enum NodeType { NonPV, PV, Root };
// Futility margin
- Value futility_margin(Depth d, bool improving) {
- return Value(140 * (d - improving));
+ Value futility_margin(Depth d, bool noTtCutNode, bool improving) {
+ return Value((140 - 40 * noTtCutNode) * (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) {
// 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) {
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;
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);
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);
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;
+ int moveCount, captureCount, quietCount;
// 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
&& !excludedMove
- && tte->depth() > depth - (tte->bound() == BOUND_EXACT)
+ && tte->depth() > depth
&& ttValue != VALUE_NONE // Possible in case of TT access race or if !ttHit
&& (tte->bound() & (ttValue >= beta ? BOUND_LOWER : BOUND_UPPER)))
{
// Skip early pruning when in check
ss->staticEval = eval = VALUE_NONE;
improving = false;
- improvement = 0;
goto moves_loop;
}
else if (excludedMove)
else
{
ss->staticEval = eval = evaluate(pos);
- // Save static evaluation into transposition table
+ // Save static evaluation into the transposition table
tte->save(posKey, VALUE_NONE, ss->ttPv, BOUND_NONE, DEPTH_NONE, MOVE_NONE, eval);
}
thisThread->mainHistory[~us][from_to((ss-1)->currentMove)] << bonus;
}
- // Set up the improvement variable, which is the difference between the current
- // static evaluation and the previous static evaluation at our turn (if we were
- // in check at our previous move we look at the move prior to it). The improvement
- // 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
- : 173;
- improving = improvement > 0;
+ // Set up the improving flag, which is true if current static evaluation is
+ // bigger than the previous static evaluation at our turn (if we were in
+ // check at our previous move we look at static evaluation at move prior to it
+ // and if we were in check at move prior to it flag is set to true) and is
+ // false otherwise. The improving flag is used in various pruning heuristics.
+ improving = (ss-2)->staticEval != VALUE_NONE ? ss->staticEval > (ss-2)->staticEval
+ : (ss-4)->staticEval != VALUE_NONE ? ss->staticEval > (ss-4)->staticEval
+ : true;
// Step 7. Razoring (~1 Elo).
// If eval is really low check with qsearch if it can exceed alpha, if it can't,
// The depth condition is important for mate finding.
if ( !ss->ttPv
&& depth < 9
- && eval - futility_margin(depth, improving) - (ss-1)->statScore / 306 >= beta
+ && eval - futility_margin(depth, cutNode && !ss->ttHit, improving) - (ss-1)->statScore / 306 >= beta
&& eval >= beta
&& eval < 24923) // larger than VALUE_KNOWN_WIN, but smaller than TB wins
return eval;
&& (ss-1)->statScore < 17329
&& eval >= beta
&& eval >= ss->staticEval
- && ss->staticEval >= beta - 21 * depth - improvement / 13 + 258
+ && ss->staticEval >= beta - 21 * depth + 258
&& !excludedMove
&& pos.non_pawn_material(us)
- && (ss->ply >= thisThread->nmpMinPly))
+ && ss->ply >= thisThread->nmpMinPly
+ && beta > VALUE_TB_LOSS_IN_MAX_PLY)
{
assert(eval - beta >= 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
if ( !PvNode
&& 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
+ // So effective depth is equal to depth - 3
&& !( tte->depth() >= depth - 3
&& ttValue != VALUE_NONE
&& ttValue < probCutBeta))
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))
if ( !givesCheck
&& lmrDepth < 7
&& !ss->inCheck
- && ss->staticEval + 197 + 248 * lmrDepth + PieceValue[EG][pos.piece_on(to_sq(move))]
+ && ss->staticEval + 197 + 248 * lmrDepth + PieceValue[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(-205) * depth))
- {
- 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;
- // don't consider pieces which 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;
- }
+ // SEE based pruning for captures and checks (~11 Elo)
+ if (!pos.see_ge(move, Value(-205) * depth))
+ continue;
}
else
{
lmrDepth = std::max(lmrDepth, 0);
// Prune moves with negative SEE (~4 Elo)
- if (!pos.see_ge(move, Value(-27 * lmrDepth * lmrDepth - 16 * lmrDepth)))
+ if (!pos.see_ge(move, Value(-31 * lmrDepth * lmrDepth)))
continue;
}
}
// 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 < 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;
-
- // If the eval of ttMove is less than alpha, we reduce it (negative extension) (~1 Elo)
- else if (ttValue <= alpha)
- extension = -1;
}
// Check extensions (~1 Elo)
if (ttCapture)
r++;
- // Decrease reduction for PvNodes based on depth (~2 Elo)
+ // Decrease reduction for PvNodes (~2 Elo)
if (PvNode)
- r -= 1 + 12 / (3 + depth);
+ r--;
// Decrease reduction if ttMove has been singularly extended (~1 Elo)
if (singularQuietLMR)
// 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 > (bestValue + 64 + 11 * (newDepth - d));
const bool doEvenDeeperSearch = value > alpha + 711 && ss->doubleExtensions <= 6;
}
}
- // 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)
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
- // high (in the latter case search only if value < beta), otherwise let the
- // parent node fail low with value <= alpha and try another move.
- if (PvNode && (moveCount == 1 || (value > alpha && (rootNode || value < beta))))
+ // For PV nodes only, do a full PV search on the first move or after a fail high,
+ // otherwise let the parent node fail low with value <= alpha and try another move.
+ if (PvNode && (moveCount == 1 || value > alpha))
{
(ss+1)->pv = pv;
(ss+1)->pv[0] = MOVE_NONE;
++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;
}
else
{
- // Reduce other moves if we have found at least one score improvement (~1 Elo)
- // Reduce more for depth > 3 and depth < 12 (~1 Elo)
- if ( depth > 1
+ // Reduce other moves if we have found at least one score improvement (~2 Elo)
+ if ( depth > 2
+ && depth < 12
&& beta < 14362
&& value > -12393)
- depth -= depth > 3 && depth < 12 ? 2 : 1;
+ 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 - 113 * depth) + ((ss-1)->moveCount > 12);
+ int bonus = (depth > 5) + (PvNode || cutNode) + (bestValue < alpha - 800) + ((ss-1)->moveCount > 12);
update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, stat_bonus(depth) * bonus);
+ thisThread->mainHistory[~us][from_to((ss-1)->currentMove)] << stat_bonus(depth) * bonus / 2;
}
if (PvNode)
assert(PvNode || (alpha == beta - 1));
assert(depth <= 0);
+ // Check if we have an upcoming move that draws by repetition, or
+ // if the opponent had an alternative move earlier to this position.
+ if ( depth < 0
+ && pos.rule50_count() >= 3
+ && alpha < VALUE_DRAW
+ && pos.has_game_cycle(ss->ply))
+ {
+ alpha = value_draw(pos.this_thread());
+ if (alpha >= beta)
+ return alpha;
+ }
+
Move pv[MAX_PLY+1];
StateInfo st;
ASSERT_ALIGNED(&st, Eval::NNUE::CacheLineSize);
return bestValue;
}
- if (PvNode && bestValue > alpha)
+ if (bestValue > alpha)
alpha = bestValue;
- futilityBase = bestValue + 200;
+ futilityBase = std::min(ss->staticEval, bestValue) + 200;
}
const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,
if (moveCount > 2)
continue;
- futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
+ futilityValue = futilityBase + PieceValue[pos.piece_on(to_sq(move))];
if (futilityValue <= alpha)
{
if (PvNode) // Update pv even in fail-high case
update_pv(ss->pv, move, (ss+1)->pv);
- if (PvNode && value < beta) // Update alpha here!
+ if (value < beta) // Update alpha here!
alpha = value;
else
break; // Fail high
Piece moved_piece = pos.moved_piece(bestMove);
PieceType captured;
- int bonus1 = stat_bonus(depth + 1);
+ int quietMoveBonus = stat_bonus(depth + 1);
if (!pos.capture_stage(bestMove))
{
- int bonus2 = bestValue > beta + 145 ? bonus1 // larger bonus
- : stat_bonus(depth); // smaller bonus
+ int bestMoveBonus = bestValue > beta + 145 ? quietMoveBonus // larger bonus
+ : stat_bonus(depth); // smaller bonus
// Increase stats for the best move in case it was a quiet move
- update_quiet_stats(pos, ss, bestMove, bonus2);
+ update_quiet_stats(pos, ss, bestMove, bestMoveBonus);
// Decrease stats for all non-best quiet moves
for (int i = 0; i < quietCount; ++i)
{
- thisThread->mainHistory[us][from_to(quietsSearched[i])] << -bonus2;
- update_continuation_histories(ss, pos.moved_piece(quietsSearched[i]), to_sq(quietsSearched[i]), -bonus2);
+ thisThread->mainHistory[us][from_to(quietsSearched[i])] << -bestMoveBonus;
+ update_continuation_histories(ss, pos.moved_piece(quietsSearched[i]), to_sq(quietsSearched[i]), -bestMoveBonus);
}
}
else
{
// Increase stats for the best move in case it was a capture move
captured = type_of(pos.piece_on(to_sq(bestMove)));
- captureHistory[moved_piece][to_sq(bestMove)][captured] << bonus1;
+ captureHistory[moved_piece][to_sq(bestMove)][captured] << quietMoveBonus;
}
// Extra penalty for a quiet early move that was not a TT move or
if ( prevSq != SQ_NONE
&& ((ss-1)->moveCount == 1 + (ss-1)->ttHit || ((ss-1)->currentMove == (ss-1)->killers[0]))
&& !pos.captured_piece())
- update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -bonus1);
+ update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -quietMoveBonus);
// Decrease stats for all non-best capture moves
for (int i = 0; i < captureCount; ++i)
{
moved_piece = pos.moved_piece(capturesSearched[i]);
captured = type_of(pos.piece_on(to_sq(capturesSearched[i])));
- captureHistory[moved_piece][to_sq(capturesSearched[i])][captured] << -bonus1;
+ captureHistory[moved_piece][to_sq(capturesSearched[i])][captured] << -quietMoveBonus;
}
}
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) {
// RootMoves are already sorted by score in descending order
Value topScore = rootMoves[0].score;
- int delta = std::min(topScore - rootMoves[multiPV - 1].score, PawnValueMg);
+ int delta = std::min(topScore - rootMoves[multiPV - 1].score, PawnValue);
int maxScore = -VALUE_INFINITE;
double weakness = 120 - 2 * level;
return;
// When using nodes, ensure checking rate is not lower than 0.1% of nodes
- callsCnt = Limits.nodes ? std::min(1024, int(Limits.nodes / 1024)) : 1024;
+ callsCnt = Limits.nodes ? std::min(512, int(Limits.nodes / 1024)) : 512;
static TimePoint lastInfoTime = now();
if (ponder)
return;
- if ( (Limits.use_time_management() && (elapsed > Time.maximum() - 10 || stopOnPonderhit))
+ if ( (Limits.use_time_management() && (elapsed > Time.maximum() || stopOnPonderhit))
|| (Limits.movetime && elapsed >= Limits.movetime)
|| (Limits.nodes && Threads.nodes_searched() >= (uint64_t)Limits.nodes))
Threads.stop = true;
/// 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) {
projects / stockfish / blobdiff