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Simplify KBNK endgame implementation

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We do not need to change the winnerKSq variable, so we can simplify
a little bit the logic of the code by changing only the loserKSq
variable when it is necessary. Also consolidate and clarify comments.

See the pull request thread for a proof that the code is correct:
https://github.com/official-stockfish/Stockfish/pull/1854

No functional change
This commit is contained in:
protonspring 2018-12-08 10:08:59 -07:00 • committed by Stéphane Nicolet
parent 0f2df4e4af
commit b54bcfddaa
1 changed files with 4 additions and 10 deletions
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14
src/endgame.cpp
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@ -120,7 +120,7 @@ Value Endgame<KXK>::operator()(const Position& pos) const {
/// Mate with KBN vs K. This is similar to KX vs K, but we have to drive the /// Mate with KBN vs K. This is similar to KX vs K, but we have to drive the
/// defending king towards a corner square of the right color. /// defending king towards a corner square that our bishop attacks.
template<> template<>
Value Endgame<KBNK>::operator()(const Position& pos) const { Value Endgame<KBNK>::operator()(const Position& pos) const {
@ -131,18 +131,12 @@ Value Endgame<KBNK>::operator()(const Position& pos) const {
Square loserKSq = pos.square<KING>(weakSide); Square loserKSq = pos.square<KING>(weakSide);
Square bishopSq = pos.square<BISHOP>(strongSide); Square bishopSq = pos.square<BISHOP>(strongSide);
// kbnk_mate_table() tries to drive toward corners A1 or H8. If we have a // If our Bishop does not attack A1/H8, we flip the enemy king square
// bishop that cannot reach the above squares, we flip the kings in order // to drive to opposite corners (A8/H1).
// to drive the enemy toward corners A8 or H1.
if (opposite_colors(bishopSq, SQ_A1))
{
winnerKSq = ~winnerKSq;
loserKSq = ~loserKSq;
}
Value result = VALUE_KNOWN_WIN Value result = VALUE_KNOWN_WIN
+ PushClose[distance(winnerKSq, loserKSq)] + PushClose[distance(winnerKSq, loserKSq)]
+ PushToCorners[loserKSq]; + PushToCorners[opposite_colors(bishopSq, SQ_A1) ? ~loserKSq : loserKSq];
return strongSide == pos.side_to_move() ? result : -result; return strongSide == pos.side_to_move() ? result : -result;
} }