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  1. Forum
  2. General Draughts Discussion
  3. puzzle 602

the final position in puzzle 602 leads to what seems to a drawn endgame. I got a draw from that position against ai level 8 at least.

lidraughts.org/Zr1jcPuL/black#1

There's something weird going on here. If you play 21. 15-33 instead of 15-42 then Scan will indicate that the value of the position is 0 and that the best move is 16-27, no matter how long you let it analyse – but after playing 16-27, the score is +3 again. Looks like a bug to me.

For some unknown reason Scan doesn't play the right moves. After 7. ... 27-31 ? the position is lost for Black (Dragon Draughts database). Maybe Scan doesn't use endgame database in lidraughts ?
Anyway, Black gets more defense by playing 7. ... 13-18! then it's much more difficult for White to win.
For example :
8. 19-14 27-31 9. 14-9 (48-42 25-30! 24x35 15-20! 14x25 18-22 and then 22-27-32 =) 31-37 10. 9-4 18-23 11. 4-36 23-28 12. 36-47 28-33 13. 47x29 37-41 14. 29-47 41-46 15. 47-33 46-23 16. 33x6 23x45 17. 6-1 W+ (database)

Author here.

@Jeffrey

Congratulations on the draw! Scan probably made mistakes when answering you, however.

The version I'm using is slightly different, and plays those moves (even with only 5-piece endgame tables):
12. 40-34 (not important)
16. 49-32 (win)
19. 33-39 (win)

Your game is a draw after move 19.

@Tim

16-27 repeats a position. During Scan's search, a repetition immediately ends the game, and is scored as a draw. This makes search more efficient, by avoiding searching moves sequences that have cycles in them (which can never improve the result).

@Faraday

In your line, Scan proposes 12. 24-19. Here is a blitz game:
1... 6-11 2. 7x16 8-12 3. 35-30 20-25 4. 30-24 12-17 5. 34-29 18-22 6. 29-23
22-27 7. 23-19 13-18 8. 19-14 27-31 9. 14-9 31-37 10. 9-4 18-23 11. 4-36 23-28
12. 24-19 25-30 13. 19-13 17-22 14. 36x18 30-35 15. 40-34 37-41 16. 16-11 28-32
17. 11-7 15-20 18. 18-36 41-47 19. 7-2 20-24 20. 2-11 24-30 21. 34x25 32-37

Fabien Letouzey.

Nice to see you posting here Fabien

To add to @Faraday's question, when you run Scan in the browser on the lidraughts analysis boards, no tablebases are used (too large to download and run locally). When requesting server-side analysis Scan does use 5-piece tablebases.

Hi Fabien, kind of you to personnaly answer ! I used to follow your discussions about draughts computing in the World Draughts Forum ; I wish I had more skills on coding to understand you :)

@RoebStoep : thanks for the explanation !