Introduce separate counter-move tables for captures

Enhance counter-move history table by adding a capture/no-capture dimension,
depending wether the previous move was a quiet move or a capture. This doubles
the size of the table but provides more accurate move ordering.

STC:
LLR: 2.95 (-2.94,2.94) [0.50,4.50]
Total: 79702 W: 17720 L: 17164 D: 44818
http://tests.stockfishchess.org/tests/view/5d97945e0ebc590c21aa724b

LTC:
LLR: 2.96 (-2.94,2.94) [0.00,3.50]
Total: 29147 W: 4907 L: 4651 D: 19589
http://tests.stockfishchess.org/tests/view/5d97ccb90ebc590c21aa7bc0

Closes https://github.com/official-stockfish/Stockfish/pull/2344

Bench: 4131643

src/movepick.cpp

@@ -111,11 +111,11 @@ void MovePicker::score() {
                    + (*captureHistory)[pos.moved_piece(m)][to_sq(m)][type_of(pos.piece_on(to_sq(m)))];
 
       else if (Type == QUIETS)
-          m.value =  (*mainHistory)[pos.side_to_move()][from_to(m)]
-                   + (*continuationHistory[0])[pos.moved_piece(m)][to_sq(m)]
-                   + (*continuationHistory[1])[pos.moved_piece(m)][to_sq(m)]
-                   + (*continuationHistory[3])[pos.moved_piece(m)][to_sq(m)]
-                   + (*continuationHistory[5])[pos.moved_piece(m)][to_sq(m)] / 2;
+          m.value =      (*mainHistory)[pos.side_to_move()][from_to(m)]
+                   + 2 * (*continuationHistory[0])[pos.moved_piece(m)][to_sq(m)]
+                   + 2 * (*continuationHistory[1])[pos.moved_piece(m)][to_sq(m)]
+                   + 2 * (*continuationHistory[3])[pos.moved_piece(m)][to_sq(m)]
+                   +     (*continuationHistory[5])[pos.moved_piece(m)][to_sq(m)];
 
       else // Type == EVASIONS
       {
@@ -206,7 +206,7 @@ top:
           endMoves = generate<QUIETS>(pos, cur);
 
           score<QUIETS>();
-          partial_insertion_sort(cur, endMoves, -4000 * depth);
+          partial_insertion_sort(cur, endMoves, -3000 * depth);
       }
 
       ++stage;

src/movepick.h

@@ -80,7 +80,7 @@ struct Stats<T, D, Size> : public std::array<StatsEntry<T, D>, Size> {};
 
 /// In stats table, D=0 means that the template parameter is not used
 enum StatsParams { NOT_USED = 0 };
-
+enum StatsType { NoCaptures, Captures };
 
 /// ButterflyHistory records how often quiet moves have been successful or
 /// unsuccessful during the current search, and is used for reduction and move

src/search.cpp

@@ -334,7 +334,8 @@ void Thread::search() {
 
   std::memset(ss-7, 0, 10 * sizeof(Stack));
   for (int i = 7; i > 0; i--)
-     (ss-i)->continuationHistory = &this->continuationHistory[NO_PIECE][0]; // Use as sentinel
+      (ss-i)->continuationHistory = &this->continuationHistory[0][NO_PIECE][0]; // Use as a sentinel
+
   ss->pv = pv;
 
   bestValue = delta = alpha = -VALUE_INFINITE;
@@ -595,12 +596,13 @@ namespace {
     Value bestValue, value, ttValue, eval, maxValue;
     bool ttHit, ttPv, inCheck, givesCheck, improving, doLMR;
     bool captureOrPromotion, doFullDepthSearch, moveCountPruning, ttCapture;
-    Piece movedPiece;
+    Piece movedPiece, priorCapture;
     int moveCount, captureCount, quietCount, singularLMR;
 
     // Step 1. Initialize node
     Thread* thisThread = pos.this_thread();
     inCheck = pos.checkers();
+    priorCapture = pos.captured_piece();
     Color us = pos.side_to_move();
     moveCount = captureCount = quietCount = singularLMR = ss->moveCount = 0;
     bestValue = -VALUE_INFINITE;
@@ -680,7 +682,7 @@ namespace {
                     update_quiet_stats(pos, ss, ttMove, nullptr, 0, stat_bonus(depth));
 
                 // Extra penalty for early quiet moves of the previous ply
-                if ((ss-1)->moveCount <= 2 && !pos.captured_piece())
+                if ((ss-1)->moveCount <= 2 && !priorCapture)
                     update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + 1));
             }
             // Penalty for a quiet ttMove that fails low
@@ -814,7 +816,7 @@ namespace {
         Depth R = (835 + 70 * depth) / 256 + std::min(int(eval - beta) / 185, 3);
 
         ss->currentMove = MOVE_NULL;
-        ss->continuationHistory = &thisThread->continuationHistory[NO_PIECE][0];
+        ss->continuationHistory = &thisThread->continuationHistory[0][NO_PIECE][0];
 
         pos.do_null_move(st);
 
@@ -865,7 +867,7 @@ namespace {
                 probCutCount++;
 
                 ss->currentMove = move;
-                ss->continuationHistory = &thisThread->continuationHistory[pos.moved_piece(move)][to_sq(move)];
+                ss->continuationHistory = &thisThread->continuationHistory[!!priorCapture][pos.moved_piece(move)][to_sq(move)];
 
                 assert(depth >= 5);
 
@@ -1066,7 +1068,7 @@ moves_loop: // When in check, search starts from here
 
       // Update the current move (this must be done after singular extension search)
       ss->currentMove = move;
-      ss->continuationHistory = &thisThread->continuationHistory[movedPiece][to_sq(move)];
+      ss->continuationHistory = &thisThread->continuationHistory[!!priorCapture][movedPiece][to_sq(move)];
 
       // Step 15. Make the move
       pos.do_move(move, st, givesCheck);
@@ -1279,13 +1281,13 @@ moves_loop: // When in check, search starts from here
 
         // Extra penalty for a quiet TT or main killer move in previous ply when it gets refuted
         if (   ((ss-1)->moveCount == 1 || ((ss-1)->currentMove == (ss-1)->killers[0]))
-            && !pos.captured_piece())
+            && !priorCapture)
                 update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + 1));
 
     }
     // Bonus for prior countermove that caused the fail low
     else if (   (depth >= 3 || PvNode)
-             && !pos.captured_piece())
+             && !priorCapture)
         update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, stat_bonus(depth));
 
     if (PvNode)
@@ -1321,6 +1323,7 @@ moves_loop: // When in check, search starts from here
     Move ttMove, move, bestMove;
     Depth ttDepth;
     Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
+    Piece priorCapture;
     bool ttHit, pvHit, inCheck, givesCheck, evasionPrunable;
     int moveCount;
 
@@ -1335,6 +1338,7 @@ moves_loop: // When in check, search starts from here
     (ss+1)->ply = ss->ply + 1;
     bestMove = MOVE_NONE;
     inCheck = pos.checkers();
+    priorCapture = pos.captured_piece();
     moveCount = 0;
 
     // Check for an immediate draw or maximum ply reached
@@ -1472,7 +1476,7 @@ moves_loop: // When in check, search starts from here
       }
 
       ss->currentMove = move;
-      ss->continuationHistory = &thisThread->continuationHistory[pos.moved_piece(move)][to_sq(move)];
+      ss->continuationHistory = &thisThread->continuationHistory[!!priorCapture][pos.moved_piece(move)][to_sq(move)];
 
       // Make and search the move
       pos.do_move(move, st, givesCheck);

src/thread.cpp

@@ -70,11 +70,13 @@ void Thread::clear() {
   mainHistory.fill(0);
   captureHistory.fill(0);
 
-  for (auto& to : continuationHistory)
-      for (auto& h : to)
-          h->fill(0);
+  for (StatsType c : { NoCaptures, Captures })
+      for (auto& to : continuationHistory[c])
+          for (auto& h : to)
+              h->fill(0);
 
-  continuationHistory[NO_PIECE][0]->fill(Search::CounterMovePruneThreshold - 1);
+  for (StatsType c : { NoCaptures, Captures })
+      continuationHistory[c][NO_PIECE][0]->fill(Search::CounterMovePruneThreshold - 1);
 }
 
 /// Thread::start_searching() wakes up the thread that will start the search

src/thread.h

@@ -71,7 +71,7 @@ public:
   CounterMoveHistory counterMoves;
   ButterflyHistory mainHistory;
   CapturePieceToHistory captureHistory;
-  ContinuationHistory continuationHistory;
+  ContinuationHistory continuationHistory[2];
   Score contempt;
 };