How does lzw compression work

For instance, the memory problem. Most LZW programs handle this by taking advantage of the redundant nature of the code table.

For example, look at line 29 in Table , where code is defined to be ainl. This pattern always holds: every code can be expressed as a previous code plus one new character. The execution time of the compression algorithm is limited by searching the code table to determine if a match is present. As an analogy, imagine you want to find if a friend's name is listed in the telephone directory. The catch is, the only directory you have is arranged by telephone number, not alphabetical order.

This requires you to search page after page trying to find the name you want. This inefficient situation is exactly the same as searching all codes for a match to a specific character string. The answer: organize the code table so that what you are looking for tells you where to look like a partially alphabetized telephone directory. In other words, don't assign the codes to sequential locations in memory. Rather, divide the memory into sections based on what sequences will be stored there.

The code table should be organized so that the " x " indicates where to starting looking. There are many schemes for this type of code table management, and they can become quite complicated. This brings up the last comment on LZW and similar compression schemes: it is a very competitive field.

While the basics of data compression are relatively simple, the kinds of programs sold as commercial products are extremely sophisticated. Companies make money by selling you programs that perform compression, and jealously protect their trade-secrets through patents and the like. Don't expect to achieve the same level of performance as these programs in a few hours work.

Smith, Ph. Suppose you once again want to compress the following string of text: the quick brown fox jumps over the lazy dog. In , Terry Welch was working on a compression algorithm for high-performance disk controllers. He developed a rather simple algorithm that was based on the LZ78 algorithm and that is now called LZW. 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.

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 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. Skip to content. Change Language. Related Articles. Computer Network Fundamentals. Physical layer. Data Link layer.

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