7-Zip is a popular version of the archiver for the Windows operating system with increased compression rates. He is the leader among the family of paid and free programs. The distinctive features of this archiver include the fact that it supports 265-bit AES encryption. Can work with popular RAR archives, including RAR 3, CAB and…
Archiver – software used to merge, in some cases also compression of information.
When saving, backing up information, etc., it is often desirable to compress the files so that they occupy as little space as possible. This is done using programs called archivers. These programs not only compress information into a separate file, but can also combine a group of files into one archive .
There are many archivers. Among them are the most famous: DIET, ICE, LHA, LHARC, LZH, LZEXE, NARC, PAK, PKARC, PKLITE, PKXARC, PKPAK, PKZIP, PKUNZIP, RAR, ZOO. Recently, there were programs that, while in the memory of the computer resident, archived and unzipped “on the fly” all the files that you work with, which allows you to significantly save space on your hard disk. Such features are provided, for example, by the dblspace utility for the MS-DOS operating system and the DIET program (T.Matsumoto, Japan).
There are several methods of compressing information contained in files. Perhaps the easiest of them is the Huffman algorithm , which is to replace standard 8-bit ASCII codes with bit lengths of variable length, depending on the frequency of the use of the character, so that the characters that are used more often have a shorter length. By the way, it is easy to understand that in the texts most often there is a “space” symbol, whose ASCII code has a number 32. You can extend this idea to pairs, triples, etc. of characters. In this case, you can get a significant gain. Indeed, take, for example, two pairs of characters for and. You can name a lot of words with the first combination, while the other is very rare. And with standard ASCII encoding, each of the connections consumes evenly bits – by 16. Among the other methods that are widely used in archivers to compress information into files, one can note the Lempel-Ziva algorithm .
Finally, note that the computer does not “understand” any other character encodings other than ASCII- encoding (or related encodings). Therefore, before using, the archived file must be unzipped.
Functions and characteristics
The simplest archivers simply consistently combine (pack) the contents of files into an archive. The archive should also contain information about the names and length of the original files to restore them, so most archivers also store file metadata provided by the operating system , such as creation time and access rights. This functionality is implemented by the tar – standard archiver of UNIX type systems . If necessary, reduce the size of a tar-file used lossless compression programs the gzip , the bzip2 and so on.. Most other modern archives contain compression as a built-in feature by default.
Characteristics of archivers:
- By the degree of compression.
- By the speed of compression.
These characteristics are inversely dependent quantities. That is, the higher the compression rate, the smaller the compression ratio, and vice versa.
The program, creating an archive, processes both text files and binary files . The former are always compressed several times (depending on the algorithm used).Compression of binary files depends on their format . Some binary files can be compressed dozens of times, while compression of others can not reduce the volume they occupy.
Finding an algorithm with the smallest possible output size for any input file is an algorithmically unsolvable task .
Data compression usually occurs much slower than the reverse operation.