programming-for-dummies

Lossless Data Compression Algorithms 439
Dictionary encoding
Run-length encoding is a simple algorithm that works well with redundant
characters grouped together but doesn’t work as well with redundant data
scattered throughout. An alternative to RLE is dictionary coding. The basic
idea behind dictionary coding is to replace large data chunks with much
smaller data chunks. Suppose you had the following text:
See Dick. See Jane.
You could replace the redundant text See by a simple code, such as 1, and
create a new string:
1 Dick. 1 Jane.
Now you could replace Dick with 1 code and Jane with another code, such
as 2 and 3 respectively, to create a compressed version, like this:
1 2. 1 3.
Uncompressing this data means replacing the codes with the actual data,
using a dictionary. Each code refers to the actual data, so looking up 1
retrieves the See string, 2 retrieves Dick, and 3 retrieves Jane, as shown in
Figure 4-2.
Figure 4-2:
Uncompressing
data
requires
using a
dictionary to
replace
codes with
actual data.
1 See
2 Dick
3 Jane
1 2 1 3
Book IV
Chapter 4
If you know the data you want to compress ahead of time, such as condensing
the entire contents of an encyclopedia on a DVD, you can optimize the dictionary
to create the smallest codes, which represent the most common data chunks,
such as the word the. In most cases, you don’t know the data to compress ahead
of time, so you need an algorithm that can analyze data, create a dictionary on
the fly, and then compress data using that dictionary. Three popular dictionary
encoding algorithms include LZ77, LZ78, and LZW.
Data Compression
Algorithms