Author: Ricardo Gibert
Date: 12:32:31 05/31/00
Go up one level in this thread
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. 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... > >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. Oops! I duplicated! Oh well.
This page took 0 seconds to execute
Last modified: Thu, 15 Apr 21 08:11:13 -0700
Current Computer Chess Club Forums at Talkchess. This site by Sean Mintz.