A|H pawn && wrong bishop v K

[op12no2]

I've noticed that Lozza often finds herself left with a A|H pawn and a bishop of the opposite colour to the promotion square, against a bare King. And thinks she is up when in fact it's a draw and can be evaluated as such.

I'm about to code this in eval, but just wondering if it's a specific case of something more generic; any buzz words/phrases I can search for in the wiki/forums etc...?

[hgm]

It is one of the infamous cases where the naive evaluation does not work, and which is encountered reasonably often. (Especially against opponents that know about it!) Just like KNNK or KBK. But the latter can be recognized by material composition only, while this one requires you to look where the pieces are.

There is a somewhat similar case in KNPK, when a rook-Pawn is already on 7th rank and the defending King before it. The attacking King can then never protect the Pawn without stalemating you, so the Knight can never move without losing the Pawn to drive you away. But because it requires the Knight and Pawn to be in a very specific place, it is very rare compared to KBPK. One funny thing about this: engines that do not know about under-promotion can lose this, because they sooner or later leave the promotion square unprotected, counting on being stalemated when the opponent promotes to Q.

Other often encountered end-games where the engine 'reckons itself rich' are the drawn positions in KQKP and KPK. So it also pays to have recognizers for those.

[op12no2]

OK thx HG I'll hardwire it with some twiddly bits for king distance etc. And then look the others you mention.

[hgm]

I found that it is pretty tricky to recognize any drawn cases other than where the defending King is in front of the Pawn on the rook- or knight file. In particular
[d]8/8/P1k5/B7/8/8/8/7K b

forces you to leave the Pawn's 'square' by zugzwang. And
[d]2k5/8/P7/4B3/8/8/8/7K b

likewise. So you can lose even when you are within the square, and much closer to the promotion square than the attacking King.

[op12no2]

Ah - yes, thanks, not as simple as I thought..

[Sven]

I've noticed that Lozza often finds herself left with a A|H pawn and a bishop of the opposite colour to the promotion square, against a bare King. And thinks she is up when in fact it's a draw and can be evaluated as such.

I'm about to code this in eval, but just wondering if it's a specific case of something more generic; any buzz words/phrases I can search for in the wiki/forums etc...?

The buzz phrase would be "blind bishop". I don't think it's a specific case of something more generic.

[mjlef]

I've noticed that Lozza often finds herself left with a A|H pawn and a bishop of the opposite colour to the promotion square, against a bare King. And thinks she is up when in fact it's a draw and can be evaluated as such.

I'm about to code this in eval, but just wondering if it's a specific case of something more generic; any buzz words/phrases I can search for in the wiki/forums etc...?

This is true no matter how many pawns are on the rook file. So you might want to make your conditions general enough to cover that.

[xmas79]

When I wanted to code some endgame knowledge in my engine, I remember someone gave me a list of known endgames. There were positions drawn and won, and some were pretty crazy.... I remember this one [d]8/8/8/n7/R5K1/8/8/k7 b - -
and the others were along this wave length.... Well, even before trying to write any single line of code I resigned, because there were too much variations on the theme, and I thought it would not worth the effort... Since it's very hard to evaluate correctly all these positions, I preferred the easier route of recognizing the known draw positions (eg KBPk with bishop wrong corner color and king already in the corner) and let the search do the rest by scaling down the (winning) score down to around 50cp... If the engine can promote the pawn, well, it will see the score jump and will win the game. However, landing in such positions in qsearch at hight depths will lead to errors, because the engine will not have the time to figure out (with search) the real game outcome.... but at least your engine will keep on trying to win... Or at least this is how I think it is when the engine has advantage.... On the other hand, it will lead to a disaster if the position of the engine is lost (eg the bare king) because it will see a score jump from -4.00 up to -0.50 and will of course take that route.........

[op12no2]

Thanks for the replies. I did a quick experiment by adding an else clause to unstoppable passers:-

Code: Select all

else if (numPieces == 4 && wNumBishops && wNumPawns && bKingRank > rank) {
  if (file == 1 && wBBishop && (bKingFile == 1 || bKingFile == 2))
    return CONTEMPT;
  if (file == 8 && wWBishop && (bKingFile == 7 || bKingFile == 8))
    return CONTEMPT;
}

[hgm]

When the white King can keep you away from the promotion square it is also lost:

[d]8/K7/8/8/kB6/P7/8/8 w - -- 0 1

[d]6K1/8/5k2/8/1B6/P7/8/8 w - -- 0 1