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Subject: Re: Does your program understand castling/en passant rights on 3x repetition

Author: leonid

Date: 08:13:21 02/27/00

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On February 27, 2000 at 00:15:14, Richard A. Fowell ( wrote:

>I noticed today that six of the seven chess programs that I tried
>(three freeware, four commercial) failed to pass the two tests below.
>(my congratulations to Will Bryant, who programmed Screamer right!)
>The others don't understand how castling rights and en passant capture
>rights effect the ability to claim draw by three-fold repetition of position.

Mistake in castling and "en passant" are the real mistakes. The last one, clame
draw after three fold repetition, is a minor one. This last one is nothing more
that addition to the complet game some final touches. The first two mistakes are
serious and basic one. They will prevent the game to solve the mate, draw and
play game after the strict initial logic.


>The FIDE Laws of Chess (1997, but unchanged from when I learned chess 26 years
>ago) say:
>9.2. The game is drawn, upon a correct claim by the player having the move, when
>the same position, for at least the third time (not necessarily by repetition of
>       (a) is about to appear, if he first writes his move on his scoresheet and
>declares to the arbiter his intention to make this move, or
>       (b) has just appeared.
>       Positions as in (a) and (b) are considered the same, if the same player
>has the move, pieces of the same kind and colour occupy the same squares, and
>       the possible moves of all the pieces of both players are the same.
>Positions are not the same if a pawn could have been captured en passant or if
>theright to castle immediately or in the future has been changed.
>The part programmers seem to have difficulty with is that last sentence.
>This citation is from the Malta Chess Federation site:
>Try these two games out on your program.
>A draw cannot be claimed in either sequence, due to that last
>sentence (and, again - it isn't new).
>#1 Check for knowledge of castling rights on triple repetition draw =
>1. Nf3 Nf6
>2. Rg1 Rg8
>3. Rh1 Rh8   (second occurrence of this visible piece pattern, but first
>occurrence w/o castling rights)
>4. Rg1 Rg8
>5. Rh1 Rh8 ( No draw, yet - this is the original piece pattern, but there
>             both sides had castling rights. To put it another way, the EPD
>             strings for the starting position and this one are different.)
>6. Rg1 Rg8 - draw by triple repetition of position.
>#2 Check for knowledge of en passant opportunity of triple repetition draw =
>1.  a4  a6
>2.  a5  b4
>3. Nf3 Nf6
>4. Ng1 Ng8 (second occurrence of this visible piece pattern, but first
>occurrence w/o en passant capture option)
>5. Nf3 Nf6
>6. Ng1 Ng8 (again, no draw yet - there is no en passant capture possible here,
>            as there was after Black's second move).
>7. Nf3 - draw by triple repetition of position.
>This can be done properly, by keeping track of the
>castling rights and en passant situation.
>Which chess programs (other than Screamer) get these right?
>Richard A. Fowell (
>P.S.: Of course, programs (or humans) don't HAVE to claim the draw.
>      But, for programs that do, they can't claim it
>P.P.S: Screamer is a freeware Macintosh program at:

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