More accurate pawn attack span definition

[stockfish] / src / pawns.cpp

diff --git a/src/pawns.cpp b/src/pawns.cpp
index 50eb3aa32c3871649cf242833b657ceec73ee497..8022ae51c20d811adac0d0cb22d8208af8515e1b 100644 (file)
--- a/src/pawns.cpp
+++ b/src/pawns.cpp
@@ -71,10 +71,10 @@ namespace {
     constexpr Color     Them = (Us == WHITE ? BLACK : WHITE);
     constexpr Direction Up   = (Us == WHITE ? NORTH : SOUTH);
 
-    Bitboard neighbours, stoppers, support, phalanx;
+    Bitboard neighbours, stoppers, support, phalanx, opposed;
     Bitboard lever, leverPush;
     Square s;
-    bool opposed, backward, passed, doubled;
+    bool backward, passed, doubled;
     Score score = SCORE_ZERO;
     const Square* pl = pos.squares<PAWN>(Us);
 
@@ -94,8 +94,6 @@ namespace {
 
         Rank r = relative_rank(Us, s);
 
-        e->pawnAttacksSpan[Us] |= pawn_attack_span(Us, s);
-
         // Flag the pawn
         opposed    = theirPawns & forward_file_bb(Us, s);
         stoppers   = theirPawns & passed_pawn_span(Us, s);
@@ -112,6 +110,17 @@ namespace {
         backward =  !(neighbours & forward_ranks_bb(Them, s))
                   && (stoppers & (leverPush | (s + Up)));
 
+        // Span of backward pawns and span behind opposing pawns are not included
+        // in the pawnAttacksSpan bitboard.
+        if (!backward || phalanx)
+        {
+            if (opposed)
+                e->pawnAttacksSpan[Us] |=  pawn_attack_span(Us, s) &
+                                          ~pawn_attack_span(Us, frontmost_sq(Them, opposed));
+            else
+                e->pawnAttacksSpan[Us] |= pawn_attack_span(Us, s);
+        }
+
         // A pawn is passed if one of the three following conditions is true:
         // (a) there is no stoppers except some levers
         // (b) the only stoppers are the leverPush, but we outnumber them
@@ -130,17 +139,19 @@ namespace {
         // Score this pawn
         if (support | phalanx)
         {
-            int v =  Connected[r] * (2 + bool(phalanx) - opposed)
+            int v =  Connected[r] * (2 + bool(phalanx) - bool(opposed))
                    + 21 * popcount(support);
 
             score += make_score(v, v * (r - 2) / 4);
         }
 
         else if (!neighbours)
-            score -= Isolated + WeakUnopposed * !opposed;
+            score -=   Isolated
+                     + WeakUnopposed * !opposed;
 
         else if (backward)
-            score -= Backward + WeakUnopposed * !opposed;
+            score -=   Backward
+                     + WeakUnopposed * !opposed;
 
         if (!support)
             score -=   Doubled * doubled