[Computer-go] Narrow wins

Mark Goldfain MarkGoldfain at comcast.net
Thu Jun 6 20:44:04 PDT 2013


Forgive me if this is too basic an observation that most all of you 
already know full well.

It seemed there was a question out there as to why MCTS bots often win 
by a very narrow margin.  And I did not see an answer such as this 
posted (again, perhaps because everybody knows this).  But if I am 
correct, then at least I'd like to know that.  I have not programmed 
such a bot, so I would like it if one of you affirmed or refuted this.

I believe it is a simple interaction between the goal for these bots and 
the nature of the game of go.  The bots, as has been pointed out, care 
nothing for the margin of victory.  They will count a 0.5-pt win as just 
as good as a 200-pt win.  Put that together with what we all know as 
human players:  There are always moves out there that eliminate a cut 
point, or secure a given territory beyond all dispute.  We spend the 
whole game knowing about these moves, but not making them, because we 
have a higher priority.  Our well-matched opponent is doing everything 
he/she can to push us and outrace us in achieving the building of live 
shapes containing the maximum number of points.

If we were really good at reading out the rest of the game and were able 
to confidently say we were 10 points ahead, and we did not need to make 
the maximum point-gain at every move, we might be able to make one or 
more "insurance" moves.  I think the bots are led naturally by their 
calculations to make such moves.

This is not how human players play.  I think human players learn to 
mostly always consider what next move will be of the most value. They do 
not fluidly switch when they are ahead to conservative moves, because we 
are never certain we are ahead until it is pretty clear.  I was recently 
being thrashed by a much stronger player.  I think he was up by about 40 
points.  What did he do next?  He knew full well that his groups were 
safe enough.  He attacked and reduced one of my territories, and went on 
to have about a 55-point margin of victory.  I think that a bot, looking 
at the probabilities of victory, would instead have been led to take it 
easy on me, and go for a result that was closer, but left no chance for 
any attack on my part.  Oddly enough, I know some human players get very 
angry about this behavior, they do not generally consider it as a polite 
way to act.  I've heard comments like "The bot will just toy with you, 
giving you a feeling you might yet win."  If I'm correct above, it is 
nothing of the sort.




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