[Computer-go] Tactical position

David Fotland fotland at smart-games.com
Sun Aug 29 17:58:30 PDT 2010


This position identified a few playout bugs, and the fixes increased the win
rate vs Gnugo from 90% to 92%. (19x19, 8000 playouts, Gnugo 3.7.10, level
10, Japanese rules, 2000 game test).

Thanks,

David

> -----Original Message-----
> From: computer-go-bounces at dvandva.org [mailto:computer-go-
> bounces at dvandva.org] On Behalf Of David Fotland
> Sent: Monday, August 23, 2010 9:44 PM
> To: computer-go at dvandva.org
> Subject: Re: [Computer-go] Tactical position
> 
> I ran Many Faces for 90 seconds and it likes D3 for X with 75% win rate,
21
> ply PV, 1.3M playouts.
> 
> Nice position.
> 
> 
> David
> 
> > -----Original Message-----
> > From: computer-go-bounces at dvandva.org [mailto:computer-go-
> > bounces at dvandva.org] On Behalf Of Brian Sheppard
> > Sent: Monday, August 23, 2010 8:06 PM
> > To: computer-go at dvandva.org
> > Subject: [Computer-go] Tactical position
> >
> > This situation taught me a lot. It is obviously a win for O, but things
> > get complicated in playouts and UCT.
> >
> >   1 2 3 4 5 6 7 8 9
> > A - - - - X - - - -
> > B - - - O X - O O -
> > C - - - O X - O X O
> > D - X - O X X O X -
> > E - O X O X O X - X
> > F - - O - X O X X X
> > G - - - - X O O O O
> > H - - - X X X O - -
> > J - - - - - O O - -
> > X to play; O wins
> >
> > O wins this position even with X to play, and despite having a 1-eye
> > group at bottom.
> >
> > The O group at bottom is cut off, but the X group immediately above is
> > also one-eyed, and can be captured in 3 moves: B9-D9-E8. The O group
> > at bottom can be captured with J5-J8-H9-H8, so O wins by 4 to 3.
> >
> > But your playout engine will need a lot of guidance for that advantage
> > to be realized, because
> >
> >     1) O's moves can be played in any order.
> >     2) X's moves must be played in the specific order B9-D9-E8.
> >     3) Proximity don't help; after X plays J5, no rule suggests B9.
> >     4) Proximity rules hurt; after X plays H8, patterns suggest J8!
> >
> > Moreover, X can win a semeai against the O group at *top*. For example,
> > if X starts with B9, and O captures with A9, then X's A7 leaves O only
one
> > move ahead: O must play B9-D9-E8 before X plays B6-C6-A8.
> >
> > In the semeai at top, again there are problems:
> >
> >     1) O might play A6, B6, C6, or A8 filling its own outside liberties.
> >     2) X cannot fill its outside liberties, because it has none.
> >     3) O must play in a specific order
> >     4) X can play in any order that ends with A8.
> >     5) Local patterns do not hurt X, since B9 and D9 match no patterns.
> >     6) Local patterns can hurt O, since local replies to A7,B6,C8 lose.
> >
> > Even if your program implements self-atari-vs-atari logic that help it
to
> > win the base cases of semeai, it will lose for o in almost every trial.
> >
> > Now, the left-hand side is also tricky. You would think that O's 6
stones
> > will easily chase and kill O's two loose stones and establish life, but
> > it isn't so simple.
> >
> >   1 2 3 4 5 6 7 8 9
> > A - - - - X - - - -
> > B - - - O X - O O -
> > C - - - O X - O X O
> > D - X - O X X O X -
> > E - O X O X O X - X
> > F - - O - X O X X X
> > G - - - - X O O O O
> > H - - - X X X O - -
> > J - - - - - O O - -
> > X to play; O wins (Diagram repeated)
> >
> > First, X's first turn is D3, which is suggested by escape-atari logic.
> > So X finds the path of stiffest resistance on many (most) trials.
> >
> > Second, in reply to D3, O has to play F2, which is not a local play.
> > (That is, it is not in the 3x3 neighborhood of D3.) Because of this, O
> > will usually *not* find the path of stiffest resistance.
> >
> > Third, that situation is likely to continue! That is, X's local patterns
> > are often helpful at string fighting, whereas O's are often wrong.
Often,
> > the error is basically that O needs to fill outside liberties, and these
> > are located outside the 3x3 neighborhood of X's last move. (Pebbles
> > matches patterns around the last two turns, but that just creates a
> > larger pool of potentially bad moves to play. Since O should win the
> > fight, it is O's errors that matter; X cannot make an error.)
> >
> > Fourth, as long as X is alive and making larger strings, O is getting
> > squeezed on both sides. If X happens to cut on F4, then O is likely to
> > capture something.
> >
> > Fifth, whenever the UCT tree figures out how to handle a situation,
> > X can play a forcing move someplace to distract O. For example, if O
> > figures out that F2 is a good reply to D3, then X can start with B9.
> > Now X has to reply with A9, and then O can return to D3 and force X
> > to figure out everything again. There are more than enough forcing
> > moves to push the resolution of D3 into the playouts.
> >
> > X isn't a favorite to live on the left in Pebbles playouts, but it is
> > a lot closer to 50-50 than I would like.
> >
> > To win this game, O has to live on both sides. O doesn't have to save
> > everything, but there have to be stones on both sides. So having low
> > probabilities on *either* side makes O an underdog.
> >
> > RAVE has surprisingly little effect on this. The key moves for O are
> > situational, so they often do not appear beneficial. RAVE is hurt when
> > O loses globally even though it makes a "winning" local play. For
> > example, in one test, after X played D3, O lost faith in F2 because
> > it started out with particularly bad luck on the right and won only
> > 3 out of the first 30 games.
> >
> > One of the key parameters for this position is the number of trials
> > required to find B9 in response to O's initial play of J5, J8 or H9.
> > If O needs thousands of trials to discover B9 then the UCT search will
> > have a terrible time refuting every forcing sequence that X can play.
> >
> > Programs that count semeais better than Pebbles might solve this
> > quickly. A UCT program can deal with one problem, so if Pebbles was
> > accurate at move ordering *either* side of this battle, then it
> > might be "easy" for UCT to see that O wins. But that isn't what
> > happens, in fact.
> >
> > The bottom line: Pebbles rated X as a 90% favorite when this position
> > occurred in a game. I am working through the many, many tactical issues
> > that have to be addressed to evaluate this situation correctly.
> >
> > It is a rich position because solving it will require upgrades to
> > many aspects of the system: semeai logic, ladders, big eye, nakade,
> > hole-of-three, approach moves, RAVE, and maybe reworking patterns
> > in the UCT portion of the tree. Testing has already revealed bugs
> > and new opportunities.
> >
> > Best,
> > Brian
> >
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> > Computer-go at dvandva.org
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> 
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