Better document null search window

[stockfish] / src / search.cpp

diff --git a/src/search.cpp b/src/search.cpp
index d59fc521eedcba2815ae3cdeaf92f4bdb6e80730..3e6e38bd1273e4a94e67c39403ffb5448d697610 100644 (file)
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -600,11 +600,9 @@ namespace {
         &&  abs(beta) < VALUE_MATE_IN_MAX_PLY
         && !pos.pawn_on_7th(pos.side_to_move()))
     {
-        Value rbeta = beta - razor_margin(depth);
-        Value v = qsearch<NonPV, false>(pos, ss, rbeta-1, rbeta, DEPTH_ZERO);
-        if (v < rbeta)
-            // Logically we should return (v + razor_margin(depth)), but
-            // surprisingly this performed slightly weaker in tests.
+        Value ralpha = alpha - razor_margin(depth);
+        Value v = qsearch<NonPV, false>(pos, ss, ralpha, ralpha+1, DEPTH_ZERO);
+        if (v <= ralpha)
             return v;
     }
 
@@ -628,17 +626,17 @@ namespace {
     {
         ss->currentMove = MOVE_NULL;
 
-        // Null move dynamic reduction based on depth
-        Depth R = 3 * ONE_PLY + depth / 4;
+        assert(eval - beta >= 0);
 
-        // Null move dynamic reduction based on value
-        if (eval - PawnValueMg > beta)
-            R += ONE_PLY;
+        // Null move dynamic reduction based on depth and value
+        Depth R =  3 * ONE_PLY
+                 + depth / 4
+                 + int(eval - beta) / PawnValueMg * ONE_PLY;
 
         pos.do_null_move(st);
         (ss+1)->skipNullMove = true;
-        nullValue = depth-R < ONE_PLY ? -qsearch<NonPV, false>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
-                                      : - search<NonPV>(pos, ss+1, -beta, -alpha, depth-R, !cutNode);
+        nullValue = depth-R < ONE_PLY ? -qsearch<NonPV, false>(pos, ss+1, -beta, -beta+1, DEPTH_ZERO)
+                                      : - search<NonPV>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode);
         (ss+1)->skipNullMove = false;
         pos.undo_null_move();
 
@@ -653,7 +651,8 @@ namespace {
 
             // Do verification search at high depths
             ss->skipNullMove = true;
-            Value v = search<NonPV>(pos, ss, alpha, beta, depth-R, false);
+            Value v = depth-R < ONE_PLY ? qsearch<NonPV, false>(pos, ss, beta-1, beta, DEPTH_ZERO)
+                                        :  search<NonPV>(pos, ss, beta-1, beta, depth-R, false);
             ss->skipNullMove = false;
 
             if (v >= beta)