How does WinRAR perform compression verification?

Question

eg. how can he say that a 4 GB text file can be compressed, say, 200 MB? Obviously, it does not read all the contents in 2 or so seconds ... so what kind of forecasting algorithm does it use?

Answer 1

They use a partial matching (PPM) prediction option called PPMd. Check out the wiki

Answer 2

To compress x bits, the -log (x) + log (2) bit is required. However, this is a very theoretical value, and it depends on the data you want to compress. For your data, you need to write down each character and frequency and insert it into the formula. For example, first try only 3 characters. You want to find the shannon code.