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/movepick.cpp

@@ -29,10 +29,11 @@ namespace {
 
   enum MovegenPhase {
     PH_TT_MOVE,       // Transposition table move
-    PH_GOOD_CAPTURES, // Queen promotions and captures with SEE values >= 0
+    PH_GOOD_CAPTURES, // Queen promotions and captures with SEE values >= captureThreshold (captureThreshold <= 0)
+    PH_GOOD_PROBCUT,  // Queen promotions and captures with SEE values > captureThreshold (captureThreshold >= 0)
     PH_KILLERS,       // Killer moves from the current ply
     PH_NONCAPTURES,   // Non-captures and underpromotions
-    PH_BAD_CAPTURES,  // Queen promotions and captures with SEE values < 0
+    PH_BAD_CAPTURES,  // Queen promotions and captures with SEE values < captureThreshold (captureThreshold <= 0)
     PH_EVASIONS,      // Check evasions
     PH_QCAPTURES,     // Captures in quiescence search
     PH_QCHECKS,       // Non-capture checks in quiescence search
@@ -44,11 +45,10 @@ namespace {
   const uint8_t EvasionTable[] = { PH_TT_MOVE, PH_EVASIONS, PH_STOP };
   const uint8_t QsearchWithChecksTable[] = { PH_TT_MOVE, PH_QCAPTURES, PH_QCHECKS, PH_STOP };
   const uint8_t QsearchWithoutChecksTable[] = { PH_TT_MOVE, PH_QCAPTURES, PH_STOP };
+  const uint8_t ProbCutTable[] = { PH_TT_MOVE, PH_GOOD_PROBCUT, PH_STOP };
 }
 
-bool MovePicker::isBadCapture() const { return phase == PH_BAD_CAPTURES; }
+/// Constructors for the MovePicker class. As arguments we pass information
-
-/// Constructor for the MovePicker class. As arguments we pass information
 /// to help it to return the presumably good moves first, to decide which
 /// moves to return (in the quiescence search, for instance, we only want to
 /// search captures, promotions and some checks) and about how important good
@@ -56,7 +56,7 @@ bool MovePicker::isBadCapture() const { return phase == PH_BAD_CAPTURES; }
 
 MovePicker::MovePicker(const Position& p, Move ttm, Depth d, const History& h,
                        SearchStack* ss, Value beta) : pos(p), H(h) {
-  badCaptureThreshold = 0;
+  captureThreshold = 0;
   badCaptures = moves + MAX_MOVES;
 
   assert(d > DEPTH_ZERO);
@@ -74,7 +74,7 @@ MovePicker::MovePicker(const Position& p, Move ttm, Depth d, const History& h,
       // Consider sligtly negative captures as good if at low
       // depth and far from beta.
       if (ss && ss->eval < beta - PawnValueMidgame && d < 3 * ONE_PLY)
-          badCaptureThreshold = -PawnValueMidgame;
+          captureThreshold = -PawnValueMidgame;
 
       phasePtr = MainSearchTable;
   }
@@ -109,6 +109,24 @@ MovePicker::MovePicker(const Position& p, Move ttm, Depth d, const History& h)
   go_next_phase();
 }
 
+MovePicker::MovePicker(const Position& p, Move ttm, const History& h, int parentCapture)
+                       : pos(p), H(h) {
+
+  assert (!pos.in_check());
+
+  // In ProbCut we consider only captures better than parent's move
+  captureThreshold = parentCapture;
+  phasePtr = ProbCutTable;
+
+  if (   ttm != MOVE_NONE
+      && (!pos.move_is_capture(ttm) ||  pos.see(ttm) <= captureThreshold))
+      ttm = MOVE_NONE;
+
+  ttMove = (ttm && pos.move_is_pl(ttm) ? ttm : MOVE_NONE);
+  phasePtr += int(ttMove == MOVE_NONE) - 1;
+  go_next_phase();
+}
+
 
 /// MovePicker::go_next_phase() generates, scores and sorts the next bunch
 /// of moves when there are no more moves to try for the current phase.
@@ -124,6 +142,7 @@ void MovePicker::go_next_phase() {
       return;
 
   case PH_GOOD_CAPTURES:
+  case PH_GOOD_PROBCUT:
       lastMove = generate<MV_CAPTURE>(pos, moves);
       score_captures();
       return;
@@ -270,9 +289,11 @@ Move MovePicker::get_next_move() {
           move = pick_best(curMove++, lastMove).move;
           if (move != ttMove)
           {
+              assert(captureThreshold <= 0); // Otherwise we must use see instead of see_sign
+
               // Check for a non negative SEE now
               int seeValue = pos.see_sign(move);
-              if (seeValue >= badCaptureThreshold)
+              if (seeValue >= captureThreshold)
                   return move;
 
               // Losing capture, move it to the tail of the array, note
@@ -282,6 +303,13 @@ Move MovePicker::get_next_move() {
           }
           break;
 
+     case PH_GOOD_PROBCUT:
+          move = pick_best(curMove++, lastMove).move;
+          if (   move != ttMove
+              && pos.see(move) > captureThreshold)
+              return move;
+          break;
+
       case PH_KILLERS:
           move = (curMove++)->move;
           if (   move != MOVE_NONE

src/movepick.h

@@ -42,8 +42,8 @@ class MovePicker {
 public:
   MovePicker(const Position&, Move, Depth, const History&, SearchStack*, Value);
   MovePicker(const Position&, Move, Depth, const History&);
+  MovePicker(const Position&, Move, const History&, int parentCapture);
   Move get_next_move();
-  bool isBadCapture() const;
 
 private:
   void score_captures();
@@ -55,7 +55,7 @@ private:
   const History& H;
   Move ttMove;
   MoveStack killers[2];
-  int badCaptureThreshold, phase;
+  int captureThreshold, phase;
   const uint8_t* phasePtr;
   MoveStack *curMove, *lastMove, *lastGoodNonCapture, *badCaptures;
   MoveStack moves[MAX_MOVES];

src/position.h

@@ -213,6 +213,7 @@ public:
   // Static exchange evaluation
   int see(Move m) const;
   int see_sign(Move m) const;
+  static int see_value(PieceType pt);
 
   // Accessing hash keys
   Key get_key() const;
@@ -466,6 +467,10 @@ inline bool Position::square_is_weak(Square s, Color c) const {
   return !(pieces(PAWN, opposite_color(c)) & attack_span_mask(c, s));
 }
 
+inline int Position::see_value(PieceType pt) {
+  return seeValues[pt];
+}
+
 inline Key Position::get_key() const {
   return st->key;
 }

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
+          // Step 16. Full depth search
-          // 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
           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())