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Null capture pruning

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Null move can fail low because of a capture artifact due
to the side to move change. Try to detect this condition
and fail high instead.

This pruning is very powerful, around 7% of nodes, but is
still experimental so is disabled by default.

Set UseNullCapturePruning to true to enable.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>
This commit is contained in:
Marco Costalba 2008-12-06 11:31:14 +01:00
parent dc2302b701
commit b4fcfed55b
1 changed files with 37 additions and 0 deletions
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37
src/search.cpp
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@ -133,6 +133,9 @@ namespace {
// evaluation of the position is more than NullMoveMargin below beta.
const Value NullMoveMargin = Value(0x300);
// Use null capture pruning?
const bool UseNullCapturePruning = false;
// Pruning criterions. See the code and comments in ok_to_prune() to
// understand their precise meaning.
const bool PruneEscapeMoves = false;
@ -1130,6 +1133,7 @@ namespace {
Value approximateEval = quick_evaluate(pos);
bool mateThreat = false;
bool nullCapturePruning = false;
bool isCheck = pos.is_check();
// Null move search
@ -1143,7 +1147,20 @@ namespace {
UndoInfo u;
pos.do_null_move(u);
int R = (depth > 7 ? 4 : 3);
Value nullValue = -search(pos, ss, -(beta-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
// later if we are really under threat.
if ( UseNullCapturePruning
&& nullValue < beta
&& depth < 5 * OnePly
&& ss[ply + 1].currentMove != MOVE_NONE
&& pos.move_is_capture(ss[ply + 1].currentMove)
&& pos.see(ss[ply + 1].currentMove) * PawnValueMidgame + nullValue > beta)
nullCapturePruning = true;
pos.undo_null_move(u);
if (nullValue >= beta)
@ -1170,6 +1187,26 @@ namespace {
&& ss[ply - 1].reduction
&& connected_moves(pos, ss[ply - 1].currentMove, ss[ply].threatMove))
return beta - 1;
if (nullCapturePruning && !mateThreat)
{
// The null move failed low due to a suspicious capture. Verify if
// position is really dangerous or we are facing a null capture
// artifact due to the side to move change. So search this
// position with a reduced depth and see if we still fail low.
Move tm = ss[ply].threatMove;
assert(tm != MOVE_NONE);
Value v = search(pos, ss, beta, depth-3*OnePly, ply, false, threadID);
if (v >= beta)
return beta;
// Restore stack and update ttMove if was empty
ss[ply].threatMove = tm;
if (ttMove == MOVE_NONE)
ttMove = ss[ply].pv[ply];
}
}
}
// Null move search not allowed, try razoring