Use cycle detection to bound search value

[stockfish] / src / search.cpp

diff --git a/src/search.cpp b/src/search.cpp
index 2072c4bc241d7590d8dca47ba04e3e2c6985146b..025c1141c97546e6e98693877fad9afff26336d5 100644 (file)
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -577,6 +577,17 @@ 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);
@@ -682,12 +693,12 @@ namespace {
         }
     }
 
-    // Step 6. Evaluate the position statically
+    // Step 6. Static evaluation of the position
     if (inCheck)
     {
         ss->staticEval = eval = VALUE_NONE;
         improving = false;
-        goto moves_loop;
+        goto moves_loop;  // Skip early pruning when in check
     }
     else if (ttHit)
     {
@@ -710,13 +721,7 @@ namespace {
                   ss->staticEval, TT.generation());
     }
 
-    improving =   ss->staticEval >= (ss-2)->staticEval
-               ||(ss-2)->staticEval == VALUE_NONE;
-
-    if (ss->excludedMove || !pos.non_pawn_material(pos.side_to_move()))
-        goto moves_loop;
-
-    // Step 7. Razoring (skipped when in check, ~2 Elo)
+    // Step 7. Razoring (~2 Elo)
     if (  !PvNode
         && depth < 3 * ONE_PLY
         && eval <= alpha - RazorMargin[depth / ONE_PLY])
@@ -727,7 +732,10 @@ namespace {
             return v;
     }
 
-    // Step 8. Futility pruning: child node (skipped when in check, ~30 Elo)
+    improving =   ss->staticEval >= (ss-2)->staticEval
+               || (ss-2)->staticEval == VALUE_NONE;
+
+    // Step 8. Futility pruning: child node (~30 Elo)
     if (   !rootNode
         &&  depth < 7 * ONE_PLY
         &&  eval - futility_margin(depth, improving) >= beta
@@ -737,8 +745,11 @@ namespace {
     // Step 9. Null move search with verification search (~40 Elo)
     if (   !PvNode
         && (ss-1)->currentMove != MOVE_NULL
+        && (ss-1)->statScore < 22500
         &&  eval >= beta
         &&  ss->staticEval >= beta - 36 * depth / ONE_PLY + 225
+        && !excludedMove
+        &&  pos.non_pawn_material(pos.side_to_move())
         && (ss->ply >= thisThread->nmp_ply || ss->ply % 2 != thisThread->nmp_odd))
     {
         assert(eval - beta >= 0);
@@ -778,7 +789,7 @@ namespace {
         }
     }
 
-    // Step 10. ProbCut (skipped when in check, ~10 Elo)
+    // Step 10. ProbCut (~10 Elo)
     // If we have a good enough capture and a reduced search returns a value
     // much above beta, we can (almost) safely prune the previous move.
     if (   !PvNode
@@ -816,7 +827,7 @@ namespace {
             }
     }
 
-    // Step 11. Internal iterative deepening (skipped when in check, ~2 Elo)
+    // Step 11. Internal iterative deepening (~2 Elo)
     if (    depth >= 8 * ONE_PLY
         && !ttMove)
     {
@@ -1153,7 +1164,7 @@ moves_loop: // When in check, search starts from here
             update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + ONE_PLY));
     }
     // Bonus for prior countermove that caused the fail low
-    else if (    depth >= 3 * ONE_PLY
+    else if (   (depth >= 3 * ONE_PLY || PvNode)
              && !pos.captured_piece()
              && is_ok((ss-1)->currentMove))
         update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, stat_bonus(depth));