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Use fail soft in null search

If null search fails high return null value instead of beta.

With TT hash there may be a small advantage for fail-soft since
storing slightly better bounds may cause slightly more hash hits.

After 990 games at 1+0
Mod vs Orig +171 =665 -154  +6 ELO

Signed-off-by: Marco Costalba <mcostalba@gmail.com>
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1 parent b638f6b commit 426f55b78d8d83ad779772dc46ad4f645177c4c4 @mcostalba committed Mar 20, 2010
Showing with 2 additions and 2 deletions.
  1. +2 −2 src/search.cpp
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@@ -1379,12 +1379,12 @@ namespace {
if (nullValue >= beta)
{
if (depth < 6 * OnePly)
- return beta;
+ return nullValue;
// Do zugzwang verification search
Value v = search(pos, ss, beta, depth-5*OnePly, ply, false, threadID);
if (v >= beta)
- return beta;
+ return nullValue;
} else {
// The null move failed low, which means that we may be faced with
// some kind of threat. If the previous move was reduced, check if

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