Detect drawish KQKP endings

[stockfish] / src / endgame.cpp

diff --git a/src/endgame.cpp b/src/endgame.cpp
index dbf10582ac8964fdf3f047d6d93c54b202274c5d..c0d317b779234f4ae5cca323c221ffb5be8c4c93 100644 (file)
--- a/src/endgame.cpp
+++ b/src/endgame.cpp
@@ -31,7 +31,7 @@ namespace {
 
   // Table used to drive the defending king towards the edge of the board
   // in KX vs K and KQ vs KR endgames.
-  const int MateTable[64] = {
+  const int MateTable[SQUARE_NB] = {
     100, 90, 80, 70, 70, 80, 90, 100,
      90, 70, 60, 50, 50, 60, 70,  90,
      80, 60, 40, 30, 30, 40, 60,  80,
@@ -44,7 +44,7 @@ namespace {
 
   // Table used to drive the defending king towards a corner square of the
   // right color in KBN vs K endgames.
-  const int KBNKMateTable[64] = {
+  const int KBNKMateTable[SQUARE_NB] = {
     200, 190, 180, 170, 160, 150, 140, 130,
     190, 180, 170, 160, 150, 140, 130, 140,
     180, 170, 155, 140, 140, 125, 140, 150,
@@ -95,6 +95,7 @@ Endgames::Endgames() {
   add<KRKP>("KRKP");
   add<KRKB>("KRKB");
   add<KRKN>("KRKN");
+  add<KQKP>("KQKP");
   add<KQKR>("KQKR");
   add<KBBKN>("KBBKN");
 
@@ -169,12 +170,12 @@ Value Endgame<KBNK>::operator()(const Position& pos) const {
 
   Square winnerKSq = pos.king_square(strongerSide);
   Square loserKSq = pos.king_square(weakerSide);
-  Square bishopSquare = pos.piece_list(strongerSide, BISHOP)[0];
+  Square bishopSq = pos.piece_list(strongerSide, BISHOP)[0];
 
   // kbnk_mate_table() tries to drive toward corners A1 or H8,
   // if we have a bishop that cannot reach the above squares we
   // mirror the kings so to drive enemy toward corners A8 or H1.
-  if (opposite_colors(bishopSquare, SQ_A1))
+  if (opposite_colors(bishopSq, SQ_A1))
   {
       winnerKSq = mirror(winnerKSq);
       loserKSq = mirror(loserKSq);
@@ -326,6 +327,37 @@ Value Endgame<KRKN>::operator()(const Position& pos) const {
 }
 
 
+/// KQ vs KP.  In general, a win for the stronger side, however, there are a few
+/// important exceptions.  Pawn on 7th rank, A,C,F or H file, with king next can
+/// be a draw, so we scale down to distance between kings only.
+template<>
+Value Endgame<KQKP>::operator()(const Position& pos) const {
+
+  assert(pos.non_pawn_material(strongerSide) == QueenValueMg);
+  assert(pos.piece_count(strongerSide, PAWN) == 0);
+  assert(pos.non_pawn_material(weakerSide) == 0);
+  assert(pos.piece_count(weakerSide, PAWN) == 1);
+
+  Square winnerKSq = pos.king_square(strongerSide);
+  Square loserKSq = pos.king_square(weakerSide);
+  Square pawnSq = pos.piece_list(weakerSide, PAWN)[0];
+
+  Value result =  QueenValueEg
+                - PawnValueEg
+                + DistanceBonus[square_distance(winnerKSq, loserKSq)];
+
+  if (   square_distance(loserKSq, pawnSq) == 1
+      && relative_rank(weakerSide, pawnSq) == RANK_7)
+  {
+      File f = file_of(pawnSq);
+
+      if (f == FILE_A || f == FILE_C || f == FILE_F || f == FILE_H)
+          result = Value(DistanceBonus[square_distance(winnerKSq, loserKSq)]);
+  }
+  return strongerSide == pos.side_to_move() ? result : -result;
+}
+
+
 /// KQ vs KR.  This is almost identical to KX vs K:  We give the attacking
 /// king a bonus for having the kings close together, and for forcing the
 /// defending king towards the edge.  If we also take care to avoid null move
@@ -648,7 +680,7 @@ ScaleFactor Endgame<KPsK>::operator()(const Position& pos) const {
       // Does the defending king block the pawns?
       if (   square_distance(ksq, relative_square(strongerSide, SQ_A8)) <= 1
           || (    file_of(ksq) == FILE_A
-              && !in_front_bb(strongerSide, ksq) & pawns))
+              && !(in_front_bb(strongerSide, ksq) & pawns)))
           return SCALE_FACTOR_DRAW;
   }
   // Are all pawns on the 'h' file?
@@ -657,7 +689,7 @@ ScaleFactor Endgame<KPsK>::operator()(const Position& pos) const {
     // Does the defending king block the pawns?
     if (   square_distance(ksq, relative_square(strongerSide, SQ_H8)) <= 1
         || (    file_of(ksq) == FILE_H
-            && !in_front_bb(strongerSide, ksq) & pawns))
+            && !(in_front_bb(strongerSide, ksq) & pawns)))
         return SCALE_FACTOR_DRAW;
   }
   return SCALE_FACTOR_NONE;