38.1 Introduction to Compression

As an introduction to compression, consider the processes of creating and unzipping a zip file.

$ zip mobydick.zip mobydick.txt 
adding: mobydick.txt (deflated 59%)

$ ls -l
-rw-rw-r-- 1 jug jug 643207 Apr 24 10:55 mobydick.txt
-rw-rw-r-- 1 jug jug 261375 Apr 24 10:55 mobydick.zip

Note that before and after unzipping, the file size changes!

Compression Model 1: Algorithms on Bits

In our first model of compression, we consider compression as applying a compression algorithm on a sequence of bits. To reverse the compression, we apply the inverse decompression algorithm.

Compression and decompression.

Say you had a text file called example.txt. If you were to zip that text file, you'd get example.zip, a zip file with a size much lesser than the original example.txt file. This is the main idea behind compression--a technique used to reduce file size.

Then, if you were to unzip example.zip into a file called unzippedexample.txt, you would notice no difference between example.txt and unzippedexample.txt. This is an indicator of lossless compression, where no information is lost.