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Old February 10th 04, 02:52 PM
Spur
 
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Default 3-move repetition and FEN notation

FEN notation is supposed to represent a game state fully. It does
represent castling permissions, enpassant, 50-move draw. But it
doesn't represent 3-move repetition draw. How come ?
Given some position in FEN notation, there's no way to know whether
we're in the middle of a 3-move repetition, so FEN doesn't fully
represent a position. Or am I misunderstanding something ?

Thks

Eli
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Old February 10th 04, 07:41 PM
Arnold Meijster
 
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Default 3-move repetition and FEN notation


Hi,

Actually it is not a 3-move repetition but a repetition of
3 times the same board situation. This means that you
do need a transcription of the entire game (or at least
up till the last capture or pawn move), which FEN
indeed does not include.

Cheers,

Arnold

"Spur" wrote in message
om...
FEN notation is supposed to represent a game state fully. It does
represent castling permissions, enpassant, 50-move draw. But it
doesn't represent 3-move repetition draw. How come ?
Given some position in FEN notation, there's no way to know whether
we're in the middle of a 3-move repetition, so FEN doesn't fully
represent a position. Or am I misunderstanding something ?

Thks

Eli



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Old February 10th 04, 11:23 PM
David Richerby
 
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Default 3-move repetition and FEN notation

Spur wrote:
FEN notation is supposed to represent a game state fully. It does
represent castling permissions, enpassant, 50-move draw. But it
doesn't represent 3-move repetition draw. How come ?


FEN only represents a position up to telling you what moves are legal. It
also tells you when the 50-move counter expires but, as Arnold Meijster
says in his followup to your post, you need to know every move since the
last irrevocable move (pawn moves or captures) to know whether there's a
draw by threefold repetition in the offing.


Dave.

--
David Richerby Strange Metal Lotion (TM): it's like
www.chiark.greenend.org.uk/~davidr/ a soothing hand lotion that's made of
steel but it's totally weird!
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Old February 11th 04, 07:10 AM
Spur
 
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Default 3-move repetition and FEN notation

David Richerby wrote in message ...
Spur wrote:
FEN notation is supposed to represent a game state fully. It does
represent castling permissions, enpassant, 50-move draw. But it
doesn't represent 3-move repetition draw. How come ?


FEN only represents a position up to telling you what moves are legal. It
also tells you when the 50-move counter expires but, as Arnold Meijster
says in his followup to your post, you need to know every move since the
last irrevocable move (pawn moves or captures) to know whether there's a
draw by threefold repetition in the offing.


FEN is indeed enough for legal move generation, but why does it include
the 50-ply and full-move counters ? These are of no importance for move
generation.

It just seems a bit incomplete.
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Old February 11th 04, 08:09 AM
Mikko Nummelin
 
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Default 3-move repetition and FEN notation

On Wed, 10 Feb 2004, Spur wrote:

David Richerby wrote:


Spur wrote:


FEN notation is supposed to represent a game state fully. It does
represent castling permissions, enpassant, 50-move draw. But it
doesn't represent 3-move repetition draw. How come ?


FEN only represents a position up to telling you what moves are legal.
It also tells you when the 50-move counter expires but, as Arnold
Meijster says in his followup to your post, you need to know every move
since the last irrevocable move (pawn moves or captures) to know
whether there's a draw by threefold repetition in the offing.


FEN is indeed enough for legal move generation, but why does it include
the 50-ply and full-move counters ? These are of no importance for move
generation.


It just seems a bit incomplete.


These are both valuable information. For example a chess engine might want
to quicky test for the 50-move case like this:

if(brd-half_moves=100)
return DRAW;

rather than browsing through all the stuff in a PGN file. Full moves is
important information when chess clocks are used. Indeed, FEN is somewhat
incomplete, and the following additional fields would perhaps be quite
appropriate:

Earlier repetitions of this positions, value '0', '1', '2' or '-', where 0
and 1 are 0/1 earlier repetitions, 2 is threefold repetition and therefore
immediate draw, '-' is unknown.

Clock type, value 'c' (conventional), 'i' (incremental), 's' (stmove) or
'-' (none) and numeric values mps, base and inc and numeric values, how
much white and black have time left in their clocks measured in seconds.
Again '-' if this information is not known.


Mikko Nummelin


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Old February 11th 04, 11:40 AM
David Richerby
 
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Default 3-move repetition and FEN notation

Mikko Nummelin wrote:
Indeed, FEN is somewhat incomplete, and the following additional fields
would perhaps be quite appropriate:

Earlier repetitions of this positions, value '0', '1', '2' or '-', where
0 and 1 are 0/1 earlier repetitions, 2 is threefold repetition and
therefore immediate draw, '-' is unknown.


EPD (Extended Position Description) already does this.


Clock type, value 'c' (conventional), 'i' (incremental), 's' (stmove) or
'-' (none) and numeric values mps, base and inc and numeric values, how
much white and black have time left in their clocks measured in seconds.
Again '-' if this information is not known.


You need more information than that to specify a time control. Consider
the case where you have 90 minutes for the first forty moves and then
thirty minutes plus thirty seconds per move for the remainder of the game.


Dave.

--
David Richerby Hungry Sadistic Cat (TM): it's like
www.chiark.greenend.org.uk/~davidr/ a cuddly pet but it wants to hurt you
and it'll eat you!
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Old February 11th 04, 02:12 PM
Mikko Nummelin
 
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Default 3-move repetition and FEN notation

On Wed, 11 Feb 2004, David Richerby wrote:

Mikko Nummelin wrote:


Indeed, FEN is somewhat incomplete, and the following additional fields
would perhaps be quite appropriate:


Earlier repetitions of this positions, value '0', '1', '2' or '-', where
0 and 1 are 0/1 earlier repetitions, 2 is threefold repetition and
therefore immediate draw, '-' is unknown.


EPD (Extended Position Description) already does this.


That sounds fine!

Clock type, value 'c' (conventional), 'i' (incremental), 's' (stmove)
or '-' (none) and numeric values mps, base and inc and numeric values,
how much white and black have time left in their clocks measured in
seconds. Again '-' if this information is not known.


You need more information than that to specify a time control. Consider
the case where you have 90 minutes for the first forty moves and then
thirty minutes plus thirty seconds per move for the remainder of the
game.


Oh yes, the guillotine. I left it out as it is not part of XBoard Chess
Engine Protocol and neither recognised in FICS. It would be easy to add
the "simple" guillotine type to the clock type list, but more complex
definitions would need a full language specification.


Mikko Nummelin
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Old February 11th 04, 03:42 PM
David Richerby
 
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Default 3-move repetition and FEN notation

Mikko Nummelin wrote:
On Wed, 11 Feb 2004, David Richerby wrote:
Mikko Nummelin wrote:
Earlier repetitions of this positions, value '0', '1', '2' or '-',
where 0 and 1 are 0/1 earlier repetitions, 2 is threefold repetition
and therefore immediate draw, '-' is unknown.


EPD (Extended Position Description) already does this.


That sounds fine!


(I should clarify: EPD achieves the same effect but by a slightly
different mechanism.)


You need more information than that to specify a time control.
Consider the case where you have 90 minutes for the first forty moves
and then thirty minutes plus thirty seconds per move for the remainder
of the game.


Oh yes, the guillotine. I left it out as it is not part of XBoard Chess
Engine Protocol and neither recognised in FICS. It would be easy to add
the "simple" guillotine type to the clock type list, but more complex
definitions would need a full language specification.


PGN has enough of a language for this in the TimeControl tag. It's
important as, while FICS doesn't support it, pretty much all tournament
and match play has some sort of multi-part time control.


Dave.

--
David Richerby Pickled Hi-Fi (TM): it's like a music
www.chiark.greenend.org.uk/~davidr/ system but it's preserved in vinegar!
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Old February 12th 04, 01:50 PM
Dr A. N. Walker
 
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Default 3-move repetition and FEN notation

In article ,
Mikko Nummelin wrote:
[...]. For example a chess engine might want
to quicky test for the 50-move case like this:
if(brd-half_moves=100)
return DRAW;
rather than browsing through all the stuff in a PGN file.


Of course, the 50-move rule is not "DRAW" but "DRAW can
be claimed". The difference is unimportant in analysis, but
matters in a game.

[...]. Indeed, FEN is somewhat
incomplete, and the following additional fields would perhaps be quite
appropriate:
Earlier repetitions of this positions,


If you really want FEN to be complete, then we need this info
not just for *this* position, but for any position that can be reached
without pawn moves and captures. Too much like hard work ....

value '0', '1', '2' or '-', where 0
and 1 are 0/1 earlier repetitions, 2 is threefold repetition and therefore
immediate draw, '-' is unknown.


Again, 3-fold repetition is a *claimable* draw, not an actual
draw. Even in computer play, it is conceivable that in a time scramble
each side thinks it is winning [or could be winning] and therefore
wants to play on until the time control is reached, allowing a longer
[deeper] look at the position. But for general analysis purposes,
even a two-fold repetition is a draw, of course.

In the days when we had adjudications in the local league, I
once had to adjudicate a position where White had an obvious win "on
the board", but that winning move led to a position that had already
occurred four times [presumably in a scramble]. So the question was
whether White still had a win given that several key positions had
to be avoided.

For computer analysis, all the stuff about 50-move rules and
3-fold repetition is seriously compromised by transposition tables.
You analyse position A, and it's a forced win. So you move on to
position B, and you find that it's a win by playing into A. But
no-one bothers to tell you that the win from A involves a position
C [no longer in the TT] that is part of the path from B to A. Later,
you analyse C itself, and you find that C is a win by playing to B,
then back to C, thence to A [playing back to B now marked "drawn"!],
and now the only "win" is to go back to C. Oops. Or else you find
that A is a draw because from it you can reach a position that already
occurred in the path to A. Later, you analyse B, and assume you can
draw by playing to A; but the route via B doesn't go through the
repeating position. Oops. Or that A is a win, but it takes 49 moves
to conversion, so that the longer route to A via B leads to a draw
instead. Getting this absolutely right is a nightmare.

--
Andy Walker, School of MathSci., Univ. of Nott'm, UK.

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Old February 12th 04, 05:30 PM
Mikko Nummelin
 
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Default 3-move repetition and FEN notation

On Thu, 12 Feb 2004, Dr A. N. Walker wrote:

Of course, the 50-move rule is not "DRAW" but "DRAW can
be claimed". The difference is unimportant in analysis, but
matters in a game.


To be precise, FIDE has ruled that in some situations, i.e. kbbkq and
krbkr, draw can't even be claimed before 100 moves each side due to
possibilities of forceful mate lines having over 50 moves before next
capture.


Mikko Nummelin
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