The program-archivers, or the program-packers, are intended for archiving files for the purpose of their long-term storage. Under archiving files is usually understood as placing one or more source files in an archive file (archive) in a compressed (packed) form. Such file contains the service information about file names, dates and time of their creation or modification, volumes, ways of information compression. Compression of information is called the process of reducing its redundancy and, consequently, reducing the amount of memory for storage. The reduction in redundancy is achieved in various ways: by simplifying codes, by removing from the codes of constant bits or by repetitive symbols in the form of a repetition factor.
File-archiver programs are of interest from several points of view:
• allow you to update the archive file with new files or remove the required files from the archive file. At completion of archive in it only modified (updated) files are brought in;
• Have the ability to create self-extracting archives that do not require the archiver program to extract files;
• Packaged files take up a small amount (an average of 20 to 90% of their original volume);
• Reduce the time and cost of transferring information over communication channels in computer networks, when copying files from one disc to another, simplifying the transfer of files to another computer, helping to protect against infection with viruses, significantly increasing the number of files stored on hard or flexible disks;
• When archiving, the directory names are stored in the archive along with file names, which allows you to save the entire file structure when you unzip the file.
One of the most important parameters of the quality of file archiving is the compression ratio
where V c , V 0 are the volumes of compressed and source files, respectively.
The compression ratio depends on the archiver program used, the compression method, the type of the source file, and other factors. High compression ratio (small value of K s) has graphic and text files, data files for which K C is 5-40%. Files of executable programs and loading modules for which K s have a lower compression ratio are 60-90%. Archival files are almost not compressed.
Currently, a large number of programs are used to archive files, among which is the multifunctional integrated WinRAR archiver developed in our country.
WinRAR is a 32-bit version of the RAR archiver for Windows, a powerful tool for creating and managing archive files.
WinRAR features :
• Full support for RAR and ZIP archives;
• Original high-performance data compression algorithm;
• A special algorithm for multimedia compression;
• the shell with the support of the technology drag & drop & quot ;;
• command-line interface;
• management of archives of other formats (CAB, ARJ, LZH);
• Support for continuous archives in which the compression ratio can be 10-50% more than with conventional compression methods, especially when packing a large number of small similar files;
• Support for multi-volume archives;
• Create self-extracting (SFX) regular and multi-volume archives using standard or optional SFX modules;
• restoration of physically damaged archives;
• Other advanced features, such as encryption, adding archive comments (with support for ANSI ESC sequences), error logging, etc.
In WinRAR, there is a menu: "File", "Commands", "Parameters", "Favorites", "?" (Help) and the toolbar below which the list of files is located.
The WinRAR shell has two main modes: file management and archive management, which lists the files and folders in the current folder or open archive. For each file, the following information is displayed: name, size, type and date of change. The files in the archive show one more parameter - the size after the backup.
You can select files and folders with the mouse or keyboard and perform various operations with the selected files, for example, archive them or delete them; unpack, test, or comment.
Comparison of RAR and ZIP formats. WinRAR can create archives of two different formats: RAR and ZIP. The merits of each are described below.
RAR format in most cases provides significantly better compression than ZIP, especially when creating continuous archives. Another important feature of RAR is the support for multi-volume archives. They are much more convenient and easier to use than the so-called disks (span disks) ZIP archives. The RAR format has several important features that are not available in the ZIP, for example, adding information that allows you to restore a physically damaged file, and locking important archives to prevent their accidental modification.
The advantages of the ZIP format are its popularity (most of the archives on the Internet are in ZIP format) and speed (ZIP archives are usually created faster than RAR archives).
Types of archives. A continuous archive is a RAR archive packed in a special way, in which all compressible files are treated as one serial data stream. Such archives are only supported in the RAR format. The compression method for RAR archives is normal or continuous - the user chooses. Continuous archiving significantly increases the compression ratio, especially when adding a large number of small similar files. However, continuous archives have some disadvantages: the update is slower than for regular archives; encrypted continuous archives can not be changed; extracting individual files from the middle of a continuous archive is slower than extracting from a regular archive; if a file is damaged in a continuous archive, you will not be able to extract any files that follow it either.
Self-extracting archive (SFX - SelF-eXtracting) is the archive to which the executable module is attached. This module allows you to extract files by running the archive as a normal program. SFX archives, like any other executable files, usually have the .exe extension. To extract the contents of the SFX archive, no additional external programs are required.
The WinRAR archiver allows you to:
• Select the archive format. The ZIP format can be used if the recipient does not have a WinRAR program. Otherwise, it's better to choose RAR, which has more functions and better compression;
• Select the compression method. In the formats RAR and ZIP, six methods of archiving are supported: "Without compression", "Fast", "Quick", "Normal", "Good" and Best & quot ;;
• the size of the dictionary. This parameter can take the values 64, 128, 256, 512 and 1024 Kbytes. Large values of this parameter lead to better, but slower compression;
• Continuous compression, which is used only if the maximum compression ratio is required;
• Multimedia compression. When archiving in this mode, RAR uses an additional algorithm to improve the compression of data such as digitized audio containing four 8-bit or two 16-bit channels, or full-color (24-bit) graphics in BMP format.
Both RAR and ZIP formats support encryption. To encrypt files, you must specify a password before archiving. Passwords are case-sensitive. You can enter the password in the WinRAR shell in various ways, for example by pressing Ctrl + P. When extracting encrypted files, you can enter the password in advance or at the request of the WinRAR program.
You can add optional text information to the RAR archives (ZIP and ARJ), called the comment of the archive.
The RAR archives format supports archive locking, for which the Archive Lock in WinRAR shell mode. A locked archive can not be changed using WinRAR. You can lock important data to prevent accidental changes.