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Enhanced verify search (#1338)

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by disabling null-move-pruning for the side to move for first part of
the remaining search tree. This helps to better recognize zugzwang.

STC:
LLR: 2.96 (-2.94,2.94) [-3.00,1.00]
Total: 18220 W: 3379 L: 3253 D: 11588
http://tests.stockfishchess.org/tests/view/5a2fa6460ebc590ccbb8bc2f

LTC:
LLR: 2.96 (-2.94,2.94) [-3.00,1.00]
Total: 41899 W: 5359 L: 5265 D: 31275
http://tests.stockfishchess.org/tests/view/5a2fcf440ebc590ccbb8bc47

For further detail see commit notes and discussion at 
6401a80ab9

bench: 5776193
This commit is contained in:
Günther Demetz
2017-12-18 16:30:27 +01:00
committed by Marco Costalba
parent 83e829c9dc
commit b53239d641
3 changed files with 14 additions and 2 deletions
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12
src/search.cpp
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@@ -681,7 +681,8 @@ namespace {
// Step 8. Null move search with verification search (is omitted in PV nodes)
if ( !PvNode
&& eval >= beta
&& ss->staticEval >= beta - 36 * depth / ONE_PLY + 225)
&& ss->staticEval >= beta - 36 * depth / ONE_PLY + 225
&& (ss->ply >= thisThread->nmp_ply || ss->ply % 2 == thisThread->pair))
{
assert(eval - beta >= 0);
@@ -707,8 +708,17 @@ namespace {
return nullValue;
// Do verification search at high depths
R += ONE_PLY;
// disable null move pruning for side to move for the first part of the remaining search tree
int nmp_ply = thisThread->nmp_ply;
int pair = thisThread->pair;
thisThread->nmp_ply = ss->ply + 3 * (depth-R) / 4;
thisThread->pair = (ss->ply % 2) == 0;
Value v = depth-R < ONE_PLY ? qsearch<NonPV, false>(pos, ss, beta-1, beta)
: search<NonPV>(pos, ss, beta-1, beta, depth-R, false, true);
thisThread->pair = pair;
thisThread->nmp_ply = nmp_ply;
if (v >= beta)
return nullValue;