Introduce ProbCut for check evasions

The idea of this patch can be described as follows: if we are in check
and the transposition table move is a capture that returns a value
far above beta, we can assume that the opponent just blundered a piece
by giving check, and we return the transposition table value. This is
similar to the usual probCut logic for quiet moves, but with a different
threshold.

Passed STC
LLR: 2.94 (-2.94,2.94) {-0.25,1.25}
Total: 33440 W: 3056 L: 2891 D: 27493
Ptnml(0-2): 110, 2338, 11672, 2477, 123
https://tests.stockfishchess.org/tests/view/602cd1087f517a561bc49bda

Passed LTC
LLR: 2.98 (-2.94,2.94) {0.25,1.25}
Total: 10072 W: 401 L: 309 D: 9362
Ptnml(0-2): 2, 288, 4365, 378, 3
https://tests.stockfishchess.org/tests/view/602ceea57f517a561bc49bf0

The committed version has an additional fix to never return unproven wins
in the tablebase range or the mate range. This fix passed tests for non-
regression at STC and LTC:

STC:
LLR: 2.93 (-2.94,2.94) {-1.25,0.25}
Total: 26240 W: 2354 L: 2280 D: 21606
Ptnml(0-2): 85, 1763, 9372, 1793, 107
https://tests.stockfishchess.org/tests/view/602d86a87f517a561bc49c7a

LTC:
LLR: 2.95 (-2.94,2.94) {-0.75,0.25}
Total: 35304 W: 1299 L: 1256 D: 32749
Ptnml(0-2): 14, 1095, 15395, 1130, 18
https://tests.stockfishchess.org/tests/view/602d98d17f517a561bc49c83

Closes https://github.com/official-stockfish/Stockfish/pull/3362

Bench: 3830215

diff --git a/src/search.cpp b/src/search.cpp
index 8f01f785a9e6596c1999289747ccfbdcb4038660..c33bc9143920324d229dd34ae050072d1079a963 100644 (file)
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -969,6 +969,23 @@ namespace {
 
 moves_loop: // When in check, search starts from here
 
+    ttCapture = ttMove && pos.capture_or_promotion(ttMove);
+
+    // Step 11. A small Probcut idea, when we are in check
+    probCutBeta = beta + 400;
+    if (   ss->inCheck
+        && !PvNode
+        && depth >= 4
+        && ttCapture
+        && (tte->bound() & BOUND_LOWER)
+        && tte->depth() >= depth - 3
+        && ttValue >= probCutBeta
+        && abs(ttValue) <= VALUE_KNOWN_WIN
+        && abs(beta) <= VALUE_KNOWN_WIN
+       )
+        return probCutBeta;
+
+
     const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,
                                           nullptr                   , (ss-4)->continuationHistory,
                                           nullptr                   , (ss-6)->continuationHistory };
@@ -985,12 +1002,11 @@ moves_loop: // When in check, search starts from here
 
     value = bestValue;
     singularQuietLMR = moveCountPruning = false;
-    ttCapture = ttMove && pos.capture_or_promotion(ttMove);
 
     // Mark this node as being searched
     ThreadHolding th(thisThread, posKey, ss->ply);
 
-    // Step 11. Loop through all pseudo-legal moves until no moves remain
+    // Step 12. Loop through all pseudo-legal moves until no moves remain
     // or a beta cutoff occurs.
     while ((move = mp.next_move(moveCountPruning)) != MOVE_NONE)
     {
@@ -1036,7 +1052,7 @@ moves_loop: // When in check, search starts from here
       // Calculate new depth for this move
       newDepth = depth - 1;
 
-      // Step 12. Pruning at shallow depth (~200 Elo)
+      // Step 13. Pruning at shallow depth (~200 Elo)
       if (  !rootNode
           && pos.non_pawn_material(us)
           && bestValue > VALUE_TB_LOSS_IN_MAX_PLY)
@@ -1084,7 +1100,7 @@ moves_loop: // When in check, search starts from here
           }
       }
 
-      // Step 13. Extensions (~75 Elo)
+      // Step 14. Extensions (~75 Elo)
 
       // Singular extension search (~70 Elo). If all moves but one fail low on a
       // search of (alpha-s, beta-s), and just one fails high on (alpha, beta),
@@ -1156,10 +1172,10 @@ moves_loop: // When in check, search starts from here
                                                                 [movedPiece]
                                                                 [to_sq(move)];
 
-      // Step 14. Make the move
+      // Step 15. Make the move
       pos.do_move(move, st, givesCheck);
 
-      // Step 15. Reduced depth search (LMR, ~200 Elo). If the move fails high it will be
+      // Step 16. Reduced depth search (LMR, ~200 Elo). If the move fails high it will be
       // re-searched at full depth.
       if (    depth >= 3
           &&  moveCount > 1 + 2 * rootNode
@@ -1266,7 +1282,7 @@ moves_loop: // When in check, search starts from here
           didLMR = false;
       }
 
-      // Step 16. Full depth search when LMR is skipped or fails high
+      // Step 17. Full depth search when LMR is skipped or fails high
       if (doFullDepthSearch)
       {
           value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode);
@@ -1293,12 +1309,12 @@ moves_loop: // When in check, search starts from here
                               std::min(maxNextDepth, newDepth), false);
       }
 
-      // Step 17. Undo move
+      // Step 18. Undo move
       pos.undo_move(move);
 
       assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
 
-      // Step 18. Check for a new best move
+      // Step 19. Check for a new best move
       // Finished searching the move. If a stop occurred, the return value of
       // the search cannot be trusted, and we return immediately without
       // updating best move, PV and TT.
@@ -1375,7 +1391,7 @@ moves_loop: // When in check, search starts from here
         return VALUE_DRAW;
     */
 
-    // Step 19. Check for mate and stalemate
+    // Step 20. Check for mate and stalemate
     // All legal moves have been searched and if there are no legal moves, it
     // must be a mate or a stalemate. If we are in a singular extension search then
     // return a fail low score.