. The source code implementing the KMP algorithm is algoritnm in the file kmp.

LZSS Compression Functions

If I encode the offset and length of a string in a 16 bit word, that leaves me with 4 bits for the length. My e-mail address is: Based on the discussion above, encoding input requires the following steps: A copy of the archives may be obtained by clicking on the links below. Any failed match results in advancing the compare to the string starting with the next character in the dictionary.

Read an uncoded string that is the length of the maximum allowable match. Here is the beginning of Dr. Seuss’s Green Eggs and Hamwith character numbers at the beginning of lines for convenience.

The pointers are for each node’s left child, right child, and parent.

The worst case occurs when the binary search tree resembles a linked list and each node only has one child. KMP is smart enough to skip ahead and resume by comparing string[0] to dictionary[3]which happens to be a match in this example. In my implementation, all pointers list head and next are int indices into the sliding window dictionary.

Shift a copy of the symbols written to the decoded output into the dictionary. Uses bitfile library for reading and writing encoded files. Since the minimum encoded length is three, I am able to add three to the 4 bit value stored in the length field, resulting in encoded string lengths of 3 to Archived on February 3, Storer and Szymanski observed that individual unmatched symbols or matched strings of one or two symbols take up more space to encode than they do to leave uncoded.


Information on downloading the source code for all of my LZSS implementations may be found here. No searching is required. If the flag indicates an encoded string:. The size of the hash table used to search the dictionary is now based on the size of the dictionary.

The larger Nthe longer it takes to search the whole dictionary for a match and the more bits will be required to store the offset into the dictionary. The rest of this section documents some of what I have tried so far. Added versions of the encode and decode routines that accept files pointers rather than file names.

Index O’Stuff

However, as Storer and Szymanski observed, it only takes 1 byte to write out characters that match dictionary strings 0 or 1 character long, and 2 bytes to write out characters that match dictionary strings 2 characters long.

This implementation might be useful to those developing on systems that do algoritnm include a file system. There’s only one additional complication. The preprocessing generates a look-up table used to determine how far back from algorihhm failed comparison, the algorithm must go to resume comparisons.

The first search I implemented was a sequential search. A symbol dictionary would require 9 bits to represent all possible offsets.

By using this site, you agree to the Terms of Use and Privacy Policy. For my implementation alphabet size is and M is 3, so the hash table would need 3 entries that’s 16, for anybody without a calculator. The encoding process requires that a the dictionary is searched for matches to algorihtm string to be encoding.