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Use prob cut search to prune bad captures

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The idea is to try a shallow search with reduced beta
on bad captures so to quickly prune them out in case
are really bad.

After 5529 games 966 - 868 - 3695  ELO +6 (+- 5.4) LOS 91%

Tested also version without upper limitation to 8 plies:

After 8780 games 1537 - 1398 - 5850  ELO +5 (+- 4.3) LOS 93%

Signed-off-by: Marco Costalba <mcostalba@gmail.com>
This commit is contained in:
Joona Kiiski 2011-04-02 08:15:05 +01:00 • committed by Marco Costalba
parent 8402b40341
commit 2e8998eac9
1 changed files with 24 additions and 1 deletions
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25
src/search.cpp
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@ -825,7 +825,7 @@ namespace {
ValueType vt;
Value bestValue, value, oldAlpha;
Value refinedValue, nullValue, futilityBase, futilityValueScaled; // Non-PV specific
bool isPvMove, isCheck, singularExtensionNode, moveIsCheck, captureOrPromotion, dangerous;
bool isPvMove, isCheck, singularExtensionNode, moveIsCheck, captureOrPromotion, dangerous, isBadCap;
bool mateThreat = false;
int moveCount = 0, playedMoveCount = 0;
int threadID = pos.thread();
@ -1172,6 +1172,16 @@ split_point_start: // At split points actual search starts from here
}
}
// Bad capture detection. Will be used by prob-cut search
isBadCap = depth >= 3 * ONE_PLY
&& depth < 8 * ONE_PLY
&& captureOrPromotion
&& move != ttMove
&& !dangerous
&& !move_is_promotion(move)
&& abs(alpha) < VALUE_MATE_IN_PLY_MAX
&& pos.see_sign(move) < 0;
// Step 13. Make the move
pos.do_move(move, st, ci, moveIsCheck);
@ -1193,6 +1203,7 @@ split_point_start: // At split points actual search starts from here
// Step 14. Reduced depth search
// If the move fails high will be re-searched at full depth.
bool doFullDepthSearch = true;
alpha = SpNode ? sp->alpha : alpha;
if ( depth >= 3 * ONE_PLY
&& !captureOrPromotion
@ -1213,6 +1224,18 @@ split_point_start: // At split points actual search starts from here
ss->reduction = DEPTH_ZERO; // Restore original reduction
}
// Probcut search for bad captures. If a reduced search returns a value
// very below beta then we can (almost) safely prune the bad capture.
if (isBadCap)
{
ss->reduction = 3 * ONE_PLY;
Value redAlpha = alpha - 300;
Depth d = newDepth - ss->reduction;
value = -search<NonPV>(pos, ss+1, -(redAlpha+1), -redAlpha, d, ply+1);
doFullDepthSearch = (value > redAlpha);
ss->reduction = DEPTH_ZERO; // Restore original reduction
}
// Step 15. Full depth search
if (doFullDepthSearch)
{