chessprogramming - Backward Pawns (Bitboards)

Older Version Newer Version
GerdIsenberg May 5, 2017
**[[Home]] * [[Board Representation]] * [[Bitboards]] * [[Pawn Pattern and Properties]] * Backward Pawns**
[[toc]]

[[Hans Kmoch|Kmoch's]] definition of a [[Backward Pawn|backward pawn]] or straggler: A half-free pawn on the second or third rank whose stop square lacks pawn protection but is controlled by a [[Sentry|sentry]]. The definition of a backward pawn seems a bit ambiguous. May an [[Isolated Pawns (Bitboards)|isolated pawn]] also be a backward one? Or an pawn on a [[Pawns and Files (Bitboards)#ClosedFiles|closed file]]? What about a backward [[Pawn Rams (Bitboards)|ram]] or [[Candidates (Bitboards)|faked candidates]] or even [[Pawn Levers (Bitboards)|levers]]?
[[#Backward]]
=Backward= 
We may use a more general definition of backwardness, to consider certain subsets independently. All pawns, which [[Pawn Spans#StopandDistantStop|stop]] is not member of own [[Attack Spans|front-attackspans]] but controlled by a sentry are **backward**:
[[code format="cpp"]]
U64 wBackward(U64 wpawns, U64 bpawns) {
   U64 stops = wpawns << 8;
   U64 wAttackSpans = wEastAttackFrontSpans(wpawns)
                    | wWestAttackFrontSpans(wpawns);
   U64 bAttacks     = bPawnEastAttacks(bpawns)
                    | bPawnWestAttacks(bpawns);
   return (stops & bAttacks & ~wAttackSpans) >> 8;
}
[[code]]
[[#Straggler]]
=Straggler= 
Stragglers are the intersection of backward with [[Open Pawns (Bitboards)#OpenPawnsSetwise|open pawns]] and the second or third rank:
[[code format="cpp"]]
U64 wStraggler(U64 wpawns, U64 bpawns) {
   return wBackward (wpawns, bpawns)
        & wOpenPawns(wpawns, bpawns)
        & 0xffff00; // rank 2,3
}
[[code]]
In the strict sense, we still need to exclude [[Pawn Levers (Bitboards)|levers]]. On the other hand, levers are subject of [[quiescence search]] anyway.

=Telestop Weakness=
As pointed out by [[Sam Hamilton]] <ref>[[http://www.talkchess.com/forum/viewtopic.php?topic_view=threads&p=290991&t=29689|Re: Doubled and Backward Pawn Engine "Definitions"]] by [[Sam Hamilton]], [[CCC]], September 13, 2009</ref> considering stop squares might be insufficient for pawns which may actually push, but have a permanent weakened [[Pawn Spans#StopandDistantStop|telestop]], f.i.:
> [[image:http://webchess.freehostia.com/diag/chessdiag.php?fen=8/5p2/6p1/p1p3P1/P1P5/7P/1P6/8%20w%20-%20-&size=medium&coord=yes&cap=no&stm=no&fb=no&theme=classic&color1=E3CEAA&color2=635147&color3=000000 caption="8/5p2/6p1/p1p3P1/P1P5/7P/1P6/8 w - -"]]

On the other hand, such a backward prospective pawn has a vital [[Tempo|tempo]], which is often decisive in certain [[Pawn Endgame|pawn endings]], so one should be careful to don't make the penalty too worse. Anyway, one may either apply the above [[Backward Pawns (Bitboards)#Backward|wBackward]] algorithm with so far movable pawns shifted one rank up, or alternatively determine a //backward square area//, which is the black [[Attack Spans|attack span]] of all black attacks which are outside the white [[Attack Spans|front attack spans]]:

[[code]]
white pawns         black pawns
. . . . . . . .     . . . . . . . .
. . . . . . . .     . . . . . 1 . .
. . . . . . . .     . . . . . . 1 .
. . . . . . 1 .     1 . 1 . . . . .
1 . 1 . . . . .     . . . . . . . .
. . . . . . . 1     . . . . . . . .
. 1 . . . . . .     . . . . . . . .
. . . . . . . .     . . . . . . . .

~white front     &  black attacks    -> stop squares
attackspans                             dominated by black 
. . . . 1 . . .     . . . . . . . .     . . . . . . . .
. . . . 1 . . .     . . . . . . . .     . . . . . . . .
. . . . 1 . . .     . . . . 1 . 1 .     . . . . 1 . . .
. . . . 1 1 . 1     . . . . . 1 . 1     . . . . . 1 . 1
. 1 . 1 1 1 . 1  &  . 1 . 1 . . . .     . 1 . 1 . . . .
. 1 . 1 1 1 1 1     . . . . . . . .     . . . . . . . .
1 1 1 1 1 1 1 1     . . . . . . . .     . . . . . . . .
1 1 1 1 1 1 1 1     . . . . . . . .     . . . . . . . .
stop squares
dominated by black
filled down
-> white         &  white pawns      -> backward pawns
backward area
. . . . . . . .     . . . . . . . .     . . . . . . . .
. . . . . . . .     . . . . . . . .     . . . . . . . .
. . . . 1 . . .     . . . . . . . .     . . . . . . . .
. . . . 1 1 . 1     . . . . . . 1 .     . . . . . . . .
. 1 . 1 1 1 . 1  &  1 . 1 . . . . .     . . . . . . . .
. 1 . 1 1 1 . 1     . . . . . . . 1     . . . . . . . 1
. 1 . 1 1 1 . 1     . 1 . . . . . .     . 1 . . . . . .
. 1 . 1 1 1 . 1     . . . . . . . .     . . . . . . . .
[[code]]
=Dynamic Backwardness=
==A further Pass==
Another issue is that some pawns or their stop squares are statically member of their own [[Attack Spans|attack spans]], but those spans could not be realized, since their pawns are either backward or [[Pawn Rams (Bitboards)|rammed]] (or both). Those pawns (f.i. white pawn c4, and mutually black pawn a4 in the diagram) may determined in a second pass, only considering attack spans of movable pawns.
> [[image:http://webchess.freehostia.com/diag/chessdiag.php?fen=8/8/1p6/8/p1P1p3/3pP3/1P1P4/8%20w%20-%20-&size=medium&coord=yes&cap=no&stm=no&fb=no&theme=classic&color1=E3CEAA&color2=635147&color3=000000 caption="8/8/1p6/8/p1P1p3/3pP3/1P1P4/8 w - -"]]

==Stops with negative SEE==
A more sophisticated definition of backwardness was given by [[Vincent Diepeveen]] <ref>[[http://www.stmintz.com/ccc/index.php?id=272903|Re: WHAT is the definition of a backward pawn?]] by [[Vincent Diepeveen]], [[CCC]], December 24, 2002</ref>, even a member of a [[Duo Trio Quart (Bitboards)|pawn duo]] may be backward, if its stop square has a negative [[Static Exchange Evaluation|SEE]] score, also considering pieces. Here b7 is considered backward, no matter whether the other friendly pawn is on c7 or c6, since it's stop square b6 has a negative SEE in both cases.
> [[image:http://webchess.freehostia.com/diag/chessdiag.php?fen=8/1pp5/8/2P5/8/8/8/1R6%20w%20-%20-&size=medium&coord=yes&cap=no&stm=no&fb=no&theme=classic&color1=E3CEAA&color2=635147&color3=000000 caption="8/1pp5/8/2P5/8/8/8/1R6 w - -"]]

=See also=
* [[Backward Pawn]]
* [[Open Pawns (Bitboards)|Open Pawns]]
* [[Sentry]]
* [[Attack Spans]]
* [[Candidates (Bitboards)#Sneaker|Sneaker]] from [[Candidates (Bitboards)]]
* [[Hidden Passed Pawn]]
* [[Faker]]
* [[Hans Kmoch]]

=Publications=
* [[Hans Kmoch]] (**1959, 1990**). //Pawn Power in Chess//. New York: Dover, 1990. Previous ed.: New York: McKay, 1959. ISBN 0-486-26486-6

=Forum Posts=
* [[http://www.stmintz.com/ccc/index.php?id=272739|WHAT is the definition of a backward pawn?]] by [[Bas Hamstra]], [[CCC]], December 23, 2002
* [[http://www.talkchess.com/forum/viewtopic.php?t=20320|Backward Pawns]] by [[Mark Lefler]], [[CCC]], March 24, 2008
* [[http://www.talkchess.com/forum/viewtopic.php?t=29689|Doubled and Backward Pawn Engine "Definitions"]] by [[Brian Richardson]], [[CCC]], September 07, 2009
* [[http://www.talkchess.com/forum/viewtopic.php?t=52300|What is a backward pawn?]] by [[Lyudmil Tsvetkov]], [[CCC]], May 12, 2014

=External Links=
* [[http://en.wikipedia.org/wiki/Backward_pawn|Backward pawn from Wikipedia]]

=References=
<references />
=What links here?= 
[[include page="Backward Pawns (Bitboards)" component="backlinks" limit="40" ]]
**[[Pawn Pattern and Properties|Up one Level]]**