X-Git-Url: https://git.sesse.net/?a=blobdiff_plain;f=src%2Fsearch.cpp;h=b9ca3961053d435b2207ebe30d1ae6970ecb4ac3;hb=6d85f43e26cb8632337e67cea5ef88bab78121f3;hp=2fd8245ec2c1eecbaa7bef97e33860794ff335c7;hpb=0b41887527f1d7264ee5644dfa53e00ab64441b1;p=stockfish diff --git a/src/search.cpp b/src/search.cpp index 2fd8245e..a745d3bf 100644 --- a/src/search.cpp +++ b/src/search.cpp @@ -1,6 +1,6 @@ /* Stockfish, a UCI chess playing engine derived from Glaurung 2.1 - Copyright (C) 2004-2021 The Stockfish developers (see AUTHORS file) + Copyright (C) 2004-2023 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 @@ -16,24 +16,34 @@ along with this program. If not, see . */ +#include "search.h" + #include +#include +#include #include #include -#include // For std::memset +#include +#include +#include #include #include +#include +#include +#include "bitboard.h" #include "evaluate.h" #include "misc.h" #include "movegen.h" #include "movepick.h" +#include "nnue/evaluate_nnue.h" +#include "nnue/nnue_common.h" #include "position.h" -#include "search.h" +#include "syzygy/tbprobe.h" #include "thread.h" #include "timeman.h" #include "tt.h" #include "uci.h" -#include "syzygy/tbprobe.h" namespace Stockfish { @@ -62,64 +72,45 @@ namespace { enum NodeType { NonPV, PV, Root }; // Futility margin - Value futility_margin(Depth d, bool improving) { - return Value(214 * (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) { - int r = Reductions[d] * Reductions[mn]; - return (r + 1358 - int(delta) * 1024 / int(rootDelta)) / 1024 + (!i && r > 904); + int reductionScale = Reductions[d] * Reductions[mn]; + return (reductionScale + 1372 - int(delta) * 1073 / int(rootDelta)) / 1024 + + (!i && reductionScale > 936); } constexpr int futility_move_count(bool improving, Depth depth) { - return (3 + depth * depth) / (2 - improving); + return improving ? (3 + depth * depth) + : (3 + depth * depth) / 2; } // History and stats update bonus, based on depth int stat_bonus(Depth d) { - return std::min((6 * d + 229) * d - 215 , 2000); + return std::min(336 * d - 547, 1561); } // 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); - } - - // Check if the current thread is in a search explosion - ExplosionState search_explosion(Thread* thisThread) { - - uint64_t nodesNow = thisThread->nodes; - bool explosive = thisThread->doubleExtensionAverage[WHITE].is_greater(2, 100) - || thisThread->doubleExtensionAverage[BLACK].is_greater(2, 100); - - if (explosive) - thisThread->nodesLastExplosive = nodesNow; - else - thisThread->nodesLastNormal = nodesNow; - - if ( explosive - && thisThread->state == EXPLOSION_NONE - && nodesNow - thisThread->nodesLastNormal > 6000000) - thisThread->state = MUST_CALM_DOWN; - - if ( thisThread->state == MUST_CALM_DOWN - && nodesNow - thisThread->nodesLastExplosive > 6000000) - thisThread->state = EXPLOSION_NONE; - - return thisThread->state; + Value value_draw(const Thread* thisThread) { + return VALUE_DRAW - 1 + Value(thisThread->nodes & 0x2); } // 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) { if (uci_elo) - level = std::clamp(std::pow((uci_elo - 1346.6) / 143.4, 1 / 0.806), 0.0, 20.0); + { + double e = double(uci_elo - 1320) / (3190 - 1320); + level = std::clamp((((37.2473 * e - 40.8525) * e + 22.2943) * e - 0.311438), 0.0, 19.0); + } else level = double(skill_level); } @@ -139,7 +130,7 @@ namespace { Value value_to_tt(Value v, int ply); Value value_from_tt(Value v, int ply, int r50c); - void update_pv(Move* pv, Move move, Move* childPv); + void update_pv(Move* pv, Move move, const Move* childPv); void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus); void update_quiet_stats(const Position& pos, Stack* ss, Move move, int bonus); void update_all_stats(const Position& pos, Stack* ss, Move bestMove, Value bestValue, Value beta, Square prevSq, @@ -181,7 +172,7 @@ namespace { void Search::init() { for (int i = 1; i < MAX_MOVES; ++i) - Reductions[i] = int((21.9 + std::log(Threads.size()) / 2) * std::log(i)); + Reductions[i] = int((20.57 + std::log(Threads.size()) / 2) * std::log(i)); } @@ -264,7 +255,7 @@ void MainThread::search() { // Send again PV info if we have a new best thread if (bestThread != this) - sync_cout << UCI::pv(bestThread->rootPos, bestThread->completedDepth, -VALUE_INFINITE, VALUE_INFINITE) << sync_endl; + sync_cout << UCI::pv(bestThread->rootPos, bestThread->completedDepth) << sync_endl; sync_cout << "bestmove " << UCI::move(bestThread->rootMoves[0].pv[0], rootPos.is_chess960()); @@ -296,16 +287,18 @@ void Thread::search() { int iterIdx = 0; std::memset(ss-7, 0, 10 * sizeof(Stack)); - for (int i = 7; i > 0; i--) + for (int i = 7; i > 0; --i) + { (ss-i)->continuationHistory = &this->continuationHistory[0][0][NO_PIECE][0]; // Use as a sentinel + (ss-i)->staticEval = VALUE_NONE; + } for (int i = 0; i <= MAX_PLY + 2; ++i) (ss+i)->ply = i; ss->pv = pv; - bestValue = delta = alpha = -VALUE_INFINITE; - beta = VALUE_INFINITE; + bestValue = -VALUE_INFINITE; if (mainThread) { @@ -321,22 +314,12 @@ void Thread::search() { 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); multiPV = std::min(multiPV, rootMoves.size()); - doubleExtensionAverage[WHITE].set(0, 100); // initialize the running average at 0% - doubleExtensionAverage[BLACK].set(0, 100); // initialize the running average at 0% - - nodesLastExplosive = nodes; - nodesLastNormal = nodes; - state = EXPLOSION_NONE; - trend = SCORE_ZERO; - optimism[ us] = Value(25); - optimism[~us] = -optimism[us]; - int searchAgainCounter = 0; // Iterative deepening loop until requested to stop or the target depth is reached @@ -348,7 +331,7 @@ void Thread::search() { 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; @@ -357,7 +340,7 @@ void Thread::search() { pvLast = 0; if (!Threads.increaseDepth) - searchAgainCounter++; + searchAgainCounter++; // MultiPV loop. We perform a full root search for each PV line for (pvIdx = 0; pvIdx < multiPV && !Threads.stop; ++pvIdx) @@ -374,22 +357,15 @@ void Thread::search() { selDepth = 0; // Reset aspiration window starting size - if (rootDepth >= 4) - { - Value prev = rootMoves[pvIdx].averageScore; - delta = Value(17) + int(prev) * prev / 16384; - alpha = std::max(prev - delta,-VALUE_INFINITE); - beta = std::min(prev + delta, VALUE_INFINITE); - - // Adjust trend and optimism based on root move's previousScore - int tr = sigmoid(prev, 0, 0, 147, 113, 1); - trend = (us == WHITE ? make_score(tr, tr / 2) - : -make_score(tr, tr / 2)); - - int opt = sigmoid(prev, 0, 25, 147, 14464, 256); - 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 @@ -397,14 +373,16 @@ void Thread::search() { int failedHighCnt = 0; while (true) { - Depth adjustedDepth = std::max(1, rootDepth - failedHighCnt - searchAgainCounter); + // 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(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); @@ -420,7 +398,7 @@ void Thread::search() { && multiPV == 1 && (bestValue <= alpha || bestValue >= beta) && Time.elapsed() > 3000) - sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl; + sync_cout << UCI::pv(rootPos, rootDepth) << sync_endl; // In case of failing low/high increase aspiration window and // re-search, otherwise exit the loop. @@ -441,7 +419,7 @@ void Thread::search() { else break; - delta += delta / 4 + 5; + delta += delta / 3; assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE); } @@ -451,15 +429,16 @@ void Thread::search() { if ( mainThread && (Threads.stop || pvIdx + 1 == multiPV || Time.elapsed() > 3000)) - sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl; + sync_cout << UCI::pv(rootPos, rootDepth) << sync_endl; } if (!Threads.stop) completedDepth = rootDepth; - if (rootMoves[0].pv[0] != lastBestMove) { - lastBestMove = rootMoves[0].pv[0]; - lastBestMoveDepth = rootDepth; + if (rootMoves[0].pv[0] != lastBestMove) + { + lastBestMove = rootMoves[0].pv[0]; + lastBestMoveDepth = rootDepth; } // Have we found a "mate in x"? @@ -471,7 +450,7 @@ void Thread::search() { 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); @@ -487,15 +466,15 @@ void Thread::search() { && !Threads.stop && !mainThread->stopOnPonderhit) { - double fallingEval = (142 + 12 * (mainThread->bestPreviousAverageScore - bestValue) - + 6 * (mainThread->iterValue[iterIdx] - bestValue)) / 825.0; + double fallingEval = (69 + 13 * (mainThread->bestPreviousAverageScore - bestValue) + + 6 * (mainThread->iterValue[iterIdx] - bestValue)) / 619.6; fallingEval = std::clamp(fallingEval, 0.5, 1.5); // If the bestMove is stable over several iterations, reduce time accordingly - timeReduction = lastBestMoveDepth + 9 < completedDepth ? 1.92 : 0.95; - double reduction = (1.47 + mainThread->previousTimeReduction) / (2.32 * timeReduction); - double bestMoveInstability = 1.073 + std::max(1.0, 2.25 - 9.9 / rootDepth) - * totBestMoveChanges / Threads.size(); + timeReduction = lastBestMoveDepth + 8 < completedDepth ? 1.57 : 0.65; + double reduction = (1.4 + mainThread->previousTimeReduction) / (2.08 * timeReduction); + double bestMoveInstability = 1 + 1.8 * 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 @@ -513,12 +492,11 @@ void Thread::search() { else Threads.stop = true; } - else if ( Threads.increaseDepth - && !mainThread->ponder - && Time.elapsed() > totalTime * 0.58) - Threads.increaseDepth = false; + else if ( !mainThread->ponder + && Time.elapsed() > totalTime * 0.50) + Threads.increaseDepth = false; else - Threads.increaseDepth = true; + Threads.increaseDepth = true; } mainThread->iterValue[iterIdx] = bestValue; @@ -530,7 +508,7 @@ void Thread::search() { 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))); @@ -544,22 +522,12 @@ namespace { template Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) { - Thread* thisThread = pos.this_thread(); - - // Step 0. Limit search explosion - if ( ss->ply > 10 - && search_explosion(thisThread) == MUST_CALM_DOWN - && depth > (ss-1)->depth) - depth = (ss-1)->depth; - 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 + // 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 && alpha < VALUE_DRAW && pos.has_game_cycle(ss->ply)) { @@ -586,16 +554,17 @@ namespace { Move ttMove, move, excludedMove, bestMove; Depth extension, newDepth; Value bestValue, value, ttValue, eval, maxValue, probCutBeta; - bool givesCheck, improving, didLMR, priorCapture; - bool captureOrPromotion, doFullDepthSearch, moveCountPruning, ttCapture; + bool givesCheck, improving, priorCapture, singularQuietLMR; + bool capture, moveCountPruning, ttCapture; Piece movedPiece; - int moveCount, captureCount, quietCount, bestMoveCount, improvement; + int moveCount, captureCount, quietCount; // Step 1. Initialize node + Thread* thisThread = pos.this_thread(); ss->inCheck = pos.checkers(); priorCapture = pos.captured_piece(); Color us = pos.side_to_move(); - moveCount = bestMoveCount = captureCount = quietCount = ss->moveCount = 0; + moveCount = captureCount = quietCount = ss->moveCount = 0; bestValue = -VALUE_INFINITE; maxValue = VALUE_INFINITE; @@ -620,8 +589,8 @@ namespace { // 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) @@ -632,56 +601,45 @@ namespace { assert(0 <= ss->ply && ss->ply < MAX_PLY); - (ss+1)->ttPv = false; (ss+1)->excludedMove = bestMove = MOVE_NONE; (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE; + (ss+2)->cutoffCnt = 0; ss->doubleExtensions = (ss-1)->doubleExtensions; - ss->depth = depth; - Square prevSq = to_sq((ss-1)->currentMove); + Square prevSq = is_ok((ss-1)->currentMove) ? to_sq((ss-1)->currentMove) : SQ_NONE; + ss->statScore = 0; - // Update the running average statistics for double extensions - thisThread->doubleExtensionAverage[us].update(ss->depth > (ss-1)->depth); - - // Initialize statScore to zero for the grandchildren of the current position. - // So statScore is shared between all grandchildren and only the first grandchild - // starts with statScore = 0. Later grandchildren start with the last calculated - // statScore of the previous grandchild. This influences the reduction rules in - // LMR which are based on the statScore of parent position. - if (!rootNode) - (ss+2)->statScore = 0; - - // Step 4. Transposition table lookup. We don't want the score of a partial - // search to overwrite a previous full search TT value, so we use a different - // position key in case of an excluded move. + // Step 4. Transposition table lookup. excludedMove = ss->excludedMove; - posKey = excludedMove == MOVE_NONE ? pos.key() : pos.key() ^ make_key(excludedMove); + posKey = pos.key(); tte = TT.probe(posKey, ss->ttHit); ttValue = ss->ttHit ? value_from_tt(tte->value(), ss->ply, pos.rule50_count()) : VALUE_NONE; ttMove = rootNode ? thisThread->rootMoves[thisThread->pvIdx].pv[0] : ss->ttHit ? tte->move() : MOVE_NONE; - ttCapture = ttMove && pos.capture_or_promotion(ttMove); + ttCapture = ttMove && pos.capture_stage(ttMove); + + // At this point, if excluded, skip straight to step 6, static eval. However, + // to save indentation, we list the condition in all code between here and there. if (!excludedMove) ss->ttPv = PvNode || (ss->ttHit && tte->is_pv()); // At non-PV nodes we check for an early TT cutoff if ( !PvNode - && ss->ttHit - && tte->depth() > depth - (thisThread->id() % 2 == 1) - && ttValue != VALUE_NONE // Possible in case of TT access race - && (ttValue >= beta ? (tte->bound() & BOUND_LOWER) - : (tte->bound() & BOUND_UPPER))) + && !excludedMove + && tte->depth() > depth + && 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 (~1 Elo) + // If ttMove is quiet, update move sorting heuristics on TT hit (~2 Elo) if (ttMove) { if (ttValue >= beta) { - // Bonus for a quiet ttMove that fails high (~3 Elo) + // Bonus for a quiet ttMove that fails high (~2 Elo) if (!ttCapture) update_quiet_stats(pos, ss, ttMove, stat_bonus(depth)); - // Extra penalty for early quiet moves of the previous ply (~0 Elo) - if ((ss-1)->moveCount <= 2 && !priorCapture) + // Extra penalty for early quiet moves of the previous ply (~0 Elo on STC, ~2 Elo on LTC) + if (prevSq != SQ_NONE && (ss-1)->moveCount <= 2 && !priorCapture) update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + 1)); } // Penalty for a quiet ttMove that fails low (~1 Elo) @@ -700,7 +658,7 @@ namespace { } // Step 5. Tablebases probe - if (!rootNode && TB::Cardinality) + if (!rootNode && !excludedMove && TB::Cardinality) { int piecesCount = pos.count(); @@ -759,21 +717,24 @@ namespace { // Skip early pruning when in check ss->staticEval = eval = VALUE_NONE; improving = false; - improvement = 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; + } 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); + else if (PvNode) + Eval::NNUE::hint_common_parent_position(pos); - // Randomize draw evaluation - if (eval == VALUE_DRAW) - eval = value_draw(thisThread); - - // ttValue can be used as a better position evaluation (~4 Elo) + // ttValue can be used as a better position evaluation (~7 Elo) if ( ttValue != VALUE_NONE && (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER))) eval = ttValue; @@ -781,52 +742,61 @@ namespace { else { ss->staticEval = eval = evaluate(pos); - - // Save static evaluation into transposition table - if (!excludedMove) - tte->save(posKey, VALUE_NONE, ss->ttPv, BOUND_NONE, DEPTH_NONE, MOVE_NONE, eval); + // 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 (~3 Elo) + // 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(-16 * int((ss-1)->staticEval + ss->staticEval), -2000, 2000); + int bonus = std::clamp(-18 * int((ss-1)->staticEval + ss->staticEval), -1817, 1817); 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 - : 200; - - 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, + // return a fail low. + if (eval < alpha - 456 - 252 * depth * depth) + { + value = qsearch(pos, ss, alpha - 1, alpha); + if (value < alpha) + return value; + } - // Step 7. Futility pruning: child node (~25 Elo). + // Step 8. Futility pruning: child node (~40 Elo). // The depth condition is important for mate finding. if ( !ss->ttPv && depth < 9 - && eval - futility_margin(depth, improving) >= beta - && eval < 15000) // 50% larger than VALUE_KNOWN_WIN, but smaller than TB wins. + && 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; - // Step 8. Null move search with verification search (~22 Elo) + // Step 9. Null move search with verification search (~35 Elo) if ( !PvNode && (ss-1)->currentMove != MOVE_NULL - && (ss-1)->statScore < 23767 + && (ss-1)->statScore < 17329 && eval >= beta && eval >= ss->staticEval - && ss->staticEval >= beta - 20 * depth - improvement / 15 + 204 + && 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 and value - Depth R = std::min(int(eval - beta) / 205, 3) + depth / 3 + 4; + // 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]; @@ -840,18 +810,16 @@ namespace { 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(pos, ss, beta-1, beta, depth-R, false); @@ -862,40 +830,49 @@ namespace { } } - probCutBeta = beta + 209 - 44 * 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(pos, ss, alpha, beta); + + if ( cutNode + && depth >= 8 + && !ttMove) + depth -= 2; + + probCutBeta = beta + 168 - 61 * improving; - // Step 9. ProbCut (~4 Elo) - // If we have a good enough capture and a reduced search returns a value + // 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)) { assert(probCutBeta < VALUE_INFINITE); MovePicker mp(pos, ttMove, probCutBeta - ss->staticEval, &captureHistory); - bool ttPv = ss->ttPv; - ss->ttPv = false; while ((move = mp.next_move()) != MOVE_NONE) if (move != excludedMove && pos.legal(move)) { - assert(pos.capture_or_promotion(move)); - assert(depth >= 5); - - captureOrPromotion = true; + assert(pos.capture_stage(move)); ss->currentMove = move; ss->continuationHistory = &thisThread->continuationHistory[ss->inCheck] - [captureOrPromotion] + [true] [pos.moved_piece(move)] [to_sq(move)]; @@ -912,52 +889,34 @@ namespace { if (value >= probCutBeta) { - // if transposition table doesn't have equal or more deep info write probCut data into it - if ( !(ss->ttHit - && tte->depth() >= depth - 3 - && ttValue != VALUE_NONE)) - tte->save(posKey, value_to_tt(value, ss->ply), ttPv, - BOUND_LOWER, - depth - 3, move, ss->staticEval); + // Save ProbCut data into transposition table + tte->save(posKey, value_to_tt(value, ss->ply), ss->ttPv, BOUND_LOWER, depth - 3, move, ss->staticEval); return value; } } - ss->ttPv = ttPv; - } - - // Step 10. If the position is not in TT, decrease depth by 2 or 1 depending on node type (~3 Elo) - if ( PvNode - && depth >= 6 - && !ttMove) - depth -= 2; - if ( cutNode - && depth >= 9 - && !ttMove) - depth--; + Eval::NNUE::hint_common_parent_position(pos); + } moves_loop: // When in check, search starts here - // Step 11. A small Probcut idea, when we are in check (~0 Elo) - probCutBeta = beta + 409; + // Step 12. A small Probcut idea, when we are in check (~4 Elo) + probCutBeta = beta + 413; if ( ss->inCheck && !PvNode - && depth >= 4 && 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 - ) + && abs(beta) <= VALUE_KNOWN_WIN) return probCutBeta; - const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory, nullptr , (ss-4)->continuationHistory, nullptr , (ss-6)->continuationHistory }; - Move countermove = thisThread->counterMoves[pos.piece_on(prevSq)][prevSq]; + Move countermove = prevSq != SQ_NONE ? thisThread->counterMoves[pos.piece_on(prevSq)][prevSq] : MOVE_NONE; MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory, &captureHistory, @@ -966,16 +925,16 @@ moves_loop: // When in check, search starts here ss->killers); value = bestValue; - moveCountPruning = false; + 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) && tte->depth() >= depth; - // Step 12. Loop through all pseudo-legal moves until no moves remain + // Step 13. Loop through all pseudo-legal moves until no moves remain // or a beta cutoff occurs. while ((move = mp.next_move(moveCountPruning)) != MOVE_NONE) { @@ -985,8 +944,8 @@ moves_loop: // When in check, search starts here 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)) @@ -1006,7 +965,7 @@ moves_loop: // When in check, search starts here (ss+1)->pv = nullptr; extension = 0; - captureOrPromotion = pos.capture_or_promotion(move); + capture = pos.capture_stage(move); movedPiece = pos.moved_piece(move); givesCheck = pos.gives_check(move); @@ -1015,32 +974,32 @@ moves_loop: // When in check, search starts here Value delta = beta - alpha; - // Step 13. Pruning at shallow depth (~98 Elo). Depth conditions are important for mate finding. + Depth r = reduction(improving, depth, moveCount, delta, thisThread->rootDelta); + + // Step 14. Pruning at shallow depth (~120 Elo). Depth conditions are important for mate finding. if ( !rootNode && pos.non_pawn_material(us) && bestValue > VALUE_TB_LOSS_IN_MAX_PLY) { - // Skip quiet moves if movecount exceeds our FutilityMoveCount threshold (~7 Elo) + // Skip quiet moves if movecount exceeds our FutilityMoveCount threshold (~8 Elo) moveCountPruning = moveCount >= futility_move_count(improving, depth); // Reduced depth of the next LMR search - int lmrDepth = std::max(newDepth - reduction(improving, depth, moveCount, delta, thisThread->rootDelta), 0); + int lmrDepth = newDepth - r; - if ( captureOrPromotion + if ( capture || givesCheck) { - // Futility pruning for captures (~0 Elo) - if ( !pos.empty(to_sq(move)) - && !givesCheck - && !PvNode - && lmrDepth < 6 + // Futility pruning for captures (~2 Elo) + if ( !givesCheck + && lmrDepth < 7 && !ss->inCheck - && ss->staticEval + 342 + 238 * lmrDepth + PieceValue[EG][pos.piece_on(to_sq(move))] - + captureHistory[movedPiece][to_sq(move)][type_of(pos.piece_on(to_sq(move)))] / 8 < alpha) + && 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; - // SEE based pruning (~9 Elo) - if (!pos.see_ge(move, Value(-218) * depth)) + // SEE based pruning for captures and checks (~11 Elo) + if (!pos.see_ge(move, Value(-205) * depth)) continue; } else @@ -1050,83 +1009,105 @@ moves_loop: // When in check, search starts here + (*contHist[3])[movedPiece][to_sq(move)]; // Continuation history based pruning (~2 Elo) - if ( lmrDepth < 5 - && history < -3000 * depth + 3000) + if ( lmrDepth < 6 + && history < -3832 * depth) continue; - history += thisThread->mainHistory[us][from_to(move)]; + history += 2 * thisThread->mainHistory[us][from_to(move)]; + + lmrDepth += history / 7011; + lmrDepth = std::max(lmrDepth, -2); - // Futility pruning: parent node (~9 Elo) + // Futility pruning: parent node (~13 Elo) if ( !ss->inCheck - && lmrDepth < 8 - && ss->staticEval + 142 + 139 * lmrDepth + history / 64 <= alpha) + && lmrDepth < 12 + && ss->staticEval + 112 + 138 * lmrDepth <= alpha) continue; - // Prune moves with negative SEE (~3 Elo) - if (!pos.see_ge(move, Value(-21 * lmrDepth * lmrDepth - 21 * lmrDepth))) + lmrDepth = std::max(lmrDepth, 0); + + // Prune moves with negative SEE (~4 Elo) + if (!pos.see_ge(move, Value(-31 * lmrDepth * lmrDepth))) continue; } } - // Step 14. Extensions (~66 Elo) - - // Singular extension search (~58 Elo). If all moves but one fail low on a - // search of (alpha-s, beta-s), and just one fails high on (alpha, beta), - // 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 ( !rootNode - && depth >= 6 + 2 * (PvNode && tte->is_pv()) - && move == ttMove - && !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) + // Step 15. Extensions (~100 Elo) + // We take care to not overdo to avoid search getting stuck. + if (ss->ply < thisThread->rootDepth * 2) { - Value singularBeta = ttValue - 3 * depth; - Depth singularDepth = (depth - 1) / 2; + // Singular extension search (~94 Elo). If all moves but one fail low on a + // search of (alpha-s, beta-s), and just one fails high on (alpha, beta), + // 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 > 22) + 2 * (PvNode && tte->is_pv()) + && move == ttMove + && !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 - (82 + 65 * (ss->ttPv && !PvNode)) * depth / 64; + Depth singularDepth = (depth - 1) / 2; - ss->excludedMove = move; - value = search(pos, ss, singularBeta - 1, singularBeta, singularDepth, cutNode); - ss->excludedMove = MOVE_NONE; + ss->excludedMove = move; + value = search(pos, ss, singularBeta - 1, singularBeta, singularDepth, cutNode); + ss->excludedMove = MOVE_NONE; - if (value < singularBeta) - { - extension = 1; + if (value < singularBeta) + { + extension = 1; + singularQuietLMR = !ttCapture; + + // Avoid search explosion by limiting the number of double extensions + if ( !PvNode + && value < singularBeta - 21 + && ss->doubleExtensions <= 11) + { + extension = 2; + depth += depth < 13; + } + } - // Avoid search explosion by limiting the number of double extensions - if ( !PvNode - && value < singularBeta - 75 - && ss->doubleExtensions <= 6) - extension = 2; + // Multi-cut pruning + // 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 softbound. + else if (singularBeta >= beta) + return singularBeta; + + // If the eval of ttMove is greater than beta, we reduce it (negative extension) (~7 Elo) + 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; } - // Multi-cut pruning - // 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. - else if (singularBeta >= beta) - return singularBeta; - - // If the eval of ttMove is greater than beta, we reduce it (negative extension) - else if (ttValue >= beta) - extension = -2; - } - - // Check extensions (~1 Elo) - else if ( givesCheck - && depth > 6 - && abs(ss->staticEval) > 100) - extension = 1; + // Check extensions (~1 Elo) + else if ( givesCheck + && depth > 9) + extension = 1; - // Quiet ttMove extensions (~0 Elo) - else if ( PvNode - && move == ttMove - && move == ss->killers[0] - && (*contHist[0])[movedPiece][to_sq(move)] >= 10000) - extension = 1; + // Quiet ttMove extensions (~1 Elo) + else if ( PvNode + && move == ttMove + && move == ss->killers[0] + && (*contHist[0])[movedPiece][to_sq(move)] >= 5168) + extension = 1; + } // Add extension to new depth newDepth += extension; @@ -1138,116 +1119,128 @@ moves_loop: // When in check, search starts here // Update the current move (this must be done after singular extension search) ss->currentMove = move; ss->continuationHistory = &thisThread->continuationHistory[ss->inCheck] - [captureOrPromotion] + [capture] [movedPiece] [to_sq(move)]; - // Step 15. Make the move + // Step 16. Make the move pos.do_move(move, st, givesCheck); - bool doDeeperSearch = false; - - // Step 16. Late moves reduction / extension (LMR, ~98 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 - // cases where we extend a son if it has good chances to be "interesting". - if ( depth >= 3 - && moveCount > 1 + 2 * rootNode - && ( !ss->ttPv - || !captureOrPromotion - || (cutNode && (ss-1)->moveCount > 1))) - { - Depth r = reduction(improving, depth, moveCount, delta, thisThread->rootDelta); + // 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 -= cutNode && tte->depth() >= depth ? 3 : 2; - // Decrease reduction at some PvNodes (~2 Elo) - if ( PvNode - && bestMoveCount <= 3) - r--; + // Decrease reduction if opponent's move count is high (~1 Elo) + if ((ss-1)->moveCount > 8) + r--; - // Decrease reduction if position is or has been on the PV - // and node is not likely to fail low. (~3 Elo) - if ( ss->ttPv - && !likelyFailLow) - r -= 2; + // Increase reduction for cut nodes (~3 Elo) + if (cutNode) + r += 2; - // Decrease reduction if opponent's move count is high (~1 Elo) - if ((ss-1)->moveCount > 13) - r--; + // Increase reduction if ttMove is a capture (~3 Elo) + if (ttCapture) + r++; - // Increase reduction for cut nodes (~3 Elo) - if (cutNode && move != ss->killers[0]) - r += 2; + // Decrease reduction for PvNodes (~2 Elo) + if (PvNode) + r--; - // Increase reduction if ttMove is a capture (~3 Elo) - if (ttCapture) - r++; + // Decrease reduction if ttMove has been singularly extended (~1 Elo) + if (singularQuietLMR) + r--; - ss->statScore = thisThread->mainHistory[us][from_to(move)] - + (*contHist[0])[movedPiece][to_sq(move)] - + (*contHist[1])[movedPiece][to_sq(move)] - + (*contHist[3])[movedPiece][to_sq(move)] - - 4923; + // Increase reduction on repetition (~1 Elo) + if ( move == (ss-4)->currentMove + && pos.has_repeated()) + r += 2; - // Decrease/increase reduction for moves with a good/bad history (~30 Elo) - r -= ss->statScore / 14721; + // Increase reduction if next ply has a lot of fail high (~5 Elo) + if ((ss+1)->cutoffCnt > 3) + r++; - // In general we want to cap the LMR depth search at newDepth. But if reductions - // are really negative and movecount is low, we allow this move to be searched - // deeper than the first move (this may lead to hidden double extensions). - int deeper = r >= -1 ? 0 - : moveCount <= 5 ? 2 - : PvNode && depth > 6 ? 1 - : cutNode && moveCount <= 7 ? 1 - : 0; + else if (move == ttMove) + r--; - Depth d = std::clamp(newDepth - r, 1, newDepth + deeper); + 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)] + - 4006; - value = -search(pos, ss+1, -(alpha+1), -alpha, d, true); + // Decrease/increase reduction for moves with a good/bad history (~25 Elo) + r -= ss->statScore / (11124 + 4740 * (depth > 5 && depth < 22)); - // If the son is reduced and fails high it will be re-searched at full depth - doFullDepthSearch = value > alpha && d < newDepth; - doDeeperSearch = value > (alpha + 62 + 20 * (newDepth - d)); - didLMR = true; - } - else + // 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 + // cases where we extend a son if it has good chances to be "interesting". + if ( depth >= 2 + && moveCount > 1 + (PvNode && ss->ply <= 1) + && ( !ss->ttPv + || !capture + || (cutNode && (ss-1)->moveCount > 1))) { - doFullDepthSearch = !PvNode || moveCount > 1; - didLMR = false; - } + // In general we want to cap the LMR depth search at newDepth, but when + // reduction is negative, we allow this move a limited search extension + // beyond the first move depth. This may lead to hidden double extensions. + Depth d = std::clamp(newDepth - r, 1, newDepth + 1); - // Step 17. Full depth search when LMR is skipped or fails high - if (doFullDepthSearch) - { - value = -search(pos, ss+1, -(alpha+1), -alpha, newDepth + doDeeperSearch, !cutNode); + value = -search(pos, ss+1, -(alpha+1), -alpha, d, true); - // If the move passed LMR update its stats - if (didLMR && !captureOrPromotion) + // Do a full-depth search when reduced LMR search fails high + if (value > alpha && d < newDepth) { - int bonus = value > alpha ? stat_bonus(newDepth) - : -stat_bonus(newDepth); + // 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; + const bool doShallowerSearch = value < bestValue + newDepth; + + ss->doubleExtensions = ss->doubleExtensions + doEvenDeeperSearch; + + newDepth += doDeeperSearch - doShallowerSearch + doEvenDeeperSearch; + + if (newDepth > d) + value = -search(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode); + + int bonus = value <= alpha ? -stat_bonus(newDepth) + : value >= beta ? stat_bonus(newDepth) + : 0; update_continuation_histories(ss, movedPiece, to_sq(move), bonus); } } - // 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)))) + // 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(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, + // 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; - value = -search(pos, ss+1, -beta, -alpha, - std::min(maxNextDepth, newDepth), false); + value = -search(pos, ss+1, -beta, -alpha, newDepth, false); } - // Step 18. Undo move + // Step 19. Undo move pos.undo_move(move); assert(value > -VALUE_INFINITE && value < VALUE_INFINITE); - // Step 19. Check for a new best move + // Step 20. 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. @@ -1264,8 +1257,21 @@ moves_loop: // When in check, search starts here // PV move or new best move? if (moveCount == 1 || value > alpha) { - rm.score = value; + rm.score = rm.uciScore = value; rm.selDepth = thisThread->selDepth; + rm.scoreLowerbound = rm.scoreUpperbound = false; + + if (value >= beta) + { + rm.scoreLowerbound = true; + rm.uciScore = beta; + } + else if (value <= alpha) + { + rm.scoreUpperbound = true; + rm.uciScore = alpha; + } + rm.pv.resize(1); assert((ss+1)->pv); @@ -1281,7 +1287,7 @@ moves_loop: // When in check, search starts here ++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; @@ -1298,39 +1304,48 @@ moves_loop: // When in check, search starts here 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) { - alpha = value; - bestMoveCount++; + ss->cutoffCnt += 1 + !ttMove; + assert(value >= beta); // Fail high + break; } else { - 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 (captureOrPromotion && captureCount < 32) + if (capture && captureCount < 32) capturesSearched[captureCount++] = move; - else if (!captureOrPromotion && quietCount < 64) + else if (!capture && quietCount < 64) quietsSearched[quietCount++] = move; } } // 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) return VALUE_DRAW; */ - // Step 20. Check for mate and stalemate + // Step 21. 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. @@ -1342,35 +1357,26 @@ moves_loop: // When in check, search starts here 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 ( (depth >= 3 || PvNode) - && !priorCapture) + else if (!priorCapture && prevSq != SQ_NONE) { - //Assign extra bonus if current node is PvNode or cutNode - //or fail low was really bad - bool extraBonus = PvNode - || cutNode - || bestValue < alpha - 94 * depth; - - update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, stat_bonus(depth) * (1 + extraBonus)); + 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) bestValue = std::min(bestValue, maxValue); // If no good move is found and the previous position was ttPv, then the previous - // opponent move is probably good and the new position is added to the search tree. + // opponent move is probably good and the new position is added to the search tree. (~7 Elo) if (bestValue <= alpha) ss->ttPv = ss->ttPv || ((ss-1)->ttPv && depth > 3); - // Otherwise, a counter move has been found and if the position is the last leaf - // in the search tree, remove the position from the search tree. - else if (depth > 3) - ss->ttPv = ss->ttPv && (ss+1)->ttPv; // Write gathered information in transposition table if (!excludedMove && !(rootNode && thisThread->pvIdx)) @@ -1387,6 +1393,7 @@ moves_loop: // When in check, search starts here // 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. + // (~155 Elo) template Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) { @@ -1397,6 +1404,17 @@ moves_loop: // When in check, search starts here 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 + && 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); @@ -1406,9 +1424,10 @@ moves_loop: // When in check, search starts here Move ttMove, move, bestMove; Depth ttDepth; Value bestValue, value, ttValue, futilityValue, futilityBase; - bool pvHit, givesCheck, captureOrPromotion; + bool pvHit, givesCheck, capture; int moveCount; + // Step 1. Initialize node if (PvNode) { (ss+1)->pv = pv; @@ -1420,7 +1439,7 @@ moves_loop: // When in check, search starts here ss->inCheck = pos.checkers(); moveCount = 0; - // Check for an immediate draw or maximum ply reached + // Step 2. Check for an immediate draw or maximum ply reached if ( pos.is_draw(ss->ply) || ss->ply >= MAX_PLY) return (ss->ply >= MAX_PLY && !ss->inCheck) ? evaluate(pos) : VALUE_DRAW; @@ -1431,28 +1450,25 @@ moves_loop: // When in check, search starts here // TT entry depth that we are going to use. Note that in qsearch we use // only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS. ttDepth = ss->inCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS - : DEPTH_QS_NO_CHECKS; - // Transposition table lookup + : DEPTH_QS_NO_CHECKS; + + // Step 3. Transposition table lookup posKey = pos.key(); tte = TT.probe(posKey, ss->ttHit); ttValue = ss->ttHit ? value_from_tt(tte->value(), ss->ply, pos.rule50_count()) : VALUE_NONE; ttMove = ss->ttHit ? tte->move() : MOVE_NONE; pvHit = ss->ttHit && tte->is_pv(); + // 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 >= beta ? (tte->bound() & BOUND_LOWER) - : (tte->bound() & BOUND_UPPER))) + && ttValue != VALUE_NONE // Only in case of TT access race or if !ttHit + && (tte->bound() & (ttValue >= beta ? BOUND_LOWER : BOUND_UPPER))) return ttValue; - // Evaluate the position statically + // Step 4. Static evaluation of the position if (ss->inCheck) - { - ss->staticEval = VALUE_NONE; bestValue = futilityBase = -VALUE_INFINITE; - } else { if (ss->ttHit) @@ -1461,16 +1477,15 @@ moves_loop: // When in check, search starts here if ((ss->staticEval = bestValue = tte->eval()) == VALUE_NONE) ss->staticEval = bestValue = evaluate(pos); - // ttValue can be used as a better position evaluation (~7 Elo) + // ttValue can be used as a better position evaluation (~13 Elo) if ( ttValue != VALUE_NONE && (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER))) bestValue = ttValue; } 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) @@ -1483,10 +1498,10 @@ moves_loop: // When in check, search starts here return bestValue; } - if (PvNode && bestValue > alpha) + if (bestValue > alpha) alpha = bestValue; - futilityBase = bestValue + 155; + futilityBase = std::min(ss->staticEval, bestValue) + 200; } const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory, @@ -1497,100 +1512,113 @@ moves_loop: // When in check, search starts here // 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 = to_sq((ss-1)->currentMove); + Square prevSq = is_ok((ss-1)->currentMove) ? to_sq((ss-1)->currentMove) : SQ_NONE; MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory, &thisThread->captureHistory, contHist, prevSq); - // Loop through the moves until no moves remain or a beta cutoff occurs + int quietCheckEvasions = 0; + + // Step 5. Loop through all pseudo-legal moves until no moves remain + // or a beta cutoff occurs. while ((move = mp.next_move()) != MOVE_NONE) { - assert(is_ok(move)); - - // Check for legality - if (!pos.legal(move)) - continue; + assert(is_ok(move)); - givesCheck = pos.gives_check(move); - captureOrPromotion = pos.capture_or_promotion(move); + // Check for legality + if (!pos.legal(move)) + continue; - moveCount++; + givesCheck = pos.gives_check(move); + capture = pos.capture_stage(move); - // Futility pruning and moveCount pruning (~5 Elo) - if ( bestValue > VALUE_TB_LOSS_IN_MAX_PLY - && !givesCheck - && to_sq(move) != prevSq - && futilityBase > -VALUE_KNOWN_WIN - && type_of(move) != PROMOTION) - { + moveCount++; - if (moveCount > 2) - 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; - futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))]; + futilityValue = futilityBase + PieceValue[pos.piece_on(to_sq(move))]; - if (futilityValue <= alpha) - { - bestValue = std::max(bestValue, futilityValue); - continue; - } + if (futilityValue <= alpha) + { + bestValue = std::max(bestValue, futilityValue); + continue; + } - if (futilityBase <= alpha && !pos.see_ge(move, VALUE_ZERO + 1)) - { - bestValue = std::max(bestValue, futilityBase); - continue; - } - } + if (futilityBase <= alpha && !pos.see_ge(move, VALUE_ZERO + 1)) + { + bestValue = std::max(bestValue, futilityBase); + continue; + } + } - // Do not search moves with negative SEE values (~5 Elo) - if ( bestValue > VALUE_TB_LOSS_IN_MAX_PLY - && !pos.see_ge(move)) - 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))); - ss->currentMove = move; - ss->continuationHistory = &thisThread->continuationHistory[ss->inCheck] - [captureOrPromotion] - [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)]; - // Continuation history based pruning (~2 Elo) - if ( !captureOrPromotion - && bestValue > VALUE_TB_LOSS_IN_MAX_PLY - && (*contHist[0])[pos.moved_piece(move)][to_sq(move)] < CounterMovePruneThreshold - && (*contHist[1])[pos.moved_piece(move)][to_sq(move)] < CounterMovePruneThreshold) - continue; + quietCheckEvasions += !capture && ss->inCheck; - // Make and search the move - pos.do_move(move, st, givesCheck); - value = -qsearch(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(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); - // 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 (value < beta) // Update alpha here! + alpha = value; + else + break; // Fail high + } + } } + // Step 9. Check for mate // All legal moves have been searched. A special case: if we're in check // and no legal moves were found, it is checkmate. if (ss->inCheck && bestValue == -VALUE_INFINITE) @@ -1657,7 +1685,7 @@ moves_loop: // When in check, search starts here // update_pv() adds current move and appends child pv[] - void update_pv(Move* pv, Move move, Move* childPv) { + void update_pv(Move* pv, Move move, const Move* childPv) { for (*pv++ = move; childPv && *childPv != MOVE_NONE; ) *pv++ = *childPv++; @@ -1670,45 +1698,49 @@ moves_loop: // When in check, search starts here void update_all_stats(const Position& pos, Stack* ss, Move bestMove, Value bestValue, Value beta, Square prevSq, Move* quietsSearched, int quietCount, Move* capturesSearched, int captureCount, Depth depth) { - int bonus1, bonus2; Color us = pos.side_to_move(); Thread* thisThread = pos.this_thread(); CapturePieceToHistory& captureHistory = thisThread->captureHistory; Piece moved_piece = pos.moved_piece(bestMove); - PieceType captured = type_of(pos.piece_on(to_sq(bestMove))); + PieceType captured; - bonus1 = stat_bonus(depth + 1); - bonus2 = bestValue > beta + PawnValueMg ? bonus1 // larger bonus - : stat_bonus(depth); // smaller bonus + int quietMoveBonus = stat_bonus(depth + 1); - if (!pos.capture_or_promotion(bestMove)) + if (!pos.capture_stage(bestMove)) { + 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 - captureHistory[moved_piece][to_sq(bestMove)][captured] << bonus1; + captured = type_of(pos.piece_on(to_sq(bestMove))); + captureHistory[moved_piece][to_sq(bestMove)][captured] << quietMoveBonus; + } // 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])) + 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; } } @@ -1720,7 +1752,7 @@ moves_loop: // When in check, search starts here 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)) @@ -1753,7 +1785,7 @@ moves_loop: // When in check, search starts here } } - // 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) { @@ -1763,7 +1795,7 @@ moves_loop: // When in check, search starts here // 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; @@ -1798,7 +1830,7 @@ void MainThread::check_time() { 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(); @@ -1815,7 +1847,7 @@ void MainThread::check_time() { 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; @@ -1825,7 +1857,7 @@ void MainThread::check_time() { /// UCI::pv() formats PV information according to the UCI protocol. UCI requires /// that all (if any) unsearched PV lines are sent using a previous search score. -string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) { +string UCI::pv(const Position& pos, Depth depth) { std::stringstream ss; TimePoint elapsed = Time.elapsed() + 1; @@ -1843,7 +1875,7 @@ string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) { continue; Depth d = updated ? depth : std::max(1, depth - 1); - Value v = updated ? rootMoves[i].score : rootMoves[i].previousScore; + Value v = updated ? rootMoves[i].uciScore : rootMoves[i].previousScore; if (v == -VALUE_INFINITE) v = VALUE_ZERO; @@ -1863,16 +1895,13 @@ string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) { if (Options["UCI_ShowWDL"]) ss << UCI::wdl(v, pos.game_ply()); - if (!tb && i == pvIdx) - ss << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : ""); + if (i == pvIdx && !tb && updated) // tablebase- and previous-scores are exact + ss << (rootMoves[i].scoreLowerbound ? " lowerbound" : (rootMoves[i].scoreUpperbound ? " upperbound" : "")); ss << " nodes " << nodesSearched - << " nps " << nodesSearched * 1000 / elapsed; - - if (elapsed > 1000) // Earlier makes little sense - ss << " hashfull " << TT.hashfull(); - - ss << " tbhits " << tbHits + << " nps " << nodesSearched * 1000 / elapsed + << " hashfull " << TT.hashfull() + << " tbhits " << tbHits << " time " << elapsed << " pv"; @@ -1887,7 +1916,7 @@ string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) { /// 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) {