From: Marco Costalba <mcostalba@gmail.com>
Date: Wed, 12 Feb 2014 13:16:21 +0000 (+0100)
Subject: Revert "Retire null search verification"
X-Git-Url: https://git.sesse.net/?p=stockfish;a=commitdiff_plain;h=b534176d4acc15ca5d7fdf3f44d1afb4e33c899a;ds=sidebyside

Revert "Retire null search verification"

Although does not change ELO level, it seems
verification is useful in many zugzwang positions
as reported by many sources.

So revert this simplification.

bench: 8430785
---

diff --git a/src/search.cpp b/src/search.cpp
index 097faf90..7188925c 100644
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -640,8 +640,24 @@ namespace {
         (ss+1)->skipNullMove = false;
         pos.undo_null_move();
 
-        if (nullValue >= beta) // Do not return unproven mate scores
-            return nullValue >= VALUE_MATE_IN_MAX_PLY ? beta : nullValue;
+        if (nullValue >= beta)
+        {
+            // Do not return unproven mate scores
+            if (nullValue >= VALUE_MATE_IN_MAX_PLY)
+                nullValue = beta;
+
+            if (depth < 12 * ONE_PLY)
+                return nullValue;
+
+            // Do verification search at high depths
+            ss->skipNullMove = true;
+            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)
+                return nullValue;
+        }
     }
 
     // Step 9. ProbCut (skipped when in check)