New extended probcut implementation

Here the idea is to test probcut not only after bad
captures, but after any bad move, i.e. any move that
leaves the opponent with a good capture.

Ported by a patch from Onno, the difference from
original version is that we have moved probcut after
null search.

After 7917 games 4 threads 20"+0.1
Mod vs Orig: 1261 - 1095 - 5561 ELO +7 (+- 4.2) LOS 96%

Signed-off-by: Marco Costalba <mcostalba@gmail.com>

src/search.cpp

@@ -677,6 +677,7 @@ namespace {
     StateInfo st;
     const TTEntry *tte;
     Key posKey;
+    Bitboard pinned;
     Move ttMove, move, excludedMove, threatMove;
     Depth ext, newDepth;
     ValueType vt;
@@ -862,7 +863,37 @@ namespace {
         }
     }
 
-    // Step 9. Internal iterative deepening
+    // Step 9. ProbCut (is omitted in PV nodes)
+    // If we have a very good capture (i.e. SEE > seeValues[captured_piece_type])
+    // and a reduced search returns a value much above beta, we can (almost) safely
+    // prune the previous move.
+    if (   !PvNode
+        &&  depth >= RazorDepth + ONE_PLY
+        && !inCheck
+        && !ss->skipNullMove
+        &&  excludedMove == MOVE_NONE
+        &&  abs(beta) < VALUE_MATE_IN_PLY_MAX)
+    {
+        Value rbeta = beta + 200;
+        Depth rdepth = depth - ONE_PLY - 3 * ONE_PLY;
+
+        assert(rdepth >= ONE_PLY);
+
+        MovePicker mp(pos, ttMove, H, Position::see_value(pos.captured_piece_type()));
+        pinned = pos.pinned_pieces(pos.side_to_move());
+
+        while ((move = mp.get_next_move()) != MOVE_NONE)
+            if (pos.pl_move_is_legal(move, pinned))
+            {
+                pos.do_move(move, st);
+                value = -search<NonPV>(pos, ss+1, -rbeta, -rbeta+1, rdepth);
+                pos.undo_move(move);
+                if (value >= rbeta)
+                    return value;
+            }
+    }
+
+    // Step 10. Internal iterative deepening
     if (   depth >= IIDDepth[PvNode]
         && ttMove == MOVE_NONE
         && (PvNode || (!inCheck && ss->eval + IIDMargin >= beta)))
@@ -882,7 +913,7 @@ split_point_start: // At split points actual search starts from here
     // Initialize a MovePicker object for the current position
     MovePickerExt<NT> mp(pos, ttMove, depth, H, ss, PvNode ? -VALUE_INFINITE : beta);
     CheckInfo ci(pos);
-    Bitboard pinned = pos.pinned_pieces(pos.side_to_move());
+    pinned = pos.pinned_pieces(pos.side_to_move());
     ss->bestMove = MOVE_NONE;
     futilityBase = ss->eval + ss->evalMargin;
     singularExtensionNode =   !RootNode
@@ -898,7 +929,7 @@ split_point_start: // At split points actual search starts from here
         bestValue = sp->bestValue;
     }
 
-    // Step 10. Loop through moves
+    // Step 11. Loop through moves
     // Loop through all pseudo-legal moves until no moves remain or a beta cutoff occurs
     while (   bestValue < beta
            && (move = mp.get_next_move()) != MOVE_NONE
@@ -947,7 +978,7 @@ split_point_start: // At split points actual search starts from here
       givesCheck = pos.move_gives_check(move, ci);
       captureOrPromotion = pos.move_is_capture(move) || move_is_promotion(move);
 
-      // Step 11. Decide the new search depth
+      // Step 12. Decide the new search depth
       ext = extension<PvNode>(pos, move, captureOrPromotion, givesCheck, &dangerous);
 
       // Singular extension search. If all moves but one fail low on a search of
@@ -979,7 +1010,7 @@ split_point_start: // At split points actual search starts from here
       // Update current move (this must be done after singular extension search)
       newDepth = depth - ONE_PLY + ext;
 
-      // Step 12. Futility pruning (is omitted in PV nodes)
+      // Step 13. Futility pruning (is omitted in PV nodes)
       if (   !PvNode
           && !captureOrPromotion
           && !inCheck
@@ -1040,7 +1071,7 @@ split_point_start: // At split points actual search starts from here
 
       ss->currentMove = move;
 
-      // Step 13. Make the move
+      // Step 14. Make the move
       pos.do_move(move, st, ci, givesCheck);
 
       if (!SpNode && !captureOrPromotion)
@@ -1059,7 +1090,7 @@ split_point_start: // At split points actual search starts from here
       }
       else
       {
-          // Step 14. Reduced depth search
+          // Step 15. Reduced depth search
           // If the move fails high will be re-searched at full depth.
           bool doFullDepthSearch = true;
           alpha = SpNode ? sp->alpha : alpha;
@@ -1082,26 +1113,7 @@ split_point_start: // At split points actual search starts from here
               ss->reduction = DEPTH_ZERO; // Restore original reduction
           }
 
-          // Probcut search for bad captures. If a reduced search returns a value
-          // very below beta then we can (almost) safely prune the bad capture.
-          if (   depth >= 3 * ONE_PLY
-              && depth < 8 * ONE_PLY
-              && mp.isBadCapture()
-              && move != ttMove
-              && !dangerous
-              && !move_is_promotion(move)
-              &&  abs(alpha) < VALUE_MATE_IN_PLY_MAX)
-          {
-              ss->reduction = 3 * ONE_PLY;
-              Value rAlpha = alpha - 300;
-              Depth d = newDepth - ss->reduction;
-              value = d < ONE_PLY ? -qsearch<NonPV>(pos, ss+1, -(rAlpha+1), -rAlpha, DEPTH_ZERO)
-                                  : - search<NonPV>(pos, ss+1, -(rAlpha+1), -rAlpha, d);
-              doFullDepthSearch = (value > rAlpha);
-              ss->reduction = DEPTH_ZERO; // Restore original reduction
-          }
-
-          // Step 15. Full depth search
+          // Step 16. Full depth search
           if (doFullDepthSearch)
           {
               alpha = SpNode ? sp->alpha : alpha;
@@ -1117,12 +1129,12 @@ split_point_start: // At split points actual search starts from here
           }
       }
 
-      // Step 16. Undo move
+      // Step 17. Undo move
       pos.undo_move(move);
 
       assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
 
-      // Step 17. Check for new best move
+      // Step 18. Check for new best move
       if (SpNode)
       {
           lock_grab(&(sp->lock));
@@ -1197,7 +1209,7 @@ split_point_start: // At split points actual search starts from here
 
       } // RootNode
 
-      // Step 18. Check for split
+      // Step 19. Check for split
       if (   !RootNode
           && !SpNode
           && depth >= Threads.min_split_depth()
@@ -1209,14 +1221,14 @@ split_point_start: // At split points actual search starts from here
                                    threatMove, moveCount, &mp, PvNode);
     }
 
-    // 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 mate or stalemate.
     // If one move was excluded return fail low score.
     if (!SpNode && !moveCount)
         return excludedMove ? oldAlpha : inCheck ? value_mated_in(ss->ply) : VALUE_DRAW;
 
-    // Step 20. Update tables
+    // Step 21. Update tables
     // If the search is not aborted, update the transposition table,
     // history counters, and killer moves.
     if (!SpNode && !StopRequest && !Threads[threadID].cutoff_occurred())