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LZW Compression

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

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LZW Compression -

Tag(s) : Compression

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# LZW Compression

## LZW Decoder

 Dictionary size 128 (ASCII values) 26 (only for letters from A to Z) 10 (only digits from 0 to 9)

## LZW Encoder

 Dictionary size 128 (ASCII values) 26 (only for letters from A to Z) 10 (only digits from 0 to 9)
 Output display Table of values (decimal) Binary Code Dictionary Content Compression Ratio

### How to compress/encrypt using LZW compression?

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)

### How to decompress/decrypt with LZW compression?

LZW decompression/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.

### How to recognize LZW code?

The ciphered message (generally in binary) is rather short (compressed). The first values are generally simple values of the dictionary (non compressed), usually ASCII.

LZW is used in several file formats like GIF or TIFF.

### What are the variants of the LZW?

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.

### When LZW was invented?

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

## Source code

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