Author: Ricardo Gibert
Date: 13:05:55 05/31/00
Go up one level in this thread
On May 31, 2000 at 15:46:15, blass uri wrote: >On May 31, 2000 at 15:23:28, Robert Hyatt wrote: > >>On May 31, 2000 at 13:22:34, blass uri wrote: >> >>>On May 30, 2000 at 18:11:51, Robert Hyatt wrote: >>> >>>>On May 30, 2000 at 15:24:36, Ed Schröder wrote: >>>> >>>>>On May 30, 2000 at 00:28:47, Robert Hyatt wrote: >>>>> >>>>>>On May 28, 2000 at 16:37:32, Gian-Carlo Pascutto wrote: >>>>>> >>>>>>>On May 28, 2000 at 10:02:05, Georg v. Zimmermann wrote: >>>>>>> >>>>>>>>From my tests it shows that it sticks with the hash-move about 50% of the time. >>>>>>>>Should this number be higher ? >>>>>>> >>>>>>>Hmm...if this number is also effectively your 'move ordering percentage', >>>>>>>which I assume it is, it is quite low. I'd expect it to be at least about 75%. >>>>>>> >>>>>>>> >>>>>> >>>>>> >>>>>> >>>>>>The classic definition of a "strongly-ordered tree" is this: If, for every >>>>>>node where you fail high, you fail high on the first move at least 90% of the >>>>>>time, then your move ordering is good." If you are much below 90% and already >>>>>>have a serious problem that is not hard to fix. The traditional ordering ideas >>>>>>holds Crafty at 92% and better for most of the game. >>>>> >>>>>I can't understand the 92%. A perfect mini-max search requires many many >>>>>nodes an alpha-beta cutoff will not work and you are forced to search all >>>>>the nodes of the ply in question. And this number is certainly much higher >>>>>than 8%. >>>> >>>>You have to re-read the definition again, _very carefully_ to avoid the semantic >>>>trap you just fell into. >>>> >>>>For every position where you fail high, if you fail high on the first move you >>>>try, you increment a counter "right++". You always increment a counter "fh++". >>>>When you finish the search, you compute percent=right/fh. That number needs to >>>>be over 90% to consider your tree strongly ordered. Notice that this 92% number >>>>(in crafty) simply says this: >>>> >>>> "if we look at _all_ the positions in the tree where the search fails high, >>>> then 92% of those fail highs happen on the first move searched in that >>>> position, which is known as 'optimal move ordering'. >>> >>> >>>I do not agree that failing high on the first move is optimal move ordering. >>> >>>Here is an example: >> >>That particular idea isn't open to debate. Alpha/beta is all about minimizing >>the number of nodes searched. It is easy to prove mathematically that if I >>get the best move first every time, and you don't, I am going to search fewer >>total nodes than you are to get the exact same score. >> >> >> >> >> >>> >>>[D]8/6k1/rp3ppp/8/N7/8/4RPPP/6K1 w - - 0 1 >>> >>>My understanding of optimal move ordering is that after the moves Nxb6 or Nc5 >>>the first move to search will be Ra1+(at least in cases that you are going to >>>search more than few plies after these moves because Ra1+ Re1 Rxe1# is the >>>faster way to prove that Nxb6 or Nc5 is wrong) >>> >>>If you start with taking the knight than your first move may fail high but you >>>waste more time to prove that Nxb6 or Nc5 are wrong. >>> >>>Uri >> >> >>No... that is the wrong way to think about alpha/beta. In any given position, >>the 'best' move is the one which produces the best score _in that position_. It >>doesn't matter a dime what has happened in similar positions, or at shallower >>search depths. Ie it doesn't matter if a move looks "best" to a human, in the >>context of alpha/beta, or anything else. > > >If a program rejects Nxb6 or Nc5 because of Ra1+ Re1 Rxe1# it is using less time >about Nxb6 and Nc5. > >If a program is using less time to get to the same depth the order of moves is >better. > >The point is not to search first the best move but to search first the move that >you need the minimal number of nodes to create a cutoff. > > > > It is all about the move that produces >>a move that causes a cutoff. It is _not_ necessary that alpha search the "best" >>move first, ever. It is only necessary that alpha/beta searches a move good >>enough to cause a cutoff... > >If you search first the move that you need less time to get the cutoff then the >order of moves is better. > > >> >>My original statement is still on target: If, at every move where you get a >>cutoff, you get it on the _first_ move, you are searching the "minimal tree" >>which is the goal of alpha/beta. > >No >You are not searching the minimal tree because after Nxb6 because you search >more nodes in the line Rxb6 then in the line Ra1+ with the only exception of a >very shallow search. > >Uri Line A Line B 1 Nxb6 1 Nxb6 2 Rxb6 2 Ra1+ 3 No captures for white 3 Re1 4 Rxe1 5 No captures for white Which is shorter Line A or Line B?
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