Author: GuyHaworth
Date: 15:34:11 04/28/03
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Well, I haven't read the paper you cite and am not familiar with their 'language', but I will say something about retrograde EGT(B) generation. There are two ways of generating EGTs, both perfectly ok, though differing in efficiency. The 'Edwards/Nalimov' way looks at every (say, wtm) position - and for each position examines its successors. It backs up the best option for White - which might be 'nothing known' or 'win in N1'. This is not to say that, on the next occasion, this 'win in N1' will not be improved to 'win in N2' with N2 < N1. The 'Thompson/Wirth/Wu-Beal' way is to literally 'retro' a 'frontier' of 'newly evaluated positions'. So, if we back up btm losses, we know automatically that they back up to wtm wins - and the only question is whether they are quicker wins than already found. With this fully retro approach ... For DTC, and DTM if you use the Wu/Beal technique of not backing up 'mate in N' until cycle N, and DTZ if you are cleverer still ( but this hasn't been done yet), you can say that wins in N will be set in cycle N so, in fact, they never get reduced. They just get their value set once and it is correct. So, the integrity of EGT generation depends on how you manage successors-of-P (or in the retro approach, predecessors of P and the known set of 'latest results'). The fundamental reason these methods work is that they set interim depth values which, if necessary, are open to being reduced. If you have the efficiency that you know the 'interim' depths cannot be reduced, e.g. because DTC or DTM = cycle_number when set, then you do not need to try to reduce them. I hope that is clear. I have to say that the reasons why EGT generation works are non-trivial, and I once tripped up and defined an incorrect algorithm for DTR calculations. So I do sympathise with your question. Guy
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