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Merge Joona Kiiski NULL search beta correction

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Prune more moves after a null search because of
a lower beta limit then in main search.

In test positions reduces the searched nodes of 30% !!!!

Signed-off-by: Marco Costalba <mcostalba@gmail.com>
This commit is contained in:
Marco Costalba
2009-03-13 15:26:02 +01:00
parent 3ed603cd64
commit c8773c720a
3 changed files with 35 additions and 20 deletions
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20
src/search.cpp
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@@ -152,6 +152,16 @@ namespace {
// evaluation of the position is more than NullMoveMargin below beta.
const Value NullMoveMargin = Value(0x300);
//Null move search refutes move when Nullvalue >= Beta - Delta. Index is depth
//in full plies. Last index is 9+.
const Value NullMoveDeltaMidgame[] =
{ Value(-8), Value( 6), Value(-15), Value( 9), Value(21),
Value(34), Value(54), Value( 59), Value(61), Value(61) };
const Value NullMoveDeltaEndgame[] =
{ Value( 6), Value( 0), Value(-13), Value(-9), Value(-35),
Value(12), Value(24), Value( 9), Value( 5), Value( 5) };
// Pruning criterions. See the code and comments in ok_to_prune() to
// understand their precise meaning.
const bool PruneEscapeMoves = false;
@@ -1202,13 +1212,19 @@ namespace {
&& ok_to_do_nullmove(pos)
&& approximateEval >= beta - NullMoveMargin)
{
//Calculate correct delta. Idea and tuning from Joona Kiiski.
ScaleFactor factor[2] = { SCALE_FACTOR_NORMAL, SCALE_FACTOR_NORMAL };
Phase phase = pos.game_phase();
int i = Min(depth / OnePly, 9);
Value delta = scale_by_game_phase(NullMoveDeltaMidgame[i], NullMoveDeltaEndgame[i], phase, factor);
ss[ply].currentMove = MOVE_NULL;
StateInfo st;
pos.do_null_move(st);
int R = (depth >= 4 * OnePly ? 4 : 3); // Null move dynamic reduction
Value nullValue = -search(pos, ss, -(beta-1), depth-R*OnePly, ply+1, false, threadID);
Value nullValue = -search(pos, ss, -(beta-delta-1), depth-R*OnePly, ply+1, false, threadID);
// Check for a null capture artifact, if the value without the null capture
// is above beta then mark the node as a suspicious failed low. We will verify
@@ -1229,7 +1245,7 @@ namespace {
{
/* Do not return unproven mates */
}
else if (nullValue >= beta)
else if (nullValue >= beta - delta)
{
if (depth < 6 * OnePly)
return beta;