G'day.
Quoting Bulat Ziganshin
sorry, but you really misunderstand how PPM works and why it's so efficient.
I understand PPM. I really do. I think what you don't understand is why LZW does as well as it does considering how simple it is. It's because it's a poor but cheap approximation to what PPM does. Part of the misunderstanding is that you're looking at what PPM does algorithmically, whereas I'm talking about what it's equivalent to mathematically. It turns out that pretty much all known text compression algorithms are "equivalent" in the sense that they give the same compression ration on the same text to within a (possibly large) constant factor. BZip, for example, gives a compression ratio that's almost always within 5% or so of the best PPM variants given an input text. How could that be, since they're so different? The answer is that they're actually _not_ that different, mathematically speaking. Cheers, Andrew Bromage