namespace {
// Different node types, used as a template parameter
- enum NodeType { NonPV, PV };
+ enum NodeType { NonPV, PV, Root };
constexpr uint64_t TtHitAverageWindow = 4096;
constexpr uint64_t TtHitAverageResolution = 1024;
Move best = MOVE_NONE;
};
- template <NodeType NT>
+ template <NodeType nodeType>
Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode);
- template <NodeType NT>
+ template <NodeType nodeType>
Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth = 0);
Value value_to_tt(Value v, int ply);
void Search::init() {
for (int i = 1; i < MAX_MOVES; ++i)
- Reductions[i] = int((21.3 + 2 * std::log(Threads.size())) * std::log(i + 0.25 * std::log(i)));
+ Reductions[i] = int(21.3 * std::log(i + 0.25 * std::log(i)));
}
// To allow access to (ss-7) up to (ss+2), the stack must be oversized.
// The former is needed to allow update_continuation_histories(ss-1, ...),
// which accesses its argument at ss-6, also near the root.
- // The latter is needed for statScores and killer initialization.
+ // The latter is needed for statScore and killer initialization.
Stack stack[MAX_PLY+10], *ss = stack+7;
Move pv[MAX_PLY+1];
Value bestValue, alpha, beta, delta;
for (int i = 7; i > 0; i--)
(ss-i)->continuationHistory = &this->continuationHistory[0][0][NO_PIECE][0]; // Use as a sentinel
+ for (int i = 0; i <= MAX_PLY + 2; ++i)
+ (ss+i)->ply = i;
+
ss->pv = pv;
bestValue = delta = alpha = -VALUE_INFINITE;
while (true)
{
Depth adjustedDepth = std::max(1, rootDepth - failedHighCnt - searchAgainCounter);
- bestValue = Stockfish::search<PV>(rootPos, ss, alpha, beta, adjustedDepth, false);
+ bestValue = Stockfish::search<Root>(rootPos, ss, alpha, beta, adjustedDepth, false);
// Bring the best move to the front. It is critical that sorting
// is done with a stable algorithm because all the values but the
totBestMoveChanges += th->bestMoveChanges;
th->bestMoveChanges = 0;
}
- double bestMoveInstability = 1 + 2 * totBestMoveChanges / Threads.size();
-
+ double bestMoveInstability = 1.073 + std::max(1.0, 2.25 - 9.9 / rootDepth)
+ * totBestMoveChanges / Threads.size();
double totalTime = Time.optimum() * fallingEval * reduction * bestMoveInstability;
// Cap used time in case of a single legal move for a better viewer experience in tournaments
// search<>() is the main search function for both PV and non-PV nodes
- template <NodeType NT>
+ template <NodeType nodeType>
Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
- constexpr bool PvNode = NT == PV;
- const bool rootNode = PvNode && ss->ply == 0;
+ constexpr bool PvNode = nodeType != NonPV;
+ constexpr bool rootNode = nodeType == Root;
const Depth maxNextDepth = rootNode ? depth : depth + 1;
// Check if we have an upcoming move which draws by repetition, or
// if the opponent had an alternative move earlier to this position.
- if ( pos.rule50_count() >= 3
+ if ( !rootNode
+ && pos.rule50_count() >= 3
&& alpha < VALUE_DRAW
- && !rootNode
&& pos.has_game_cycle(ss->ply))
{
alpha = value_draw(pos.this_thread());
// Dive into quiescence search when the depth reaches zero
if (depth <= 0)
- return qsearch<NT>(pos, ss, alpha, beta);
+ return qsearch<PvNode ? PV : NonPV>(pos, ss, alpha, beta);
assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
assert(PvNode || (alpha == beta - 1));
Move ttMove, move, excludedMove, bestMove;
Depth extension, newDepth;
Value bestValue, value, ttValue, eval, maxValue, probCutBeta;
- bool formerPv, givesCheck, improving, didLMR, priorCapture;
+ bool givesCheck, improving, didLMR, priorCapture;
bool captureOrPromotion, doFullDepthSearch, moveCountPruning,
ttCapture, singularQuietLMR;
Piece movedPiece;
assert(0 <= ss->ply && ss->ply < MAX_PLY);
- (ss+1)->ply = ss->ply + 1;
- (ss+1)->ttPv = false;
+ (ss+1)->ttPv = false;
(ss+1)->excludedMove = bestMove = MOVE_NONE;
- (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
- Square prevSq = to_sq((ss-1)->currentMove);
+ (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
+ ss->doubleExtensions = (ss-1)->doubleExtensions;
+ Square prevSq = to_sq((ss-1)->currentMove);
// Initialize statScore to zero for the grandchildren of the current position.
// So statScore is shared between all grandchildren and only the first grandchild
: ss->ttHit ? tte->move() : MOVE_NONE;
if (!excludedMove)
ss->ttPv = PvNode || (ss->ttHit && tte->is_pv());
- formerPv = ss->ttPv && !PvNode;
// Update low ply history for previous move if we are near root and position is or has been in PV
if ( ss->ttPv
// then that move is singular and should be extended. To verify this we do
// a reduced search on all the other moves but the ttMove and if the
// result is lower than ttValue minus a margin, then we will extend the ttMove.
- if ( depth >= 7
+ if ( !rootNode
+ && depth >= 7
&& move == ttMove
- && !rootNode
&& !excludedMove // Avoid recursive singular search
/* && ttValue != VALUE_NONE Already implicit in the next condition */
&& abs(ttValue) < VALUE_KNOWN_WIN
&& (tte->bound() & BOUND_LOWER)
&& tte->depth() >= depth - 3)
{
- Value singularBeta = ttValue - ((formerPv + 4) * depth) / 2;
- Depth singularDepth = (depth - 1 + 3 * formerPv) / 2;
+ Value singularBeta = ttValue - 2 * depth;
+ Depth singularDepth = (depth - 1) / 2;
ss->excludedMove = move;
value = search<NonPV>(pos, ss, singularBeta - 1, singularBeta, singularDepth, cutNode);
{
extension = 1;
singularQuietLMR = !ttCapture;
- if (!PvNode && value < singularBeta - 93)
+
+ // Avoid search explosion by limiting the number of double extensions to at most 3
+ if ( !PvNode
+ && value < singularBeta - 93
+ && ss->doubleExtensions < 3)
extension = 2;
}
return beta;
}
}
+ else if ( givesCheck
+ && depth > 6
+ && abs(ss->staticEval) > Value(100))
+ extension = 1;
// Add extension to new depth
newDepth += extension;
+ ss->doubleExtensions = (ss-1)->doubleExtensions + (extension == 2);
// Speculative prefetch as early as possible
prefetch(TT.first_entry(pos.key_after(move)));
if ( depth >= 3
&& moveCount > 1 + 2 * rootNode
&& ( !captureOrPromotion
- || cutNode
- || (!PvNode && !formerPv))
+ || (cutNode && (ss-1)->moveCount > 1)
+ || !ss->ttPv)
&& (!PvNode || ss->ply > 1 || thisThread->id() % 4 != 3))
{
Depth r = reduction(improving, depth, moveCount);
+ if (PvNode)
+ r--;
+
// Decrease reduction if the ttHit running average is large (~0 Elo)
if (thisThread->ttHitAverage > 537 * TtHitAverageResolution * TtHitAverageWindow / 1024)
r--;
if (singularQuietLMR)
r--;
+ // Increase reduction for cut nodes (~3 Elo)
+ if (cutNode)
+ r += 1 + !captureOrPromotion;
+
if (!captureOrPromotion)
{
// Increase reduction if ttMove is a capture (~3 Elo)
if (ttCapture)
r++;
- // Increase reduction for cut nodes (~3 Elo)
- if (cutNode)
- r += 2;
-
ss->statScore = thisThread->mainHistory[us][from_to(move)]
+ (*contHist[0])[movedPiece][to_sq(move)]
+ (*contHist[1])[movedPiece][to_sq(move)]
else
{
assert(value >= beta); // Fail high
- ss->statScore = 0;
break;
}
}
// qsearch() is the quiescence search function, which is called by the main search
// function with zero depth, or recursively with further decreasing depth per call.
- template <NodeType NT>
+ template <NodeType nodeType>
Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
- constexpr bool PvNode = NT == PV;
+ static_assert(nodeType != Root);
+ constexpr bool PvNode = nodeType == PV;
assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
assert(PvNode || (alpha == beta - 1));
}
Thread* thisThread = pos.this_thread();
- (ss+1)->ply = ss->ply + 1;
bestMove = MOVE_NONE;
ss->inCheck = pos.checkers();
moveCount = 0;
// Initialize a MovePicker object for the current position, and prepare
// to search the moves. Because the depth is <= 0 here, only captures,
- // queen and checking knight promotions, and other checks(only if depth >= DEPTH_QS_CHECKS)
+ // queen promotions, and other checks (only if depth >= DEPTH_QS_CHECKS)
// will be generated.
MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
&thisThread->captureHistory,
// Make and search the move
pos.do_move(move, st, givesCheck);
- value = -qsearch<NT>(pos, ss+1, -beta, -alpha, depth - 1);
+ value = -qsearch<nodeType>(pos, ss+1, -beta, -alpha, depth - 1);
pos.undo_move(move);
assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);