Simplify capture pruning margin formula

[stockfish] / src / search.cpp

diff --git a/src/search.cpp b/src/search.cpp
index b87c689479d2a7909667a591d31c49b6b6cc21d4..3a80e161ff42238a3eb77c396e9a0feb80159fcc 100644 (file)
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -71,16 +71,6 @@ namespace {
     return Value((175 - 50 * improving) * d / ONE_PLY);
   }
 
-  // Margin for pruning capturing moves: almost linear in depth
-  constexpr int CapturePruneMargin[] = { 0,
-                                         1 * PawnValueEg * 1055 / 1000,
-                                         2 * PawnValueEg * 1042 / 1000,
-                                         3 * PawnValueEg * 963  / 1000,
-                                         4 * PawnValueEg * 1038 / 1000,
-                                         5 * PawnValueEg * 950  / 1000,
-                                         6 * PawnValueEg * 930  / 1000
-                                       };
-
   // Futility and reductions lookup tables, initialized at startup
   int FutilityMoveCounts[2][16]; // [improving][depth]
   int Reductions[2][2][64][64];  // [pv][improving][depth][moveNumber]
@@ -296,6 +286,7 @@ void Thread::search() {
   MainThread* mainThread = (this == Threads.main() ? Threads.main() : nullptr);
   double timeReduction = 1.0;
   Color us = rootPos.side_to_move();
+  bool failedLow;
 
   std::memset(ss-4, 0, 7 * sizeof(Stack));
   for (int i = 4; i > 0; i--)
@@ -305,7 +296,7 @@ void Thread::search() {
   beta = VALUE_INFINITE;
 
   if (mainThread)
-      mainThread->bestMoveChanges = 0, mainThread->failedLow = false;
+      mainThread->bestMoveChanges = 0, failedLow = false;
 
   size_t multiPV = Options["MultiPV"];
   Skill skill(Options["Skill Level"]);
@@ -346,7 +337,7 @@ void Thread::search() {
 
       // Age out PV variability metric
       if (mainThread)
-          mainThread->bestMoveChanges *= 0.517, mainThread->failedLow = false;
+          mainThread->bestMoveChanges *= 0.517, failedLow = false;
 
       // Save the last iteration's scores before first PV line is searched and
       // all the move scores except the (new) PV are set to -VALUE_INFINITE.
@@ -423,7 +414,7 @@ void Thread::search() {
 
                   if (mainThread)
                   {
-                      mainThread->failedLow = true;
+                      failedLow = true;
                       Threads.stopOnPonderhit = false;
                   }
               }
@@ -471,7 +462,7 @@ void Thread::search() {
           && !Threads.stop
           && !Threads.stopOnPonderhit)
           {
-              const int F[] = { mainThread->failedLow,
+              const int F[] = { failedLow,
                                 bestValue - mainThread->previousScore };
 
               int improvingFactor = std::max(246, std::min(832, 306 + 119 * F[0] - 6 * F[1]));
@@ -519,13 +510,25 @@ namespace {
   template <NodeType NT>
   Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
 
-    // Use quiescence search when needed
-    if (depth < ONE_PLY)
-        return qsearch<NT>(pos, ss, alpha, beta);
-
     constexpr bool PvNode = NT == PV;
     const bool rootNode = PvNode && ss->ply == 0;
 
+    // Check if we have an upcoming move which draws by repetition, or
+    // if the opponent had an alternative move earlier to this position.
+    if (   pos.rule50_count() >= 3
+        && alpha < VALUE_DRAW
+        && !rootNode
+        && pos.has_game_cycle(ss->ply))
+    {
+        alpha = VALUE_DRAW;
+        if (alpha >= beta)
+            return alpha;
+    }
+
+    // Dive into quiescence search when the depth reaches zero
+    if (depth < ONE_PLY)
+        return qsearch<NT>(pos, ss, alpha, beta);
+
     assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
     assert(PvNode || (alpha == beta - 1));
     assert(DEPTH_ZERO < depth && depth < DEPTH_MAX);
@@ -578,17 +581,6 @@ namespace {
         beta = std::min(mate_in(ss->ply+1), beta);
         if (alpha >= beta)
             return alpha;
-
-        // Check if there exists a move which draws by repetition, or an alternative
-        // earlier move to this position.
-        if (   pos.rule50_count() >= 3
-            && alpha < VALUE_DRAW
-            && pos.has_game_cycle(ss->ply))
-        {
-            alpha = VALUE_DRAW;
-            if (alpha >= beta)
-                return alpha;
-        }
     }
 
     assert(0 <= ss->ply && ss->ply < MAX_PLY);
@@ -751,7 +743,7 @@ namespace {
         &&  ss->staticEval >= beta - 36 * depth / ONE_PLY + 225
         && !excludedMove
         &&  pos.non_pawn_material(us)
-        && (ss->ply > thisThread->nmpMinPly || us != thisThread->nmpColor))
+        && (ss->ply >= thisThread->nmpMinPly || us != thisThread->nmpColor))
     {
         assert(eval - beta >= 0);
 
@@ -780,7 +772,7 @@ namespace {
 
             // Do verification search at high depths, with null move pruning disabled
             // for us, until ply exceeds nmpMinPly.
-            thisThread->nmpMinPly = ss->ply + 3 * (depth-R) / 4 - 1;
+            thisThread->nmpMinPly = ss->ply + 3 * (depth-R) / 4;
             thisThread->nmpColor = us;
 
             Value v = search<NonPV>(pos, ss, beta-1, beta, depth-R, false);
@@ -961,7 +953,7 @@ moves_loop: // When in check, search starts from here
           }
           else if (    depth < 7 * ONE_PLY // (~20 Elo)
                    && !extension
-                   && !pos.see_ge(move, -Value(CapturePruneMargin[depth / ONE_PLY])))
+                   && !pos.see_ge(move, -Value(PawnValueEg * (depth / ONE_PLY))))
                   continue;
       }
 
@@ -996,8 +988,7 @@ moves_loop: // When in check, search starts from here
           if (captureOrPromotion) // (~5 Elo)
           {
               // Increase reduction by comparing opponent's stat score
-              if (   (ss-1)->statScore >= 0
-                  && thisThread->captureHistory[movedPiece][to_sq(move)][type_of(pos.captured_piece())] < 0)
+              if ((ss-1)->statScore >= 0)
                   r += ONE_PLY;
 
               r -= r ? ONE_PLY : DEPTH_ZERO;
@@ -1164,9 +1155,10 @@ moves_loop: // When in check, search starts from here
     {
         // Quiet best move: update move sorting heuristics
         if (!pos.capture_or_promotion(bestMove))
-            update_quiet_stats(pos, ss, bestMove, quietsSearched, quietCount, stat_bonus(depth));
+            update_quiet_stats(pos, ss, bestMove, quietsSearched, quietCount,
+                               stat_bonus(depth + (bestValue > beta + PawnValueMg ? ONE_PLY : DEPTH_ZERO)));
         else
-            update_capture_stats(pos, bestMove, capturesSearched, captureCount, stat_bonus(depth));
+            update_capture_stats(pos, bestMove, capturesSearched, captureCount, stat_bonus(depth + ONE_PLY));
 
         // Extra penalty for a quiet TT move in previous ply when it gets refuted
         if ((ss-1)->moveCount == 1 && !pos.captured_piece())