LZW compression is a lossless data compression algorithm that replaces repeated sequences of bytes in a file with a shorthand code. The algorithm builds a dictionary of strings and their associated code as it encodes the input data. It parses the input into strings, looks up each string in its dictionary, outputs the code for matching strings, and adds new strings to the dictionary. For decoding, it expands the codes back into strings using the reverse dictionary lookup process. The example shows how the input string "AABABBBABAABABBBABBABB" is encoded into codes representing the parsed substrings.

1. LZW COMPRESSION
BY:PRASANNA KUMAR G
CSE-2 ; 21C070

2. LEMPEL ZE ENCODING
IT IS A LOSSLESS COMPRESSION.
ALGORITHM:
LZWencoding()
enter all letters to the dictionary;
initialize string s to the first letter from input;
while any input is left
real symbol c;
if s+c is in the dictionary
s=s+c;
else output codeword(s);
enter s+c to the dictionary;;
s=c;
output codeword(s);

3. EXPLANATION WITH EXAMPLE (ENCODING)
EG:AABABBBABAABABBBABBABB A=0;B=1
A|AB|ABB|B|ABA|ABAB|BB|ABBA|BB|
1 2 3 4 5 6 7 8 9
POSITION 1 2 3 4 5 6 7 8 9
SEQUENCE A AB ABB B ABA ABAB BB ABBA BB
NUMERICAL REP 0A 1B 2B 0B 2A 5B 4B 3A 7
CODE 0000 0011 0101 0001 0100 1011 1001 0110 0111

4. LEMPEL ZE DECODING
ALGORITHM:
LZWdecoding()
Enter all the source letters to the dictionary;
Read priorCodeword and output one symbol corresponding to it;
While codewords are still left
Read codeword;
priorString = string(priorCodeWord);
If codeword is in the dictionary;
Enter in dictionary priorString + firstSymbol(String(codeword));
Output string(codeword);
Else
Enter in directory priorString + firstSymbol(priorString);
Output priorString+firstSymbol(priorString);
PriorCodeword=codeword;

5. EXPLANATION WITH EXAMPLE DECODING
EG:112613
DECODER DICTIONARY
INPUT OUTPUT INDEX ENTRY
1 a
2 b
3 c
4 d
1 a
1 a 5 aa
2 b 6 ab
6 ab 7 ba
1 a 8 aba
3 c 9 ac

6. THANK YOU…