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Null move driven internal iterative deepening 

When a null move fails low due to a capture, try
to detect if without the capture we are above beta,
in this case there is a good possibility this is
a cut-node and the capture is just a null move
artifact due to side to move change. So if we still
don't have a TT move it's a good time to start an IID.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>
sf_2.3.1_base
Marco Costalba 2008-12-08 10:46:52 +01:00
parent b7c36d078b
commit c172af1b61
1 changed files with 28 additions and 32 deletions
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60
src/search.cpp
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@ -106,6 +106,9 @@ namespace {
const bool UseIIDAtPVNodes = true;
const bool UseIIDAtNonPVNodes = false;
// Use null move driven internal iterative deepening?
bool UseNullDrivenIID = true;
// Internal iterative deepening margin. At Non-PV moves, when
// UseIIDAtNonPVNodes is true, we do an internal iterative deepening search
// when the static evaluation is at most IIDMargin below beta.
@ -133,9 +136,6 @@ 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;
@ -1127,13 +1127,13 @@ namespace {
if (tte && ok_to_use_TT(tte, depth, beta, ply))
{
ss[ply].currentMove = ttMove; // can be MOVE_NONE ?
ss[ply].currentMove = ttMove; // can be MOVE_NONE
return value_from_tt(tte->value(), ply);
}
Value approximateEval = quick_evaluate(pos);
bool mateThreat = false;
bool nullCapturePruning = false;
bool nullDrivenIID = false;
bool isCheck = pos.is_check();
// Null move search
@ -1151,15 +1151,16 @@ namespace {
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
// is above beta then there is a good possibility that this is a cut-node.
// We will do an IID later to find a ttMove.
if ( UseNullDrivenIID
&& nullValue < beta
&& depth < 5 * OnePly
&& depth > 6 * OnePly
&& ttMove == MOVE_NONE
&& 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.see(ss[ply + 1].currentMove) * PawnValueMidgame + nullValue > beta - IIDMargin)
nullDrivenIID = true;
pos.undo_null_move(u);
@ -1180,33 +1181,15 @@ namespace {
// low score (which will cause the reduced move to fail high in the
// parent node, which will trigger a re-search with full depth).
if (nullValue == value_mated_in(ply + 2))
{
mateThreat = true;
nullDrivenIID = false;
}
ss[ply].threatMove = ss[ply + 1].currentMove;
if ( depth < ThreatDepth
&& 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
@ -1225,6 +1208,19 @@ namespace {
search(pos, ss, beta, Min(depth/2, depth-2*OnePly), ply, false, threadID);
ttMove = ss[ply].pv[ply];
}
else if (nullDrivenIID)
{
// The null move failed low due to a suspicious capture. Perhaps we
// are facing a null capture artifact due to the side to move change
// and this is a cut-node. So it's a good time to search for a ttMove.
Move tm = ss[ply].threatMove;
assert(tm != MOVE_NONE);
search(pos, ss, beta, Min(depth/2, depth-3*OnePly), ply, false, threadID);
ttMove = ss[ply].pv[ply];
ss[ply].threatMove = tm;
}
// Initialize a MovePicker object for the current position, and prepare
// to search all moves: