Author: Dan Homan
Date: 09:04:05 12/15/99
In a game recently sent to me by a tester, EXchess reached the following
position against another program
8/8/8/7r/2K5/8/6kp/4R3 b - -
+---+---+---+---+---+---+---+---+
8 | |:::| |:::| |:::| |:::| Black to move
+---+---+---+---+---+---+---+---+
7 |:::| |:::| |:::| |:::| | castle: -
+---+---+---+---+---+---+---+---+
6 | |:::| |:::| |:::| |:::|
+---+---+---+---+---+---+---+---+
5 |:::| |:::| |:::| |:::|<R>|
+---+---+---+---+---+---+---+---+
4 | |:::| K |:::| |:::| |:::| fifty: 0
+---+---+---+---+---+---+---+---+
3 |:::| |:::| |:::| |:::| |
+---+---+---+---+---+---+---+---+
2 | |:::| |:::| |:::|<K>|<P>|
+---+---+---+---+---+---+---+---+
1 |:::| |:::| | R | |:::| |
+---+---+---+---+---+---+---+---+
a b c d e f g h
In this position, EXchess probed tablebases, found a mate and moved
Rh4+?
It turns out that the tester has the KRKRP tablebase, but not the
KRKRQ tablebase. So as long as the pawn was on the board, EXchess
saw a mate, but with the pawn promoted to a queen - EXchess would
have to search several plies to see the mate (after an exchange into
one of the 3 or 4 man tablebases). As a result, EXchess will
prefer Rh4+ in this position for several iterations - unfortunately,
I have the search set to terminate if a mate is found and held after
the 3rd iteration.
Even if I didn't have the search set to terminate under these conditions,
selecting a move like this would still be a problem if EXchess was under
any kind of time pressure.
Has anyone else dealt with problems like this in their programs? I
assume this kind of thing has come up before... One quick fix would
be to turn off all tablebases if a promotion is possible at the root
position and the corresponding tablebase doesn't exist. There might be
other variations on this problem that this fix doesn't cover, however.
- Dan
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