Author: Robert Hyatt
Date: 06:55:34 11/24/97
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On November 24, 1997 at 00:47:37, Jouni Uski wrote: >Solution times for Mchess 6 in K6/200 harware: > >1. 0 sec >2. 0 sec >3. 17 sec >4. 4 sec >5. 41 sec (found Na5 after 10 ply only!) > >Jouni The first cut thru crafty found #5 quickly. It turned out to be a bug. Here's why finding this quickly is bad: after giving up the knight, the opponent has two tasks: 1. Keep the king away from the queening square because of the wrong bishop; 2. capture both pawns that are blocking the promotion. Winning the pawns can only be done at the expense of letting the other king reach the queening square and draw. And there is no way to capture the pawns and prevent this. But the program can try for a long time. My bug was "assuming" the king could reach a1 and draw. Which was correct. But there were also positions where it was just barely prevented from reaching, and it still thought draw. I'd be concerned for any program that finds this quickly, as that program is likely making an assumption that works in this position, but fails in others, where the pawn and bishop just barely prevent the king from getting to the queening square. This happened in crafty a couple of months back and someone sent me the position where it failed. It turned out that I *had* to make sure the king physically reached the queening square before calling it a draw. So solving #5 at shallow depths is most likely wrong, because it takes *many* plies to actually see that black can't win the pawns and hold the king out. So black uses his king and bishop to drive the white king away, then runs back over to grab one pawn right at the horizon and thinks he is still winning. Each additional ply is used to extend the win of one or both pawns to right at the horizon without giving white a chance to reach a1...
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