Tool to apply LZW compression. Lempel-Ziv-Welch (LZW) is a lossless data compression algorithm created by Abraham Lempel, Jacob Ziv, et Terry Welch.

LZW Compression - dCode

Tag(s) : Compression

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Tool to apply LZW compression. Lempel-Ziv-Welch (LZW) is a lossless data compression algorithm created by Abraham Lempel, Jacob Ziv, et Terry Welch.

LZW encoding algorithm uses a predefined dictionary, such as 128 ASCII values, and encodes characters with their entry number in the dictionary.

Example: The dictionary is 0:A,1:B,2:C,...,25:Z and the plain text is DECODED which can be written 3,4,2,14,3,4,3 (made of 7 items) in the dictionary.

At each step, look for a substring in the dictionary, if it does not exists, the dictionary evolves and stores a new entry constituted of the last two entries found.

Example: Step 1, look for DE, which is not in the dictionary. Store DE (position 26) and save the position of D (position 3) as output.

Step 2, look for EC, which is not in the dictionary. Store EC (position 27) and save the position of E (position 4) as output. And so on with other steps 3 and 4.

Step 5, look for DE again, this time DE exists in the dictionary, go to step 6.

Step 6, look for DED, which is not in the dictionary. Store DED (position 30) and save the position of DE (position 26).

Dictionary has become 0:A,1:B,...,25:Z,26:DE,27:EC,28:CO,29:OD,DED:30

The ciphertext is made up of numbers saved for output.

Example: The ciphertext is 3,4,2,14,26,3 (made of 6 items, the message have been compressed)

LZW decoding/decryption requires to know the dictionary used and the sequence of values from the compression.

Example: The cipher text is 3,4,2,14,26,3 and the dictionary be 0:A,1:B,2:C,...,25:Z

For each value, check for the corresponding character in the dictionary.

At each step, the dictionary evolves like in the compression part (see above).

Example: Step 1: 3 corresponds to D

Step 2: 4 corresponds to E, add DE in the dictionary in position 26,

Step 3: 2 corresponds to C, add EC in the dictionary in position 27, the same for step 4

Step 5: 26 corresponds to DE, etc.

The decompressed plain text is DECODED.

Many variants exist for LZW improving the compression such as LZ77 and LZ78, LZMA, LZSS, or the algorithm Deflate. It is also interesting to combine this compression with Burrows-Wheeler or Huffman coding.

In 1987 by Abraham Lempel, Jacob Ziv, and Terry Welch

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