Data program is always sliding all around

Data Compression just sounds complicated.

Dont beafraid, compression is our good friend for many reasons. It saves hard drivespace. It makes data files to handle. It also cuts those immense file downloadtimes from the Internet.

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Wouldnt it be nice if we could compress all filesdown to just a few bytes?There is a limit to how much you can compressa file. How random the file is, is the determining factor to how far it canbe 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 actualproof that proves this is at the end of my paper.

The key to compressing afile is to find some sort of exploitable pattern. Most of this paper willbe explaining those patterns that are commonly used.Null suppression isthe 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 spreadsheet), and any of them have only zeros in them, then the program just eliminatesthe data and goes straight from the empty data set to the next.

Only onestep up from null suppression is Run Length Encoding. Run length encodingsimply tells you how many of what you have in a row. It would change a setof 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 findinga series of 0s (null suppression) and 1s in this case too and abbreviatingthem.Once the whole idea of data compression caught on, more people startedworking on programs for it.

From these people we got some new premises towork with. Substitutional encoding is a big one. It was invented jointlyby two people: Abraham Lempel and Jakob Ziv. Most compression algorithms (bigword meaning roughly program) using substitutional encoding start with LZfor Lempel-Ziv.

LZ-77 is a really neat compression in which the programstarts off just copying the source file over to the new target file, but whenit recognizes a phrase of data that it has previously written, it replacesthe second set of data in the target file with directions on how to get tothe first occurrence of it and copy it in the directions place. This is morecommonly called a sliding-window compression because the focus of the programis always sliding all around the file.LZ-78 is the compression that mostpeople have in their homes. Some of the more common ones are ZIP, LHA, ARJ,ZOO, and GZIP. The main idea behind LZ-78 is a dictionary. Yet it worksquite a bit like the LZ-77. For every phrase it comes across, it indexes thestring by a number and writes it in a dictionary.

When the program comesacross the same string, it uses the associated number in the dictionary insteadof the string. The dictionary is then written along side the compressedfile to be used in decoding. There is a combined version of LZ-77 anLZ-78. It is called LZFG.

It only writes to the dictionary when it findsthe repeated phrase, not on every phrase. Then instead of LZFG replacing thesecond set of data with directions on how to get to the first occurrence ofit, the program puts in the number reference for the dictionarys translation.Not only is it faster, but it compresses better because of the fact that itdoesnt have as big of a dictionary attached.Statistical encoding is anotherone of the new compression concepts. It is an offshoot of the LZ family ofcompressors; It uses basically the same style as LZFG, but instead of assigningthe numbers in order that the strings come out of the source file, statisticalcompressors do some research.

It calculates the number of times each stringis used and then ranks the string with the most number of uses at the top ofthe hash table. The string with the least is ranked at the bottom. (A hashtable is where the rank is figured) The higher up a string is on this list,the smaller of a reference number it gets to minimize the total bit usage.This gives this compression just a slight edge on the others, but every littlebit helps.

(ha ha -bit- )Beware! There are a few compression programsout there that claim wonderful compression


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