Lempel-Ziv-Storer-Szymanski

From Wikipedia, the free encyclopedia

Lempel-Ziv-Storer-Szymanski (LZSS) is a lossless data compression algorithm, a derivative of LZ77, that was created in 1982 by James Storer and Thomas Szymanski.

LZSS is a dictionary encoding technique. It attempts to replace a string of symbols with a reference to a dictionary location of the same string.

The main difference between LZ77 and LZSS is that in LZ77 the dictionary reference could actually be longer than the string it was replacing. In LZSS, such references are omitted if the length is less than the "break even" point. Furthermore, LZSS uses one-bit flags to indicate whether the next chunk of data is a literal (byte) or a reference to an offset/length pair.

[edit] Example

Here is the beginning of Dr. Seuss's Green eggs and ham, with character numbers at the beginning of lines for convenience.

0: I am Sam
9:
10: Sam I am
19:
20: That Sam-I-am!
35: That Sam-I-am!
50: I do not like
64: that Sam-I-am!
79: 
80: Do you like green eggs and ham?
112:
113: I do not like them, Sam-I-am.
143: I do not like green eggs and ham.

This text takes 177 bytes in uncompressed form. Assuming a break even point of 2 bytes (and thus 2 byte pointer/offset pairs), and one byte newlines, this text compressed with LZSS becomes 98 bytes long:

0: I am Sam
9:
10: (6,3) (0,4)
16:
17: That (6,3)-I-am!
31: (20,15)I do not like
47: t(21,14)Do you(58,5) green eggs and ham?
79:
80: (50,13) them,(24,9).
92: (50,13)(93,18).

Note: this does not include a few bytes (3-4) of flags indicating whether the next chunk of text is a pointer or a literal. Adding it, the text becomes just over 100 bytes long, which is much shorter than the original 177 bytes.