

(1) Posted by Andrew Buchanan [Sunday, Apr 25, 2021 05:47] 
8Men Tablebase Hi,
An important update on 8Men Tablebase by Marc Bourzutschky.
http://arves.org/arves/index.php/en/2ongecategoriseerd/15098mentablebasefirstexplorations?fbclid=IwAR2BwvtDqDsu_2HIZjzHKWGgzJg42e1SX1SugBV_fLxr8KfQ3rfp3iOt1C4
This is worth reading in detail, but one point demands quotation:
QUOTE An important question is at what point the chess board becomes so crowded that adding more pieces does not lead to longer winning lines due to the increased likelihood of shortening captures. My results suggest that we may already be at or close to this saturation point: the longest winning line for 8man endgames without pawns appears to be “only” 400 moves. [...] After generating about 15% of the pawnless endings I’m quite confident to have captured the longest ones. While 15% seems like a small subset at first blush, most other piece configurations have large material differences between White and Black so that long lines are unlikely. 

(2) Posted by Eric Huber [Sunday, Apr 25, 2021 13:40] 
One of the interesting new outcomes obtained from 8Men Tablebase:
(= 4+4 )
White to move loses
Black to move loses
Marc Bourzutschky provides some incredible variations and welcome explanations at the link above. 

(3) Posted by Torsten Linß [Sunday, Apr 25, 2021 18:15] 
BTW, the idea of 8men tablebases is not very original:
https://pdb.dieschwalbe.de/search.jsp?expression=a%3D%27pali%27+and+wpieces%3D2+and+bpieces%3D6+and+g%3D%27h%23%27+and+year%3D%27201%25%27
Even 7mentablebase problems had been published 16 year prior to the Lomonossov tablebases:
https://pdb.dieschwalbe.de/search.jsp?expression=PROBID%20IN%20%27P1014870;%20P1014888%27 

(4) Posted by Viktoras Paliulionis [Monday, Apr 26, 2021 00:55] 
Torsten, I think such tablebases require a supercomputer, which I unfortunately don’t have (I created the above problems without using any tablebases). What computer resources did you need to create 7men tablebases? 

(5) Posted by Torsten Linß [Monday, Apr 26, 2021 07:48] 
There are subsets of the tablebase that can be generated independently of the rest of the tablebase. Bourzutschky picked the material KQRRkqrr with at most 2222255026372 position. That requires about 2 TB of storage. That's less than the complete 6men tablebases. These days that's doable on a mediumsized workstation or even an upperend desktop PC. It gets more interesting when pawns are on the board.
In 1996, when generating P1014870 and P1014888 I used a PC with at most 8MB main memory. The material I picked (without Queen) was just particularly suitable. 

(6) Posted by Joost de Heer [Monday, Apr 26, 2021 08:57] 
QUOTE BTW, the idea of 8men tablebases is not very original:
The idea of 8man tablebases exists ever since the first idea of tablebases, it's just that until recently the resources needed for 8men TBs weren't available. s# tablebases (and h# tablebases) are probably easier because they're a lot less deep (compare max depth of the 7piece s# TBs to the max depth of # TBs...), and therefore require less resources. 

(7) Posted by Torsten Linß [Monday, Apr 26, 2021 10:47] 
Wrong #1: The resources for doing the eg KQRRkqrr (with only 2T positions, or other pawnless 8men tablebases) were available 10 years ago (the hardware would have been about 12.000 EUR in those days).
Wrong #2: The crucial point is not depth (move length), but total number of positions, and the *number of legal positions* does not depend on the stipulation (h#, s#, #/eg, ...). 

(8) Posted by Viktoras Paliulionis [Monday, Apr 26, 2021 12:26] 
KQRRkqrr is very simple case since it is symmetrical and has repeating pieces. For example, KQRNkqbn has more than 10T of legal positions (there are more of them with illegal ones in total). In addition, this number should be multiplied by two (White to move, Black to move). Minor tables should also be included (when at least one piece is captured). This would require more than one HDD for just one tablebase. 

(9) Posted by Torsten Linß [Monday, Apr 26, 2021 12:53] 
Indeed, also you need more than one byte to store a DTM > 256... ;) Some disc space can be saved by using compression. I pipe my outputs through GNUzip. That saves about 7580% of disk space. [Though packing and unpacking takes extra time...] 

(10) Posted by Dmitri Turevski [Wednesday, Apr 28, 2021 17:17] 
QUOTE In 1996, when generating P1014870 and P1014888 I used a PC with at most 8MB main memory. The material I picked (without Queen) was just particularly suitable.
Took me a while to realize how exactly not having a queen is beneficial. I guess the point is that w/o a queen any capture would lead to an a priori nonsolvable selfmate, so you didn't have to generate all the minor 6men & 5men tables at all?
Doing that with only 8M of RAM is really impressive. Table generation itself could be pretty streamlined, but searching them with this little memory should have taken weeks if not months! 

(11) Posted by Torsten Linß [Thursday, Apr 29, 2021 22:05] 
I've just redone that particular table base. It has only 64241 positions. 7 byte suffice to code the position, 1 byte suffices for depth and correctness. That's less than 502KB to store the whole table base. ;) 

(12) Posted by Dmitri Turevski [Friday, Apr 30, 2021 08:57] 
QUOTE It has only 64241 positions.
I must be missing something obvious.
There are something like 462 (I guess) ways to legally place two kings if you count mirrored and rotated positions as one.
The white knight can be placed on any of the remaining 62 squares, exclude 8 from where it would check the opponent king (we go for lower bound estimate of legal positions)
Ditto the black knight: 61  8.
462 * (62  8) * (61  8) = 1322244
It seems like there are more than a million of meaningfully different positions for KSKS alone.
Edit:
Unless you mean that there are 64241 positions with a finite DTM. But then this statement is misleading:
QUOTE The crucial point is not depth (move length), but total number of positions, and the *number of legal positions* does not depend on the stipulation (h#, s#, #/eg, ...).
If the crucial point is the number of positions with a finite DTM then this number drastically depends on the stipulation: in, for example, KSKS there are no solvable selfmates, but every legal position is a solvable helpmate. 

(13) Posted by Torsten Linß [Monday, May 3, 2021 17:02] 
Indeed, KRRBSkb is rather sparse, which made its computation possible 25 years ago. Other s#tablebases are more dense. I've encountered some where more then 30% of the legal positions have a solution. In h#tablebases this ratio is close to 100%. [No idea what that ratio is for egtablebases.] 

(14) Posted by Andrew Buchanan [Thursday, May 6, 2021 07:03] 
So experts, please:
If the longest mate with 8 pieces is less than the longest with 7, where does it go from here in 9,10,...,32? Might we already know the longest mate in all of chess? 

(15) Posted by James Malcom (Rewan Demontay) [Thursday, May 6, 2021 15:33] 
Andrew, I'm fairly sure we just need to wait a bit for them to generate a longer mate. :)
Tim Krabbe, Journal Entry #393: https://timkr.home.xs4all.nl/chess2/diarytxt.htm
After the revealing of the mate in 549, this is said: "A scary part of Haworth's article is where he extrapolates the findings in sub8man Endgame Tables to predictions for 8 to 10man endgame.
(cue graph, estimates for 8, 9, and 10 checkmate lengths in plies along with the known 17 lengths)
It is "pretty awesome that maxDTM seems to be doubling with each man," Haworth writes  in this logarithmic graph, we see what that might mean: an 80% probability that a 10man endgame exists where a forced mate takes more than 5,000 moves." 

(16) Posted by Andrew Buchanan [Thursday, May 6, 2021 15:45] 
James that expectation for 8 pieces that you wrote about has not been fulfilled. That's the news. There are some reasons to speculate about more complexity with 9 pieces, due to their being a few more unbalanced but close matchups, but the underlying idea is now that more pieces means more space to find a speedier mate. 

(17) Posted by Peter Wong [Tuesday, May 11, 2021 06:16] 
Thanks for the tip about this important update, Andrew!
Here's another rare fullpoint mutual zugzwang position from the article:
(= 4+4 )
"wtm lost in 1; btm lost in 13"
For WTM, although there's a dual mate after 1.S~, this is just like the set play of a complete block 2mover.
There doesn't seem to be a PGN file for the BTM win (not sure what's in the EPD files). Can anyone find the win? Since the metric used is Distance to Conversion, presumably it takes White 13 moves to win a black piece. Stockfish without access to tablebases can't find it. 

(18) Posted by Geir Sune Tallaksen Østmoe [Tuesday, May 11, 2021 09:31] 
Hard to prove without a tablebase. My best guess is that White can force a knight exchange in 13 moves, but Stockfish without tablebases doesn't know that a knight exchange proves a win for White.
Since a knight exchange is a general win, I suppose this material is also a general win, with exceptions like this zugzwang. 

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