Compression is the process of processing data using a mathematical algorithm resulting in new data containing the same information, but smaller.

The key to most compression algorithms is that when decompressed, the original data is recovered, exactly. So-called “lossy” algorithms, like jpeg, can be used where a further trade-off between absolute visual accuracy and size can be made.

Most often we compress data contained in files on our computers. A compression program, such as WinZip, 7-zip, gzip, WinRAR, and many others, reads the file to be compressed, runs the data it contains through the compression algorithm, and writes the result to a new, typically smaller, file.

A compression algorithm is a mathematical function, of which there are many. Any program that understands a specific algorithm can typically compress or decompress files interchangeably with other programs that understand the same algorithm. Many programs, 7-Zip being one good example, have the ability to understand several different compression algorithms.

Many compression programs, WinZip and similar being examples, double as file archiving programs. In addition to compressing files, these programs bundle multiple compressed files into a single output file, referred to as a compressed archive.

It’s worth noting that all compression algorithms have worst-case scenarios, where “compressing” data produces a result that is larger than the original. With most common algorithms it’s relatively rare.

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