Give bonus for rooks that are alligned with enemy kingring

The idea of this patch is that if rooks are not directly attacking the opponent king,
they can support king attacks staying behind pawns or minor pieces and be really
deadly if position slightly opens up at enemy king ring ranks. Loosely based on
some stockfish games where it underestimated attacks on it king when enemy has one
or two rooks supporting pawn pushes towards it king.

passed STC
https://tests.stockfishchess.org/tests/view/5ecb093680f2c838b96550f9
LLR: 2.93 (-2.94,2.94) {-0.50,1.50}
Total: 53672 W: 10535 L: 10265 D: 32872
Ptnml(0-2): 952, 6210, 12258, 6448, 968

passed LTC
https://tests.stockfishchess.org/tests/view/5ecb639f80f2c838b9655117
LLR: 2.94 (-2.94,2.94) {0.25,1.75}
Total: 62424 W: 8094 L: 7748 D: 46582
Ptnml(0-2): 426, 5734, 18565, 6042, 445

closes https://github.com/official-stockfish/Stockfish/pull/2700

Bench: 4663220

diff --git a/src/evaluate.cpp b/src/evaluate.cpp
index 7aa67f26db6c4d0a2f916f97a822d2efb6a812fe..8e8cc0912e57ddd4e37c12cb579631eaee9de289 100644 (file)
--- a/src/evaluate.cpp
+++ b/src/evaluate.cpp
@@ -143,6 +143,7 @@ namespace {
   constexpr Score PassedFile          = S( 11,  8);
   constexpr Score PawnlessFlank       = S( 17, 95);
   constexpr Score RestrictedPiece     = S(  7,  7);
+  constexpr Score RookOnKingRing      = S( 16,  0);
   constexpr Score RookOnQueenFile     = S(  5,  9);
   constexpr Score SliderOnQueen       = S( 59, 18);
   constexpr Score ThreatByKing        = S( 24, 89);
@@ -287,6 +288,8 @@ namespace {
             kingAttackersWeight[Us] += KingAttackWeights[Pt];
             kingAttacksCount[Us] += popcount(b & attackedBy[Them][KING]);
         }
+        else if (Pt == ROOK && (file_bb(s) & kingRing[Them]))
+            score += RookOnKingRing;
 
         int mob = popcount(b & mobilityArea[Us]);