Hello ajb, Friday, July 13, 2007, 12:10:54 PM, you wrote:
it seems that you either misunderstood PPM or make too wide assumptions.
More likely I'm not explaining myself well.
in classical fixed-order ppm each char probability found in context of previous N chars. i.e. when encoding abcdef.. with order 3, 'd' encoded in context of 'abc', 'e' encoded in context of 'bcd' and so on
Let's suppose that "abcdef" is a context in PPM, and we're starting from the "start" state (which usually isn't the case, of course). Then to encode it, you encode the probability of finding an "a" in the start state, followed by the probability of finding a "b" in the "a" state, followed by the probability of finding a "c" in the "ab" state...
sorry, but you really misunderstand how PPM works and why it's so efficient. it searches probability of each symbol in context of previous N chars where N is fixed. lzw can be directly compared with ppm modification you described here, but this modification isn't ppm itself. also, because ppm encodes each symbol separately, making its probabilities flat effectively disables compression - the key why flat probabilities still work in lzw is that it encodes whole word with this flat model -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com