Lz77 (sliding window)
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The document describes the LZ77 data compression algorithm. It uses a sliding window approach where the text is searched for matches within a search buffer. When a match is found, it is encoded as an offset and length rather than encoding the full text. This allows duplication or repetition within the text to be replaced with smaller and more efficient offset/length codes. An example is provided to demonstrate how the text "sir sid eastman easily teases sea sick seals" would be encoded using this approach.
Lz77 (sliding window)
- 1. LZ77 (Sliding Window) Data Compression
- 2. LZ77 (Sliding Window) • Search Buffer (Thousands of bytes long) • Look ahead Buffer (Tens of bytes long) • Text should be read from right to left in the buffer • The encoder check the symbol in Look ahead buffer with search buffer considered here as dictionary • Offset is the count from the dividing line between search and look ahead buffer. • Length is the span of string matched in look ahead buffer corresponding to search buffer • Symbol is the next character after the string match • The encoder select the longest match or if they are all the same length the last one found because the last one will be leaving the search buffer soon.
- 3. LZ77 (Sliding Window) Search Buffer (Dictionary) Look ahead Buffer Dividing Line Text Movement Input Text : sir˽sid˽eastman˽easily˽teases˽sea˽sick˽seals Note:- ˽ It represents space
- 4. LZ77 (Sliding Window) sir˽sid˽eastman˽easily˽teases˽sea˽sick˽se als sir˽sid˽eastman˽easily˽teases˽sea˽sick˽se als Search Buffer (Dictionary) Size = 30 symbols (app) Look ahead Buffer Size = 10 symbols (app) Dividing Line Text Movement Encoding scheme (offset, length, next symbol)
- 5. LZ77 (Sliding Window) s ir˽sid˽eastman˽easily˽teases˽sea˽sick˽seal s ir˽sid˽eastman˽easily˽teases˽sea˽sick˽seal s Search Buffer (Dictionary) Look ahead Buffer Dividing Line Text Movement Encoding scheme: (0, 0, s)
- 6. LZ77 (Sliding Window) si r˽sid˽eastman˽easily˽teases˽sea˽sick˽seal s r˽sid˽eastman˽easily˽teases˽sea˽sick˽seal s Search Buffer (Dictionary) Look ahead Buffer Dividing Line Text Movement Encoding scheme: (0, 0, s) (0, 0, i )
- 7. LZ77 (Sliding Window) sir ˽sid˽eastman˽easily˽teases˽sea˽sick˽seals˽sid˽eastman˽easily˽teases˽sea˽sick˽seals Search Buffer (Dictionary) Look ahead Buffer Dividing Line Text Movement Encoding scheme: (0, 0, s) (0, 0, i ) (0, 0, r )
- 8. LZ77 (Sliding Window) sir ˽ sid˽eastman˽easily˽teases˽sea˽sick˽sealssid˽eastman˽easily˽teases˽sea˽sick˽seals Search Buffer (Dictionary) Look ahead Buffer Dividing Line Text Movement Encoding scheme: (0, 0, s) (0, 0, i ) (0, 0, r ) (0, 0, ˽ )
- 9. LZ77 (Sliding Window) sir ˽sid ˽eastman˽easily˽teases˽sea˽sick˽seals˽eastman˽easily˽teases˽sea˽sick˽seals Search Buffer (Dictionary) Look ahead Buffer Dividing Line Text Movement Encoding scheme: (0, 0, s) (0, 0, i ) (0, 0, r ) (0, 0, ˽ ) (4, 2, d )
- 10. LZ77 (Sliding Window) sir ˽sid ˽e astman˽easily˽teases˽sea˽sick˽sealsastman˽easily˽teases˽sea˽sick˽seals Search Buffer (Dictionary) Look ahead Buffer Dividing Line Text Movement Encoding scheme: (0, 0, s) (4, 1, e) (0, 0, i ) (0, 0, r ) (0, 0, ˽ ) (4, 2, d )
- 11. LZ77 (Sliding Window) sir ˽sid ˽ea stman˽easily˽teases˽sea˽sick˽sealsstman˽easily˽teases˽sea˽sick˽seals Search Buffer (Dictionary) Look ahead Buffer Dividing Line Text Movement Encoding scheme: (0, 0, s) (4, 1, e) (0, 0, i ) (0, 0, a ) (0, 0, r ) (0, 0, ˽ ) (4, 2, d )
- 12. LZ77 (Sliding Window) sir ˽sid ˽east man˽easily˽teases˽sea˽sick˽sealsman˽easily˽teases˽sea˽sick˽seals Search Buffer (Dictionary) Look ahead Buffer Dividing Line Text Movement Encoding scheme: (0, 0, s) (4, 1, e) (0, 0, i ) (0, 0, a ) (0, 0, r ) (10 ,1, t ) (0, 0, ˽ ) (4, 2, d ) Take last occurrence of s and not (6 ,1, t ) if length is same for both
- 13. LZ77 (Sliding Window) sir ˽sid ˽eastm an˽easily˽teases˽sea˽sick˽sealsan˽easily˽teases˽sea˽sick˽seals Search Buffer (Dictionary) Look ahead Buffer Dividing Line Text Movement Encoding scheme: (0, 0, s) (4, 1, e) (0, 0, i ) (0, 0, a ) (0, 0, r ) (10 ,1, t ) (0, 0, ˽ ) (0 ,0, m ) (4, 2, d )
- 14. LZ77 (Sliding Window) sir ˽sid ˽eastman ˽easily˽teases˽sea˽sick˽seals˽easily˽teases˽sea˽sick˽seals Search Buffer (Dictionary) Look ahead Buffer Dividing Line Text Movement Encoding scheme: (0, 0, s) (4, 1, e) (0, 0, i ) (0, 0, a ) (0, 0, r ) (10 ,1, t ) (0, 0, ˽ ) (0 ,0, m ) (4, 2, d ) (4, 1, n )
- 15. LZ77 (Sliding Window) sir ˽sid ˽eastman˽easi ly˽teases˽sea˽sick˽sealsly˽teases˽sea˽sick˽seals Search Buffer (Dictionary) Look ahead Buffer Dividing Line Text Movement Encoding scheme: (0, 0, s) (4, 1, e) (8, 4, i) (0, 0, i ) (0, 0, a ) (0, 0, r ) (10 ,1, t ) (0, 0, ˽ ) (0 ,0, m ) (4, 2, d ) (4, 1, n )
- 16. LZ77 (Sliding Window) sir ˽sid ˽eastman˽easil y˽teases˽sea˽sick˽sealsy˽teases˽sea˽sick˽seals Search Buffer (Dictionary) Look ahead Buffer Dividing Line Text Movement Encoding scheme: (0, 0, s) (4, 1, e) (8, 4, i) (0, 0, i ) (0, 0, a ) (0, 0, l ) (0, 0, r ) (10 ,1, t ) (0, 0, ˽ ) (0 ,0, m ) (4, 2, d ) (4, 1, n )
- 17. LZ77 (Sliding Window) sir ˽sid ˽eastman˽easily ˽teases˽sea˽sick˽seals˽teases˽sea˽sick˽seals Search Buffer (Dictionary) Look ahead Buffer Dividing Line Text Movement Encoding scheme: (0, 0, s) (4, 1, e) (8, 4, i) (0, 0, i ) (0, 0, a ) (0, 0, l ) (0, 0, r ) (10 ,1, t ) (0, 0, y ) (0, 0, ˽ ) (0 ,0, m ) (4, 2, d ) (4, 1, n )
- 18. LZ77 (Sliding Window) sir ˽sid ˽eastman˽easily˽t eases˽sea˽sick˽sealseases˽sea˽sick˽seals Search Buffer (Dictionary) Look ahead Buffer Dividing Line Text Movement Encoding scheme: (0, 0, s) (4, 1, e) (8, 4, i) (0, 0, i ) (0, 0, a ) (0, 0, l ) (0, 0, r ) (10 ,1, t ) (0, 0, y ) (0, 0, ˽ ) (0 ,0, m ) (19, 1, t) (4, 2, d ) (4, 1, n )
- 19. LZ77 (Sliding Window) sir ˽sid ˽eastman˽easily˽tease s˽sea˽sick˽sealss˽sea˽sick˽seals Search Buffer (Dictionary) Look ahead Buffer Dividing Line Text Movement Encoding scheme: (0, 0, s) (4, 1, e) (8, 4, i) (0, 0, i ) (0, 0, a ) (0, 0, l ) (0, 0, r ) (10 ,1, t ) (0, 0, y ) (0, 0, ˽ ) (0 ,0, m ) (19, 1, t) (4, 2, d ) (4, 1, n ) (16, 3, e)
- 20. LZ77 (Sliding Window) sir˽sid˽eastman˽easily˽teases˽ sea˽sick˽sealssea˽sick˽seals Search Buffer (Dictionary) Look ahead Buffer Dividing Line Text Movement Encoding scheme: (0, 0, s) (4, 1, e) (8, 4, i) (30,1, ˽) (0, 0, i ) (0, 0, a ) (0, 0, l ) (0, 0, r ) (10 ,1, t ) (0, 0, y ) (0, 0, ˽ ) (0 ,0, m ) (19, 1, t) (4, 2, d ) (4, 1, n ) (16, 3, e)
- 21. LZ77 (Sliding Window) sir˽sid˽eastman˽easily˽teases˽sea ˽sick˽seals˽sick˽seals Search Buffer (Dictionary) Look ahead Buffer Dividing Line Text Movement Encoding scheme: (0, 0, s) (4, 1, e) (8, 4, i) (30,1, ˽) (0, 0, i ) (0, 0, a ) (0, 0, l ) (4, 2, a) (0, 0, r ) (10 ,1, t ) (0, 0, y ) (0, 0, ˽ ) (0 ,0, m ) (19, 1, t) (4, 2, d ) (4, 1, n ) (16, 3, e)
- 22. LZ77 (Sliding Window) sir˽sid˽eastman˽easily˽teases˽sea˽si c k˽sealsk˽seals Search Buffer (Dictionary) Look ahead Buffer Dividing Line Text Movement Encoding scheme: (0, 0, s) (4, 1, e) (8, 4, i) (30,1, ˽) (0, 0, i ) (0, 0, a ) (0, 0, l ) (4, 2, a) (0, 0, r ) (10 ,1, t ) (0, 0, y ) (30, 3,c) (0, 0, ˽ ) (0 ,0, m ) (19, 1, t )
- 23. LZ77 (Sliding Window) sir˽sid˽eastman˽easily˽teases˽sea˽si ck ˽seals˽seals Search Buffer (Dictionary) Look ahead Buffer Dividing Line Text Movement Encoding scheme: (0, 0, s) (4, 1, e) (8, 4, i) (30,1, ˽) (0, 0, i ) (0, 0, a ) (0, 0, l ) (4, 2, a) (0, 0, r ) (10 ,1, t ) (0,0,y) (30, 3,c) (0, 0, ˽ ) (0 ,0, m ) (19, 1, t ) (0,0,k)
- 24. LZ77 (Sliding Window) sir˽sid˽eastman˽easily˽teases˽sea˽sick˽s eal ss Search Buffer (Dictionary) Look ahead Buffer Dividing Line Text Movement Encoding scheme: (0, 0, s) (4, 1, e) (8, 4, i) (30,1, ˽) (0, 0, i ) (0, 0, a ) (0, 0, l ) (4, 2, a) (0, 0, r ) (10 ,1, t ) (0, 0, y) (30, 3,c) (0, 0, ˽ ) (0 ,0, m ) (19, 1, t) (0,0,k) (4, 2, d ) (4, 1, n ) (16, 3, e) (9, 4, l)
- 25. LZ77 (Sliding Window) sir˽sid˽eastman˽easily˽teases˽sea˽sick˽s eals Search Buffer (Dictionary) Look ahead Buffer Dividing Line Text Movement Encoding scheme: (0, 0, s) (4, 1, e) (8, 4, i) (30,1, ˽) (25, 1,eos) (0, 0, i ) (0, 0, a ) (0, 0, l ) (4, 2, a) (0, 0, r ) (10 ,1, t ) (0, 0, y ) (30, 3,c) (0, 0, ˽ ) (0 ,0, m ) (19, 1, t) (0,0,k) (4, 2, d ) (4, 1, n ) (16, 3, e) (9, 4, l)