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Fix (zugzwang) verification to be shallower then null search

Currently starting from depth 12*OnePly on we have a verification
search deeper then the null search.

Note that, although reduction is R we start from one ply less then
null search, so actually we reach a depth that is OnePly shallower
then null search.

After 1130 games at 1'+0 on QUAD
Mod vs Orig +202 =756 -172  +9 ELO

Signed-off-by: Marco Costalba <>
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commit f4e98dbe0b07604b0ea3969c042b1bb189b95e9f 1 parent 379d4c4
@mcostalba authored
Showing with 2 additions and 2 deletions.
  1. +2 −2 src/search.cpp
4 src/search.cpp
@@ -1155,12 +1155,12 @@ namespace {
if (nullValue >= value_mate_in(PLY_MAX))
nullValue = beta;
- // Do zugzwang verification search at high depths
if (depth < 6 * OnePly)
return nullValue;
+ // Do verification search at high depths
ss->skipNullMove = true;
- Value v = search<NonPV>(pos, ss, alpha, beta, depth-5*OnePly, ply);
+ Value v = search<NonPV>(pos, ss, alpha, beta, depth-R*OnePly, ply);
ss->skipNullMove = false;
if (v >= beta)

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