mmt Vs. Ovyron (G4 D5 BG2)

[Graham Banks]

So is this game finished or just permanently on hold?

[Alayan]

Ovyron finally played a move recently.

I apologize for all the circus that has been mounted in our game. But these have been the most interesting discussions I've had in a while! I'm picking up the pace and playing much faster now:

1. g4 d5 2. Bg2 Bxg4 3. c4 c6 4. Qb3 e6 5. Qxb7 Nd7 6. Nc3 Ne7 7. cxd5 exd5 8. d4 Rb8 9. Qa6 Rb6 10. Qd3 Ng6 11. h3 Be6 12. Nf3 Bd6 13. h4 h5 14. b3 Nf6 15. Bg5 O-O 16. e3 Re8 17. Kf1 Bg4 18. Ne1 Bb4 19. Na4 Rb8 20. Nc2 Be7 21. f3 Be6 22. Nc5 Bc8 23. Kf2 Nd7 24. Ne6 Qa5 25. Bxe7 Rxe7 26. b4 Qb6

[d]1rb3k1/p2nrpp1/1qp1N1n1/3p3p/1P1P3P/3QPP2/P1N2KB1/R6R w - -

[Graham Banks]

Ovyron finally played a move recently.

Thanks. Easy to miss amongst all the other posts.

[Ovyron]

Yeah, I acknowledge that and apologize about it in that post. The irony is that if you miss the post you also miss that!

[mmt]

1. g4 d5 2. Bg2 Bxg4 3. c4 c6 4. Qb3 e6 5. Qxb7 Nd7 6. Nc3 Ne7 7. cxd5 exd5 8. d4 Rb8 9. Qa6 Rb6 10. Qd3 Ng6 11. h3 Be6 12. Nf3 Bd6 13. h4 h5 14. b3 Nf6 15. Bg5 O-O 16. e3 Re8 17. Kf1 Bg4 18. Ne1 Bb4 19. Na4 Rb8 20. Nc2 Be7 21. f3 Be6 22. Nc5 Bc8 23. Kf2 Nd7 24. Ne6 Qa5 25. Bxe7 Rxe7 26. b4 Qb6 27.Ng5

[d]1rb3k1/p2nrpp1/1qp3n1/3p2Np/1P1P3P/3QPP2/P1N2KB1/R6R b - - 2 4

[jp]

Mate-in-n is not calculated correctly in many cases because of pruning. If the shortest mating move is a sacrifice for example, but there's another mate that is a bit a longer, the longer mate is likely to be selected because the tree with the sac looks bad compared to the existing mate score. Turning off pruning weakens the engine in general. Stockfish also does aggressive depth reduction in some cases.

May I ask what Arasan does? If it makes mate announcements, does it switch off pruning for the defence (not the attack) and verify every defensive line before making the announcement? (Can you point us to the relevant section of the source code?)

[Ovyron]

jp, all engines do that, you're just wasting time asking.

If the engine hasn't done that, it doesn't have a mate to report, so it'll just show a high score.

It's like asking each engine author if they use minimax in their engine...

In other words: engines that don't do this would just report high scores instead of mate scores in all cases. Fearing that a reported mate may not be maximal or may not be real is just paranoia.

[jp]

Please just let others respond, if they are happy to. I doubt that in the entire history of chess engines, they've all always verified mate announcements. When, almost a quarter of a century ago, an engine reported a mate in 150 (as was reported on this board), it would have had to have made TBs do most of the work for that to be possible (and maybe it did, but who knows?).

It's funny, because I do not remember SF announcing mates in 150 or 250 moves in the last few years (and Sesse, for example, does have TB access). Maybe, just maybe, it's because SF does try to verify its announcements (so, TB or not, it's not easy to get to 250 moves) and other engines do not. (There are other possible explanations, but surely this is one.)

I mean, to make such a sweeping claim that all engines have always verified, when most of them are not open source, is more you wanting to believe something than having strong evidence for it.

[Ovyron]

1. g4 d5 2. Bg2 Bxg4 3. c4 c6 4. Qb3 e6 5. Qxb7 Nd7 6. Nc3 Ne7 7. cxd5 exd5 8. d4 Rb8 9. Qa6 Rb6 10. Qd3 Ng6 11. h3 Be6 12. Nf3 Bd6 13. h4 h5 14. b3 Nf6 15. Bg5 O-O 16. e3 Re8 17. Kf1 Bg4 18. Ne1 Bb4 19. Na4 Rb8 20. Nc2 Be7 21. f3 Be6 22. Nc5 Bc8 23. Kf2 Nd7 24. Ne6 Qa5 25. Bxe7 Rxe7 26. b4 Qb6 27.Ng5

[d]1rb3k1/p2nrpp1/1qp3n1/3p2Np/1P1P3P/3QPP2/P1N2KB1/R6R b - - 2 4

1. g4 d5 2. Bg2 Bxg4 3. c4 c6 4. Qb3 e6 5. Qxb7 Nd7 6. Nc3 Ne7 7. cxd5 exd5 8. d4 Rb8 9. Qa6 Rb6 10. Qd3 Ng6 11. h3 Be6 12. Nf3 Bd6 13. h4 h5 14. b3 Nf6 15. Bg5 O-O 16. e3 Re8 17. Kf1 Bg4 18. Ne1 Bb4 19. Na4 Rb8 20. Nc2 Be7 21. f3 Be6 22. Nc5 Bc8 23. Kf2 Nd7 24. Ne6 Qa5 25. Bxe7 Rxe7 26. b4 Qb6 27. Ng5 Ba6

[d]1r4k1/p2nrpp1/bqp3n1/3p2Np/1P1P3P/3QPP2/P1N2KB1/R6R w - -

[Ovyron]

Please just let others respond, if they are happy to.

I'm not stopping them from responding, I just want to make sure you understand how chess engines work.

Normally after some iteration the engine has a PV with some score, it will try to find improvements for both sides from the tail of the variation to its root, and if it can't it'll show the score to the user and start the next iteration.

What happens on a mate announcement is that the very last move in the PV is a mate position. The engine will register it as a Mate in 0, and it will go back a ply and see if the defending side has a move that avoids mate. If it does then the engine will go back to producing a normal score and try bringing it to the root. If it doesn't it'll register it as a Mate in 1 and go back a ply.

It'll search for a move from the defending side that doesn't get mated and if it finds it it'll go back to a normal score, but if it doesn't find it it'll register a Mate in 3 and go back a ply. And so on, so when a Mate in 25 announcement makes it to the root it's because after following this procedure the engine was unable to find any move from the defending side that had normal non-mate scoring, and all other lines checked from the defending side were a mate in 25 or worse. That's the only way a Mate in 25 can make it to the root.

So there's no possible scenario where a mate announcement made it to the root when it was not real, and there's no possible scenario where a mate announcement that was shorter than reality made it to the root. By design (if so, there's a bug or some extraordinary hash collision that made the engine confuse one position with another one, but you'd see that once in your lifetime.)