// Futility margin
Value futility_margin(Depth d, bool improving) {
- return Value(154 * (d - improving));
+ return Value(140 * (d - improving));
}
- // Reductions lookup table, initialized at startup
+ // Reductions lookup table initialized at startup
int Reductions[MAX_MOVES]; // [depth or moveNumber]
Depth reduction(bool i, Depth d, int mn, Value delta, Value rootDelta) {
int r = Reductions[d] * Reductions[mn];
- return (r + 1449 - int(delta) * 937 / int(rootDelta)) / 1024 + (!i && r > 941);
+ return (r + 1372 - int(delta) * 1073 / int(rootDelta)) / 1024 + (!i && r > 936);
}
constexpr int futility_move_count(bool improving, Depth depth) {
// History and stats update bonus, based on depth
int stat_bonus(Depth d) {
- return std::min(341 * d - 470, 1710);
+ return std::min(336 * d - 547, 1561);
}
// Add a small random component to draw evaluations to avoid 3-fold blindness
// Skill structure is used to implement strength limit. If we have an uci_elo then
// we convert it to a suitable fractional skill level using anchoring to CCRL Elo
- // (goldfish 1.13 = 2000) and a fit through Ordo derived Elo for match (TC 60+0.6)
+ // (goldfish 1.13 = 2000) and a fit through Ordo derived Elo for a match (TC 60+0.6)
// results spanning a wide range of k values.
struct Skill {
Skill(int skill_level, int uci_elo) {
void Search::init() {
for (int i = 1; i < MAX_MOVES; ++i)
- Reductions[i] = int((19.47 + std::log(Threads.size()) / 2) * std::log(i));
+ Reductions[i] = int((20.57 + std::log(Threads.size()) / 2) * std::log(i));
}
ss->pv = pv;
- bestValue = delta = alpha = -VALUE_INFINITE;
- beta = VALUE_INFINITE;
- optimism[WHITE] = optimism[BLACK] = VALUE_ZERO;
+ bestValue = -VALUE_INFINITE;
if (mainThread)
{
-
- int rootComplexity;
- Eval::evaluate(rootPos, &rootComplexity);
-
- mainThread->complexity = std::min(1.03 + (rootComplexity - 241) / 1552.0, 1.45);
-
if (mainThread->bestPreviousScore == VALUE_INFINITE)
for (int i = 0; i < 4; ++i)
mainThread->iterValue[i] = VALUE_ZERO;
Skill skill(Options["Skill Level"], Options["UCI_LimitStrength"] ? int(Options["UCI_Elo"]) : 0);
// When playing with strength handicap enable MultiPV search that we will
- // use behind the scenes to retrieve a set of possible moves.
+ // use behind-the-scenes to retrieve a set of possible moves.
if (skill.enabled())
multiPV = std::max(multiPV, (size_t)4);
if (mainThread)
totBestMoveChanges /= 2;
- // Save the last iteration's scores before first PV line is searched and
+ // Save the last iteration's scores before the first PV line is searched and
// all the move scores except the (new) PV are set to -VALUE_INFINITE.
for (RootMove& rm : rootMoves)
rm.previousScore = rm.score;
selDepth = 0;
// Reset aspiration window starting size
- if (rootDepth >= 4)
- {
- Value prev = rootMoves[pvIdx].averageScore;
- delta = Value(10) + int(prev) * prev / 16502;
- alpha = std::max(prev - delta,-VALUE_INFINITE);
- beta = std::min(prev + delta, VALUE_INFINITE);
-
- // Adjust optimism based on root move's previousScore
- int opt = 120 * prev / (std::abs(prev) + 161);
- optimism[ us] = Value(opt);
- optimism[~us] = -optimism[us];
- }
+ Value prev = rootMoves[pvIdx].averageScore;
+ delta = Value(10) + int(prev) * prev / 15799;
+ alpha = std::max(prev - delta,-VALUE_INFINITE);
+ beta = std::min(prev + delta, VALUE_INFINITE);
+
+ // Adjust optimism based on root move's previousScore
+ int opt = 109 * prev / (std::abs(prev) + 141);
+ optimism[ us] = Value(opt);
+ optimism[~us] = -optimism[us];
// Start with a small aspiration window and, in the case of a fail
// high/low, re-search with a bigger window until we don't fail
int failedHighCnt = 0;
while (true)
{
- // Adjust the effective depth searched, but ensuring at least one effective increment for every
+ // Adjust the effective depth searched, but ensure at least one effective increment for every
// four searchAgain steps (see issue #2717).
Depth adjustedDepth = std::max(1, rootDepth - failedHighCnt - 3 * (searchAgainCounter + 1) / 4);
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
- // first and eventually the new best one are set to -VALUE_INFINITE
+ // first and eventually the new best one is set to -VALUE_INFINITE
// and we want to keep the same order for all the moves except the
- // new PV that goes to the front. Note that in case of MultiPV
+ // new PV that goes to the front. Note that in the case of MultiPV
// search the already searched PV lines are preserved.
std::stable_sort(rootMoves.begin() + pvIdx, rootMoves.begin() + pvLast);
else
break;
- delta += delta / 4 + 2;
+ delta += delta / 3;
assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
}
if (!mainThread)
continue;
- // If skill level is enabled and time is up, pick a sub-optimal best move
+ // If the skill level is enabled and time is up, pick a sub-optimal best move
if (skill.enabled() && skill.time_to_pick(rootDepth))
skill.pick_best(multiPV);
double reduction = (1.4 + mainThread->previousTimeReduction) / (2.08 * timeReduction);
double bestMoveInstability = 1 + 1.8 * totBestMoveChanges / Threads.size();
- double totalTime = Time.optimum() * fallingEval * reduction * bestMoveInstability * mainThread->complexity;
+ double totalTime = Time.optimum() * fallingEval * reduction * bestMoveInstability;
// Cap used time in case of a single legal move for a better viewer experience in tournaments
// yielding correct scores and sufficiently fast moves.
mainThread->previousTimeReduction = timeReduction;
- // If skill level is enabled, swap best PV line with the sub-optimal one
+ // If the skill level is enabled, swap the best PV line with the sub-optimal one
if (skill.enabled())
std::swap(rootMoves[0], *std::find(rootMoves.begin(), rootMoves.end(),
skill.best ? skill.best : skill.pick_best(multiPV)));
constexpr bool PvNode = nodeType != NonPV;
constexpr bool rootNode = nodeType == Root;
- // Check if we have an upcoming move which draws by repetition, or
+ // Check if we have an upcoming move that draws by repetition, or
// if the opponent had an alternative move earlier to this position.
if ( !rootNode
&& pos.rule50_count() >= 3
bool givesCheck, improving, priorCapture, singularQuietLMR;
bool capture, moveCountPruning, ttCapture;
Piece movedPiece;
- int moveCount, captureCount, quietCount, improvement, complexity;
+ int moveCount, captureCount, quietCount, improvement;
// Step 1. Initialize node
Thread* thisThread = pos.this_thread();
// would be at best mate_in(ss->ply+1), but if alpha is already bigger because
// a shorter mate was found upward in the tree then there is no need to search
// because we will never beat the current alpha. Same logic but with reversed
- // signs applies also in the opposite condition of being mated instead of giving
- // mate. In this case return a fail-high score.
+ // signs apply also in the opposite condition of being mated instead of giving
+ // mate. In this case, return a fail-high score.
alpha = std::max(mated_in(ss->ply), alpha);
beta = std::min(mate_in(ss->ply+1), beta);
if (alpha >= beta)
// At non-PV nodes we check for an early TT cutoff
if ( !PvNode
- && ss->ttHit
&& !excludedMove
&& tte->depth() > depth - (tte->bound() == BOUND_EXACT)
- && ttValue != VALUE_NONE // Possible in case of TT access race
+ && ttValue != VALUE_NONE // Possible in case of TT access race or if !ttHit
&& (tte->bound() & (ttValue >= beta ? BOUND_LOWER : BOUND_UPPER)))
{
// If ttMove is quiet, update move sorting heuristics on TT hit (~2 Elo)
ss->staticEval = eval = VALUE_NONE;
improving = false;
improvement = 0;
- complexity = 0;
goto moves_loop;
}
else if (excludedMove)
// Providing the hint that this node's accumulator will be used often brings significant Elo gain (13 Elo)
Eval::NNUE::hint_common_parent_position(pos);
eval = ss->staticEval;
- complexity = abs(ss->staticEval - pos.psq_eg_stm());
}
else if (ss->ttHit)
{
// Never assume anything about values stored in TT
ss->staticEval = eval = tte->eval();
if (eval == VALUE_NONE)
- ss->staticEval = eval = evaluate(pos, &complexity);
- else // Fall back to (semi)classical complexity for TT hits, the NNUE complexity is lost
- {
- complexity = abs(ss->staticEval - pos.psq_eg_stm());
- if (PvNode)
- Eval::NNUE::hint_common_parent_position(pos);
- }
+ ss->staticEval = eval = evaluate(pos);
+ else if (PvNode)
+ Eval::NNUE::hint_common_parent_position(pos);
// ttValue can be used as a better position evaluation (~7 Elo)
if ( ttValue != VALUE_NONE
}
else
{
- ss->staticEval = eval = evaluate(pos, &complexity);
- // Save static evaluation into transposition table
+ ss->staticEval = eval = evaluate(pos);
+ // Save static evaluation into the transposition table
tte->save(posKey, VALUE_NONE, ss->ttPv, BOUND_NONE, DEPTH_NONE, MOVE_NONE, eval);
}
// Use static evaluation difference to improve quiet move ordering (~4 Elo)
if (is_ok((ss-1)->currentMove) && !(ss-1)->inCheck && !priorCapture)
{
- int bonus = std::clamp(-19 * int((ss-1)->staticEval + ss->staticEval), -1920, 1920);
+ int bonus = std::clamp(-18 * int((ss-1)->staticEval + ss->staticEval), -1817, 1817);
thisThread->mainHistory[~us][from_to((ss-1)->currentMove)] << bonus;
}
// margin and the improving flag are used in various pruning heuristics.
improvement = (ss-2)->staticEval != VALUE_NONE ? ss->staticEval - (ss-2)->staticEval
: (ss-4)->staticEval != VALUE_NONE ? ss->staticEval - (ss-4)->staticEval
- : 156;
+ : 173;
improving = improvement > 0;
// Step 7. Razoring (~1 Elo).
// If eval is really low check with qsearch if it can exceed alpha, if it can't,
// return a fail low.
- if (eval < alpha - 426 - 256 * depth * depth)
+ if (eval < alpha - 456 - 252 * depth * depth)
{
value = qsearch<NonPV>(pos, ss, alpha - 1, alpha);
if (value < alpha)
// The depth condition is important for mate finding.
if ( !ss->ttPv
&& depth < 9
- && eval - futility_margin(depth, improving) - (ss-1)->statScore / 280 >= beta
+ && eval - futility_margin(depth, improving) - (ss-1)->statScore / 306 >= beta
&& eval >= beta
- && eval < 25128) // larger than VALUE_KNOWN_WIN, but smaller than TB wins
+ && eval < 24923) // larger than VALUE_KNOWN_WIN, but smaller than TB wins
return eval;
// Step 9. Null move search with verification search (~35 Elo)
if ( !PvNode
&& (ss-1)->currentMove != MOVE_NULL
- && (ss-1)->statScore < 18755
+ && (ss-1)->statScore < 17329
&& eval >= beta
&& eval >= ss->staticEval
- && ss->staticEval >= beta - 20 * depth - improvement / 13 + 253 + complexity / 25
+ && ss->staticEval >= beta - 21 * depth + 258
&& !excludedMove
&& pos.non_pawn_material(us)
- && (ss->ply >= thisThread->nmpMinPly || us != thisThread->nmpColor))
+ && ss->ply >= thisThread->nmpMinPly
+ && beta > VALUE_TB_LOSS_IN_MAX_PLY)
{
assert(eval - beta >= 0);
- // Null move dynamic reduction based on depth, eval and complexity of position
- Depth R = std::min(int(eval - beta) / 172, 6) + depth / 3 + 4 - (complexity > 825);
+ // Null move dynamic reduction based on depth and eval
+ Depth R = std::min(int(eval - beta) / 173, 6) + depth / 3 + 4;
ss->currentMove = MOVE_NULL;
ss->continuationHistory = &thisThread->continuationHistory[0][0][NO_PIECE][0];
if (nullValue >= beta)
{
// Do not return unproven mate or TB scores
- if (nullValue >= VALUE_TB_WIN_IN_MAX_PLY)
- nullValue = beta;
+ nullValue = std::min(nullValue, VALUE_TB_WIN_IN_MAX_PLY-1);
- if (thisThread->nmpMinPly || (abs(beta) < VALUE_KNOWN_WIN && depth < 14))
+ if (thisThread->nmpMinPly || depth < 14)
return nullValue;
assert(!thisThread->nmpMinPly); // Recursive verification is not allowed
// Do verification search at high depths, with null move pruning disabled
- // for us, until ply exceeds nmpMinPly.
+ // until ply exceeds nmpMinPly.
thisThread->nmpMinPly = ss->ply + 3 * (depth-R) / 4;
- thisThread->nmpColor = us;
Value v = search<NonPV>(pos, ss, beta-1, beta, depth-R, false);
}
}
- probCutBeta = beta + 186 - 54 * improving;
+ // Step 10. If the position doesn't have a ttMove, decrease depth by 2
+ // (or by 4 if the TT entry for the current position was hit and the stored depth is greater than or equal to the current depth).
+ // Use qsearch if depth is equal or below zero (~9 Elo)
+ if ( PvNode
+ && !ttMove)
+ depth -= 2 + 2 * (ss->ttHit && tte->depth() >= depth);
+
+ if (depth <= 0)
+ return qsearch<PV>(pos, ss, alpha, beta);
+
+ if ( cutNode
+ && depth >= 8
+ && !ttMove)
+ depth -= 2;
- // Step 10. ProbCut (~10 Elo)
+ probCutBeta = beta + 168 - 61 * improving;
+
+ // Step 11. ProbCut (~10 Elo)
// If we have a good enough capture (or queen promotion) and a reduced search returns a value
// much above beta, we can (almost) safely prune the previous move.
if ( !PvNode
- && depth > 4
+ && depth > 3
&& abs(beta) < VALUE_TB_WIN_IN_MAX_PLY
- // if value from transposition table is lower than probCutBeta, don't attempt probCut
+ // If value from transposition table is lower than probCutBeta, don't attempt probCut
// there and in further interactions with transposition table cutoff depth is set to depth - 3
// because probCut search has depth set to depth - 4 but we also do a move before it
- // so effective depth is equal to depth - 3
- && !( ss->ttHit
- && tte->depth() >= depth - 3
+ // So effective depth is equal to depth - 3
+ && !( tte->depth() >= depth - 3
&& ttValue != VALUE_NONE
&& ttValue < probCutBeta))
{
Eval::NNUE::hint_common_parent_position(pos);
}
- // Step 11. If the position is not in TT, decrease depth by 2 (or by 4 if the TT entry for the current position was hit and the stored depth is greater than or equal to the current depth).
- // Use qsearch if depth is equal or below zero (~9 Elo)
- if ( PvNode
- && !ttMove)
- depth -= 2 + 2 * (ss->ttHit && tte->depth() >= depth);
-
- if (depth <= 0)
- return qsearch<PV>(pos, ss, alpha, beta);
-
- if ( cutNode
- && depth >= 7
- && !ttMove)
- depth -= 2;
-
moves_loop: // When in check, search starts here
// Step 12. A small Probcut idea, when we are in check (~4 Elo)
- probCutBeta = beta + 391;
+ probCutBeta = beta + 413;
if ( ss->inCheck
&& !PvNode
- && depth >= 2
&& ttCapture
&& (tte->bound() & BOUND_LOWER)
- && tte->depth() >= depth - 3
+ && tte->depth() >= depth - 4
&& ttValue >= probCutBeta
&& abs(ttValue) <= VALUE_KNOWN_WIN
&& abs(beta) <= VALUE_KNOWN_WIN)
moveCountPruning = singularQuietLMR = false;
// Indicate PvNodes that will probably fail low if the node was searched
- // at a depth equal or greater than the current depth, and the result of this search was a fail low.
+ // at a depth equal to or greater than the current depth, and the result of this search was a fail low.
bool likelyFailLow = PvNode
&& ttMove
&& (tte->bound() & BOUND_UPPER)
continue;
// At root obey the "searchmoves" option and skip moves not listed in Root
- // Move List. As a consequence any illegal move is also skipped. In MultiPV
- // mode we also skip PV moves which have been already searched and those
+ // Move List. As a consequence, any illegal move is also skipped. In MultiPV
+ // mode we also skip PV moves that have been already searched and those
// of lower "TB rank" if we are in a TB root position.
if (rootNode && !std::count(thisThread->rootMoves.begin() + thisThread->pvIdx,
thisThread->rootMoves.begin() + thisThread->pvLast, move))
moveCountPruning = moveCount >= futility_move_count(improving, depth);
// Reduced depth of the next LMR search
- int lmrDepth = std::max(newDepth - r, 0);
+ int lmrDepth = newDepth - r;
if ( capture
|| givesCheck)
{
// Futility pruning for captures (~2 Elo)
if ( !givesCheck
- && lmrDepth < 6
+ && lmrDepth < 7
&& !ss->inCheck
- && ss->staticEval + 182 + 230 * lmrDepth + PieceValue[EG][pos.piece_on(to_sq(move))]
+ && ss->staticEval + 197 + 248 * lmrDepth + PieceValue[EG][pos.piece_on(to_sq(move))]
+ captureHistory[movedPiece][to_sq(move)][type_of(pos.piece_on(to_sq(move)))] / 7 < alpha)
continue;
Bitboard occupied;
// SEE based pruning (~11 Elo)
- if (!pos.see_ge(move, occupied, Value(-206) * depth))
+ if (!pos.see_ge(move, occupied, Value(-205) * depth))
{
- if (depth < 2 - capture)
- continue;
- // don't prune move if a heavy enemy piece (KQR) is under attack after the exchanges
- Bitboard leftEnemies = (pos.pieces(~us, QUEEN, ROOK) | pos.pieces(~us, KING)) & occupied;
- Bitboard attacks = 0;
- occupied |= to_sq(move);
- while (leftEnemies && !attacks)
- {
+ if (depth < 2 - capture)
+ continue;
+ // Don't prune the move if opponent Queen/Rook is under discovered attack after the exchanges
+ // Don't prune the move if opponent King is under discovered attack after or during the exchanges
+ Bitboard leftEnemies = (pos.pieces(~us, KING, QUEEN, ROOK)) & occupied;
+ Bitboard attacks = 0;
+ occupied |= to_sq(move);
+ while (leftEnemies && !attacks)
+ {
Square sq = pop_lsb(leftEnemies);
attacks |= pos.attackers_to(sq, occupied) & pos.pieces(us) & occupied;
- // exclude Queen/Rook(s) which were already threatened before SEE
+ // Don't consider pieces that were already threatened/hanging before SEE exchanges
if (attacks && (sq != pos.square<KING>(~us) && (pos.attackers_to(sq, pos.pieces()) & pos.pieces(us))))
- attacks = 0;
- }
- if (!attacks)
- continue;
+ attacks = 0;
+ }
+ if (!attacks)
+ continue;
}
}
else
+ (*contHist[3])[movedPiece][to_sq(move)];
// Continuation history based pruning (~2 Elo)
- if ( lmrDepth < 5
- && history < -4405 * (depth - 1))
+ if ( lmrDepth < 6
+ && history < -3832 * depth)
continue;
history += 2 * thisThread->mainHistory[us][from_to(move)];
- lmrDepth += history / 7278;
+ lmrDepth += history / 7011;
lmrDepth = std::max(lmrDepth, -2);
// Futility pruning: parent node (~13 Elo)
if ( !ss->inCheck
- && lmrDepth < 13
- && ss->staticEval + 103 + 138 * lmrDepth <= alpha)
+ && lmrDepth < 12
+ && ss->staticEval + 112 + 138 * lmrDepth <= alpha)
continue;
lmrDepth = std::max(lmrDepth, 0);
// Prune moves with negative SEE (~4 Elo)
- if (!pos.see_ge(move, Value(-24 * lmrDepth * lmrDepth - 16 * lmrDepth)))
+ if (!pos.see_ge(move, Value(-27 * lmrDepth * lmrDepth - 16 * lmrDepth)))
continue;
}
}
// 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.
+ // Depth margin and singularBeta margin are known for having non-linear scaling.
+ // Their values are optimized to time controls of 180+1.8 and longer
+ // so changing them requires tests at this type of time controls.
if ( !rootNode
- && depth >= 4 - (thisThread->completedDepth > 21) + 2 * (PvNode && tte->is_pv())
+ && depth >= 4 - (thisThread->completedDepth > 22) + 2 * (PvNode && tte->is_pv())
&& move == ttMove
&& !excludedMove // Avoid recursive singular search
/* && ttValue != VALUE_NONE Already implicit in the next condition */
&& (tte->bound() & BOUND_LOWER)
&& tte->depth() >= depth - 3)
{
- Value singularBeta = ttValue - (3 + 2 * (ss->ttPv && !PvNode)) * depth / 2;
+ Value singularBeta = ttValue - (82 + 65 * (ss->ttPv && !PvNode)) * depth / 64;
Depth singularDepth = (depth - 1) / 2;
ss->excludedMove = move;
// Avoid search explosion by limiting the number of double extensions
if ( !PvNode
- && value < singularBeta - 25
- && ss->doubleExtensions <= 10)
+ && value < singularBeta - 21
+ && ss->doubleExtensions <= 11)
{
extension = 2;
depth += depth < 13;
// Our ttMove is assumed to fail high, and now we failed high also on a reduced
// search without the ttMove. So we assume this expected Cut-node is not singular,
// that multiple moves fail high, and we can prune the whole subtree by returning
- // a soft bound.
+ // a softbound.
else if (singularBeta >= beta)
return singularBeta;
else if (ttValue >= beta)
extension = -2 - !PvNode;
+ // If we are on a cutNode, reduce it based on depth (negative extension) (~1 Elo)
+ else if (cutNode)
+ extension = depth > 8 && depth < 17 ? -3 : -1;
+
// If the eval of ttMove is less than value, we reduce it (negative extension) (~1 Elo)
else if (ttValue <= value)
extension = -1;
// Check extensions (~1 Elo)
else if ( givesCheck
- && depth > 10
- && abs(ss->staticEval) > 88)
+ && depth > 9)
extension = 1;
// Quiet ttMove extensions (~1 Elo)
else if ( PvNode
&& move == ttMove
&& move == ss->killers[0]
- && (*contHist[0])[movedPiece][to_sq(move)] >= 5705)
+ && (*contHist[0])[movedPiece][to_sq(move)] >= 5168)
extension = 1;
}
// Decrease reduction if position is or has been on the PV
// and node is not likely to fail low. (~3 Elo)
+ // Decrease further on cutNodes. (~1 Elo)
if ( ss->ttPv
&& !likelyFailLow)
- r -= 2;
+ r -= cutNode && tte->depth() >= depth + 3 ? 3 : 2;
// Decrease reduction if opponent's move count is high (~1 Elo)
- if ((ss-1)->moveCount > 7)
+ if ((ss-1)->moveCount > 8)
r--;
// Increase reduction for cut nodes (~3 Elo)
// Decrease reduction for PvNodes based on depth (~2 Elo)
if (PvNode)
- r -= 1 + 12 / (3 + depth);
+ r -= 1 + (depth < 6);
// Decrease reduction if ttMove has been singularly extended (~1 Elo)
if (singularQuietLMR)
if ((ss+1)->cutoffCnt > 3)
r++;
- // Decrease reduction if move is a killer and we have a good history (~1 Elo)
- if (move == ss->killers[0]
- && (*contHist[0])[movedPiece][to_sq(move)] >= 3722)
+ else if (move == ttMove)
r--;
ss->statScore = 2 * thisThread->mainHistory[us][from_to(move)]
+ (*contHist[0])[movedPiece][to_sq(move)]
+ (*contHist[1])[movedPiece][to_sq(move)]
+ (*contHist[3])[movedPiece][to_sq(move)]
- - 4082;
+ - 4006;
// Decrease/increase reduction for moves with a good/bad history (~25 Elo)
- r -= ss->statScore / (11079 + 4626 * (depth > 6 && depth < 19));
+ r -= ss->statScore / (11124 + 4740 * (depth > 5 && depth < 22));
// Step 17. Late moves reduction / extension (LMR, ~117 Elo)
// We use various heuristics for the sons of a node after the first son has
- // been searched. In general we would like to reduce them, but there are many
+ // been searched. In general, we would like to reduce them, but there are many
// cases where we extend a son if it has good chances to be "interesting".
if ( depth >= 2
&& moveCount > 1 + (PvNode && ss->ply <= 1)
value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true);
- // Do full depth search when reduced LMR search fails high
+ // Do a full-depth search when reduced LMR search fails high
if (value > alpha && d < newDepth)
{
- // Adjust full depth search based on LMR results - if result
+ // Adjust full-depth search based on LMR results - if the result
// was good enough search deeper, if it was bad enough search shallower
- const bool doDeeperSearch = value > (alpha + 58 + 12 * (newDepth - d));
- const bool doEvenDeeperSearch = value > alpha + 588 && ss->doubleExtensions <= 5;
+ const bool doDeeperSearch = value > (bestValue + 64 + 11 * (newDepth - d));
+ const bool doEvenDeeperSearch = value > alpha + 711 && ss->doubleExtensions <= 6;
const bool doShallowerSearch = value < bestValue + newDepth;
ss->doubleExtensions = ss->doubleExtensions + doEvenDeeperSearch;
if (newDepth > d)
value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode);
- int bonus = value > alpha ? stat_bonus(newDepth)
- : -stat_bonus(newDepth);
+ int bonus = value <= alpha ? -stat_bonus(newDepth)
+ : value >= beta ? stat_bonus(newDepth)
+ : 0;
update_continuation_histories(ss, movedPiece, to_sq(move), bonus);
}
}
- // Step 18. Full depth search when LMR is skipped. If expected reduction is high, reduce its depth by 1.
+ // Step 18. Full-depth search when LMR is skipped. If expected reduction is high, reduce its depth by 1.
else if (!PvNode || moveCount > 1)
{
// Increase reduction for cut nodes and not ttMove (~1 Elo)
if (!ttMove && cutNode)
r += 2;
- value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth - (r > 4), !cutNode);
+ value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth - (r > 3), !cutNode);
}
// For PV nodes only, do a full PV search on the first move or after a fail
(ss+1)->pv[0] = MOVE_NONE;
value = -search<PV>(pos, ss+1, -beta, -alpha, newDepth, false);
-
- if (moveCount > 1 && newDepth >= depth && !capture)
- update_continuation_histories(ss, movedPiece, to_sq(move), -stat_bonus(newDepth));
}
// Step 19. Undo move
++thisThread->bestMoveChanges;
}
else
- // All other moves but the PV are set to the lowest value: this
+ // All other moves but the PV, are set to the lowest value: this
// is not a problem when sorting because the sort is stable and the
// move position in the list is preserved - just the PV is pushed up.
rm.score = -VALUE_INFINITE;
if (PvNode && !rootNode) // Update pv even in fail-high case
update_pv(ss->pv, move, (ss+1)->pv);
- if (PvNode && value < beta) // Update alpha! Always alpha < beta
+ if (value >= beta)
{
-
- // Reduce other moves if we have found at least one score improvement (~1 Elo)
- if ( depth > 1
- && ((improving && complexity > 971) || (value < (5 * alpha + 75 * beta) / 87) || depth < 6)
- && beta < 12535
- && value > -12535) {
- bool extraReduction = depth > 2 && alpha > -12535 && bestValue != -VALUE_INFINITE && (value - bestValue) > (7 * (beta - alpha)) / 8;
- depth -= 1 + extraReduction;
- }
-
- assert(depth > 0);
- alpha = value;
+ ss->cutoffCnt += 1 + !ttMove;
+ assert(value >= beta); // Fail high
+ break;
}
else
{
- ss->cutoffCnt++;
- assert(value >= beta); // Fail high
- break;
+ // Reduce other moves if we have found at least one score improvement (~2 Elo)
+ if ( depth > 2
+ && depth < 12
+ && beta < 14362
+ && value > -12393)
+ depth -= 2;
+
+ assert(depth > 0);
+ alpha = value; // Update alpha! Always alpha < beta
}
}
}
- // If the move is worse than some previously searched move, remember it to update its stats later
+ // If the move is worse than some previously searched move, remember it, to update its stats later
if (move != bestMove)
{
if (capture && captureCount < 32)
}
// The following condition would detect a stop only after move loop has been
- // completed. But in this case bestValue is valid because we have fully
+ // completed. But in this case, bestValue is valid because we have fully
// searched our subtree, and we can anyhow save the result in TT.
/*
if (Threads.stop)
ss->inCheck ? mated_in(ss->ply)
: VALUE_DRAW;
- // If there is a move which produces search value greater than alpha we update stats of searched moves
+ // If there is a move that produces search value greater than alpha we update the stats of searched moves
else if (bestMove)
update_all_stats(pos, ss, bestMove, bestValue, beta, prevSq,
quietsSearched, quietCount, capturesSearched, captureCount, depth);
// Bonus for prior countermove that caused the fail low
else if (!priorCapture && prevSq != SQ_NONE)
{
- int bonus = (depth > 5) + (PvNode || cutNode) + (bestValue < alpha - 97 * depth) + ((ss-1)->moveCount > 10);
+ int bonus = (depth > 5) + (PvNode || cutNode) + (bestValue < alpha - 113 * depth) + ((ss-1)->moveCount > 12);
update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, stat_bonus(depth) * bonus);
}
// At non-PV nodes we check for an early TT cutoff
if ( !PvNode
- && ss->ttHit
&& tte->depth() >= ttDepth
- && ttValue != VALUE_NONE // Only in case of TT access race
+ && ttValue != VALUE_NONE // Only in case of TT access race or if !ttHit
&& (tte->bound() & (ttValue >= beta ? BOUND_LOWER : BOUND_UPPER)))
return ttValue;
// Step 4. Static evaluation of the position
if (ss->inCheck)
- {
- ss->staticEval = VALUE_NONE;
bestValue = futilityBase = -VALUE_INFINITE;
- }
else
{
if (ss->ttHit)
}
else
// In case of null move search use previous static eval with a different sign
- ss->staticEval = bestValue =
- (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
- : -(ss-1)->staticEval;
+ ss->staticEval = bestValue = (ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
+ : -(ss-1)->staticEval;
// Stand pat. Return immediately if static value is at least beta
if (bestValue >= beta)
if (PvNode && bestValue > alpha)
alpha = bestValue;
- futilityBase = bestValue + 168;
+ futilityBase = bestValue + 200;
}
const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,
// to search the moves. Because the depth is <= 0 here, only captures,
// queen promotions, and other checks (only if depth >= DEPTH_QS_CHECKS)
// will be generated.
- Square prevSq = (ss-1)->currentMove != MOVE_NULL ? to_sq((ss-1)->currentMove) : SQ_NONE;
+ Square prevSq = is_ok((ss-1)->currentMove) ? to_sq((ss-1)->currentMove) : SQ_NONE;
MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
&thisThread->captureHistory,
contHist,
// or a beta cutoff occurs.
while ((move = mp.next_move()) != MOVE_NONE)
{
- assert(is_ok(move));
-
- // Check for legality
- if (!pos.legal(move))
- continue;
-
- givesCheck = pos.gives_check(move);
- capture = pos.capture_stage(move);
+ assert(is_ok(move));
- moveCount++;
+ // Check for legality
+ if (!pos.legal(move))
+ continue;
- // Step 6. Pruning.
- if (bestValue > VALUE_TB_LOSS_IN_MAX_PLY)
- {
- // Futility pruning and moveCount pruning (~10 Elo)
- if ( !givesCheck
- && to_sq(move) != prevSq
- && futilityBase > -VALUE_KNOWN_WIN
- && type_of(move) != PROMOTION)
- {
- if (moveCount > 2)
- continue;
+ givesCheck = pos.gives_check(move);
+ capture = pos.capture_stage(move);
- futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
+ moveCount++;
- if (futilityValue <= alpha)
- {
- bestValue = std::max(bestValue, futilityValue);
- continue;
- }
+ // Step 6. Pruning.
+ if (bestValue > VALUE_TB_LOSS_IN_MAX_PLY)
+ {
+ // Futility pruning and moveCount pruning (~10 Elo)
+ if ( !givesCheck
+ && to_sq(move) != prevSq
+ && futilityBase > -VALUE_KNOWN_WIN
+ && type_of(move) != PROMOTION)
+ {
+ if (moveCount > 2)
+ continue;
- if (futilityBase <= alpha && !pos.see_ge(move, VALUE_ZERO + 1))
- {
- bestValue = std::max(bestValue, futilityBase);
- continue;
- }
- }
+ futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
- // We prune after 2nd quiet check evasion where being 'in check' is implicitly checked through the counter
- // and being a 'quiet' apart from being a tt move is assumed after an increment because captures are pushed ahead.
- if (quietCheckEvasions > 1)
- break;
+ if (futilityValue <= alpha)
+ {
+ bestValue = std::max(bestValue, futilityValue);
+ continue;
+ }
- // Continuation history based pruning (~3 Elo)
- if ( !capture
- && (*contHist[0])[pos.moved_piece(move)][to_sq(move)] < 0
- && (*contHist[1])[pos.moved_piece(move)][to_sq(move)] < 0)
- continue;
+ if (futilityBase <= alpha && !pos.see_ge(move, VALUE_ZERO + 1))
+ {
+ bestValue = std::max(bestValue, futilityBase);
+ continue;
+ }
+ }
- // Do not search moves with bad enough SEE values (~5 Elo)
- if (!pos.see_ge(move, Value(-110)))
- continue;
- }
+ // We prune after the second quiet check evasion move, where being 'in check' is
+ // implicitly checked through the counter, and being a 'quiet move' apart from
+ // being a tt move is assumed after an increment because captures are pushed ahead.
+ if (quietCheckEvasions > 1)
+ break;
+
+ // Continuation history based pruning (~3 Elo)
+ if ( !capture
+ && (*contHist[0])[pos.moved_piece(move)][to_sq(move)] < 0
+ && (*contHist[1])[pos.moved_piece(move)][to_sq(move)] < 0)
+ continue;
+
+ // Do not search moves with bad enough SEE values (~5 Elo)
+ if (!pos.see_ge(move, Value(-95)))
+ continue;
+ }
- // Speculative prefetch as early as possible
- prefetch(TT.first_entry(pos.key_after(move)));
+ // Speculative prefetch as early as possible
+ prefetch(TT.first_entry(pos.key_after(move)));
- // Update the current move
- ss->currentMove = move;
- ss->continuationHistory = &thisThread->continuationHistory[ss->inCheck]
- [capture]
- [pos.moved_piece(move)]
- [to_sq(move)];
+ // Update the current move
+ ss->currentMove = move;
+ ss->continuationHistory = &thisThread->continuationHistory[ss->inCheck]
+ [capture]
+ [pos.moved_piece(move)]
+ [to_sq(move)];
- quietCheckEvasions += !capture && ss->inCheck;
+ quietCheckEvasions += !capture && ss->inCheck;
- // Step 7. Make and search the move
- pos.do_move(move, st, givesCheck);
- value = -qsearch<nodeType>(pos, ss+1, -beta, -alpha, depth - 1);
- pos.undo_move(move);
+ // Step 7. Make and search the move
+ pos.do_move(move, st, givesCheck);
+ value = -qsearch<nodeType>(pos, ss+1, -beta, -alpha, depth - 1);
+ pos.undo_move(move);
- assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
+ assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
- // Step 8. Check for a new best move
- if (value > bestValue)
- {
- bestValue = value;
+ // Step 8. Check for a new best move
+ if (value > bestValue)
+ {
+ bestValue = value;
- if (value > alpha)
- {
- bestMove = move;
+ if (value > alpha)
+ {
+ bestMove = move;
- if (PvNode) // Update pv even in fail-high case
- update_pv(ss->pv, move, (ss+1)->pv);
+ if (PvNode) // Update pv even in fail-high case
+ update_pv(ss->pv, move, (ss+1)->pv);
- if (PvNode && value < beta) // Update alpha here!
- alpha = value;
- else
- break; // Fail high
- }
- }
+ if (PvNode && value < beta) // Update alpha here!
+ alpha = value;
+ else
+ break; // Fail high
+ }
+ }
}
// Step 9. Check for mate
if (!pos.capture_stage(bestMove))
{
- int bonus2 = bestValue > beta + 153 ? bonus1 // larger bonus
+ int bonus2 = bestValue > beta + 145 ? bonus1 // larger bonus
: stat_bonus(depth); // smaller bonus
// Increase stats for the best move in case it was a quiet move
for (int i : {1, 2, 4, 6})
{
- // Only update first 2 continuation histories if we are in check
+ // Only update the first 2 continuation histories if we are in check
if (ss->inCheck && i > 2)
break;
if (is_ok((ss-i)->currentMove))
}
}
- // When playing with strength handicap, choose best move among a set of RootMoves
+ // When playing with strength handicap, choose the best move among a set of RootMoves
// using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
Move Skill::pick_best(size_t multiPV) {
/// RootMove::extract_ponder_from_tt() is called in case we have no ponder move
/// before exiting the search, for instance, in case we stop the search during a
/// fail high at root. We try hard to have a ponder move to return to the GUI,
-/// otherwise in case of 'ponder on' we have nothing to think on.
+/// otherwise in case of 'ponder on' we have nothing to think about.
bool RootMove::extract_ponder_from_tt(Position& pos) {