along with this program. If not, see <http://www.gnu.org/licenses/>.
*/
+#include "search.h"
+
#include <algorithm>
+#include <array>
+#include <atomic>
#include <cassert>
#include <cmath>
-#include <cstring> // For std::memset
+#include <cstdlib>
+#include <cstring>
+#include <initializer_list>
#include <iostream>
#include <sstream>
+#include <string>
+#include <utility>
+#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"
-#include "nnue/evaluate_nnue.h"
namespace Stockfish {
namespace Search {
- LimitsType Limits;
+LimitsType Limits;
}
namespace Tablebases {
- int Cardinality;
- bool RootInTB;
- bool UseRule50;
- Depth ProbeDepth;
+int Cardinality;
+bool RootInTB;
+bool UseRule50;
+Depth ProbeDepth;
}
namespace TB = Tablebases;
namespace {
- // Different node types, used as a template parameter
- enum NodeType { NonPV, PV, Root };
+// Different node types, used as a template parameter
+enum NodeType {
+ NonPV,
+ PV,
+ Root
+};
+
+// Futility margin
+Value futility_margin(Depth d, bool noTtCutNode, bool improving) {
+ return Value((116 - 44 * noTtCutNode) * (d - improving));
+}
- // Futility margin
- Value futility_margin(Depth d, bool improving) {
- return Value(140 * (d - improving));
- }
+// Reductions lookup table initialized at startup
+int Reductions[MAX_MOVES]; // [depth or moveNumber]
- // 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 reductionScale = Reductions[d] * Reductions[mn];
+ return (reductionScale + 1346 - int(delta) * 896 / int(rootDelta)) / 1024
+ + (!i && reductionScale > 880);
+}
- Depth reduction(bool i, Depth d, int mn, Value delta, Value rootDelta) {
- int r = Reductions[d] * Reductions[mn];
- return (r + 1372 - int(delta) * 1073 / int(rootDelta)) / 1024 + (!i && r > 936);
- }
+constexpr int futility_move_count(bool improving, Depth depth) {
+ return improving ? (3 + depth * depth) : (3 + depth * depth) / 2;
+}
- constexpr int futility_move_count(bool improving, Depth depth) {
- 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(268 * d - 352, 1153); }
- // History and stats update bonus, based on depth
- int stat_bonus(Depth d) {
- return std::min(336 * d - 547, 1561);
- }
+// History and stats update malus, based on depth
+int stat_malus(Depth d) { return std::min(400 * d - 354, 1201); }
- // Add a small random component to draw evaluations to avoid 3-fold blindness
- Value value_draw(const Thread* thisThread) {
+// Add a small random component to draw evaluations to avoid 3-fold blindness
+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 a match (TC 60+0.6)
- // results spanning a wide range of k values.
- struct Skill {
+// Skill structure is used to implement strength limit. If we have a UCI_Elo,
+// we convert it to an appropriate skill level, anchored to the Stash engine.
+// This method is based on a fit of the Elo results for games played between
+// Stockfish at various skill levels and various versions of the Stash engine.
+// Skill 0 .. 19 now covers CCRL Blitz Elo from 1320 to 3190, approximately
+// Reference: https://github.com/vondele/Stockfish/commit/a08b8d4e9711c2
+struct Skill {
Skill(int skill_level, int uci_elo) {
if (uci_elo)
{
Move pick_best(size_t multiPV);
double level;
- Move best = MOVE_NONE;
- };
-
- template <NodeType nodeType>
- Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode);
-
- template <NodeType nodeType>
- Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth = 0);
-
- 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, 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,
- Move* quietsSearched, int quietCount, Move* capturesSearched, int captureCount, Depth depth);
-
- // perft() is our utility to verify move generation. All the leaf nodes up
- // to the given depth are generated and counted, and the sum is returned.
- template<bool Root>
- uint64_t perft(Position& pos, Depth depth) {
+ Move best = MOVE_NONE;
+};
+
+template<NodeType nodeType>
+Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode);
+
+template<NodeType nodeType>
+Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth = 0);
+
+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, 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,
+ Move* quietsSearched,
+ int quietCount,
+ Move* capturesSearched,
+ int captureCount,
+ Depth depth);
+
+// Utility to verify move generation. All the leaf nodes up
+// to the given depth are generated and counted, and the sum is returned.
+template<bool Root>
+uint64_t perft(Position& pos, Depth depth) {
StateInfo st;
ASSERT_ALIGNED(&st, Eval::NNUE::CacheLineSize);
- uint64_t cnt, nodes = 0;
+ uint64_t cnt, nodes = 0;
const bool leaf = (depth == 2);
for (const auto& m : MoveList<LEGAL>(pos))
sync_cout << UCI::move(m, pos.is_chess960()) << ": " << cnt << sync_endl;
}
return nodes;
- }
-
-} // namespace
+}
+} // namespace
-/// Search::init() is called at startup to initialize various lookup tables
+// Called at startup to initialize various lookup tables
void Search::init() {
- for (int i = 1; i < MAX_MOVES; ++i)
- Reductions[i] = int((20.57 + std::log(Threads.size()) / 2) * std::log(i));
+ for (int i = 1; i < MAX_MOVES; ++i)
+ Reductions[i] = int((20.37 + std::log(Threads.size()) / 2) * std::log(i));
}
-/// Search::clear() resets search state to its initial value
-
+// Resets search state to its initial value
void Search::clear() {
- Threads.main()->wait_for_search_finished();
+ Threads.main()->wait_for_search_finished();
- Time.availableNodes = 0;
- TT.clear();
- Threads.clear();
- Tablebases::init(Options["SyzygyPath"]); // Free mapped files
+ Time.availableNodes = 0;
+ TT.clear();
+ Threads.clear();
+ Tablebases::init(Options["SyzygyPath"]); // Free mapped files
}
-/// MainThread::search() is started when the program receives the UCI 'go'
-/// command. It searches from the root position and outputs the "bestmove".
-
+// Called when the program receives the UCI 'go'
+// command. It searches from the root position and outputs the "bestmove".
void MainThread::search() {
- if (Limits.perft)
- {
- nodes = perft<true>(rootPos, Limits.perft);
- sync_cout << "\nNodes searched: " << nodes << "\n" << sync_endl;
- return;
- }
-
- Color us = rootPos.side_to_move();
- Time.init(Limits, us, rootPos.game_ply());
- TT.new_search();
-
- Eval::NNUE::verify();
-
- if (rootMoves.empty())
- {
- rootMoves.emplace_back(MOVE_NONE);
- sync_cout << "info depth 0 score "
- << UCI::value(rootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
- << sync_endl;
- }
- else
- {
- Threads.start_searching(); // start non-main threads
- Thread::search(); // main thread start searching
- }
-
- // When we reach the maximum depth, we can arrive here without a raise of
- // Threads.stop. However, if we are pondering or in an infinite search,
- // the UCI protocol states that we shouldn't print the best move before the
- // GUI sends a "stop" or "ponderhit" command. We therefore simply wait here
- // until the GUI sends one of those commands.
-
- while (!Threads.stop && (ponder || Limits.infinite))
- {} // Busy wait for a stop or a ponder reset
-
- // Stop the threads if not already stopped (also raise the stop if
- // "ponderhit" just reset Threads.ponder).
- Threads.stop = true;
-
- // Wait until all threads have finished
- Threads.wait_for_search_finished();
-
- // When playing in 'nodes as time' mode, subtract the searched nodes from
- // the available ones before exiting.
- if (Limits.npmsec)
- Time.availableNodes += Limits.inc[us] - Threads.nodes_searched();
-
- Thread* bestThread = this;
- Skill skill = Skill(Options["Skill Level"], Options["UCI_LimitStrength"] ? int(Options["UCI_Elo"]) : 0);
-
- if ( int(Options["MultiPV"]) == 1
- && !Limits.depth
- && !skill.enabled()
- && rootMoves[0].pv[0] != MOVE_NONE)
- bestThread = Threads.get_best_thread();
-
- bestPreviousScore = bestThread->rootMoves[0].score;
- bestPreviousAverageScore = bestThread->rootMoves[0].averageScore;
-
- // Send again PV info if we have a new best thread
- if (bestThread != this)
- sync_cout << UCI::pv(bestThread->rootPos, bestThread->completedDepth) << sync_endl;
-
- sync_cout << "bestmove " << UCI::move(bestThread->rootMoves[0].pv[0], rootPos.is_chess960());
-
- if (bestThread->rootMoves[0].pv.size() > 1 || bestThread->rootMoves[0].extract_ponder_from_tt(rootPos))
- std::cout << " ponder " << UCI::move(bestThread->rootMoves[0].pv[1], rootPos.is_chess960());
-
- std::cout << sync_endl;
-}
+ if (Limits.perft)
+ {
+ nodes = perft<true>(rootPos, Limits.perft);
+ sync_cout << "\nNodes searched: " << nodes << "\n" << sync_endl;
+ return;
+ }
+
+ Color us = rootPos.side_to_move();
+ Time.init(Limits, us, rootPos.game_ply());
+ TT.new_search();
+
+ Eval::NNUE::verify();
+
+ if (rootMoves.empty())
+ {
+ rootMoves.emplace_back(MOVE_NONE);
+ sync_cout << "info depth 0 score "
+ << UCI::value(rootPos.checkers() ? -VALUE_MATE : VALUE_DRAW) << sync_endl;
+ }
+ else
+ {
+ Threads.start_searching(); // start non-main threads
+ Thread::search(); // main thread start searching
+ }
+
+ // When we reach the maximum depth, we can arrive here without a raise of
+ // Threads.stop. However, if we are pondering or in an infinite search,
+ // the UCI protocol states that we shouldn't print the best move before the
+ // GUI sends a "stop" or "ponderhit" command. We therefore simply wait here
+ // until the GUI sends one of those commands.
+
+ while (!Threads.stop && (ponder || Limits.infinite))
+ {} // Busy wait for a stop or a ponder reset
+
+ // Stop the threads if not already stopped (also raise the stop if
+ // "ponderhit" just reset Threads.ponder).
+ Threads.stop = true;
+
+ // Wait until all threads have finished
+ Threads.wait_for_search_finished();
+
+ // When playing in 'nodes as time' mode, subtract the searched nodes from
+ // the available ones before exiting.
+ if (Limits.npmsec)
+ Time.availableNodes += Limits.inc[us] - Threads.nodes_searched();
+ Thread* bestThread = this;
+ Skill skill =
+ Skill(Options["Skill Level"], Options["UCI_LimitStrength"] ? int(Options["UCI_Elo"]) : 0);
-/// Thread::search() is the main iterative deepening loop. It calls search()
-/// repeatedly with increasing depth until the allocated thinking time has been
-/// consumed, the user stops the search, or the maximum search depth is reached.
+ if (int(Options["MultiPV"]) == 1 && !Limits.depth && !skill.enabled()
+ && rootMoves[0].pv[0] != MOVE_NONE)
+ bestThread = Threads.get_best_thread();
+ bestPreviousScore = bestThread->rootMoves[0].score;
+ bestPreviousAverageScore = bestThread->rootMoves[0].averageScore;
+
+ // Send again PV info if we have a new best thread
+ if (bestThread != this)
+ sync_cout << UCI::pv(bestThread->rootPos, bestThread->completedDepth) << sync_endl;
+
+ sync_cout << "bestmove " << UCI::move(bestThread->rootMoves[0].pv[0], rootPos.is_chess960());
+
+ if (bestThread->rootMoves[0].pv.size() > 1
+ || bestThread->rootMoves[0].extract_ponder_from_tt(rootPos))
+ std::cout << " ponder " << UCI::move(bestThread->rootMoves[0].pv[1], rootPos.is_chess960());
+
+ std::cout << sync_endl;
+}
+
+
+// Main iterative deepening loop. It calls search()
+// repeatedly with increasing depth until the allocated thinking time has been
+// consumed, the user stops the search, or the maximum search depth is reached.
void Thread::search() {
- // To allow access to (ss-7) up to (ss+2), the stack must be oversized.
- // The former is needed to allow update_continuation_histories(ss-1, ...),
- // which accesses its argument at ss-6, also near the root.
- // The latter is needed for statScore and killer initialization.
- Stack stack[MAX_PLY+10], *ss = stack+7;
- Move pv[MAX_PLY+1];
- Value alpha, beta, delta;
- Move lastBestMove = MOVE_NONE;
- Depth lastBestMoveDepth = 0;
- MainThread* mainThread = (this == Threads.main() ? Threads.main() : nullptr);
- double timeReduction = 1, totBestMoveChanges = 0;
- Color us = rootPos.side_to_move();
- int iterIdx = 0;
-
- std::memset(ss-7, 0, 10 * sizeof(Stack));
- 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 = -VALUE_INFINITE;
-
- if (mainThread)
- {
- if (mainThread->bestPreviousScore == VALUE_INFINITE)
- for (int i = 0; i < 4; ++i)
- mainThread->iterValue[i] = VALUE_ZERO;
- else
- for (int i = 0; i < 4; ++i)
- mainThread->iterValue[i] = mainThread->bestPreviousScore;
- }
-
- size_t multiPV = size_t(Options["MultiPV"]);
- 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.
- if (skill.enabled())
- multiPV = std::max(multiPV, (size_t)4);
-
- multiPV = std::min(multiPV, rootMoves.size());
-
- int searchAgainCounter = 0;
-
- // Iterative deepening loop until requested to stop or the target depth is reached
- while ( ++rootDepth < MAX_PLY
- && !Threads.stop
- && !(Limits.depth && mainThread && rootDepth > Limits.depth))
- {
- // Age out PV variability metric
- if (mainThread)
- totBestMoveChanges /= 2;
-
- // 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;
-
- size_t pvFirst = 0;
- pvLast = 0;
-
- if (!Threads.increaseDepth)
- searchAgainCounter++;
-
- // MultiPV loop. We perform a full root search for each PV line
- for (pvIdx = 0; pvIdx < multiPV && !Threads.stop; ++pvIdx)
- {
- if (pvIdx == pvLast)
- {
- pvFirst = pvLast;
- for (pvLast++; pvLast < rootMoves.size(); pvLast++)
- if (rootMoves[pvLast].tbRank != rootMoves[pvFirst].tbRank)
- break;
- }
-
- // Reset UCI info selDepth for each depth and each PV line
- selDepth = 0;
-
- // Reset aspiration window starting size
- 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
- // high/low anymore.
- int failedHighCnt = 0;
- while (true)
- {
- // 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 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 the case of MultiPV
- // search the already searched PV lines are preserved.
- std::stable_sort(rootMoves.begin() + pvIdx, rootMoves.begin() + pvLast);
-
- // If search has been stopped, we break immediately. Sorting is
- // safe because RootMoves is still valid, although it refers to
- // the previous iteration.
- if (Threads.stop)
- break;
-
- // When failing high/low give some update (without cluttering
- // the UI) before a re-search.
- if ( mainThread
- && multiPV == 1
- && (bestValue <= alpha || bestValue >= beta)
- && Time.elapsed() > 3000)
- sync_cout << UCI::pv(rootPos, rootDepth) << sync_endl;
-
- // In case of failing low/high increase aspiration window and
- // re-search, otherwise exit the loop.
- if (bestValue <= alpha)
- {
- beta = (alpha + beta) / 2;
- alpha = std::max(bestValue - delta, -VALUE_INFINITE);
-
- failedHighCnt = 0;
- if (mainThread)
- mainThread->stopOnPonderhit = false;
- }
- else if (bestValue >= beta)
- {
- beta = std::min(bestValue + delta, VALUE_INFINITE);
- ++failedHighCnt;
- }
- else
- break;
-
- delta += delta / 3;
-
- assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
- }
-
- // Sort the PV lines searched so far and update the GUI
- std::stable_sort(rootMoves.begin() + pvFirst, rootMoves.begin() + pvIdx + 1);
-
- if ( mainThread
- && (Threads.stop || pvIdx + 1 == multiPV || Time.elapsed() > 3000))
- 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;
- }
-
- // Have we found a "mate in x"?
- if ( Limits.mate
- && bestValue >= VALUE_MATE_IN_MAX_PLY
- && VALUE_MATE - bestValue <= 2 * Limits.mate)
- Threads.stop = true;
-
- if (!mainThread)
- continue;
-
- // 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);
-
- // Use part of the gained time from a previous stable move for the current move
- for (Thread* th : Threads)
- {
- totBestMoveChanges += th->bestMoveChanges;
- th->bestMoveChanges = 0;
- }
-
- // Do we have time for the next iteration? Can we stop searching now?
- if ( Limits.use_time_management()
- && !Threads.stop
- && !mainThread->stopOnPonderhit)
- {
- 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 + 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
- // yielding correct scores and sufficiently fast moves.
- if (rootMoves.size() == 1)
- totalTime = std::min(500.0, totalTime);
-
- // Stop the search if we have exceeded the totalTime
- if (Time.elapsed() > totalTime)
- {
- // If we are allowed to ponder do not stop the search now but
- // keep pondering until the GUI sends "ponderhit" or "stop".
- if (mainThread->ponder)
- mainThread->stopOnPonderhit = true;
- else
- Threads.stop = true;
- }
- else if ( !mainThread->ponder
- && Time.elapsed() > totalTime * 0.50)
- Threads.increaseDepth = false;
- else
- Threads.increaseDepth = true;
- }
-
- mainThread->iterValue[iterIdx] = bestValue;
- iterIdx = (iterIdx + 1) & 3;
- }
-
- if (!mainThread)
- return;
-
- mainThread->previousTimeReduction = timeReduction;
-
- // 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)));
+ // Allocate stack with extra size to allow access from (ss - 7) to (ss + 2):
+ // (ss - 7) is needed for update_continuation_histories(ss - 1) which accesses (ss - 6),
+ // (ss + 2) is needed for initialization of cutOffCnt and killers.
+ Stack stack[MAX_PLY + 10], *ss = stack + 7;
+ Move pv[MAX_PLY + 1];
+ Value alpha, beta, delta;
+ Move lastBestMove = MOVE_NONE;
+ Depth lastBestMoveDepth = 0;
+ MainThread* mainThread = (this == Threads.main() ? Threads.main() : nullptr);
+ double timeReduction = 1, totBestMoveChanges = 0;
+ Color us = rootPos.side_to_move();
+ int iterIdx = 0;
+
+ std::memset(ss - 7, 0, 10 * sizeof(Stack));
+ 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 = -VALUE_INFINITE;
+
+ if (mainThread)
+ {
+ if (mainThread->bestPreviousScore == VALUE_INFINITE)
+ for (int i = 0; i < 4; ++i)
+ mainThread->iterValue[i] = VALUE_ZERO;
+ else
+ for (int i = 0; i < 4; ++i)
+ mainThread->iterValue[i] = mainThread->bestPreviousScore;
+ }
+
+ size_t multiPV = size_t(Options["MultiPV"]);
+ 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.
+ if (skill.enabled())
+ multiPV = std::max(multiPV, size_t(4));
+
+ multiPV = std::min(multiPV, rootMoves.size());
+
+ int searchAgainCounter = 0;
+
+ // Iterative deepening loop until requested to stop or the target depth is reached
+ while (++rootDepth < MAX_PLY && !Threads.stop
+ && !(Limits.depth && mainThread && rootDepth > Limits.depth))
+ {
+ // Age out PV variability metric
+ if (mainThread)
+ totBestMoveChanges /= 2;
+
+ // 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;
+
+ size_t pvFirst = 0;
+ pvLast = 0;
+
+ if (!Threads.increaseDepth)
+ searchAgainCounter++;
+
+ // MultiPV loop. We perform a full root search for each PV line
+ for (pvIdx = 0; pvIdx < multiPV && !Threads.stop; ++pvIdx)
+ {
+ if (pvIdx == pvLast)
+ {
+ pvFirst = pvLast;
+ for (pvLast++; pvLast < rootMoves.size(); pvLast++)
+ if (rootMoves[pvLast].tbRank != rootMoves[pvFirst].tbRank)
+ break;
+ }
+
+ // Reset UCI info selDepth for each depth and each PV line
+ selDepth = 0;
+
+ // Reset aspiration window starting size
+ Value avg = rootMoves[pvIdx].averageScore;
+ delta = Value(9) + int(avg) * avg / 14847;
+ alpha = std::max(avg - delta, -VALUE_INFINITE);
+ beta = std::min(avg + delta, VALUE_INFINITE);
+
+ // Adjust optimism based on root move's averageScore (~4 Elo)
+ optimism[us] = 121 * avg / (std::abs(avg) + 109);
+ 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
+ // high/low anymore.
+ int failedHighCnt = 0;
+ while (true)
+ {
+ // 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 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 the case of MultiPV
+ // search the already searched PV lines are preserved.
+ std::stable_sort(rootMoves.begin() + pvIdx, rootMoves.begin() + pvLast);
+
+ // If search has been stopped, we break immediately. Sorting is
+ // safe because RootMoves is still valid, although it refers to
+ // the previous iteration.
+ if (Threads.stop)
+ break;
+
+ // When failing high/low give some update (without cluttering
+ // the UI) before a re-search.
+ if (mainThread && multiPV == 1 && (bestValue <= alpha || bestValue >= beta)
+ && Time.elapsed() > 3000)
+ sync_cout << UCI::pv(rootPos, rootDepth) << sync_endl;
+
+ // In case of failing low/high increase aspiration window and
+ // re-search, otherwise exit the loop.
+ if (bestValue <= alpha)
+ {
+ beta = (alpha + beta) / 2;
+ alpha = std::max(bestValue - delta, -VALUE_INFINITE);
+
+ failedHighCnt = 0;
+ if (mainThread)
+ mainThread->stopOnPonderhit = false;
+ }
+ else if (bestValue >= beta)
+ {
+ beta = std::min(bestValue + delta, VALUE_INFINITE);
+ ++failedHighCnt;
+ }
+ else
+ break;
+
+ delta += delta / 3;
+
+ assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
+ }
+
+ // Sort the PV lines searched so far and update the GUI
+ std::stable_sort(rootMoves.begin() + pvFirst, rootMoves.begin() + pvIdx + 1);
+
+ if (mainThread && (Threads.stop || pvIdx + 1 == multiPV || Time.elapsed() > 3000))
+ 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;
+ }
+
+ // Have we found a "mate in x"?
+ if (Limits.mate && bestValue >= VALUE_MATE_IN_MAX_PLY
+ && VALUE_MATE - bestValue <= 2 * Limits.mate)
+ Threads.stop = true;
+
+ if (!mainThread)
+ continue;
+
+ // 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);
+
+ // Use part of the gained time from a previous stable move for the current move
+ for (Thread* th : Threads)
+ {
+ totBestMoveChanges += th->bestMoveChanges;
+ th->bestMoveChanges = 0;
+ }
+
+ // Do we have time for the next iteration? Can we stop searching now?
+ if (Limits.use_time_management() && !Threads.stop && !mainThread->stopOnPonderhit)
+ {
+ double fallingEval = (66 + 14 * (mainThread->bestPreviousAverageScore - bestValue)
+ + 6 * (mainThread->iterValue[iterIdx] - bestValue))
+ / 616.6;
+ fallingEval = std::clamp(fallingEval, 0.51, 1.51);
+
+ // If the bestMove is stable over several iterations, reduce time accordingly
+ timeReduction = lastBestMoveDepth + 8 < completedDepth ? 1.56 : 0.69;
+ double reduction = (1.4 + mainThread->previousTimeReduction) / (2.17 * timeReduction);
+ double bestMoveInstability = 1 + 1.79 * 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
+ if (rootMoves.size() == 1)
+ totalTime = std::min(500.0, totalTime);
+
+ // Stop the search if we have exceeded the totalTime
+ if (Time.elapsed() > totalTime)
+ {
+ // If we are allowed to ponder do not stop the search now but
+ // keep pondering until the GUI sends "ponderhit" or "stop".
+ if (mainThread->ponder)
+ mainThread->stopOnPonderhit = true;
+ else
+ Threads.stop = true;
+ }
+ else if (!mainThread->ponder && Time.elapsed() > totalTime * 0.50)
+ Threads.increaseDepth = false;
+ else
+ Threads.increaseDepth = true;
+ }
+
+ mainThread->iterValue[iterIdx] = bestValue;
+ iterIdx = (iterIdx + 1) & 3;
+ }
+
+ if (!mainThread)
+ return;
+
+ mainThread->previousTimeReduction = timeReduction;
+
+ // 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)));
}
namespace {
- // search<>() is the main search function for both PV and non-PV nodes
+// Main search function for both PV and non-PV nodes
+template<NodeType nodeType>
+Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
- template <NodeType nodeType>
- Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
-
- constexpr bool PvNode = nodeType != NonPV;
+ constexpr bool PvNode = nodeType != NonPV;
constexpr bool rootNode = nodeType == Root;
+ // Dive into quiescence search when the depth reaches zero
+ if (depth <= 0)
+ return qsearch < PvNode ? PV : NonPV > (pos, ss, alpha, beta);
+
// 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))
+ if (!rootNode && alpha < VALUE_DRAW && pos.has_game_cycle(ss->ply))
{
alpha = value_draw(pos.this_thread());
if (alpha >= beta)
return alpha;
}
- // Dive into quiescence search when the depth reaches zero
- if (depth <= 0)
- return qsearch<PvNode ? PV : NonPV>(pos, ss, alpha, beta);
-
assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
assert(PvNode || (alpha == beta - 1));
assert(0 < depth && depth < MAX_PLY);
assert(!(PvNode && cutNode));
- Move pv[MAX_PLY+1], capturesSearched[32], quietsSearched[64];
+ Move pv[MAX_PLY + 1], capturesSearched[32], quietsSearched[32];
StateInfo st;
ASSERT_ALIGNED(&st, Eval::NNUE::CacheLineSize);
TTEntry* tte;
- Key posKey;
- Move ttMove, move, excludedMove, bestMove;
- Depth extension, newDepth;
- Value bestValue, value, ttValue, eval, maxValue, probCutBeta;
- bool givesCheck, improving, priorCapture, singularQuietLMR;
- bool capture, moveCountPruning, ttCapture;
- Piece movedPiece;
- int moveCount, captureCount, quietCount, improvement;
+ Key posKey;
+ Move ttMove, move, excludedMove, bestMove;
+ Depth extension, newDepth;
+ Value bestValue, value, ttValue, eval, maxValue, probCutBeta;
+ bool givesCheck, improving, priorCapture, singularQuietLMR;
+ bool capture, moveCountPruning, ttCapture;
+ Piece movedPiece;
+ 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 = captureCount = quietCount = ss->moveCount = 0;
- bestValue = -VALUE_INFINITE;
- maxValue = VALUE_INFINITE;
+ moveCount = captureCount = quietCount = ss->moveCount = 0;
+ bestValue = -VALUE_INFINITE;
+ maxValue = VALUE_INFINITE;
// Check for the available remaining time
if (thisThread == Threads.main())
if (!rootNode)
{
// Step 2. Check for aborted search and immediate draw
- if ( Threads.stop.load(std::memory_order_relaxed)
- || pos.is_draw(ss->ply)
+ if (Threads.stop.load(std::memory_order_relaxed) || pos.is_draw(ss->ply)
|| ss->ply >= MAX_PLY)
return (ss->ply >= MAX_PLY && !ss->inCheck) ? evaluate(pos)
: value_draw(pos.this_thread());
// Step 3. Mate distance pruning. Even if we mate at the next move our score
- // would be at best mate_in(ss->ply+1), but if alpha is already bigger because
+ // 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 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);
+ beta = std::min(mate_in(ss->ply + 1), beta);
if (alpha >= beta)
return alpha;
}
assert(0 <= ss->ply && ss->ply < MAX_PLY);
- (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;
- Square prevSq = is_ok((ss-1)->currentMove) ? to_sq((ss-1)->currentMove) : SQ_NONE;
- ss->statScore = 0;
+ (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;
+ Square prevSq = is_ok((ss - 1)->currentMove) ? to_sq((ss - 1)->currentMove) : SQ_NONE;
+ ss->statScore = 0;
// Step 4. Transposition table lookup.
excludedMove = ss->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;
+ 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_stage(ttMove);
// At this point, if excluded, skip straight to step 6, static eval. However,
ss->ttPv = PvNode || (ss->ttHit && tte->is_pv());
// At non-PV nodes we check for an early TT cutoff
- if ( !PvNode
- && !excludedMove
- && tte->depth() > depth - (tte->bound() == BOUND_EXACT)
- && ttValue != VALUE_NONE // Possible in case of TT access race or if !ttHit
+ if (!PvNode && !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 (~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 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));
+ // 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_malus(depth + 1));
}
// Penalty for a quiet ttMove that fails low (~1 Elo)
else if (!ttCapture)
{
- int penalty = -stat_bonus(depth);
+ int penalty = -stat_malus(depth);
thisThread->mainHistory[us][from_to(ttMove)] << penalty;
update_continuation_histories(ss, pos.moved_piece(ttMove), to_sq(ttMove), penalty);
}
{
int piecesCount = pos.count<ALL_PIECES>();
- if ( piecesCount <= TB::Cardinality
- && (piecesCount < TB::Cardinality || depth >= TB::ProbeDepth)
- && pos.rule50_count() == 0
+ if (piecesCount <= TB::Cardinality
+ && (piecesCount < TB::Cardinality || depth >= TB::ProbeDepth) && pos.rule50_count() == 0
&& !pos.can_castle(ANY_CASTLING))
{
TB::ProbeState err;
- TB::WDLScore wdl = Tablebases::probe_wdl(pos, &err);
+ TB::WDLScore wdl = Tablebases::probe_wdl(pos, &err);
// Force check of time on the next occasion
if (thisThread == Threads.main())
int drawScore = TB::UseRule50 ? 1 : 0;
- // use the range VALUE_MATE_IN_MAX_PLY to VALUE_TB_WIN_IN_MAX_PLY to score
- value = wdl < -drawScore ? VALUE_MATED_IN_MAX_PLY + ss->ply + 1
- : wdl > drawScore ? VALUE_MATE_IN_MAX_PLY - ss->ply - 1
- : VALUE_DRAW + 2 * wdl * drawScore;
+ Value tbValue = VALUE_TB - ss->ply;
+
+ // use the range VALUE_TB to VALUE_TB_WIN_IN_MAX_PLY to score
+ value = wdl < -drawScore ? -tbValue
+ : wdl > drawScore ? tbValue
+ : VALUE_DRAW + 2 * wdl * drawScore;
- Bound b = wdl < -drawScore ? BOUND_UPPER
- : wdl > drawScore ? BOUND_LOWER : BOUND_EXACT;
+ Bound b = wdl < -drawScore ? BOUND_UPPER
+ : wdl > drawScore ? BOUND_LOWER
+ : BOUND_EXACT;
- if ( b == BOUND_EXACT
- || (b == BOUND_LOWER ? value >= beta : value <= alpha))
+ if (b == BOUND_EXACT || (b == BOUND_LOWER ? value >= beta : value <= alpha))
{
tte->save(posKey, value_to_tt(value, ss->ply), ss->ttPv, b,
- std::min(MAX_PLY - 1, depth + 6),
- MOVE_NONE, VALUE_NONE);
+ std::min(MAX_PLY - 1, depth + 6), MOVE_NONE, VALUE_NONE);
return value;
}
{
// Skip early pruning when in check
ss->staticEval = eval = VALUE_NONE;
- improving = false;
- improvement = 0;
+ improving = false;
goto moves_loop;
}
else if (excludedMove)
{
- // Providing the hint that this node's accumulator will be used often brings significant Elo gain (13 Elo)
+ // 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;
}
Eval::NNUE::hint_common_parent_position(pos);
// ttValue can be used as a better position evaluation (~7 Elo)
- if ( ttValue != VALUE_NONE
- && (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER)))
+ if (ttValue != VALUE_NONE && (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER)))
eval = ttValue;
}
else
}
// Use static evaluation difference to improve quiet move ordering (~4 Elo)
- if (is_ok((ss-1)->currentMove) && !(ss-1)->inCheck && !priorCapture)
+ if (is_ok((ss - 1)->currentMove) && !(ss - 1)->inCheck && !priorCapture)
{
- int bonus = std::clamp(-18 * int((ss-1)->staticEval + ss->staticEval), -1817, 1817);
- thisThread->mainHistory[~us][from_to((ss-1)->currentMove)] << bonus;
+ int bonus = std::clamp(-13 * int((ss - 1)->staticEval + ss->staticEval), -1652, 1546);
+ thisThread->mainHistory[~us][from_to((ss - 1)->currentMove)] << bonus;
+ if (type_of(pos.piece_on(prevSq)) != PAWN && type_of((ss - 1)->currentMove) != PROMOTION)
+ thisThread->pawnHistory[pawn_structure(pos)][pos.piece_on(prevSq)][prevSq] << bonus / 4;
}
- // 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).
+ // 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)
+ // Adjust razor margin according to cutoffCnt. (~1 Elo)
+ if (eval < alpha - 472 - (284 - 165 * ((ss + 1)->cutoffCnt > 3)) * depth * depth)
{
value = qsearch<NonPV>(pos, ss, alpha - 1, alpha);
if (value < alpha)
return value;
}
- // Step 8. Futility pruning: child node (~40 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) - (ss-1)->statScore / 306 >= beta
- && eval >= beta
- && eval < 24923) // larger than VALUE_KNOWN_WIN, but smaller than TB wins
- return eval;
+ if (!ss->ttPv && depth < 9
+ && eval - futility_margin(depth, cutNode && !ss->ttHit, improving)
+ - (ss - 1)->statScore / 337
+ >= beta
+ && eval >= beta && eval < 29008 // smaller than TB wins
+ && (!ttMove || ttCapture))
+ return (eval + beta) / 2;
// Step 9. Null move search with verification search (~35 Elo)
- if ( !PvNode
- && (ss-1)->currentMove != MOVE_NULL
- && (ss-1)->statScore < 17329
- && eval >= beta
- && eval >= ss->staticEval
- && ss->staticEval >= beta - 21 * depth + 258
- && !excludedMove
- && pos.non_pawn_material(us)
- && ss->ply >= thisThread->nmpMinPly
- && beta > VALUE_TB_LOSS_IN_MAX_PLY)
+ if (!PvNode && (ss - 1)->currentMove != MOVE_NULL && (ss - 1)->statScore < 17496 && eval >= beta
+ && eval >= ss->staticEval && ss->staticEval >= beta - 23 * depth + 304 && !excludedMove
+ && pos.non_pawn_material(us) && ss->ply >= thisThread->nmpMinPly
+ && beta > VALUE_TB_LOSS_IN_MAX_PLY)
{
assert(eval - beta >= 0);
// Null move dynamic reduction based on depth and eval
- Depth R = std::min(int(eval - beta) / 173, 6) + depth / 3 + 4;
+ Depth R = std::min(int(eval - beta) / 144, 6) + depth / 3 + 4;
- ss->currentMove = MOVE_NULL;
+ ss->currentMove = MOVE_NULL;
ss->continuationHistory = &thisThread->continuationHistory[0][0][NO_PIECE][0];
pos.do_null_move(st);
- Value nullValue = -search<NonPV>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode);
+ Value nullValue = -search<NonPV>(pos, ss + 1, -beta, -beta + 1, depth - R, !cutNode);
pos.undo_null_move();
- if (nullValue >= beta)
+ // Do not return unproven mate or TB scores
+ if (nullValue >= beta && nullValue < VALUE_TB_WIN_IN_MAX_PLY)
{
- // Do not return unproven mate or TB scores
- nullValue = std::min(nullValue, VALUE_TB_WIN_IN_MAX_PLY-1);
-
- if (thisThread->nmpMinPly || depth < 14)
+ if (thisThread->nmpMinPly || depth < 15)
return nullValue;
- assert(!thisThread->nmpMinPly); // Recursive verification is not allowed
+ assert(!thisThread->nmpMinPly); // Recursive verification is not allowed
// Do verification search at high depths, with null move pruning disabled
// until ply exceeds nmpMinPly.
- thisThread->nmpMinPly = ss->ply + 3 * (depth-R) / 4;
+ thisThread->nmpMinPly = ss->ply + 3 * (depth - R) / 4;
- Value v = search<NonPV>(pos, ss, beta-1, beta, depth-R, false);
+ Value v = search<NonPV>(pos, ss, beta - 1, beta, depth - R, false);
thisThread->nmpMinPly = 0;
}
}
- // 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)
+ // Step 10. Internal iterative reductions (~9 Elo)
+ // For PV nodes without a ttMove, we decrease depth by 2,
+ // or by 4 if the current position is present in the TT and
+ // the stored depth is greater than or equal to the current depth.
+ // Use qsearch if depth <= 0.
+ if (PvNode && !ttMove)
depth -= 2 + 2 * (ss->ttHit && tte->depth() >= depth);
if (depth <= 0)
return qsearch<PV>(pos, ss, alpha, beta);
- if ( cutNode
- && depth >= 8
- && !ttMove)
+ // For cutNodes without a ttMove, we decrease depth by 2 if depth is high enough.
+ if (cutNode && depth >= 8 && !ttMove)
depth -= 2;
- probCutBeta = beta + 168 - 61 * improving;
+ probCutBeta = beta + 163 - 67 * improving;
// Step 11. ProbCut (~10 Elo)
// If we have a good enough capture (or queen promotion) and a reduced search returns a value
// much above beta, we can (almost) safely prune the previous move.
- if ( !PvNode
- && depth > 3
- && abs(beta) < VALUE_TB_WIN_IN_MAX_PLY
- // 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
- && !( tte->depth() >= depth - 3
- && ttValue != VALUE_NONE
- && ttValue < probCutBeta))
+ if (
+ !PvNode && depth > 3
+ && std::abs(beta) < VALUE_TB_WIN_IN_MAX_PLY
+ // 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
+ && !(tte->depth() >= depth - 3 && ttValue != VALUE_NONE && ttValue < probCutBeta))
{
assert(probCutBeta < VALUE_INFINITE);
{
assert(pos.capture_stage(move));
+ // Prefetch the TT entry for the resulting position
+ prefetch(TT.first_entry(pos.key_after(move)));
+
ss->currentMove = move;
- ss->continuationHistory = &thisThread->continuationHistory[ss->inCheck]
- [true]
- [pos.moved_piece(move)]
- [to_sq(move)];
+ ss->continuationHistory =
+ &thisThread
+ ->continuationHistory[ss->inCheck][true][pos.moved_piece(move)][to_sq(move)];
pos.do_move(move, st);
// Perform a preliminary qsearch to verify that the move holds
- value = -qsearch<NonPV>(pos, ss+1, -probCutBeta, -probCutBeta+1);
+ value = -qsearch<NonPV>(pos, ss + 1, -probCutBeta, -probCutBeta + 1);
// If the qsearch held, perform the regular search
if (value >= probCutBeta)
- value = -search<NonPV>(pos, ss+1, -probCutBeta, -probCutBeta+1, depth - 4, !cutNode);
+ value = -search<NonPV>(pos, ss + 1, -probCutBeta, -probCutBeta + 1, depth - 4,
+ !cutNode);
pos.undo_move(move);
if (value >= probCutBeta)
{
// 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;
+ tte->save(posKey, value_to_tt(value, ss->ply), ss->ttPv, BOUND_LOWER, depth - 3,
+ move, ss->staticEval);
+ return value - (probCutBeta - beta);
}
}
Eval::NNUE::hint_common_parent_position(pos);
}
-moves_loop: // When in check, search starts here
+moves_loop: // When in check, search starts here
// Step 12. A small Probcut idea, when we are in check (~4 Elo)
- probCutBeta = beta + 413;
- if ( ss->inCheck
- && !PvNode
- && ttCapture
- && (tte->bound() & BOUND_LOWER)
- && tte->depth() >= depth - 4
- && ttValue >= probCutBeta
- && abs(ttValue) <= VALUE_KNOWN_WIN
- && abs(beta) <= VALUE_KNOWN_WIN)
+ probCutBeta = beta + 425;
+ if (ss->inCheck && !PvNode && ttCapture && (tte->bound() & BOUND_LOWER)
+ && tte->depth() >= depth - 4 && ttValue >= probCutBeta
+ && std::abs(ttValue) < VALUE_TB_WIN_IN_MAX_PLY && std::abs(beta) < VALUE_TB_WIN_IN_MAX_PLY)
return probCutBeta;
- const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,
- nullptr , (ss-4)->continuationHistory,
- nullptr , (ss-6)->continuationHistory };
+ const PieceToHistory* contHist[] = {(ss - 1)->continuationHistory,
+ (ss - 2)->continuationHistory,
+ (ss - 3)->continuationHistory,
+ (ss - 4)->continuationHistory,
+ nullptr,
+ (ss - 6)->continuationHistory};
- Move countermove = prevSq != SQ_NONE ? thisThread->counterMoves[pos.piece_on(prevSq)][prevSq] : MOVE_NONE;
+ Move countermove =
+ prevSq != SQ_NONE ? thisThread->counterMoves[pos.piece_on(prevSq)][prevSq] : MOVE_NONE;
- MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
- &captureHistory,
- contHist,
- countermove,
- ss->killers);
+ MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory, &captureHistory, contHist,
+ &thisThread->pawnHistory, countermove, ss->killers);
- value = bestValue;
+ value = bestValue;
moveCountPruning = singularQuietLMR = false;
// Indicate PvNodes that will probably fail low if the node was searched
- // 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;
+ // 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 13. Loop through all pseudo-legal moves until no moves remain
// or a beta cutoff occurs.
while ((move = mp.next_move(moveCountPruning)) != MOVE_NONE)
{
- assert(is_ok(move));
-
- if (move == excludedMove)
- 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 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))
- continue;
-
- // Check for legality
- if (!rootNode && !pos.legal(move))
- continue;
-
- ss->moveCount = ++moveCount;
-
- if (rootNode && thisThread == Threads.main() && Time.elapsed() > 3000)
- sync_cout << "info depth " << depth
- << " currmove " << UCI::move(move, pos.is_chess960())
- << " currmovenumber " << moveCount + thisThread->pvIdx << sync_endl;
- if (PvNode)
- (ss+1)->pv = nullptr;
-
- extension = 0;
- capture = pos.capture_stage(move);
- movedPiece = pos.moved_piece(move);
- givesCheck = pos.gives_check(move);
-
- // Calculate new depth for this move
- newDepth = depth - 1;
-
- Value delta = beta - alpha;
-
- 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 (~8 Elo)
- moveCountPruning = moveCount >= futility_move_count(improving, depth);
-
- // Reduced depth of the next LMR search
- int lmrDepth = newDepth - r;
-
- if ( capture
- || givesCheck)
- {
- // Futility pruning for captures (~2 Elo)
- if ( !givesCheck
- && lmrDepth < 7
- && !ss->inCheck
- && ss->staticEval + 197 + 248 * lmrDepth + PieceValue[EG][pos.piece_on(to_sq(move))]
- + captureHistory[movedPiece][to_sq(move)][type_of(pos.piece_on(to_sq(move)))] / 7 < alpha)
- continue;
-
- Bitboard occupied;
- // SEE based pruning (~11 Elo)
- if (!pos.see_ge(move, occupied, Value(-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 that were already threatened/hanging before SEE exchanges
- if (attacks && (sq != pos.square<KING>(~us) && (pos.attackers_to(sq, pos.pieces()) & pos.pieces(us))))
- attacks = 0;
- }
- if (!attacks)
+ assert(is_ok(move));
+
+ if (move == excludedMove)
+ continue;
+
+ // Check for legality
+ if (!pos.legal(move))
+ continue;
+
+ // At root obey the "searchmoves" option and skip moves not listed in Root
+ // Move List. 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))
+ continue;
+
+ ss->moveCount = ++moveCount;
+
+ if (rootNode && thisThread == Threads.main() && Time.elapsed() > 3000)
+ sync_cout << "info depth " << depth << " currmove "
+ << UCI::move(move, pos.is_chess960()) << " currmovenumber "
+ << moveCount + thisThread->pvIdx << sync_endl;
+ if (PvNode)
+ (ss + 1)->pv = nullptr;
+
+ extension = 0;
+ capture = pos.capture_stage(move);
+ movedPiece = pos.moved_piece(move);
+ givesCheck = pos.gives_check(move);
+
+ // Calculate new depth for this move
+ newDepth = depth - 1;
+
+ Value delta = beta - alpha;
+
+ 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 (~8 Elo)
+ if (!moveCountPruning)
+ moveCountPruning = moveCount >= futility_move_count(improving, depth);
+
+ // Reduced depth of the next LMR search
+ int lmrDepth = newDepth - r;
+
+ if (capture || givesCheck)
+ {
+ // Futility pruning for captures (~2 Elo)
+ if (!givesCheck && lmrDepth < 7 && !ss->inCheck)
+ {
+ Piece capturedPiece = pos.piece_on(to_sq(move));
+ int futilityEval =
+ ss->staticEval + 238 + 305 * lmrDepth + PieceValue[capturedPiece]
+ + captureHistory[movedPiece][to_sq(move)][type_of(capturedPiece)] / 7;
+ if (futilityEval < alpha)
+ continue;
+ }
+
+ // SEE based pruning for captures and checks (~11 Elo)
+ if (!pos.see_ge(move, Value(-187) * depth))
continue;
- }
- }
- else
- {
- int history = (*contHist[0])[movedPiece][to_sq(move)]
+ }
+ else
+ {
+ int history = (*contHist[0])[movedPiece][to_sq(move)]
+ (*contHist[1])[movedPiece][to_sq(move)]
- + (*contHist[3])[movedPiece][to_sq(move)];
-
- // Continuation history based pruning (~2 Elo)
- if ( lmrDepth < 6
- && history < -3832 * depth)
- continue;
-
- history += 2 * thisThread->mainHistory[us][from_to(move)];
-
- lmrDepth += history / 7011;
- lmrDepth = std::max(lmrDepth, -2);
-
- // Futility pruning: parent node (~13 Elo)
- if ( !ss->inCheck
- && lmrDepth < 12
- && ss->staticEval + 112 + 138 * lmrDepth <= alpha)
- continue;
-
- lmrDepth = std::max(lmrDepth, 0);
-
- // Prune moves with negative SEE (~4 Elo)
- if (!pos.see_ge(move, Value(-27 * lmrDepth * lmrDepth - 16 * lmrDepth)))
- continue;
- }
- }
-
- // Step 15. Extensions (~100 Elo)
- // We take care to not overdo to avoid search getting stuck.
- if (ss->ply < thisThread->rootDepth * 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<NonPV>(pos, ss, singularBeta - 1, singularBeta, singularDepth, cutNode);
- ss->excludedMove = MOVE_NONE;
-
- 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;
- }
- }
-
- // 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 > 8 && depth < 17 ? -3 : -1;
-
- // If the eval of ttMove is less than value, we reduce it (negative extension) (~1 Elo)
- else if (ttValue <= value)
- extension = -1;
-
- // 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)
- else if ( givesCheck
- && depth > 9)
- 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;
- ss->doubleExtensions = (ss-1)->doubleExtensions + (extension == 2);
-
- // Speculative prefetch as early as possible
- prefetch(TT.first_entry(pos.key_after(move)));
-
- // Update the current move (this must be done after singular extension search)
- ss->currentMove = move;
- ss->continuationHistory = &thisThread->continuationHistory[ss->inCheck]
- [capture]
- [movedPiece]
- [to_sq(move)];
-
- // Step 16. Make the move
- pos.do_move(move, st, givesCheck);
-
- // 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 ? 3 : 2;
-
- // Decrease reduction if opponent's move count is high (~1 Elo)
- if ((ss-1)->moveCount > 8)
- r--;
-
- // Increase reduction for cut nodes (~3 Elo)
- if (cutNode)
- r += 2;
-
- // Increase reduction if ttMove is a capture (~3 Elo)
- if (ttCapture)
- r++;
-
- // Decrease reduction for PvNodes based on depth (~2 Elo)
- if (PvNode)
- r -= 1 + (depth < 6);
-
- // Decrease reduction if ttMove has been singularly extended (~1 Elo)
- if (singularQuietLMR)
- r--;
-
- // Increase reduction if next ply has a lot of fail high (~5 Elo)
- if ((ss+1)->cutoffCnt > 3)
- r++;
-
- else if (move == ttMove)
- r--;
-
- ss->statScore = 2 * thisThread->mainHistory[us][from_to(move)]
- + (*contHist[0])[movedPiece][to_sq(move)]
- + (*contHist[1])[movedPiece][to_sq(move)]
- + (*contHist[3])[movedPiece][to_sq(move)]
- - 4006;
-
- // Decrease/increase reduction for moves with a good/bad history (~25 Elo)
- r -= ss->statScore / (11124 + 4740 * (depth > 5 && depth < 22));
-
- // Step 17. Late moves reduction / extension (LMR, ~117 Elo)
- // We use various heuristics for the sons of a node after the first son has
- // been searched. In general, we would like to reduce them, but there are many
- // 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)))
- {
- // 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);
-
- value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true);
-
- // 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 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<NonPV>(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);
- }
- }
-
- // Step 18. Full-depth search when LMR is skipped. If expected reduction is high, reduce its depth by 1.
- else if (!PvNode || moveCount > 1)
- {
- // Increase reduction for cut nodes and not ttMove (~1 Elo)
- if (!ttMove && cutNode)
- r += 2;
-
- value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth - (r > 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))))
- {
- (ss+1)->pv = pv;
- (ss+1)->pv[0] = MOVE_NONE;
-
- value = -search<PV>(pos, ss+1, -beta, -alpha, newDepth, false);
- }
-
- // Step 19. Undo move
- pos.undo_move(move);
-
- assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
-
- // 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.
- if (Threads.stop.load(std::memory_order_relaxed))
- return VALUE_ZERO;
-
- if (rootNode)
- {
- RootMove& rm = *std::find(thisThread->rootMoves.begin(),
- thisThread->rootMoves.end(), move);
-
- rm.averageScore = rm.averageScore != -VALUE_INFINITE ? (2 * value + rm.averageScore) / 3 : value;
-
- // PV move or new best move?
- if (moveCount == 1 || value > alpha)
- {
- 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);
-
- for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
- 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. In MultiPV mode,
- // we must take care to only do this for the first PV line.
- if ( moveCount > 1
- && !thisThread->pvIdx)
- ++thisThread->bestMoveChanges;
- }
- else
- // All other moves but the PV, are set to the lowest value: this
- // is not a problem when sorting because the sort is stable and the
- // move position in the list is preserved - just the PV is pushed up.
- rm.score = -VALUE_INFINITE;
- }
-
- if (value > bestValue)
- {
- bestValue = value;
-
- if (value > alpha)
- {
- bestMove = move;
-
- if (PvNode && !rootNode) // Update pv even in fail-high case
- update_pv(ss->pv, move, (ss+1)->pv);
-
- if (value >= beta)
- {
- ss->cutoffCnt += 1 + !ttMove;
- assert(value >= beta); // Fail high
- break;
- }
- else
- {
- // 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 (move != bestMove)
- {
- if (capture && captureCount < 32)
- capturesSearched[captureCount++] = move;
-
- else if (!capture && quietCount < 64)
- quietsSearched[quietCount++] = move;
- }
- }
+ + (*contHist[3])[movedPiece][to_sq(move)]
+ + thisThread->pawnHistory[pawn_structure(pos)][movedPiece][to_sq(move)];
+
+ // Continuation history based pruning (~2 Elo)
+ if (lmrDepth < 6 && history < -3752 * depth)
+ continue;
+
+ history += 2 * thisThread->mainHistory[us][from_to(move)];
+
+ lmrDepth += history / 7838;
+ lmrDepth = std::max(lmrDepth, -1);
+
+ // Futility pruning: parent node (~13 Elo)
+ if (!ss->inCheck && lmrDepth < 14
+ && ss->staticEval + (bestValue < ss->staticEval - 57 ? 124 : 71)
+ + 118 * lmrDepth
+ <= alpha)
+ continue;
+
+ lmrDepth = std::max(lmrDepth, 0);
+
+ // Prune moves with negative SEE (~4 Elo)
+ if (!pos.see_ge(move, Value(-26 * lmrDepth * lmrDepth)))
+ continue;
+ }
+ }
+
+ // Step 15. Extensions (~100 Elo)
+ // We take care to not overdo to avoid search getting stuck.
+ if (ss->ply < thisThread->rootDepth * 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 the position excluding the ttMove and if the result
+ // is lower than ttValue minus a margin, then we will extend the ttMove.
+
+ // Note: the 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 these types of time controls.
+ // Recursive singular search is avoided.
+ if (!rootNode && move == ttMove && !excludedMove
+ && depth >= 4 - (thisThread->completedDepth > 27) + 2 * (PvNode && tte->is_pv())
+ && std::abs(ttValue) < VALUE_TB_WIN_IN_MAX_PLY && (tte->bound() & BOUND_LOWER)
+ && tte->depth() >= depth - 3)
+ {
+ Value singularBeta = ttValue - (66 + 58 * (ss->ttPv && !PvNode)) * depth / 64;
+ Depth singularDepth = newDepth / 2;
+
+ ss->excludedMove = move;
+ value =
+ search<NonPV>(pos, ss, singularBeta - 1, singularBeta, singularDepth, cutNode);
+ ss->excludedMove = MOVE_NONE;
+
+ if (value < singularBeta)
+ {
+ extension = 1;
+ singularQuietLMR = !ttCapture;
+
+ // Avoid search explosion by limiting the number of double extensions
+ if (!PvNode && value < singularBeta - 17 && ss->doubleExtensions <= 11)
+ {
+ extension = 2;
+ depth += depth < 15;
+ }
+ }
+
+ // Multi-cut pruning
+ // Our ttMove is assumed to fail high based on the bound of the TT entry,
+ // and if after excluding the ttMove with a reduced search we fail high over the original beta,
+ // we assume this expected cut-node is not singular (multiple moves fail high),
+ // and we can prune the whole subtree by returning a softbound.
+ else if (singularBeta >= beta)
+ return singularBeta;
+
+ // Negative extensions
+ // If other moves failed high over (ttValue - margin) without the ttMove on a reduced search,
+ // but we cannot do multi-cut because (ttValue - margin) is lower than the original beta,
+ // we do not know if the ttMove is singular or can do a multi-cut,
+ // so we reduce the ttMove in favor of other moves based on some conditions:
+
+ // If the ttMove is assumed to fail high over current beta (~7 Elo)
+ else if (ttValue >= beta)
+ extension = -2 - !PvNode;
+
+ // If we are on a cutNode but the ttMove is not assumed to fail high over current beta (~1 Elo)
+ else if (cutNode)
+ extension = depth < 19 ? -2 : -1;
+
+ // If the ttMove is assumed to fail low over the value of the reduced search (~1 Elo)
+ else if (ttValue <= value)
+ extension = -1;
+ }
+
+ // Check extensions (~1 Elo)
+ else if (givesCheck && depth > 10)
+ extension = 1;
+
+ // Quiet ttMove extensions (~1 Elo)
+ else if (PvNode && move == ttMove && move == ss->killers[0]
+ && (*contHist[0])[movedPiece][to_sq(move)] >= 4325)
+ extension = 1;
+
+ // Recapture extensions (~1 Elo)
+ else if (PvNode && move == ttMove && to_sq(move) == prevSq
+ && captureHistory[movedPiece][to_sq(move)][type_of(pos.piece_on(to_sq(move)))]
+ > 4146)
+ extension = 1;
+ }
+
+ // Add extension to new depth
+ newDepth += extension;
+ ss->doubleExtensions = (ss - 1)->doubleExtensions + (extension == 2);
+
+ // Speculative prefetch as early as possible
+ prefetch(TT.first_entry(pos.key_after(move)));
+
+ // Update the current move (this must be done after singular extension search)
+ ss->currentMove = move;
+ ss->continuationHistory =
+ &thisThread->continuationHistory[ss->inCheck][capture][movedPiece][to_sq(move)];
+
+ // Step 16. Make the move
+ pos.do_move(move, st, givesCheck);
+
+ // Decrease reduction if position is or has been on the PV (~4 Elo)
+ if (ss->ttPv && !likelyFailLow)
+ r -= cutNode && tte->depth() >= depth ? 3 : 2;
+
+ // Decrease reduction if opponent's move count is high (~1 Elo)
+ if ((ss - 1)->moveCount > 7)
+ r--;
+
+ // Increase reduction for cut nodes (~3 Elo)
+ if (cutNode)
+ r += 2;
+
+ // Increase reduction if ttMove is a capture (~3 Elo)
+ if (ttCapture)
+ r++;
+
+ // Decrease reduction for PvNodes (~2 Elo)
+ if (PvNode)
+ r--;
+
+ // Decrease reduction if a quiet ttMove has been singularly extended (~1 Elo)
+ if (singularQuietLMR)
+ r--;
+
+ // Increase reduction on repetition (~1 Elo)
+ if (move == (ss - 4)->currentMove && pos.has_repeated())
+ r += 2;
+
+ // Increase reduction if next ply has a lot of fail high (~5 Elo)
+ if ((ss + 1)->cutoffCnt > 3)
+ r++;
+
+ // Set reduction to 0 for first picked move (ttMove) (~2 Elo)
+ // Nullifies all previous reduction adjustments to ttMove and leaves only history to do them
+ else if (move == ttMove)
+ r = 0;
+
+ 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)] - 3817;
+
+ // Decrease/increase reduction for moves with a good/bad history (~25 Elo)
+ r -= ss->statScore / 14767;
+
+ // 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 + rootNode
+ && (!ss->ttPv || !capture || (cutNode && (ss - 1)->moveCount > 1)))
+ {
+ // 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.
+ // To prevent problems when the max value is less than the min value,
+ // std::clamp has been replaced by a more robust implementation.
+ Depth d = std::max(1, std::min(newDepth - r, newDepth + 1));
+
+ value = -search<NonPV>(pos, ss + 1, -(alpha + 1), -alpha, d, true);
+
+ // 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 the result
+ // was good enough search deeper, if it was bad enough search shallower.
+ const bool doDeeperSearch = value > (bestValue + 53 + 2 * newDepth); // (~1 Elo)
+ const bool doShallowerSearch = value < bestValue + newDepth; // (~2 Elo)
+
+ newDepth += doDeeperSearch - doShallowerSearch;
+
+ if (newDepth > d)
+ value = -search<NonPV>(pos, ss + 1, -(alpha + 1), -alpha, newDepth, !cutNode);
+
+ int bonus = value <= alpha ? -stat_malus(newDepth)
+ : value >= beta ? stat_bonus(newDepth)
+ : 0;
+
+ update_continuation_histories(ss, movedPiece, to_sq(move), bonus);
+ }
+ }
+
+ // Step 18. Full-depth search when LMR is skipped
+ else if (!PvNode || moveCount > 1)
+ {
+ // Increase reduction if ttMove is not present (~1 Elo)
+ if (!ttMove)
+ r += 2;
+
+ // Note that if expected reduction is high, we reduce search depth by 1 here
+ 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,
+ // 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<PV>(pos, ss + 1, -beta, -alpha, newDepth, false);
+ }
+
+ // Step 19. Undo move
+ pos.undo_move(move);
+
+ assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
+
+ // 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.
+ if (Threads.stop.load(std::memory_order_relaxed))
+ return VALUE_ZERO;
+
+ if (rootNode)
+ {
+ RootMove& rm =
+ *std::find(thisThread->rootMoves.begin(), thisThread->rootMoves.end(), move);
+
+ rm.averageScore =
+ rm.averageScore != -VALUE_INFINITE ? (2 * value + rm.averageScore) / 3 : value;
+
+ // PV move or new best move?
+ if (moveCount == 1 || value > alpha)
+ {
+ 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);
- // 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
- // searched our subtree, and we can anyhow save the result in TT.
- /*
- if (Threads.stop)
- return VALUE_DRAW;
- */
+ for (Move* m = (ss + 1)->pv; *m != MOVE_NONE; ++m)
+ 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. In MultiPV mode,
+ // we must take care to only do this for the first PV line.
+ if (moveCount > 1 && !thisThread->pvIdx)
+ ++thisThread->bestMoveChanges;
+ }
+ else
+ // All other moves but the PV, are set to the lowest value: this
+ // is not a problem when sorting because the sort is stable and the
+ // move position in the list is preserved - just the PV is pushed up.
+ rm.score = -VALUE_INFINITE;
+ }
+
+ if (value > bestValue)
+ {
+ bestValue = value;
+
+ if (value > alpha)
+ {
+ bestMove = move;
+
+ if (PvNode && !rootNode) // Update pv even in fail-high case
+ update_pv(ss->pv, move, (ss + 1)->pv);
+
+ if (value >= beta)
+ {
+ ss->cutoffCnt += 1 + !ttMove;
+ assert(value >= beta); // Fail high
+ break;
+ }
+ else
+ {
+ // Reduce other moves if we have found at least one score improvement (~2 Elo)
+ if (depth > 2 && depth < 12 && beta < 13782 && value > -11541)
+ 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 (move != bestMove && moveCount <= 32)
+ {
+ if (capture)
+ capturesSearched[captureCount++] = move;
+
+ else
+ quietsSearched[quietCount++] = move;
+ }
+ }
// Step 21. Check for mate and stalemate
// All legal moves have been searched and if there are no legal moves, it
assert(moveCount || !ss->inCheck || excludedMove || !MoveList<LEGAL>(pos).size());
if (!moveCount)
- bestValue = excludedMove ? alpha :
- ss->inCheck ? mated_in(ss->ply)
- : VALUE_DRAW;
+ bestValue = excludedMove ? alpha : ss->inCheck ? mated_in(ss->ply) : VALUE_DRAW;
// 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);
+ 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);
- update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, stat_bonus(depth) * bonus);
+ int bonus = (depth > 6) + (PvNode || cutNode) + ((ss - 1)->statScore < -18782)
+ + ((ss - 1)->moveCount > 10);
+ 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)
// 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. (~7 Elo)
if (bestValue <= alpha)
- ss->ttPv = ss->ttPv || ((ss-1)->ttPv && depth > 3);
+ ss->ttPv = ss->ttPv || ((ss - 1)->ttPv && depth > 3);
// 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 :
- PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
+ bestValue >= beta ? BOUND_LOWER
+ : PvNode && bestMove ? BOUND_EXACT
+ : BOUND_UPPER,
depth, bestMove, ss->staticEval);
assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
return bestValue;
- }
+}
- // 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 <NodeType nodeType>
- Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
+// 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<NodeType nodeType>
+Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
static_assert(nodeType != Root);
constexpr bool PvNode = nodeType == PV;
assert(PvNode || (alpha == beta - 1));
assert(depth <= 0);
- Move pv[MAX_PLY+1];
+ // Check if we have an upcoming move that draws by repetition, or
+ // if the opponent had an alternative move earlier to this position.
+ if (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);
TTEntry* tte;
- Key posKey;
- Move ttMove, move, bestMove;
- Depth ttDepth;
- Value bestValue, value, ttValue, futilityValue, futilityBase;
- bool pvHit, givesCheck, capture;
- int moveCount;
+ Key posKey;
+ Move ttMove, move, bestMove;
+ Depth ttDepth;
+ Value bestValue, value, ttValue, futilityValue, futilityBase;
+ bool pvHit, givesCheck, capture;
+ int moveCount;
+ Color us = pos.side_to_move();
// Step 1. Initialize node
if (PvNode)
{
- (ss+1)->pv = pv;
- ss->pv[0] = MOVE_NONE;
+ (ss + 1)->pv = pv;
+ ss->pv[0] = MOVE_NONE;
}
Thread* thisThread = pos.this_thread();
- bestMove = MOVE_NONE;
- ss->inCheck = pos.checkers();
- moveCount = 0;
+ bestMove = MOVE_NONE;
+ ss->inCheck = pos.checkers();
+ moveCount = 0;
+
+ // Used to send selDepth info to GUI (selDepth counts from 1, ply from 0)
+ if (PvNode && thisThread->selDepth < ss->ply + 1)
+ thisThread->selDepth = ss->ply + 1;
// Step 2. Check for an immediate draw or maximum ply reached
- if ( pos.is_draw(ss->ply)
- || ss->ply >= MAX_PLY)
+ if (pos.is_draw(ss->ply) || ss->ply >= MAX_PLY)
return (ss->ply >= MAX_PLY && !ss->inCheck) ? evaluate(pos) : VALUE_DRAW;
assert(0 <= ss->ply && ss->ply < MAX_PLY);
- // Decide whether or not to include checks: this fixes also the type of
- // 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;
+ // Decide the replacement and cutoff priority of the qsearch TT entries
+ ttDepth = ss->inCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS : DEPTH_QS_NO_CHECKS;
// Step 3. Transposition table lookup
- posKey = pos.key();
- tte = TT.probe(posKey, ss->ttHit);
+ 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();
+ 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
- && tte->depth() >= ttDepth
- && ttValue != VALUE_NONE // Only in case of TT access race or if !ttHit
+ if (!PvNode && tte->depth() >= ttDepth
+ && ttValue != VALUE_NONE // Only in case of TT access race or if !ttHit
&& (tte->bound() & (ttValue >= beta ? BOUND_LOWER : BOUND_UPPER)))
return ttValue;
ss->staticEval = bestValue = evaluate(pos);
// ttValue can be used as a better position evaluation (~13 Elo)
- if ( ttValue != VALUE_NONE
+ 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;
+ // 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;
// 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);
+ tte->save(posKey, value_to_tt(bestValue, ss->ply), false, BOUND_LOWER, DEPTH_NONE,
+ MOVE_NONE, ss->staticEval);
return bestValue;
}
- if (PvNode && bestValue > alpha)
+ if (bestValue > alpha)
alpha = bestValue;
- futilityBase = bestValue + 200;
+ futilityBase = ss->staticEval + 182;
}
- const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,
- nullptr , (ss-4)->continuationHistory,
- nullptr , (ss-6)->continuationHistory };
+ const PieceToHistory* contHist[] = {(ss - 1)->continuationHistory,
+ (ss - 2)->continuationHistory};
// Initialize a MovePicker object for the current position, and prepare
// 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 = is_ok((ss-1)->currentMove) ? to_sq((ss-1)->currentMove) : SQ_NONE;
- MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
- &thisThread->captureHistory,
- contHist,
- prevSq);
+ Square prevSq = is_ok((ss - 1)->currentMove) ? to_sq((ss - 1)->currentMove) : SQ_NONE;
+ MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory, &thisThread->captureHistory,
+ contHist, &thisThread->pawnHistory);
int quietCheckEvasions = 0;
continue;
givesCheck = pos.gives_check(move);
- capture = pos.capture_stage(move);
+ capture = pos.capture_stage(move);
moveCount++;
- // Step 6. Pruning.
- if (bestValue > VALUE_TB_LOSS_IN_MAX_PLY)
+ // Step 6. Pruning
+ if (bestValue > VALUE_TB_LOSS_IN_MAX_PLY && pos.non_pawn_material(us))
{
// Futility pruning and moveCount pruning (~10 Elo)
- if ( !givesCheck
- && to_sq(move) != prevSq
- && futilityBase > -VALUE_KNOWN_WIN
- && type_of(move) != PROMOTION)
+ if (!givesCheck && to_sq(move) != prevSq && futilityBase > VALUE_TB_LOSS_IN_MAX_PLY
+ && 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 static eval + value of piece we are going to capture is much lower
+ // than alpha we can prune this move.
if (futilityValue <= alpha)
{
bestValue = std::max(bestValue, futilityValue);
continue;
}
+ // If static eval is much lower than alpha and move is not winning material
+ // we can prune this move.
if (futilityBase <= alpha && !pos.see_ge(move, VALUE_ZERO + 1))
{
bestValue = std::max(bestValue, futilityBase);
continue;
}
+
+ // If static exchange evaluation is much worse than what is needed to not
+ // fall below alpha we can prune this move.
+ if (futilityBase > alpha && !pos.see_ge(move, (alpha - futilityBase) * 4))
+ {
+ bestValue = alpha;
+ continue;
+ }
}
// We prune after the second quiet check evasion move, where being 'in check' is
break;
// Continuation history based pruning (~3 Elo)
- if ( !capture
- && (*contHist[0])[pos.moved_piece(move)][to_sq(move)] < 0
+ 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)))
+ if (!pos.see_ge(move, Value(-77)))
continue;
}
// Update the current move
ss->currentMove = move;
- ss->continuationHistory = &thisThread->continuationHistory[ss->inCheck]
- [capture]
- [pos.moved_piece(move)]
- [to_sq(move)];
+ ss->continuationHistory =
+ &thisThread
+ ->continuationHistory[ss->inCheck][capture][pos.moved_piece(move)][to_sq(move)];
quietCheckEvasions += !capture && ss->inCheck;
// Step 7. Make and search the move
pos.do_move(move, st, givesCheck);
- value = -qsearch<nodeType>(pos, ss+1, -beta, -alpha, depth - 1);
+ value = -qsearch<nodeType>(pos, ss + 1, -beta, -alpha, depth - 1);
pos.undo_move(move);
assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
{
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!
+ if (value < beta) // Update alpha here!
alpha = value;
else
- break; // Fail high
+ break; // Fail high
}
}
}
{
assert(!MoveList<LEGAL>(pos).size());
- return mated_in(ss->ply); // Plies to mate from the root
+ return mated_in(ss->ply); // Plies to mate from the root
}
+ if (abs(bestValue) < VALUE_TB_WIN_IN_MAX_PLY)
+ bestValue = bestValue >= beta ? (3 * bestValue + beta) / 4 : bestValue;
+
// Save gathered info in transposition table
tte->save(posKey, value_to_tt(bestValue, ss->ply), pvHit,
- bestValue >= beta ? BOUND_LOWER : BOUND_UPPER,
- ttDepth, bestMove, ss->staticEval);
+ bestValue >= beta ? BOUND_LOWER : BOUND_UPPER, ttDepth, bestMove, ss->staticEval);
assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
return bestValue;
- }
-
+}
- // value_to_tt() adjusts a mate or TB score from "plies to mate from the root" to
- // "plies to mate from the current position". Standard scores are unchanged.
- // The function is called before storing a value in the transposition table.
- Value value_to_tt(Value v, int ply) {
+// Adjusts a mate or TB score from "plies to mate from the root"
+// to "plies to mate from the current position". Standard scores are unchanged.
+// The function is called before storing a value in the transposition table.
+Value value_to_tt(Value v, int ply) {
assert(v != VALUE_NONE);
- return v >= VALUE_TB_WIN_IN_MAX_PLY ? v + ply
- : v <= VALUE_TB_LOSS_IN_MAX_PLY ? v - ply : v;
- }
+ return v >= VALUE_TB_WIN_IN_MAX_PLY ? v + ply : v <= VALUE_TB_LOSS_IN_MAX_PLY ? v - ply : v;
+}
- // value_from_tt() is the inverse of value_to_tt(): it adjusts a mate or TB score
- // from the transposition table (which refers to the plies to mate/be mated from
- // current position) to "plies to mate/be mated (TB win/loss) from the root". However,
- // for mate scores, to avoid potentially false mate scores related to the 50 moves rule
- // and the graph history interaction, we return an optimal TB score instead.
+// Inverse of value_to_tt(): it adjusts a mate or TB score
+// from the transposition table (which refers to the plies to mate/be mated from
+// current position) to "plies to mate/be mated (TB win/loss) from the root".
+// However, to avoid potentially false mate or TB scores related to the 50 moves rule
+// and the graph history interaction, we return highest non-TB score instead.
- Value value_from_tt(Value v, int ply, int r50c) {
+Value value_from_tt(Value v, int ply, int r50c) {
if (v == VALUE_NONE)
return VALUE_NONE;
- if (v >= VALUE_TB_WIN_IN_MAX_PLY) // TB win or better
+ // handle TB win or better
+ if (v >= VALUE_TB_WIN_IN_MAX_PLY)
{
- if (v >= VALUE_MATE_IN_MAX_PLY && VALUE_MATE - v > 99 - r50c)
- return VALUE_MATE_IN_MAX_PLY - 1; // do not return a potentially false mate score
+ // Downgrade a potentially false mate score
+ if (v >= VALUE_MATE_IN_MAX_PLY && VALUE_MATE - v > 100 - r50c)
+ return VALUE_TB_WIN_IN_MAX_PLY - 1;
+
+ // Downgrade a potentially false TB score.
+ if (VALUE_TB - v > 100 - r50c)
+ return VALUE_TB_WIN_IN_MAX_PLY - 1;
return v - ply;
}
- if (v <= VALUE_TB_LOSS_IN_MAX_PLY) // TB loss or worse
+ // handle TB loss or worse
+ if (v <= VALUE_TB_LOSS_IN_MAX_PLY)
{
- if (v <= VALUE_MATED_IN_MAX_PLY && VALUE_MATE + v > 99 - r50c)
- return VALUE_MATED_IN_MAX_PLY + 1; // do not return a potentially false mate score
+ // Downgrade a potentially false mate score.
+ if (v <= VALUE_MATED_IN_MAX_PLY && VALUE_MATE + v > 100 - r50c)
+ return VALUE_TB_LOSS_IN_MAX_PLY + 1;
+
+ // Downgrade a potentially false TB score.
+ if (VALUE_TB + v > 100 - r50c)
+ return VALUE_TB_LOSS_IN_MAX_PLY + 1;
return v + ply;
}
return v;
- }
-
+}
- // update_pv() adds current move and appends child pv[]
- void update_pv(Move* pv, Move move, const Move* childPv) {
+// Adds current move and appends child pv[]
+void update_pv(Move* pv, Move move, const Move* childPv) {
- for (*pv++ = move; childPv && *childPv != MOVE_NONE; )
+ for (*pv++ = move; childPv && *childPv != MOVE_NONE;)
*pv++ = *childPv++;
*pv = MOVE_NONE;
- }
-
-
- // update_all_stats() updates stats at the end of search() when a bestMove is found
+}
- 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) {
- Color us = pos.side_to_move();
- Thread* thisThread = pos.this_thread();
+// Updates stats at the end of search() when a bestMove is found
+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) {
+
+ Color us = pos.side_to_move();
+ Thread* thisThread = pos.this_thread();
CapturePieceToHistory& captureHistory = thisThread->captureHistory;
- Piece moved_piece = pos.moved_piece(bestMove);
- PieceType captured;
+ Piece moved_piece = pos.moved_piece(bestMove);
+ PieceType captured;
- int bonus1 = stat_bonus(depth + 1);
+ int quietMoveBonus = stat_bonus(depth + 1);
+ int quietMoveMalus = stat_malus(depth);
if (!pos.capture_stage(bestMove))
{
- int bonus2 = bestValue > beta + 145 ? bonus1 // larger bonus
- : stat_bonus(depth); // smaller bonus
+ int bestMoveBonus = bestValue > beta + 173 ? 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);
+ thisThread->pawnHistory[pawn_structure(pos)][moved_piece][to_sq(bestMove)]
+ << quietMoveBonus;
// 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->pawnHistory[pawn_structure(pos)][pos.moved_piece(quietsSearched[i])]
+ [to_sq(quietsSearched[i])]
+ << -quietMoveMalus;
+ thisThread->mainHistory[us][from_to(quietsSearched[i])] << -quietMoveMalus;
+ update_continuation_histories(ss, pos.moved_piece(quietsSearched[i]),
+ to_sq(quietsSearched[i]), -quietMoveMalus);
}
}
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
// main killer move in previous ply when it gets refuted.
- if ( prevSq != SQ_NONE
- && ((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, -quietMoveMalus);
// 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;
+ captured = type_of(pos.piece_on(to_sq(capturesSearched[i])));
+ captureHistory[moved_piece][to_sq(capturesSearched[i])][captured] << -quietMoveMalus;
}
- }
-
+}
- // update_continuation_histories() updates histories of the move pairs formed
- // by moves at ply -1, -2, -4, and -6 with current move.
- void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus) {
+// Updates histories of the move pairs formed
+// by moves at ply -1, -2, -3, -4, and -6 with current move.
+void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus) {
- for (int i : {1, 2, 4, 6})
+ for (int i : {1, 2, 3, 4, 6})
{
// Only update the first 2 continuation histories if we are in check
if (ss->inCheck && i > 2)
break;
- if (is_ok((ss-i)->currentMove))
- (*(ss-i)->continuationHistory)[pc][to] << bonus;
+ if (is_ok((ss - i)->currentMove))
+ (*(ss - i)->continuationHistory)[pc][to] << bonus / (1 + 3 * (i == 3));
}
- }
-
+}
- // update_quiet_stats() updates move sorting heuristics
- void update_quiet_stats(const Position& pos, Stack* ss, Move move, int bonus) {
+// Updates move sorting heuristics
+void update_quiet_stats(const Position& pos, Stack* ss, Move move, int bonus) {
// Update killers
if (ss->killers[0] != move)
ss->killers[0] = move;
}
- Color us = pos.side_to_move();
+ Color us = pos.side_to_move();
Thread* thisThread = pos.this_thread();
thisThread->mainHistory[us][from_to(move)] << bonus;
update_continuation_histories(ss, pos.moved_piece(move), to_sq(move), bonus);
// Update countermove history
- if (is_ok((ss-1)->currentMove))
+ if (is_ok((ss - 1)->currentMove))
{
- Square prevSq = to_sq((ss-1)->currentMove);
+ Square prevSq = to_sq((ss - 1)->currentMove);
thisThread->counterMoves[pos.piece_on(prevSq)][prevSq] = move;
}
- }
-
- // 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) {
+// 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) {
const RootMoves& rootMoves = Threads.main()->rootMoves;
- static PRNG rng(now()); // PRNG sequence should be non-deterministic
+ static PRNG rng(now()); // PRNG sequence should be non-deterministic
// 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 maxScore = -VALUE_INFINITE;
+ Value topScore = rootMoves[0].score;
+ int delta = std::min(topScore - rootMoves[multiPV - 1].score, PawnValue);
+ int maxScore = -VALUE_INFINITE;
double weakness = 120 - 2 * level;
// Choose best move. For each move score we add two terms, both dependent on
for (size_t i = 0; i < multiPV; ++i)
{
// This is our magic formula
- int push = int(( weakness * int(topScore - rootMoves[i].score)
- + delta * (rng.rand<unsigned>() % int(weakness))) / 128);
+ int push = int((weakness * int(topScore - rootMoves[i].score)
+ + delta * (rng.rand<unsigned>() % int(weakness)))
+ / 128);
if (rootMoves[i].score + push >= maxScore)
{
maxScore = rootMoves[i].score + push;
- best = rootMoves[i].pv[0];
+ best = rootMoves[i].pv[0];
}
}
return best;
- }
-
-} // namespace
+}
+} // namespace
-/// MainThread::check_time() is used to print debug info and, more importantly,
-/// to detect when we are out of available time and thus stop the search.
+// Used to print debug info and, more importantly,
+// to detect when we are out of available time and thus stop the search.
void MainThread::check_time() {
- if (--callsCnt > 0)
- return;
+ if (--callsCnt > 0)
+ 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;
+ // When using nodes, ensure checking rate is not lower than 0.1% of nodes
+ callsCnt = Limits.nodes ? std::min(512, int(Limits.nodes / 1024)) : 512;
- static TimePoint lastInfoTime = now();
+ static TimePoint lastInfoTime = now();
- TimePoint elapsed = Time.elapsed();
- TimePoint tick = Limits.startTime + elapsed;
+ TimePoint elapsed = Time.elapsed();
+ TimePoint tick = Limits.startTime + elapsed;
- if (tick - lastInfoTime >= 1000)
- {
- lastInfoTime = tick;
- dbg_print();
- }
+ if (tick - lastInfoTime >= 1000)
+ {
+ lastInfoTime = tick;
+ dbg_print();
+ }
- // We should not stop pondering until told so by the GUI
- if (ponder)
- return;
+ // We should not stop pondering until told so by the GUI
+ if (ponder)
+ return;
- if ( (Limits.use_time_management() && (elapsed > Time.maximum() - 10 || stopOnPonderhit))
- || (Limits.movetime && elapsed >= Limits.movetime)
- || (Limits.nodes && Threads.nodes_searched() >= (uint64_t)Limits.nodes))
- Threads.stop = true;
+ 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;
}
-/// 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.
-
+// 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) {
- std::stringstream ss;
- TimePoint elapsed = Time.elapsed() + 1;
- const RootMoves& rootMoves = pos.this_thread()->rootMoves;
- size_t pvIdx = pos.this_thread()->pvIdx;
- size_t multiPV = std::min((size_t)Options["MultiPV"], rootMoves.size());
- uint64_t nodesSearched = Threads.nodes_searched();
- uint64_t tbHits = Threads.tb_hits() + (TB::RootInTB ? rootMoves.size() : 0);
+ std::stringstream ss;
+ TimePoint elapsed = Time.elapsed() + 1;
+ const RootMoves& rootMoves = pos.this_thread()->rootMoves;
+ size_t pvIdx = pos.this_thread()->pvIdx;
+ size_t multiPV = std::min(size_t(Options["MultiPV"]), rootMoves.size());
+ uint64_t nodesSearched = Threads.nodes_searched();
+ uint64_t tbHits = Threads.tb_hits() + (TB::RootInTB ? rootMoves.size() : 0);
- for (size_t i = 0; i < multiPV; ++i)
- {
- bool updated = rootMoves[i].score != -VALUE_INFINITE;
+ for (size_t i = 0; i < multiPV; ++i)
+ {
+ bool updated = rootMoves[i].score != -VALUE_INFINITE;
- if (depth == 1 && !updated && i > 0)
- continue;
+ if (depth == 1 && !updated && i > 0)
+ continue;
- Depth d = updated ? depth : std::max(1, depth - 1);
- Value v = updated ? rootMoves[i].uciScore : rootMoves[i].previousScore;
+ Depth d = updated ? depth : std::max(1, depth - 1);
+ Value v = updated ? rootMoves[i].uciScore : rootMoves[i].previousScore;
- if (v == -VALUE_INFINITE)
- v = VALUE_ZERO;
+ if (v == -VALUE_INFINITE)
+ v = VALUE_ZERO;
- bool tb = TB::RootInTB && abs(v) < VALUE_MATE_IN_MAX_PLY;
- v = tb ? rootMoves[i].tbScore : v;
+ bool tb = TB::RootInTB && std::abs(v) <= VALUE_TB;
+ v = tb ? rootMoves[i].tbScore : v;
- if (ss.rdbuf()->in_avail()) // Not at first line
- ss << "\n";
+ if (ss.rdbuf()->in_avail()) // Not at first line
+ ss << "\n";
- ss << "info"
- << " depth " << d
- << " seldepth " << rootMoves[i].selDepth
- << " multipv " << i + 1
- << " score " << UCI::value(v);
+ ss << "info"
+ << " depth " << d << " seldepth " << rootMoves[i].selDepth << " multipv " << i + 1
+ << " score " << UCI::value(v);
- if (Options["UCI_ShowWDL"])
- ss << UCI::wdl(v, pos.game_ply());
+ if (Options["UCI_ShowWDL"])
+ ss << UCI::wdl(v, pos.game_ply());
- if (i == pvIdx && !tb && updated) // tablebase- and previous-scores are exact
- ss << (rootMoves[i].scoreLowerbound ? " lowerbound" : (rootMoves[i].scoreUpperbound ? " 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
- << " hashfull " << TT.hashfull()
- << " tbhits " << tbHits
- << " time " << elapsed
- << " pv";
+ ss << " nodes " << nodesSearched << " nps " << nodesSearched * 1000 / elapsed
+ << " hashfull " << TT.hashfull() << " tbhits " << tbHits << " time " << elapsed << " pv";
- for (Move m : rootMoves[i].pv)
- ss << " " << UCI::move(m, pos.is_chess960());
- }
+ for (Move m : rootMoves[i].pv)
+ ss << " " << UCI::move(m, pos.is_chess960());
+ }
- return ss.str();
+ return ss.str();
}
-/// 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 about.
-
+// 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 about.
bool RootMove::extract_ponder_from_tt(Position& pos) {
StateInfo st;
if (ttHit)
{
- Move m = tte->move(); // Local copy to be SMP safe
+ Move m = tte->move(); // Local copy to be SMP safe
if (MoveList<LEGAL>(pos).contains(m))
pv.push_back(m);
}
void Tablebases::rank_root_moves(Position& pos, Search::RootMoves& rootMoves) {
- RootInTB = false;
- UseRule50 = bool(Options["Syzygy50MoveRule"]);
- ProbeDepth = int(Options["SyzygyProbeDepth"]);
- Cardinality = int(Options["SyzygyProbeLimit"]);
+ RootInTB = false;
+ UseRule50 = bool(Options["Syzygy50MoveRule"]);
+ ProbeDepth = int(Options["SyzygyProbeDepth"]);
+ Cardinality = int(Options["SyzygyProbeLimit"]);
bool dtz_available = true;
// Tables with fewer pieces than SyzygyProbeLimit are searched with
if (Cardinality > MaxCardinality)
{
Cardinality = MaxCardinality;
- ProbeDepth = 0;
+ ProbeDepth = 0;
}
if (Cardinality >= popcount(pos.pieces()) && !pos.can_castle(ANY_CASTLING))
{
// DTZ tables are missing; try to rank moves using WDL tables
dtz_available = false;
- RootInTB = root_probe_wdl(pos, rootMoves);
+ RootInTB = root_probe_wdl(pos, rootMoves);
}
}
{
// Sort moves according to TB rank
std::stable_sort(rootMoves.begin(), rootMoves.end(),
- [](const RootMove &a, const RootMove &b) { return a.tbRank > b.tbRank; } );
+ [](const RootMove& a, const RootMove& b) { return a.tbRank > b.tbRank; });
// Probe during search only if DTZ is not available and we are winning
if (dtz_available || rootMoves[0].tbScore <= VALUE_DRAW)
}
}
-} // namespace Stockfish
+} // namespace Stockfish
projects / stockfish / blobdiff