LZW Data Compression. Dr. Dobbs Journal. October, 1. 98. 9Note I have an updated article on LZW posted here. Please check out the new article and tell me what you think. I hope it improves on this post and makes LZW easier to understand. Thanks to Jan Hakenberg for correction of a couple of errorsIn Figure 4, the values for new table entries 2. Thanks to David Littlewood for pointing out the missing line of pseudocde in Figure 6. Thanks to Joe Snyder for pointing out a line where a macro should replace a hard coded constant. Any programmer working on mini or microcomputers in this day and age should have at least some exposure to the concept of data compression. In MS DOS world, programs like ARC, by System Enhancement Associates, and PKZIP, by PKware are ubiquitous. ARC has also been ported to quite a few other machines, running UNIX, CPM, and so on. CPM users have long had SQ and USQ to squeeze and expand programs. Unix users have the COMPRESS and COMPACT utilities. Web security tool to make fuzzing at HTTP inputs, made in C with libCurl. You can do brute force passwords in auth forms directory disclosure use PATH list to. Endianness refers to the sequential order in which bytes are arranged into larger numerical values, when stored in computer memory or secondary storage, or when. INTERACTIVE CONTROL. Yet the data compression techniques used in these programs typically only show up in two places file transfers over phone lines, and archival storage. Data compression has an undeserved reputation for being difficult to master, hard to implement, and tough to maintain. In fact, the techniques used in the previously mentioned programs are relatively simple, and can be implemented with standard utilities taking only a few lines of code. This article discusses a good all purpose data compression technique Lempel Ziv Welch, or LZW compression. The routines shown here belong in any programmers toolbox. For example, a program that has a few dozen help screens could easily chop 5. K bytes off by compressing the screens. Convert Hexdump To Binary Windows Updates' title='Convert Hexdump To Binary Windows Updates' />Or 5. K bytes of software could be distributed to end users on a single 3. K byte floppy disk. Highly redundant database files can be compressed down to 1. Once the tools are available, the applications for compression will show up on a regular basis. LZW Fundamentals. The original Lempel Ziv approach to data compression was first published in in 1. Terry Welchs refinements to the 1. The algorithm is surprisingly simple. In a nutshell, LZW compression replaces strings of characters with single codes. It does not do any analysis of the incoming text. Instead, it just adds every new string of characters it sees to a table of strings. Do you want to control your TV and your devices smart Take a look at this Kickstarter campaign. I try to get root access on my Philips PFL9703 TV. Wireshark is one of those programs that many network managers would love to be able to use, but they are often prevented from getting what they would like from. Download software utilities for Onset HOBO data logging products online including the HOBOnode Viewer Utility. Compression occurs when a single code is output instead of a string of characters. The code that the LZW algorithm outputs can be of any arbitrary length, but it must have more bits in it than a single character. The first 2. 56 codes when using eight bit characters are by default assigned to the standard character set. The remaining codes are assigned to strings as the algorithm proceeds. The sample program runs as shown with 1. This means codes 0 2. Compression. The LZW compression algorithm in its simplest form is shown in Figure 1. A quick examination of the algorithm shows that LZW is always trying to output codes for strings that are already known. And each time a new code is output, a new string is added to the string table. Routine LZWCOMPRESSCODE STRING get input character. WHILE there are still input characters DO    CHARACTER get input character    IF STRINGCHARACTER is in the string table then        STRING STRINGcharacter    ELSE        output the code for STRING        add STRINGCHARACTER to the string table        STRING CHARACTER    END of IFEND of WHILEoutput the code for STRING The Compression Algorithm. Figure 1. A sample string used to demonstrate the algorithm is shown in Figure 2. The input string is a short list of English words separated by the character. Stepping through the start of the algorithm for this string, you can see that the first pass through the loop, a check is performed to see if the string W is in the table. Since it isnt, the code for is output, and the string W is added to the table. Since we have 2. 56 characters already defined for codes 0 2. After the third letter, E, has been read in, the second string code, WE is added to the table, and the code for letter W is output. This continues until in the second word, the characters and W are read in, matching string number 2. In this case, the code 2. The process continues until the string is exhausted and all of the codes have been output. Input String WEDWEWEEWEBWETCharacter Input. Code Output. New code value. New StringW2. 56WEW2. WEDE2. 58. EDD2. DWE2. WEE2. EWEE2. 60. 26. WEEW2. EWEB2. 57. WEBB2. BWET2. 60. WETEOFTThe Compression Process. Figure 2. The sample output for the string is shown in Figure 2 along with the resulting string table. As can be seen, the string table fills up rapidly, since a new string is added to the table each time a code is output. In this highly redundant input, 5 code substitutions were output, along with 7 characters. If we were using 9 bit codes for output, the 1. Of course, this example was carefully chosen to demonstrate code substitution. In real world examples, compression usually doesnt begin until a sizable table has been built, usually after at least one hundred or so bytes have been read in. Decompression. The companion algorithm for compression is the decompression algorithm. It needs to be able to take the stream of codes output from the compression algorithm, and use them to exactly recreate the input stream. One reason for the efficiency of the LZW algorithm is that it does not need to pass the string table to the decompression code. The table can be built exactly as it was during compression, using the input stream as data. This is possible because the compression algorithm always outputs the STRING and CHARACTER components of a code before it uses it in the output stream. This means that the compressed data is not burdened with carrying a large string translation table. Routine LZWDECOMPRESSCODE Read OLDCODEoutput OLDCODEWHILE there are still input characters DO    Read NEWCODE    STRING get translation of NEWCODE    output STRING    CHARACTER first character in STRING    add OLDCODE CHARACTER to the translation table    OLDCODE NEWCODEEND of WHILE The Decompression Algorithm. Figure 3. The algorithm is shown in Figure 3. Just like the compression algorithm, it adds a new string to the string table each time it reads in a new code. All it needs to do in addition to that is translate each incoming code into a string and send it to the output. Figure 4 shows the output of the algorithm given the input created by the compression earlier in the article. The important thing to note is that the string table ends up looking exactly like the table built up during compression. The output string is identical to the input string from the compression algorithm. Note that the first 2. Input Codes W E D 2. E 2. 60 2. 61 2. 57 B 2. TInputNEWCODEOLDCODESTRINGOutput. CHARACTERNew table entryWWW2. WEWEE2. 57 WEDEDD2. ED2. 56. DW2. 59 DE2. Install Ftp Server On Centos 5 Desktop. EE2. 60 WE2. 60. EWE2. E2. EE2. 62 WEE2. WEW2. 63 EWB2. BB2. WEB2. 60. BWE2. 65 BT2. TT2. 66 WETThe Decompression Process. Figure 4. The Catch. Unfortunately, the nice simple decompression algorithm shown in Figure 4 is just a little too simple. There is a single exception case in the LZW compression algorithm that causes some trouble to the decompression side.