Data Compression

‘Data Compression’ just sounds complicated. Don’t be afraid, compression is our good friend for many reasons. It saves hard drive space. It makes data files to handle. It also cuts those immense file download times from the Internet. Wouldn’t it be nice if we could compress all files down to just a few bytes?

There is a limit to how much you can compress a file. How random the file is, is the determining factor to how far it can be compressed. If the file is completely random and no pattern can be found, then the shortest representation of the file is the file it self. The actual proof that proves this is at the end of my paper. The key to compressing a file is to find some sort of exploitable pattern. Most of this paper will be explaining those patterns that are commonly used.

Null suppression is the most primitive form of data compression that I could find. Basically, it says that if you have different fields that data is in (possibly a spread sheet), and any of them have only zeros in them, then the program just eliminates the data and goes straight from the empty data set to the next.

Only one step up from null suppression is Run Length Encoding. Run length encoding simply tells you how many of what you have in a row. It would change a set of binary data like {0011100001} into what the computer reads as (2)zeros,

(3)ones, (4)zeros, 1. As you can see, it works on the same basic idea of finding a series of 0’s (null suppression) and 1’s in this case too and abbreviating them. Once the whole idea of data compression caught on, more people started working on programs for it. From these people we got some new premises to work with. Substitutional encoding is a big one. It was invented jointly by two people: Abraham Lempel and Jakob Ziv. Most compression algorithms (big word meaning roughly ‘program’) using substitutional encoding start with ‘LZ’ for Lempel-Ziv.

LZ-77 is a really neat compression in which the program starts off just copying the source file over to the new target file, but when it recognizes a phrase of data that it has previously written, it replaces the second set of data in the target file with directions on how to get to the first occurrence of it and copy it in the directions’ place. This is more commonly called a sliding-window compression because the focus of the program is always sliding all around the file.

LZ-78 is the compression that most people have in their homes.