[Computer-go] Tactical position

David Fotland fotland at smart-games.com
Mon Aug 23 21:43:52 PDT 2010


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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