Better document null search window

Hopefully this patch makes the code more:

* Self-documenting: Null search is always a zero window search,
  because it is testing for a fail high. It should never be done
  on a full window! The current code only works because we don't
  do it at PV nodes, and therefore (alpha, beta) = (beta-1, beta):
  that's the kind of "clever" trick we should avoid.

* Idiot-proof: If we want to enable null search at PV nodes, all we
  need to do now is comment out the !PvNode condition. It's that simple!

In theory, null search should not be done at PV nodes, because PV nodes
should never fail high. But in practice, they DO fail high, because of
aspiration windows, and search inconsistencies, for example. So it makes
sense to keep that flexibility in the code.

No functional change.

diff --git a/src/search.cpp b/src/search.cpp
index 056dc250377a9ecf8ae63ab3270247f3cc331da3..3e6e38bd1273e4a94e67c39403ffb5448d697610 100644 (file)
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -635,8 +635,8 @@ namespace {
 
         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();
 
@@ -651,8 +651,8 @@ namespace {
 
             // Do verification search at high depths
             ss->skipNullMove = true;
-            Value v = depth-R < ONE_PLY ? qsearch<NonPV, false>(pos, ss, alpha, beta, DEPTH_ZERO)
-                                        :  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)