Author: James Robertson
Date: 21:14:34 05/27/99
Go up one level in this thread
On May 27, 1999 at 21:59:23, James T. Walker wrote:
>On May 27, 1999 at 19:43:43, James Robertson wrote:
>
>>One of my friends tried my program on this 'Mate in 2' from John Thursby's 75
>>mate in 2 problems.
>>
>>FEN 8/8/3PQ3/2p2r2/2N2R2/2k2N2/3RK3/8/ w - -
>>
>>My program didn't see it, supplying a mate in 3 line (my email program crashed
>>and I lost the message.) Crafty also gave a mate in 3 line, 1. Qe3+ Kb4 2. Rb2+
>>Ka4 3. Qa8#.
>>
>>I responded with this, and he said no! It is still mate in 2, and supplied this
>>line: 1.Na5! {threatens Rc4# or Qe3# or Qc4# or Qb3#} 1 ... Rxf4 2. Qb3#
>>
>>I have not had time to check on it... is it mate in 3 or 2?
>>
>>James
>>
>>P.S. He also supplied this funny mate in 2 links:
>>http://misc.traveller.com/chess/graphics/cake.htm
>>http://www.hsv.tis.net/chess/graphics/boi.htm
>
>Hello James,
>You will find this is very common and not a "Problem" as I see it. Many
>programs will find a "longer" mate due to check extensions. Because the shorter
>mates are not always all checks you may not find it until later if your program
>does not stop on finding the "Long" mate. This particular mate though, should
>only take a one ply search with check extensions. Probably your program
>searches the queen move before the Knight move and therefore finds the mate by
>moving the queen + check extensions before finding the shorter mate with the
>Knight move + check extensions. I think you could avoid this by forcing your
>program to search the full ply before accepting the first mate found. Then you
>could use the "shortest" mate found.
>Jim Walker
You are close, but that is not quite the explanation. Both Crafty and my program
will complete the ply if they find a mate (there may be a shorter one than the
fist one found, due to like you mentioned check extensions), but my program
terminates the search after if finds a mate and finishes the ply. Programs like
LambChop continue searching until the time is up, so they would find the mate.
James
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