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Rewrite KBBKN endgame

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This was thought to be a draw but the bishops generally win. However,
it takes up to 66 moves. The position in the diagram was thought to be
a draw for over one hundred years, but tablebases show that White wins
in 45 moves. All of the long wins go through this type of semi-fortress
position. It takes several moves to force Black out of the temporary
fortress in the corner; then precise play with the bishops prevents Black
from forming the temporary fortress in another corner (Nunn 1995:265ff).

Before computer analysis, Speelman listed this position as unresolved,
but "probably a draw" (Speelman 1981:109).

bench: 3453945
This commit is contained in:
Marco Costalba 2013-09-02 11:03:01 +02:00
parent 849b089a63
commit 9ff594c3a9
1 changed files with 13 additions and 8 deletions
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21
src/endgame.cpp
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@ -57,7 +57,7 @@ namespace {
// Tables used to drive a piece towards or away from another piece
const int PushClose[8] = { 0, 0, 100, 80, 60, 40, 20, 10 };
const int PushAway [8] = { 0, 10, 14, 20, 30, 42, 58, 80 };
const int PushAway [8] = { 0, 5, 20, 40, 60, 80, 90, 100 };
// Get the material key of a Position out of the given endgame key code
// like "KBPKN". The trick here is to first forge an ad-hoc fen string
@ -365,6 +365,11 @@ Value Endgame<KQKR>::operator()(const Position& pos) const {
return strongerSide == pos.side_to_move() ? result : -result;
}
/// KBB vs KN. This is almost always a win. We try to push enemy king to a corner
/// and away from his knight. For a reference of this difficult endgame see:
/// en.wikipedia.org/wiki/Chess_endgame#Effect_of_tablebases_on_endgame_theory
template<>
Value Endgame<KBBKN>::operator()(const Position& pos) const {
@ -374,14 +379,14 @@ Value Endgame<KBBKN>::operator()(const Position& pos) const {
assert(pos.count<KNIGHT>(weakerSide ) == 1);
assert(!pos.pieces(PAWN));
Square wksq = pos.king_square(strongerSide);
Square bksq = pos.king_square(weakerSide);
Square nsq = pos.list<KNIGHT>(weakerSide)[0];
Square winnerKSq = pos.king_square(strongerSide);
Square loserKSq = pos.king_square(weakerSide);
Square knightSq = pos.list<KNIGHT>(weakerSide)[0];
Value result = BishopValueEg
+ PushClose[square_distance(wksq, bksq)]
+ square_distance(bksq, nsq) * 32
+ (8 - popcount<Max15>(pos.attacks_from<KNIGHT>(nsq))) * 8;
Value result = VALUE_KNOWN_WIN
+ PushToCorners[loserKSq]
+ PushClose[square_distance(winnerKSq, loserKSq)]
+ PushAway[square_distance(loserKSq, knightSq)];
return strongerSide == pos.side_to_move() ? result : -result;
}