/*
Stockfish, a UCI chess playing engine derived from Glaurung 2.1
- Copyright (C) 2004-2020 The Stockfish developers (see AUTHORS file)
+ Copyright (C) 2004-2021 The Stockfish developers (see AUTHORS file)
Stockfish is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
constexpr uint64_t TtHitAverageWindow = 4096;
constexpr uint64_t TtHitAverageResolution = 1024;
- // Razor and futility margins
- constexpr int RazorMargin = 510;
+ // Futility margin
Value futility_margin(Depth d, bool improving) {
- return Value(223 * (d - improving));
+ return Value(234 * (d - improving));
}
// Reductions lookup table, initialized at startup
Depth reduction(bool i, Depth d, int mn) {
int r = Reductions[d] * Reductions[mn];
- return (r + 509) / 1024 + (!i && r > 894);
+ return (r + 503) / 1024 + (!i && r > 915);
}
constexpr int futility_move_count(bool improving, Depth depth) {
// History and stats update bonus, based on depth
int stat_bonus(Depth d) {
- return d > 13 ? 29 : 17 * d * d + 134 * d - 134;
+ return d > 14 ? 66 : 6 * d * d + 231 * d - 206;
}
- // Add a small random component to draw evaluations to avoid 3fold-blindness
+ // Add a small random component to draw evaluations to avoid 3-fold blindness
Value value_draw(Thread* thisThread) {
return VALUE_DRAW + Value(2 * (thisThread->nodes & 1) - 1);
}
void Search::init() {
for (int i = 1; i < MAX_MOVES; ++i)
- Reductions[i] = int((22.0 + 2 * std::log(Threads.size())) * std::log(i + 0.25 * std::log(i)));
+ Reductions[i] = int((21.3 + 2 * std::log(Threads.size())) * std::log(i + 0.25 * std::log(i)));
}
beta = std::min(prev + delta, VALUE_INFINITE);
// Adjust contempt based on root move's previousScore (dynamic contempt)
- int dct = ct + (105 - ct / 2) * prev / (abs(prev) + 149);
+ int dct = ct + (113 - ct / 2) * prev / (abs(prev) + 147);
contempt = (us == WHITE ? make_score(dct, dct / 2)
: -make_score(dct, dct / 2));
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
&& depth > 12
&& ss->ply - 1 < MAX_LPH
{
if (ttValue >= beta)
{
+ // Bonus for a quiet ttMove that fails high
if (!pos.capture_or_promotion(ttMove))
update_quiet_stats(pos, ss, ttMove, stat_bonus(depth), depth);
}
}
+ // Partial workaround for the graph history interaction problem
+ // For high rule50 counts don't produce transposition table cutoffs.
if (pos.rule50_count() < 90)
return ttValue;
}
if (eval == VALUE_NONE)
ss->staticEval = eval = evaluate(pos);
+ // Randomize draw evaluation
if (eval == VALUE_DRAW)
eval = value_draw(thisThread);
}
else
{
+ // In case of null move search use previous static eval with a different sign
+ // and addition of two tempos
if ((ss-1)->currentMove != MOVE_NULL)
ss->staticEval = eval = evaluate(pos);
else
ss->staticEval = eval = -(ss-1)->staticEval + 2 * Tempo;
+ // Save static evaluation into transposition table
tte->save(posKey, VALUE_NONE, ss->ttPv, BOUND_NONE, DEPTH_NONE, MOVE_NONE, eval);
}
- // Step 7. Razoring (~1 Elo)
- if ( !rootNode // The required rootNode PV handling is not available in qsearch
- && depth == 1
- && eval <= alpha - RazorMargin)
- return qsearch<NT>(pos, ss, alpha, beta);
+ // Use static evaluation difference to improve quiet move ordering
+ if (is_ok((ss-1)->currentMove) && !(ss-1)->inCheck && !priorCapture)
+ {
+ int bonus = std::clamp(-depth * 4 * int((ss-1)->staticEval + ss->staticEval - 2 * Tempo), -1000, 1000);
+ thisThread->mainHistory[~us][from_to((ss-1)->currentMove)] << bonus;
+ }
+ // Set up improving flag that is used in various pruning heuristics
+ // We define position as improving if static evaluation of position is better
+ // Than the previous static evaluation at our turn
+ // In case of us being in check at our previous move we look at move prior to it
improving = (ss-2)->staticEval == VALUE_NONE
? ss->staticEval > (ss-4)->staticEval || (ss-4)->staticEval == VALUE_NONE
: ss->staticEval > (ss-2)->staticEval;
- // Step 8. Futility pruning: child node (~50 Elo)
+ // Step 7. Futility pruning: child node (~50 Elo)
if ( !PvNode
- && depth < 8
+ && depth < 9
&& eval - futility_margin(depth, improving) >= beta
&& eval < VALUE_KNOWN_WIN) // Do not return unproven wins
return eval;
- // Step 9. Null move search with verification search (~40 Elo)
+ // Step 8. Null move search with verification search (~40 Elo)
if ( !PvNode
&& (ss-1)->currentMove != MOVE_NULL
- && (ss-1)->statScore < 22977
+ && (ss-1)->statScore < 22661
&& eval >= beta
&& eval >= ss->staticEval
- && ss->staticEval >= beta - 30 * depth - 28 * improving + 84 * ss->ttPv + 182
+ && ss->staticEval >= beta - 24 * depth - 34 * improving + 162 * ss->ttPv + 159
&& !excludedMove
&& pos.non_pawn_material(us)
&& (ss->ply >= thisThread->nmpMinPly || us != thisThread->nmpColor))
assert(eval - beta >= 0);
// Null move dynamic reduction based on depth and value
- Depth R = (982 + 85 * depth) / 256 + std::min(int(eval - beta) / 192, 3);
+ Depth R = (1062 + 68 * depth) / 256 + std::min(int(eval - beta) / 190, 3);
ss->currentMove = MOVE_NULL;
ss->continuationHistory = &thisThread->continuationHistory[0][0][NO_PIECE][0];
if (nullValue >= VALUE_TB_WIN_IN_MAX_PLY)
nullValue = beta;
- if (thisThread->nmpMinPly || (abs(beta) < VALUE_KNOWN_WIN && depth < 13))
+ if (thisThread->nmpMinPly || (abs(beta) < VALUE_KNOWN_WIN && depth < 14))
return nullValue;
assert(!thisThread->nmpMinPly); // Recursive verification is not allowed
}
}
- probCutBeta = beta + 176 - 49 * improving;
+ probCutBeta = beta + 209 - 44 * improving;
- // Step 10. ProbCut (~10 Elo)
+ // Step 9. ProbCut (~10 Elo)
// If we have a good enough capture and a reduced search returns a value
// much above beta, we can (almost) safely prune the previous move.
if ( !PvNode
ss->ttPv = ttPv;
}
- // Step 11. If the position is not in TT, decrease depth by 2
+ // Step 10. If the position is not in TT, decrease depth by 2
if ( PvNode
&& depth >= 6
&& !ttMove)
// Mark this node as being searched
ThreadHolding th(thisThread, posKey, ss->ply);
- // Step 12. Loop through all pseudo-legal moves until no moves remain
+ // Step 11. Loop through all pseudo-legal moves until no moves remain
// or a beta cutoff occurs.
while ((move = mp.next_move(moveCountPruning)) != MOVE_NONE)
{
movedPiece = pos.moved_piece(move);
givesCheck = pos.gives_check(move);
+ // Indicate PvNodes that will probably fail low if node was searched with non-PV search
+ // at depth equal or greater to current depth and result of this search was far below alpha
+ bool likelyFailLow = PvNode
+ && ttMove
+ && (tte->bound() & BOUND_UPPER)
+ && ttValue < alpha + 200 + 100 * depth
+ && tte->depth() >= depth;
+
// Calculate new depth for this move
newDepth = depth - 1;
- // Step 13. Pruning at shallow depth (~200 Elo)
+ // Step 12. Pruning at shallow depth (~200 Elo)
if ( !rootNode
&& pos.non_pawn_material(us)
&& bestValue > VALUE_TB_LOSS_IN_MAX_PLY)
// Reduced depth of the next LMR search
int lmrDepth = std::max(newDepth - reduction(improving, depth, moveCount), 0);
- if ( !captureOrPromotion
- && !givesCheck)
+ if ( captureOrPromotion
+ || givesCheck)
+ {
+ // Capture history based pruning when the move doesn't give check
+ if ( !givesCheck
+ && lmrDepth < 1
+ && captureHistory[movedPiece][to_sq(move)][type_of(pos.piece_on(to_sq(move)))] < 0)
+ continue;
+
+ // SEE based pruning
+ if (!pos.see_ge(move, Value(-218) * depth)) // (~25 Elo)
+ continue;
+ }
+ else
{
// Countermoves based pruning (~20 Elo)
if ( lmrDepth < 4 + ((ss-1)->statScore > 0 || (ss-1)->moveCount == 1)
// Futility pruning: parent node (~5 Elo)
if ( lmrDepth < 7
&& !ss->inCheck
- && ss->staticEval + 283 + 170 * lmrDepth <= alpha
+ && ss->staticEval + 174 + 157 * lmrDepth <= alpha
&& (*contHist[0])[movedPiece][to_sq(move)]
+ (*contHist[1])[movedPiece][to_sq(move)]
+ (*contHist[3])[movedPiece][to_sq(move)]
- + (*contHist[5])[movedPiece][to_sq(move)] / 2 < 27376)
+ + (*contHist[5])[movedPiece][to_sq(move)] / 3 < 26237)
continue;
// Prune moves with negative SEE (~20 Elo)
- if (!pos.see_ge(move, Value(-(29 - std::min(lmrDepth, 18)) * lmrDepth * lmrDepth)))
- continue;
- }
- else
- {
- // Capture history based pruning when the move doesn't give check
- if ( !givesCheck
- && lmrDepth < 1
- && captureHistory[movedPiece][to_sq(move)][type_of(pos.piece_on(to_sq(move)))] < 0)
- continue;
-
- // See based pruning
- if (!pos.see_ge(move, Value(-221) * depth)) // (~25 Elo)
+ if (!pos.see_ge(move, Value(-(30 - std::min(lmrDepth, 18)) * lmrDepth * lmrDepth)))
continue;
}
}
- // Step 14. Extensions (~75 Elo)
+ // Step 13. Extensions (~75 Elo)
// Singular extension search (~70 Elo). If all moves but one fail low on a
// search of (alpha-s, beta-s), and just one fails high on (alpha, beta),
// Check extension (~2 Elo)
else if ( givesCheck
- && (pos.is_discovery_check_on_king(~us, move) || pos.see_ge(move)))
+ && (pos.is_discovered_check_on_king(~us, move) || pos.see_ge(move)))
extension = 1;
// Last captures extension
&& pos.non_pawn_material() <= 2 * RookValueMg)
extension = 1;
- // Late irreversible move extension
- if ( move == ttMove
- && pos.rule50_count() > 80
- && (captureOrPromotion || type_of(movedPiece) == PAWN))
- extension = 2;
-
// Add extension to new depth
newDepth += extension;
[movedPiece]
[to_sq(move)];
- // Step 15. Make the move
+ // Step 14. Make the move
pos.do_move(move, st, givesCheck);
- // Step 16. Reduced depth search (LMR, ~200 Elo). If the move fails high it will be
+ // Step 15. Reduced depth search (LMR, ~200 Elo). If the move fails high it will be
// re-searched at full depth.
if ( depth >= 3
&& moveCount > 1 + 2 * rootNode
|| moveCountPruning
|| ss->staticEval + PieceValue[EG][pos.captured_piece()] <= alpha
|| cutNode
- || thisThread->ttHitAverage < 427 * TtHitAverageResolution * TtHitAverageWindow / 1024))
+ || (!PvNode && !formerPv && captureHistory[movedPiece][to_sq(move)][type_of(pos.captured_piece())] < 4506)
+ || thisThread->ttHitAverage < 432 * TtHitAverageResolution * TtHitAverageWindow / 1024))
{
Depth r = reduction(improving, depth, moveCount);
// Decrease reduction if the ttHit running average is large
- if (thisThread->ttHitAverage > 509 * TtHitAverageResolution * TtHitAverageWindow / 1024)
+ if (thisThread->ttHitAverage > 537 * TtHitAverageResolution * TtHitAverageWindow / 1024)
r--;
- // Reduction if other threads are searching this position
+ // Increase reduction if other threads are searching this position
if (th.marked())
r++;
- // Decrease reduction if position is or has been on the PV (~10 Elo)
- if (ss->ttPv)
+ // Decrease reduction if position is or has been on the PV
+ // and node is not likely to fail low. (~10 Elo)
+ if (ss->ttPv && !likelyFailLow)
r -= 2;
- if (!PvNode && depth > 10 && thisThread->bestMoveChanges <= 2)
+ // Increase reduction at root and non-PV nodes when the best move does not change frequently
+ if ((rootNode || !PvNode) && thisThread->rootDepth > 10 && thisThread->bestMoveChanges <= 2)
r++;
+ // More reductions for late moves if position was not in previous PV
if (moveCountPruning && !formerPv)
r++;
if (singularQuietLMR)
r--;
- if (!captureOrPromotion)
+ if (captureOrPromotion)
+ {
+ // Unless giving check, this capture is likely bad
+ if ( !givesCheck
+ && ss->staticEval + PieceValue[EG][pos.captured_piece()] + 210 * depth <= alpha)
+ r++;
+ }
+ else
{
// Increase reduction if ttMove is a capture (~5 Elo)
if (ttCapture)
+ (*contHist[0])[movedPiece][to_sq(move)]
+ (*contHist[1])[movedPiece][to_sq(move)]
+ (*contHist[3])[movedPiece][to_sq(move)]
- - 5287;
+ - 5337;
// Decrease/increase reduction by comparing opponent's stat score (~10 Elo)
- if (ss->statScore >= -106 && (ss-1)->statScore < -104)
+ if (ss->statScore >= -89 && (ss-1)->statScore < -116)
r--;
- else if ((ss-1)->statScore >= -119 && ss->statScore < -140)
+ else if ((ss-1)->statScore >= -112 && ss->statScore < -100)
r++;
// Decrease/increase reduction for moves with a good/bad history (~30 Elo)
- r -= ss->statScore / 14884;
- }
- else
- {
- // Increase reduction for captures/promotions if late move and at low depth
- if (depth < 8 && moveCount > 2)
- r++;
-
- // Unless giving check, this capture is likely bad
- if ( !givesCheck
- && ss->staticEval + PieceValue[EG][pos.captured_piece()] + 213 * depth <= alpha)
- r++;
+ // If we are not in check use statScore, if we are in check
+ // use sum of main history and first continuation history with an offset
+ if (ss->inCheck)
+ r -= (thisThread->mainHistory[us][from_to(move)]
+ + (*contHist[0])[movedPiece][to_sq(move)] - 4341) / 16384;
+ else
+ r -= ss->statScore / 14382;
}
Depth d = std::clamp(newDepth - r, 1, newDepth);
didLMR = false;
}
- // Step 17. Full depth search when LMR is skipped or fails high
+ // Step 16. Full depth search when LMR is skipped or fails high
if (doFullDepthSearch)
{
value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode);
+ // If the move passed LMR update its stats
if (didLMR && !captureOrPromotion)
{
int bonus = value > alpha ? stat_bonus(newDepth)
: -stat_bonus(newDepth);
- if (move == ss->killers[0])
- bonus += bonus / 4;
-
update_continuation_histories(ss, movedPiece, to_sq(move), bonus);
}
}
std::min(maxNextDepth, newDepth), false);
}
- // Step 18. Undo move
+ // Step 17. Undo move
pos.undo_move(move);
assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
- // Step 19. Check for a new best move
+ // Step 18. Check for a new best move
// Finished searching the move. If a stop occurred, the return value of
// the search cannot be trusted, and we return immediately without
// updating best move, PV and TT.
rm.pv.push_back(*m);
// We record how often the best move has been changed in each
- // iteration. This information is used for time management: when
- // the best move changes frequently, we allocate some more time.
+ // iteration. This information is used for time management and LMR
if (moveCount > 1)
++thisThread->bestMoveChanges;
}
}
}
+ // If the move is worse than some previously searched move, remember it to update its stats later
if (move != bestMove)
{
if (captureOrPromotion && captureCount < 32)
return VALUE_DRAW;
*/
- // Step 20. Check for mate and stalemate
+ // Step 19. Check for mate and stalemate
// All legal moves have been searched and if there are no legal moves, it
// must be a mate or a stalemate. If we are in a singular extension search then
// return a fail low score.
bestValue = excludedMove ? alpha
: 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
else if (bestMove)
update_all_stats(pos, ss, bestMove, bestValue, beta, prevSq,
quietsSearched, quietCount, capturesSearched, captureCount, depth);
else if (depth > 3)
ss->ttPv = ss->ttPv && (ss+1)->ttPv;
+ // Write gathered information in transposition table
if (!excludedMove && !(rootNode && thisThread->pvIdx))
tte->save(posKey, value_to_tt(bestValue, ss->ply), ss->ttPv,
bestValue >= beta ? BOUND_LOWER :
bestValue = ttValue;
}
else
+ // In case of null move search use previous static eval with a different sign
+ // and addition of two tempos
ss->staticEval = bestValue =
(ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
: -(ss-1)->staticEval + 2 * Tempo;
// Stand pat. Return immediately if static value is at least beta
if (bestValue >= beta)
{
+ // Save gathered info in transposition table
if (!ss->ttHit)
tte->save(posKey, value_to_tt(bestValue, ss->ply), false, BOUND_LOWER,
DEPTH_NONE, MOVE_NONE, ss->staticEval);
if (PvNode && bestValue > alpha)
alpha = bestValue;
- futilityBase = bestValue + 145;
+ futilityBase = bestValue + 155;
}
const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,
&& futilityBase > -VALUE_KNOWN_WIN
&& !pos.advanced_pawn_push(move))
{
- assert(type_of(move) != ENPASSANT); // Due to !pos.advanced_pawn_push
+ assert(type_of(move) != EN_PASSANT); // Due to !pos.advanced_pawn_push
// moveCount pruning
if (moveCount > 2)
// Do not search moves with negative SEE values
if ( bestValue > VALUE_TB_LOSS_IN_MAX_PLY
- && !(givesCheck && pos.is_discovery_check_on_king(~pos.side_to_move(), move))
&& !pos.see_ge(move))
continue;
return mated_in(ss->ply); // Plies to mate from the root
}
+ // Save gathered info in transposition table
tte->save(posKey, value_to_tt(bestValue, ss->ply), pvHit,
bestValue >= beta ? BOUND_LOWER :
PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
PieceType captured = type_of(pos.piece_on(to_sq(bestMove)));
bonus1 = stat_bonus(depth + 1);
- bonus2 = bestValue > beta + PawnValueMg ? bonus1 // larger bonus
- : stat_bonus(depth); // smaller bonus
+ bonus2 = bestValue > beta + PawnValueMg ? bonus1 // larger bonus
+ : std::min(bonus1, stat_bonus(depth)); // smaller bonus
if (!pos.capture_or_promotion(bestMove))
{
+ // Increase stats for the best move in case it was a quiet move
update_quiet_stats(pos, ss, bestMove, bonus2, depth);
- // Decrease all the non-best quiet moves
+ // Decrease stats for all non-best quiet moves
for (int i = 0; i < quietCount; ++i)
{
thisThread->mainHistory[us][from_to(quietsSearched[i])] << -bonus2;
}
}
else
+ // Increase stats for the best move in case it was a capture move
captureHistory[moved_piece][to_sq(bestMove)][captured] << bonus1;
- // Extra penalty for a quiet early move that was not a TT move or main killer move in previous ply when it gets refuted
+ // Extra penalty for a quiet early move that was not a TT move or
+ // main killer move in previous ply when it gets refuted.
if ( ((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);
- // Decrease all the non-best capture moves
+ // Decrease stats for all non-best capture moves
for (int i = 0; i < captureCount; ++i)
{
moved_piece = pos.moved_piece(capturesSearched[i]);
for (int i : {1, 2, 4, 6})
{
+ // Only update first 2 continuation histories if we are in check
if (ss->inCheck && i > 2)
break;
if (is_ok((ss-i)->currentMove))
void update_quiet_stats(const Position& pos, Stack* ss, Move move, int bonus, int depth) {
+ // Update killers
if (ss->killers[0] != move)
{
ss->killers[1] = ss->killers[0];
thisThread->mainHistory[us][from_to(move)] << bonus;
update_continuation_histories(ss, pos.moved_piece(move), to_sq(move), bonus);
+ // Penalty for reversed move in case of moved piece not being a pawn
if (type_of(pos.moved_piece(move)) != PAWN)
thisThread->mainHistory[us][from_to(reverse_move(move))] << -bonus;
+ // Update countermove history
if (is_ok((ss-1)->currentMove))
{
Square prevSq = to_sq((ss-1)->currentMove);
thisThread->counterMoves[pos.piece_on(prevSq)][prevSq] = move;
}
+ // Update low ply history
if (depth > 11 && ss->ply < MAX_LPH)
thisThread->lowPlyHistory[ss->ply][from_to(move)] << stat_bonus(depth - 7);
}
projects / stockfish / blobdiff