Author: Tony Werten
Date: 01:18:49 06/10/01
Go up one level in this thread
On June 10, 2001 at 03:46:02, Cheok Yan Cheng wrote: >On June 10, 2001 at 02:39:43, Christophe Theron wrote: >>The trick is that you do no re-search in the first branches, because the first >>branch in the PVS algorithm is searched with the FULL alphabeta window, not a >>"null" window (where beta=alpha+1). >> >>In any position, the first move is searched with an ]alpha;beta[ window, the >>returned score becomes the new alpha value (unless there is a cutoff in which >>case you quit), and you search the rest of the moves with an ]alpha;alpha+1[ >>window. If you get a fail high in any of these moves (and only in this case), >>you need to re-search the move with an ]alpha+1;beta[ window. > >What is the advantages of using ]alpha, alpha+1[ in the 2nd and the rest of the >child node? I don't see any point it will produce more cutoff than original >alpha beta search. Compare the two windows is > >]7, 8[ //Null Window search >]7, 100[ //original alpha beta search > >If value<=7, ignore the value. (both windows) >If value>=100, return the value (both windows) >If value>7 AND value<100, re-search arghhhh...... (only Null windows) The point is that this 3rd one should not occur very often. suppose you search with -100,100 window, 4 ply deep. At ply 4 the score of the first move=10. PVS will search the remaining moves with (10,11) and alfabeta will search with (10,100). That's why PVS has more cutoffs. But the important thing is that researches shouldn't happen very often. I you have a not so good moveordering then a-b is a better choice. Tony > >See! No advantages gain from NULL WINDOWS. This confused me a lot! >Please help me with this by providing me some example. > >>This is a rough explanation of the principle, but I think it highlights a point >>you had missed. >> >>If the move ordering is perfect, you never need to do a re-search, so you have >>saved some work because many nodes have been searched with a smaller window. >> >> >> >> Christophe
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