Reintroduce SEE verification against discovered attacks

[stockfish] / src / search.cpp

diff --git a/src/search.cpp b/src/search.cpp
index 16122315032d2303ec48d4ae3ebf2c1b48d5b78a..b2c2344ac0aaff10a249bfb7f64a22fa0e648d6f 100644 (file)
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -779,7 +779,7 @@ namespace {
         && (ss-1)->statScore < 17329
         &&  eval >= beta
         &&  eval >= ss->staticEval
-        &&  ss->staticEval >= beta - 21 * depth - improvement * 99 / 1300 + 258
+        &&  ss->staticEval >= beta - 21 * depth - improvement / 13 + 258
         && !excludedMove
         &&  pos.non_pawn_material(us)
         && (ss->ply >= thisThread->nmpMinPly))
@@ -822,9 +822,24 @@ namespace {
         }
     }
 
+    // Step 10. If the position doesn't a have ttMove, decrease depth by 2
+    // (or by 4 if the TT entry for the current position was hit and the stored depth is greater than or equal to the current depth).
+    // Use qsearch if depth is equal or below zero (~9 Elo)
+    if (    PvNode
+        && !ttMove)
+        depth -= 2 + 2 * (ss->ttHit && tte->depth() >= depth);
+
+    if (depth <= 0)
+        return qsearch<PV>(pos, ss, alpha, beta);
+
+    if (    cutNode
+        &&  depth >= 8
+        && !ttMove)
+        depth -= 2;
+
     probCutBeta = beta + 168 - 61 * improving;
 
-    // Step 10. ProbCut (~10 Elo)
+    // Step 11. ProbCut (~10 Elo)
     // If we have a good enough capture (or queen promotion) and a reduced search returns a value
     // much above beta, we can (almost) safely prune the previous move.
     if (   !PvNode
@@ -875,20 +890,6 @@ namespace {
         Eval::NNUE::hint_common_parent_position(pos);
     }
 
-    // Step 11. If the position is not in TT, decrease depth by 2 (or by 4 if the TT entry for the current position was hit and the stored depth is greater than or equal to the current depth).
-    // Use qsearch if depth is equal or below zero (~9 Elo)
-    if (    PvNode
-        && !ttMove)
-        depth -= 2 + 2 * (ss->ttHit && tte->depth() >= depth);
-
-    if (depth <= 0)
-        return qsearch<PV>(pos, ss, alpha, beta);
-
-    if (    cutNode
-        &&  depth >= 8
-        && !ttMove)
-        depth -= 2;
-
 moves_loop: // When in check, search starts here
 
     // Step 12. A small Probcut idea, when we are in check (~4 Elo)
@@ -991,22 +992,25 @@ moves_loop: // When in check, search starts here
 
               Bitboard occupied;
               // SEE based pruning (~11 Elo)
-              if (!pos.see_ge(move, occupied, Value(-212) * depth))
+              if (!pos.see_ge(move, occupied, Value(-205) * depth))
               {
-                  // Don't prune the move if opponent King/Queen/Rook gets a discovered attack during or after the exchanges
-                  Bitboard leftEnemies = pos.pieces(~us, KING, QUEEN, ROOK);
-                  Bitboard attacks = 0;
-                  occupied |= to_sq(move);
-                  while (leftEnemies && !attacks)
-                  {
+                 if (depth < 2 - capture)
+                    continue;
+                 // Don't prune the move if opponent Queen/Rook is under discovered attack after the exchanges
+                 // Don't prune the move if opponent King is under discovered attack after or during the exchanges
+                 Bitboard leftEnemies = (pos.pieces(~us, KING, QUEEN, ROOK)) & occupied;
+                 Bitboard attacks = 0;
+                 occupied |= to_sq(move);
+                 while (leftEnemies && !attacks)
+                 {
                       Square sq = pop_lsb(leftEnemies);
-                      attacks = pos.attackers_to(sq, occupied) & pos.pieces(us) & occupied;
-                      // Exclude Queen/Rook(s) which were already threatened before SEE (opponent King can't be in check when it's our turn)
-                      if (attacks && sq != pos.square<KING>(~us) && (pos.attackers_to(sq, pos.pieces()) & pos.pieces(us)))
-                          attacks = 0;
-                  }
-                  if (!attacks)
-                      continue;
+                      attacks |= pos.attackers_to(sq, occupied) & pos.pieces(us) & occupied;
+                      // don't consider pieces which were already threatened/hanging before SEE exchanges
+                      if (attacks && (sq != pos.square<KING>(~us) && (pos.attackers_to(sq, pos.pieces()) & pos.pieces(us))))
+                         attacks = 0;
+                 }
+                 if (!attacks)
+                    continue;
               }
           }
           else
@@ -1101,7 +1105,8 @@ moves_loop: // When in check, search starts here
           }
 
           // Check extensions (~1 Elo)
-          else if (   givesCheck && depth > 8)
+          else if (   givesCheck
+                   && depth > 9)
               extension = 1;
 
           // Quiet ttMove extensions (~1 Elo)
@@ -1503,7 +1508,7 @@ moves_loop: // When in check, search starts here
     // to search the moves. Because the depth is <= 0 here, only captures,
     // queen promotions, and other checks (only if depth >= DEPTH_QS_CHECKS)
     // will be generated.
-    Square prevSq = (ss-1)->currentMove != MOVE_NULL ? to_sq((ss-1)->currentMove) : SQ_NONE;
+    Square prevSq = is_ok((ss-1)->currentMove) ? to_sq((ss-1)->currentMove) : SQ_NONE;
     MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
                                       &thisThread->captureHistory,
                                       contHist,