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Lossless Data Compression Algorithms
The LZ77 algorithm
The LZ77 algorithm was created by two computer scientists — Abraham
Lempel and Jakob Ziv, who first published their algorithm in 1977 (hence the
name LZ77). The LZ77 algorithm works by looking for repetitive data. Rather
than storing this repetitive data in a separate dictionary, the LZ77 remembers
the location of this data in the original file.
When the algorithm finds this same data stored somewhere else, it removes
this data (compressing the overall information) and substitutes a pointer to
the previously recognized data, as shown in Figure 4-3.
Figure 4-3:
The LZ77
algorithm
replaces
redundant
data with
pointers.
See Dick Jane
Because pointers take up less space than the actual data, the LZ77 algorithm
compresses information. The more redundant data, the more efficient
the compression.
The LZ78 algorithm
One problem with the LZ77 algorithm is that it stores redundant data
directly in the compressed data itself. To improve compression, the same
computer scientists developed a variation of the LZ77 algorithm — the LZ78
algorithm.
The LZ78 algorithm removes all redundant data and stores it in a separate
dictionary. Then the algorithm substitutes the redundant data with much
smaller codes stored in the dictionary. By removing this data, the LZ78
algorithm can compress data even further than the LZ77 algorithm.
To uncompress this data, the computer follows each code to the dictionary
to retrieve the appropriate data chunk.
The LZW algorithm
The LZW algorithm gets its name from Terry Welch, who created his own variation
of the LZ78 algorithm, dubbed the LZW algorithm. The LZW algorithm
works by creating a dictionary, just like the LZ78 algorithm. But whereas the