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Position # 57

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Offline PersianLord

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Position # 57
« on: April 07, 2009, 12:17:16 AM »
Position information:

Pip-count:  Red 78-91 White
Score:       Red   2-2  White
Match:               3-pointer

Red to play 5-2

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The leftist's feelings of inferiority run so deep that he cannot tolerate any classification of some things as successful or superior and other things as failed or inferior. This also underlies the rejection by many leftists of the concept of mental illness and of the utility of IQ tests.  - T.K

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Position # 57
« on: April 07, 2009, 12:17:16 AM »

Offline ah_clem

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Re: Position # 57
« Reply #1 on: April 07, 2009, 04:08:51 PM »
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Re: Position # 57
« Reply #1 on: April 07, 2009, 04:08:51 PM »

Offline dropper

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Re: Position # 57
« Reply #2 on: April 07, 2009, 08:09:12 PM »
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Offline diane

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Re: Position # 57
« Reply #3 on: April 07, 2009, 10:08:53 PM »
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Re: Position # 57
« Reply #3 on: April 07, 2009, 10:08:53 PM »

Offline ah_clem

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Re: Position # 57
« Reply #4 on: April 20, 2009, 10:24:46 PM »
So, do we get a rollout/answer here? 

Offline blitzxz

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Re: Position # 57
« Reply #5 on: April 21, 2009, 07:25:06 AM »
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Offline dorbel

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Re: Position # 57
« Reply #6 on: April 21, 2009, 06:27:20 PM »
Like so many bg positions, the answer to "Should I hit if gammons count?" is, "It depends". At 2-away, crawford, should you hit? No, hitting is an error and 24/17 is correct. That play will win about 80% of the time and you may even fluke a gammon (2-3%) if White rolls 2-2 or 1-1 next and you hit. If you hit now on the other hand, your gammons will go up to about 10%, but this is not enough to compensate for the fact that your overall wins will drop to something closer to 70%.
How about 4-away, 2-away? If the cube is centred, then you should hit. If you have already doubled, then the safe 23/17 is correct! What is going on here? With the cube in the middle, you get a big cash when White dances after you hit, but if White owns the cube you won't be able to do that and you will actually have to win the game by playing it until the end. There just aren't enough gammons in this position to compensate for the fact that Red has to escape very quickly after hitting or crash.
The key to understanding this position is that Red's board can only get worse, whereas White's can only get better! 24/17 is always right with the sole exception of the 4-away, 2-away centre cube position, where hit and (hope to) cash is the order of the day. Note that at most normal scores you can double (and get a take) after the incorrect hit and a dance, but you usually have a much stronger double after making the safe play anyway.
The figures quoted here are from relatively short snowie rollouts. Gnu may differ a little and IMO will probably be a little more accurate, but the underlying principles should remain the same.
« Last Edit: April 21, 2009, 06:34:33 PM by dorbel »

Offline Zorba

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Re: Position # 57
« Reply #7 on: April 21, 2009, 08:17:47 PM »
Here's the GNUBG evaluations and quick and dirty 0-ply rollout result. Whopper to hit at DoubleMatchPoint.

    1. Cubeful 2-ply    24/17                        Eq.:  +0,5605
        78,02   2,40   0,00 -  21,98   0,29   0,00
        2-ply cubeful prune [world class]
    2. Cubeful 2-ply    17/15*/10                    Eq.:  +0,3994 ( -0,1611)
        69,97  10,04   0,02 -  30,03   2,19   0,01
        2-ply cubeful prune [world class]

    1. Cubeful 3-ply    24/17                        Eq.:  +0,5764
        78,82   1,81   0,01 -  21,18   0,40   0,00
        3-ply cubeful prune [grandmaster]
    2. Cubeful 3-ply    17/15*/10                    Eq.:  +0,4628 ( -0,1135)
        73,14   7,33   0,02 -  26,86   1,78   0,01
        3-ply cubeful prune [grandmaster]

    1. Cubeful 4-ply    24/17                        Eq.:  +0,5828
        79,14   2,47   0,00 -  20,86   0,35   0,00
        4-ply cubeful prune
    2. Cubeful 4-ply    17/15*/10                    Eq.:  +0,3987 ( -0,1841)
        69,94  11,01   0,02 -  30,06   2,13   0,01
        4-ply cubeful prune
============================================
    1. Rollout          24/17                        Eq.:  +0,5859
        79,29   4,83   0,01 -  20,71   0,48   0,00 CL  +0,5859
      [  0,12   0,38   0,01 -   0,12   0,08   0,07 CL   0,0024]
        Full cubeless rollout (trunc. at one-sided bearoff) with var.redn.
        1296 games, Mersenne Twister dice gen. with seed 818332620 and quasi-random dice
        Play: 0-ply cubeful prune
        Cube: 0-ply cubeful prune
    2. Rollout          17/15*/10                    Eq.:  +0,4464 ( -0,1395)
        72,32  18,38   0,05 -  27,68   2,29   0,05 CL  +0,4464
      [  0,22   0,70   0,02 -   0,22   0,19   0,03 CL   0,0045]
        Full cubeless rollout (trunc. at one-sided bearoff) with var.redn.
        1296 games, Mersenne Twister dice gen. with seed 818332620 and quasi-random dice
        Play: 0-ply cubeful prune
        Cube: 0-ply cubeful prune
The fascist's feelings of insecurity run so deep that he desperately needs a classification of some things as successful or superior and other things as failed or inferior. This also underlies the fascist's embracement of concepts like mental illness and IQ tests.  - R.J.V.

Luck is my main skill

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Re: Position # 57
« Reply #7 on: April 21, 2009, 08:17:47 PM »

Offline Zorba

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Re: Position # 57
« Reply #8 on: April 21, 2009, 08:51:31 PM »
How about 4-away, 2-away? If the cube is centred, then you should hit. If you have already doubled, then the safe 23/17 is correct!

I checked it with GNUBG because this seemed very unlikely to me. I think you might've misinterpreted something. It's very wrong to hit when you still have to double. Maybe even more so at 4-away 2-away. When the hit works, it's overkill, because you're gonna cash instead of winning those extra gammons, so you just get to -3,-2 and the gammons are useless. When opponent enters on the sixpoint (11 rolls), you don't have much cube leverage anymore, if at all. 6-4 and 6-6 are horror returns after which you become an instant clear underdog.

After the quiet 24/17, you have a lot of decent to highly efficient cubes (small drops or small takes), simply because of the race advantage. Only the doubles from 3-3 up (4 rolls) give your opponent decent winning chances and a good take or even no double for you.

Now, at -4,-2 with opponent holding a 2-cube, gammon wins are twice as valuable here because you need exactly four points to win the match. So that's a much better time to hit. If hitting wins 10% extra gammons, at the cost of losing 10% more single games, then at the pure Gammon-Go scores of -2,-1Cr and -4,-2 (opp owns 2-cube) the plays would be equal.

As the evaluation gets win and gammon percentages very close to that, a rollout is needed for those.

The fascist's feelings of insecurity run so deep that he desperately needs a classification of some things as successful or superior and other things as failed or inferior. This also underlies the fascist's embracement of concepts like mental illness and IQ tests.  - R.J.V.

Luck is my main skill

Offline NIHILIST

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Re: Position # 57
« Reply #9 on: April 21, 2009, 11:03:45 PM »
Isn't anyone going to tell me how brilliant I was for playing 24-17 in the actual game ?

Bob
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Offline dorbel

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Re: Position # 57
« Reply #10 on: April 22, 2009, 07:33:22 AM »
Absolutely right Zorba. 24/17 is correct at all times, for the reasons that you give, particularly the very inefficient cube after the dance. I fell victim to a Snowie bug, when for reasons of its own it looks at that position at 4-away, 2-away and evaluates it at 2-ply and gets it badly wrong. Well spotted.

Offline Zorba

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Re: Position # 57
« Reply #11 on: April 22, 2009, 01:13:45 PM »
It looks like the hit may just be right at pure gammon-go scores, where gammons are twice as valuable as usual. F.i. trailing -4,-2 with the cube already given, or trailing -2,-1 in the Crawford game: any loss loses the match, a single win makes the match equal, a gammon win wins the match:

    1. Rollout          17/15*/10                    Eq.:  +0,5952
        72,36   7,40   0,11 -  27,64   2,62   0,05 CL  +0,5952 CF  +0,5952
      [  0,10   0,21   0,04 -   0,10   0,23   0,02 CL   0,0049 CF   0,0049]
        Full cubeful rollout with var.redn.
        2592 games, Mersenne Twister dice gen. with seed 818332620 and quasi-random dice
        Play:  2-ply cubeful prune [world class]
        keep the first 0 0-ply moves and up to 12 more moves within equity 0,12
        Skip pruning for 1-ply moves.
        Cube: 0-ply cubeful prune [expert]
    2. Rollout          24/17                        Eq.:  +0,5723 ( -0,0229)
        78,08   0,53   0,02 -  21,92   0,35   0,02 CL  +0,5723 CF  +0,5723
      [  0,06   0,06   0,01 -   0,06   0,15   0,02 CL   0,0018 CF   0,0018]
        Full cubeful rollout with var.redn.
        1296 games, Mersenne Twister dice gen. with seed 818332620 and quasi-random dice
        Play:  2-ply cubeful prune [world class]
        keep the first 0 0-ply moves and up to 12 more moves within equity 0,12
        Skip pruning for 1-ply moves.
        Cube: 0-ply cubeful prune [expert]


Here, the hit loses 5.7% more games overall, but gains an extra 6.9% gammons. At Gammon-Go scores with gammon value (price) of 1, that makes the gammonish play correct.

Now if Bob (or anybody else) had gotten that one right, I might've considered it brilliant!

P.S. Tom Keith' site has a very good glossary of backgammon terms, for those of you who may wonder about some of the terms being used:
http://bkgm.com/glossary.html
« Last Edit: April 22, 2009, 01:24:20 PM by Zorba »
The fascist's feelings of insecurity run so deep that he desperately needs a classification of some things as successful or superior and other things as failed or inferior. This also underlies the fascist's embracement of concepts like mental illness and IQ tests.  - R.J.V.

Luck is my main skill

FIBS Board backgammon forum

Re: Position # 57
« Reply #11 on: April 22, 2009, 01:13:45 PM »

Offline PersianLord

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Re: Position # 57
« Reply #12 on: April 23, 2009, 03:56:41 PM »
Isn't anyone going to tell me how brilliant I was for playing 24-17 in the actual game ?

Bob

Glory be to thee, leviathan.

PL
The leftist's feelings of inferiority run so deep that he cannot tolerate any classification of some things as successful or superior and other things as failed or inferior. This also underlies the rejection by many leftists of the concept of mental illness and of the utility of IQ tests.  - T.K

Offline NIHILIST

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Re: Position # 57
« Reply #13 on: April 23, 2009, 06:17:48 PM »
Finally some respect.

Bob
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