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Remove Threat Extension

Great code simplification: - instead do not futility
prune threat refutations. allows_move() is therefore removed.

4000 games at 50,000 nodes/move:
1085-989-1926 [51.2%] LOS=98.3%

4000 games in 10"+0.1"
756-751-2493 [50.1%] LOS=55.1%

EDIT: I have retested the patch of Lucas in a slightly different form
(without pruning in PvNode) and test mre or less confirms that
60 lines of code are totally unuseful:

After 6195 games at 15"+0.05"
1333 - 1325 - 3537 ELO 0

bench 5140990
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commit 8737b26a23afb36d70cb32e3d53eeac7239685bf 1 parent 78a9531
@lucasart lucasart authored committed
Showing with 4 additions and 62 deletions.
  1. +4 −62 src/search.cpp
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66 src/search.cpp
@@ -99,7 +99,6 @@ namespace {
Value value_to_tt(Value v, int ply);
Value value_from_tt(Value v, int ply);
bool check_is_dangerous(Position& pos, Move move, Value futilityBase, Value beta);
- bool allows_move(const Position& pos, Move first, Move second);
bool prevents_move(const Position& pos, Move first, Move second);
string uci_pv(const Position& pos, int depth, Value alpha, Value beta);
@@ -499,13 +498,12 @@ namespace {
Value bestValue, value, ttValue;
Value eval, nullValue, futilityValue;
bool inCheck, givesCheck, pvMove, singularExtensionNode;
- bool captureOrPromotion, dangerous, doFullDepthSearch, threatExtension;
+ bool captureOrPromotion, dangerous, doFullDepthSearch;
int moveCount, playedMoveCount;
// Step 1. Initialize node
Thread* thisThread = pos.this_thread();
moveCount = playedMoveCount = 0;
- threatExtension = false;
inCheck = pos.checkers();
if (SpNode)
@@ -694,20 +692,9 @@ namespace {
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
- // the move that refuted the null move was somehow connected to the
- // move which was reduced. If a connection is found extend moves that
- // defend against threat.
+ // some kind of threat.
threatMove = (ss+1)->currentMove;
-
- if ( depth < 5 * ONE_PLY
- && (ss-1)->reduction
- && threatMove != MOVE_NONE
- && allows_move(pos, (ss-1)->currentMove, threatMove))
- threatExtension = true;
- }
}
// Step 9. ProbCut (is omitted in PV nodes)
@@ -824,9 +811,6 @@ namespace {
if (PvNode && dangerous)
ext = ONE_PLY;
- else if (threatExtension && prevents_move(pos, move, threatMove))
- ext = ONE_PLY;
-
else if (givesCheck && pos.see_sign(move) >= 0)
ext = ONE_PLY / 2;
@@ -863,13 +847,12 @@ namespace {
&& !inCheck
&& !dangerous
&& move != ttMove
+ && (!threatMove || !prevents_move(pos, move, threatMove))
&& (bestValue > VALUE_MATED_IN_MAX_PLY || ( bestValue == -VALUE_INFINITE
&& alpha > VALUE_MATED_IN_MAX_PLY)))
{
// Move count based pruning
- if ( depth < 16 * ONE_PLY
- && moveCount >= FutilityMoveCounts[depth]
- && (!threatMove || !prevents_move(pos, move, threatMove)))
+ if (depth < 16 * ONE_PLY && moveCount >= FutilityMoveCounts[depth])
{
if (SpNode)
sp->mutex.lock();
@@ -1368,47 +1351,6 @@ namespace {
}
- // allows_move() tests whether the move at previous ply (first) somehow makes a
- // second move possible, for instance if the moving piece is the same in both
- // moves. Normally the second move is the threat move (the best move returned
- // from a null search that fails low).
-
- bool allows_move(const Position& pos, Move first, Move second) {
-
- assert(is_ok(first));
- assert(is_ok(second));
- assert(color_of(pos.piece_on(from_sq(second))) == ~pos.side_to_move());
- assert(color_of(pos.piece_on(to_sq(first))) == ~pos.side_to_move());
-
- Square m1from = from_sq(first);
- Square m2from = from_sq(second);
- Square m1to = to_sq(first);
- Square m2to = to_sq(second);
-
- // The piece is the same or second's destination was vacated by the first move
- if (m1to == m2from || m2to == m1from)
- return true;
-
- // Second one moves through the square vacated by first one
- if (between_bb(m2from, m2to) & m1from)
- return true;
-
- // Second's destination is defended by the first move's piece
- Bitboard m1att = pos.attacks_from(pos.piece_on(m1to), m1to, pos.pieces() ^ m2from);
- if (m1att & m2to)
- return true;
-
- // Second move gives a discovered check through the first's checking piece
- if (m1att & pos.king_square(pos.side_to_move()))
- {
- assert(between_bb(m1to, pos.king_square(pos.side_to_move())) & m2from);
- return true;
- }
-
- return false;
- }
-
-
// prevents_move() tests whether a move (first) is able to defend against an
// opponent's move (second). In this case will not be pruned. Normally the
// second move is the threat move (the best move returned from a null search
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