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Subject: Re: Move Ordering at the Root

Author: Andrew Dados

Date: 09:32:12 09/16/99

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On September 16, 1999 at 04:49:15, Bas Hamstra wrote:

>On September 15, 1999 at 19:42:34, Andrew Williams wrote:
>
>>On September 15, 1999 at 17:12:16, Dan Homan wrote:
>>
>>>I was reading the crafty source again the other day and noticed that
>>>Bob has a special function to improve the move ordering at the root
>>>of the search.
>>>
>>>I really didn't feel like writing such a function last night, but I
>>>thought instead to use the values returned by the search itself to
>>>improve the move ordering at the root.  I know that I only get an
>>>accurate value for the best move, but I thought that my fail-soft
>>>search might return useful numbers for the other moves as well....
>>>
>>>Implementing this was pretty quick and easy: there were a couple
>>>of pit falls, but the total changes were about 5 lines of code.
>>>Previously I simply used the same move ordering at the root that I
>>>use at all other nodes.
>>>
>>>The improvement was amazing!  I got a full ply in many positions and
>>>about a half-ply in many more.   It improved my solution times on WAC
>>>noticably and seems even better in quiet positions.
>>>
>>>I know that my solution was a quick kludge, so I am wondering what
>>>other people do for move ordering at the root of the search.
>>>
>>> - Dan
>>
>>Hi Dan,
>>
>>This is what I used to do. (I'm also using fail-soft). It's one of those
>>things that shouldn't really work, but seems to do very well. However,
>>I later changed it to use the number of nodes in the sub-tree to order
>>root moves. If I recall, this made very little difference to my program's
>>performance; I just prefer this, because I know why it should work.
>
>Interesting. Isn't this in fact the same discussion as a few days ago?  Using
>values < Alpha for movesorting? In this case on the root.
>
>Well, if it shouldn't work, why does it, if it does, if you know what I mean?
>And if it works, I would very much like to hear Bob's reaction.
>
>Anyway I will try this too.
>
>
>Regards,
>Bas Hamstra.

   Explanation why fail-soft scores work is actually easy. Imagine 2 moves with
*true* scores for that ply of (alpha-10) and (alpha-100), respectively. Your
fail-soft will more likely return score for second move lower then for first
move (there is that <alpha-100,alpha-10> score space which *can not* be returned
from search of first move, but can return from search of second move). Second
move will also more likely require less nodes to refute. So those 2 methods are
comparable, and ordering by fail-soft scores works just because probability of
scores is non-random (higher chance that worse move will return worse score,
cause it has more 'score space' for that)... I wonder why Bob neglects that...

-Andrew-



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