Author: Robert Hyatt
Date: 12:21:09 12/25/02
Go up one level in this thread
On December 25, 2002 at 02:34:22, Uri Blass wrote: >On December 24, 2002 at 23:05:09, Robert Hyatt wrote: > >>On December 24, 2002 at 19:24:04, Vincent Diepeveen wrote: >> >>>On December 23, 2002 at 19:21:57, Uri Blass wrote: >>> >>>>On December 23, 2002 at 18:31:03, Dieter Buerssner wrote: >>>> >>>>>On December 23, 2002 at 18:08:15, Martin Bauer wrote: >>>>> >>>>>>Hello, >>>>>> >>>>>>i have a queastion about move ordering. There are many sources with move >>>>>>ordering heuristics like killer heuristic, history and so on... >>>>>> >>>>>>But I found no description _how_ to program the move ordering in an _efficient_ >>>>>>way. In my own enginge I use an integer value together with the move and put it >>>>>>on the move stack. Moves that should be searched first, become a high value and >>>>>>the less important moves a low one. Then there is a function named >>>>>>"NextBestMove" that that looks for the highest value at the actual searchdepth >>>>>>on the movestack. Therefore it must look at all possible moves in the actual >>>>>>position. When the best move is found, the value is set to -Matescore, so it can >>>>>>not get the best move the next time the function is called. >>>>> >>>>>This is the normal way to do it, I think. Instead of giving a "marker score", to >>>>>not search the move again, you could shift the move to the start or to the end >>>>>of the array, and remember the new bounds (incrementing a pointer may be enough >>>>>for this). This will save a few CPU cycles. It is essentially the inner loop of >>>>>a normal selection sort. >>>>> >>>>>>This algorithm must have a look at all possible moves in the position at the >>>>>>actual depth, even if the frist 10 best moves are searched. This look not >>>>>>efficient to me, because it is an O(n) algorithm in reading the best move and >>>>>>O(1) in storing the best move. >>>>> >>>>>I think, there is no practical better way. Sorting the whole move list can >>>>>easily be done faster (especially, when it has some considerable length, so not >>>>>just relpy to check). But often, the work will be done for nothing, because one >>>>>move will be enough for a cutoff. I experimented a bit with the following idea: >>>>>Try to guess, when we expect a fail high node: use the selection sort method >>>>>above. Whe expecting a fail low node, do a qsort (the Standard C-language qsort >>>>>would probably be a bit slow for this, because of all the calls to the compare >>>>>function, I had written my own). But, I really could not measure any performance >>>>>increase, so I gave up on the idea. It just made the code bigger ... >>>> >>>>If you expect a fail low move you can simply not care about order of moves. >>> >>>This is utter nonsense. >>> >>>==> note that it is another years 80 design issue in crafty >>> >>>For many reasons sorting is better. To just list a few >>> a) it *might* give a cutoff now. No heuristic is 100% accurate >>> going to predict it is going to get a fail low again. >>> The proof for that is obvious. If you know it for 100% sure you >>> can simply return alpha and stop searching this node! >>> b) it goes into hashtable and gets reused later. You perhaps do not >>> expect a fail low then but the best move saved in the hashtable is >>> a random move in your case >>> c) it improves positional play obviously. Suppose you pick a random >>> move giving 0.001 versus a chosen move 0.001. The chosen move on >>> average is better. Do not underestimate this effect at all. this is >>> not a 'once in a million moves i play' scenario. >> >>That is utter clap-trap. Why don't you go read Knuth/Moore's paper on >>alpha beta. There you will find that move ordering does _not_ affect the >>final score, only the size of the tree. Something every senion-level computer >>science student should know. > >I can imagine a case that it can affect the final score. > >Suppose that there are 2 moves that potentialy gives fail high that you did not >search >move A and move B. > >if you search first move A you get a score for move A and move B is pruned >by null move pruning. > >if you search first move B then move B is not pruned by null move pruning and >you get a bigger score for move B so you do not play move A. That is a bug caused by null-move, _not_ by move ordering. Null-move can introduce several different types of artifacts, but a _proper_ search is not affected by move ordering... > >It does not mean that not sorting is a mistake because being faster is an >advantage and I do not think that the quality of order by history table is good >in any case so not sorting after enough moves is an advantage for a lot of >programs. > >Uri
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