J/F Bearoff database

[Vince Mounts]

I was reading a post from a little while back that talked about a bug in J/F
when it is using its bearoff database. Basically what it came down too was
that jelly voluntarily gave up a gammon because it was blindly following its
bearoff database. In other words the move that was best overall for a fast
bear-off failed to take a chip off when the opponent was going to win the
next move. So first this gave me an assumption which perhaps someone can
confirm or refute. The assumption is that the bearoff database does not take
into account the opponents side of the board. It contains info such that
given a specific position and dice roll should be played in a specific way
to get the pieces off in the quickest fashion long term,,, without regard
for what the opponents position is.

This raised the question to me as to wether there are also other situations
where this might cause jelly to play an incorrect move. Not neccessarily
giving up gammons but reducing its overall chances(equity). I was thinking
that this would occur in situations where only a number of miracle rolls
will win the game. I seem to remember a comment in an match that Kit Woolsey
commented(either Woolsey-Bagai or the demo match in the match quiz series)
that he made a certain move because only miracles could win it so may as
well play for the miracles. This leads me to believe that there must be
positions where jellyfish would play from its database and not notice that
its best or only chances to win were to play for miracle doubles. Or in
other words the best chances lie in making a move where smaller sets of
doubles(miracles) will win it.

Granted this cannot be a loss of much equity but hey I was thrilled when I
found out that jelly made a blunder in a situation that even I would easily
see what to do so thought I would see what everyone else thinks or if
someone could come up with a concrete example. I tried to dream one up using
a 4 chip end game but there didnt seem to be any situations there that would
meet these requirements. After I post this I think I am gonna try to find
the specific position that I was thinking of and check it out. In the
meantime would love to hear if anyone else can think of such a situation.
Its a small victory but against jelly I'll take any I can get.

[Vince Mounts]

I found it and it didnt take long. Below is the position I was speaking of
in my previous post.

X to play 6-3
13 14 15 16 17 18 19 20 21 22 23 24
+-----------------------------------------+
| . . . . . . | | O . O O O . |
| . . . . . . | | O . O O O . |
| . . . . . . | | . . O . O. . |
| . . . . . . | | . . . . . . |
| . . . . . . | | . . . . . . |
| | | |
| . . . . . . | | . . . . . . |
| . . . . . . | | . . . . . . |
| . . . . . . | | . X X X . . |
| . . . . . . | | X X X X X . | +-+
| . . . . X . | | X X X X X X | |2|
+-----------------------------------------+ +-+
12 11 10 9 8 7 6 5 4 3 2 1

Jellyfish with database turned on plays 8/2 3/off. So presumably in a close
race this is the fastest way to get all men off and is based on the most
likely sequence of rolls to occur. In the match 8/5 6/off was played and it
was suggestd by Kit that he played this because only miracle doubles would
win so may as well play for miracles. This play obviously makes 5-5 more
productive next roll taking 4 men off instead of only 3. 6-6 obviously does
this with either move but this move makes another set of doubles better. In
a close playing for big doubles is not best but when trailing it is.

So wooo wooo,,, yet another Jelly flaw :)... though I should mention that if
you turn the bear-off database off Jelly appears to recognize this fact and
makes Kit's play. Which raises the question. When is jelly stronger,,, with
or without this database. Does it gain enuogh equity in close races to
justify the equity it gives up when it needs miracles? My guess would be yes
but I dont know for sure. What is the minimum for Jelly to be behind in a
race before it starts giving up equity? 4 chips behind? 5?

[Fredrik Dahl]

>
> I imagine the biggest difference this can ever make is 5/216, or about
> 2.3% in wins, or .046 in equity. This occurs when your opponent is off
> in one roll on a double and two rolls on a non-double, and you can make
> your play now to make one extra double work for you on the next turn.
> So you gain when your opponent doesn't roll doubles (5/6) and you roll
> that one specific double (1/36). We can see that in this particular
> position, 8 rolls from bearing off (before moving) and 2-3 rolls (4-6
> checkers) down in the race, it only makes a difference of about .4% or
> .008 in equity.

JF has another database for short bearoffs which kicks in when both sides
have 6 or fewer checkers (read the manual ;-)).
This base gives probability distribution for the number of rolls to bear
off,
and these are used to calculate equities by convolution.
I didn't really do any research into this, but I figured that for bearoffs
longer than
this, playing for minimizing the expected number of rolls to get off
couldn't cost much.

Fredrik Dahl.

[Olivier Egger]

Hi,

according to Snowie Pro the difference in equity between 8/5 6/off and 8/2
3/off is 0.008. Snowie uses a large bearoff database so these numbers are
exact:

X to play (6 3)
+24-23-22-21-20-19-------18-17-16-15-14-13-+
| O O O O | | |
| O O O O | | |
| O O | | | S
| | | | n
| | | | o
| |BAR| | w
| | | | i
| | | | e
| X X X | | |
| X X X X X | | |
| X X X X X X | | X |
+-1--2--3--4--5--6--------7--8--9-10-11-12-+
Pipcount X: 61 O: 36
Men Off X: 0 O: 5
CubeValue: 1

1. D 8/5 6/off Eq.: -0.920
0.0% 0.0% 4.0% 96.0% 0.0% 0.0%
2. D 8/2 3/off Eq.: -0.928 (-0.008)
0.0% 0.0% 3.6% 96.4% 0.0% 0.0%
3. D 8/2 6/3 Eq.: -0.957 (-0.037)
0.0% 0.0% 2.2% 97.8% 0.0% 0.0%
4. D 8/2 4/1 Eq.: -0.963 (-0.043)
0.0% 0.0% 1.9% 98.1% 0.0% 0.0%
5. D 8/2 5/2 Eq.: -0.963 (-0.043)
0.0% 0.0% 1.8% 98.2% 0.0% 0.0%

Olivier Egger, Oasya