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search.cpp
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search.cpp
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/*
Stockfish, a UCI chess playing engine derived from Glaurung 2.1
Copyright (C) 2004-2024 The Stockfish developers (see AUTHORS file)
Stockfish is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.
Stockfish is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
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 <cstdint>
#include <cstdlib>
#include <initializer_list>
#include <string>
#include <utility>
#include "evaluate.h"
#include "misc.h"
#include "movegen.h"
#include "movepick.h"
#include "nnue/nnue_common.h"
#include "nnue/nnue_misc.h"
#include "position.h"
#include "syzygy/tbprobe.h"
#include "thread.h"
#include "timeman.h"
#include "tt.h"
#include "uci.h"
#include "ucioption.h"
namespace Stockfish {
namespace TB = Tablebases;
using Eval::evaluate;
using namespace Search;
namespace {
static constexpr double EvalLevel[10] = {1.043, 1.017, 0.952, 1.009, 0.971,
1.002, 0.992, 0.947, 1.046, 1.001};
// Futility margin
Value futility_margin(Depth d, bool noTtCutNode, bool improving, bool oppWorsening) {
Value futilityMult = 118 - 44 * noTtCutNode;
Value improvingDeduction = 52 * improving * futilityMult / 32;
Value worseningDeduction = (310 + 48 * improving) * oppWorsening * futilityMult / 1024;
return futilityMult * d - improvingDeduction - worseningDeduction;
}
constexpr int futility_move_count(bool improving, Depth depth) {
return improving ? (3 + depth * depth) : (3 + depth * depth) / 2;
}
// Add correctionHistory value to raw staticEval and guarantee evaluation does not hit the tablebase range
Value to_corrected_static_eval(Value v, const Worker& w, const Position& pos) {
auto cv = w.correctionHistory[pos.side_to_move()][pawn_structure_index<Correction>(pos)];
v += cv * std::abs(cv) / 9260;
return std::clamp(v, VALUE_TB_LOSS_IN_MAX_PLY + 1, VALUE_TB_WIN_IN_MAX_PLY - 1);
}
// History and stats update bonus, based on depth
int stat_bonus(Depth d) { return std::clamp(211 * d - 315, 0, 1291); }
// History and stats update malus, based on depth
int stat_malus(Depth d) { return (d < 4 ? 572 * d - 285 : 1372); }
// Add a small random component to draw evaluations to avoid 3-fold blindness
Value value_draw(size_t nodes) { return VALUE_DRAW - 1 + Value(nodes & 0x2); }
// 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)
{
double e = double(uci_elo - 1320) / (3190 - 1320);
level = std::clamp((((37.2473 * e - 40.8525) * e + 22.2943) * e - 0.311438), 0.0, 19.0);
}
else
level = double(skill_level);
}
bool enabled() const { return level < 20.0; }
bool time_to_pick(Depth depth) const { return depth == 1 + int(level); }
Move pick_best(const RootMoves&, size_t multiPV);
double level;
Move best = Move::none();
};
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, Search::Worker& workerThread, Move move, int bonus);
void update_all_stats(const Position& pos,
Stack* ss,
Search::Worker& workerThread,
Move bestMove,
Value bestValue,
Value beta,
Square prevSq,
Move* quietsSearched,
int quietCount,
Move* capturesSearched,
int captureCount,
Depth depth);
} // namespace
Search::Worker::Worker(SharedState& sharedState,
std::unique_ptr<ISearchManager> sm,
size_t thread_id) :
// Unpack the SharedState struct into member variables
thread_idx(thread_id),
manager(std::move(sm)),
options(sharedState.options),
threads(sharedState.threads),
tt(sharedState.tt),
networks(sharedState.networks) {
clear();
}
void Search::Worker::start_searching() {
// Non-main threads go directly to iterative_deepening()
if (!is_mainthread())
{
iterative_deepening();
return;
}
main_manager()->tm.init(limits, rootPos.side_to_move(), rootPos.game_ply(), options);
tt.new_search();
if (rootMoves.empty())
{
rootMoves.emplace_back(Move::none());
main_manager()->updates.onUpdateNoMoves(
{0, {rootPos.checkers() ? -VALUE_MATE : VALUE_DRAW, rootPos}});
}
else
{
threads.start_searching(); // start non-main threads
iterative_deepening(); // 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 && (main_manager()->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)
main_manager()->tm.advance_nodes_time(limits.inc[rootPos.side_to_move()]
- threads.nodes_searched());
Worker* bestThread = this;
Skill skill =
Skill(options["Skill Level"], options["UCI_LimitStrength"] ? int(options["UCI_Elo"]) : 0);
if (int(options["MultiPV"]) == 1 && !limits.depth && !limits.mate && !skill.enabled()
&& rootMoves[0].pv[0] != Move::none())
bestThread = threads.get_best_thread()->worker.get();
main_manager()->bestPreviousScore = bestThread->rootMoves[0].score;
main_manager()->bestPreviousAverageScore = bestThread->rootMoves[0].averageScore;
// Send again PV info if we have a new best thread
if (bestThread != this)
main_manager()->pv(*bestThread, threads, tt, bestThread->completedDepth);
std::string ponder;
if (bestThread->rootMoves[0].pv.size() > 1
|| bestThread->rootMoves[0].extract_ponder_from_tt(tt, rootPos))
ponder = UCIEngine::move(bestThread->rootMoves[0].pv[1], rootPos.is_chess960());
auto bestmove = UCIEngine::move(bestThread->rootMoves[0].pv[0], rootPos.is_chess960());
main_manager()->updates.onBestmove(bestmove, ponder);
}
// 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 Search::Worker::iterative_deepening() {
SearchManager* mainThread = (is_mainthread() ? main_manager() : nullptr);
Move pv[MAX_PLY + 1];
Depth lastBestMoveDepth = 0;
Value lastBestScore = -VALUE_INFINITE;
auto lastBestPV = std::vector{Move::none()};
Value alpha, beta;
Value bestValue = -VALUE_INFINITE;
Color us = rootPos.side_to_move();
double timeReduction = 1, totBestMoveChanges = 0;
int delta, iterIdx = 0;
// 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] = {};
Stack* ss = stack + 7;
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;
if (mainThread)
{
if (mainThread->bestPreviousScore == VALUE_INFINITE)
mainThread->iterValue.fill(VALUE_ZERO);
else
mainThread->iterValue.fill(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 = 11 + avg * avg / 11254;
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] = 125 * avg / (std::abs(avg) + 91);
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 = 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)
&& elapsed() > 3000)
main_manager()->pv(*this, threads, tt, rootDepth);
// 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 || elapsed() > 3000)
// A thread that aborted search can have mated-in/TB-loss PV and score
// that cannot be trusted, i.e. it can be delayed or refuted if we would have
// had time to fully search other root-moves. Thus we suppress this output and
// below pick a proven score/PV for this thread (from the previous iteration).
&& !(threads.abortedSearch && rootMoves[0].uciScore <= VALUE_TB_LOSS_IN_MAX_PLY))
main_manager()->pv(*this, threads, tt, rootDepth);
}
if (!threads.stop)
completedDepth = rootDepth;
// We make sure not to pick an unproven mated-in score,
// in case this thread prematurely stopped search (aborted-search).
if (threads.abortedSearch && rootMoves[0].score != -VALUE_INFINITE
&& rootMoves[0].score <= VALUE_TB_LOSS_IN_MAX_PLY)
{
// Bring the last best move to the front for best thread selection.
Utility::move_to_front(rootMoves, [&lastBestPV = std::as_const(lastBestPV)](
const auto& rm) { return rm == lastBestPV[0]; });
rootMoves[0].pv = lastBestPV;
rootMoves[0].score = rootMoves[0].uciScore = lastBestScore;
}
else if (rootMoves[0].pv[0] != lastBestPV[0])
{
lastBestPV = rootMoves[0].pv;
lastBestScore = rootMoves[0].score;
lastBestMoveDepth = rootDepth;
}
if (!mainThread)
continue;
// Have we found a "mate in x"?
if (limits.mate && rootMoves[0].score == rootMoves[0].uciScore
&& ((rootMoves[0].score >= VALUE_MATE_IN_MAX_PLY
&& VALUE_MATE - rootMoves[0].score <= 2 * limits.mate)
|| (rootMoves[0].score != -VALUE_INFINITE
&& rootMoves[0].score <= VALUE_MATED_IN_MAX_PLY
&& VALUE_MATE + rootMoves[0].score <= 2 * limits.mate)))
threads.stop = true;
// 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(rootMoves, multiPV);
// Use part of the gained time from a previous stable move for the current move
for (Thread* th : threads)
{
totBestMoveChanges += th->worker->bestMoveChanges;
th->worker->bestMoveChanges = 0;
}
// Do we have time for the next iteration? Can we stop searching now?
if (limits.use_time_management() && !threads.stop && !mainThread->stopOnPonderhit)
{
int nodesEffort = rootMoves[0].effort * 100 / std::max(size_t(1), size_t(nodes));
double fallingEval = (1067 + 223 * (mainThread->bestPreviousAverageScore - bestValue)
+ 97 * (mainThread->iterValue[iterIdx] - bestValue))
/ 10000.0;
fallingEval = std::clamp(fallingEval, 0.580, 1.667);
// If the bestMove is stable over several iterations, reduce time accordingly
timeReduction = lastBestMoveDepth + 8 < completedDepth ? 1.495 : 0.687;
double reduction = (1.48 + mainThread->previousTimeReduction) / (2.17 * timeReduction);
double bestMoveInstability = 1 + 1.88 * totBestMoveChanges / threads.size();
int el = std::clamp((bestValue + 750) / 150, 0, 9);
double totalTime = mainThread->tm.optimum() * fallingEval * reduction
* bestMoveInstability * EvalLevel[el];
// 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);
auto elapsedTime = elapsed();
if (completedDepth >= 10 && nodesEffort >= 97 && elapsedTime > totalTime * 0.739
&& !mainThread->ponder)
threads.stop = true;
// Stop the search if we have exceeded the totalTime
if (elapsedTime > 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
threads.increaseDepth = mainThread->ponder || elapsedTime <= totalTime * 0.506;
}
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(rootMoves, multiPV)));
}
void Search::Worker::clear() {
counterMoves.fill(Move::none());
mainHistory.fill(0);
captureHistory.fill(0);
pawnHistory.fill(0);
correctionHistory.fill(0);
for (bool inCheck : {false, true})
for (StatsType c : {NoCaptures, Captures})
for (auto& to : continuationHistory[inCheck][c])
for (auto& h : to)
h->fill(-65);
for (size_t i = 1; i < reductions.size(); ++i)
reductions[i] = int((20.14 + std::log(size_t(options["Threads"])) / 2) * std::log(i));
}
// Main search function for both PV and non-PV nodes.
template<NodeType nodeType>
Value Search::Worker::search(
Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
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 && alpha < VALUE_DRAW && pos.has_game_cycle(ss->ply))
{
alpha = value_draw(this->nodes);
if (alpha >= beta)
return alpha;
}
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[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, opponentWorsening;
bool capture, moveCountPruning, ttCapture;
Piece movedPiece;
int moveCount, captureCount, quietCount;
// Step 1. Initialize node
Worker* thisThread = this;
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;
// Check for the available remaining time
if (is_mainthread())
main_manager()->check_time(*thisThread);
// 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;
if (!rootNode)
{
// Step 2. Check for aborted search and immediate draw
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(networks, pos, thisThread->optimism[us])
: value_draw(thisThread->nodes);
// 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
// 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);
if (alpha >= beta)
return alpha;
}
else
thisThread->rootDelta = beta - alpha;
assert(0 <= ss->ply && ss->ply < MAX_PLY);
bestMove = Move::none();
(ss + 2)->killers[0] = (ss + 2)->killers[1] = Move::none();
(ss + 2)->cutoffCnt = 0;
ss->multipleExtensions = (ss - 1)->multipleExtensions;
Square prevSq = ((ss - 1)->currentMove).is_ok() ? ((ss - 1)->currentMove).to_sq() : 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();
ttCapture = ttMove && pos.capture_stage(ttMove);
// At this point, if excluded, skip straight to step 6, static eval. However,
// to save indentation, we list the condition in all code between here and there.
if (!excludedMove)
ss->ttPv = PvNode || (ss->ttHit && tte->is_pv());
// At non-PV nodes we check for an early TT cutoff
if (!PvNode && !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 (ttMove && ttValue >= beta)
{
// Bonus for a quiet ttMove that fails high (~2 Elo)
if (!ttCapture)
update_quiet_stats(pos, ss, *this, ttMove, stat_bonus(depth));
// Extra penalty for early quiet moves of
// the previous ply (~1 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));
}
// Partial workaround for the graph history interaction problem
// For high rule50 counts don't produce transposition table cutoffs.
if (pos.rule50_count() < 90)
return ttValue >= beta && std::abs(ttValue) < VALUE_TB_WIN_IN_MAX_PLY
? (ttValue * 3 + beta) / 4
: ttValue;
}
// Step 5. Tablebases probe
if (!rootNode && !excludedMove && tbConfig.cardinality)
{
int piecesCount = pos.count<ALL_PIECES>();
if (piecesCount <= tbConfig.cardinality
&& (piecesCount < tbConfig.cardinality || depth >= tbConfig.probeDepth)
&& pos.rule50_count() == 0 && !pos.can_castle(ANY_CASTLING))
{
TB::ProbeState err;
TB::WDLScore wdl = Tablebases::probe_wdl(pos, &err);
// Force check of time on the next occasion
if (is_mainthread())
main_manager()->callsCnt = 0;
if (err != TB::ProbeState::FAIL)
{
thisThread->tbHits.fetch_add(1, std::memory_order_relaxed);
int drawScore = tbConfig.useRule50 ? 1 : 0;
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;
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,
tt.generation());
return value;
}
if (PvNode)
{
if (b == BOUND_LOWER)
bestValue = value, alpha = std::max(alpha, bestValue);
else
maxValue = value;
}
}
}
}
// Step 6. Static evaluation of the position
Value unadjustedStaticEval = VALUE_NONE;
if (ss->inCheck)
{
// Skip early pruning when in check
ss->staticEval = eval = VALUE_NONE;
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).
Eval::NNUE::hint_common_parent_position(pos, networks);
unadjustedStaticEval = eval = ss->staticEval;
}
else if (ss->ttHit)
{
// Never assume anything about values stored in TT
unadjustedStaticEval = tte->eval();
if (unadjustedStaticEval == VALUE_NONE)
unadjustedStaticEval = evaluate(networks, pos, thisThread->optimism[us]);
else if (PvNode)
Eval::NNUE::hint_common_parent_position(pos, networks);
ss->staticEval = eval = to_corrected_static_eval(unadjustedStaticEval, *thisThread, pos);
// ttValue can be used as a better position evaluation (~7 Elo)
if (ttValue != VALUE_NONE && (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER)))
eval = ttValue;
}
else
{
unadjustedStaticEval = evaluate(networks, pos, thisThread->optimism[us]);
ss->staticEval = eval = to_corrected_static_eval(unadjustedStaticEval, *thisThread, pos);
// Static evaluation is saved as it was before adjustment by correction history
tte->save(posKey, VALUE_NONE, ss->ttPv, BOUND_NONE, DEPTH_NONE, Move::none(),
unadjustedStaticEval, tt.generation());
}
// Use static evaluation difference to improve quiet move ordering (~9 Elo)
if (((ss - 1)->currentMove).is_ok() && !(ss - 1)->inCheck && !priorCapture)
{
int bonus = std::clamp(-14 * int((ss - 1)->staticEval + ss->staticEval), -1644, 1384);
bonus = bonus > 0 ? 2 * bonus : bonus / 2;
thisThread->mainHistory[~us][((ss - 1)->currentMove).from_to()] << bonus;
if (type_of(pos.piece_on(prevSq)) != PAWN && ((ss - 1)->currentMove).type_of() != PROMOTION)
thisThread->pawnHistory[pawn_structure_index(pos)][pos.piece_on(prevSq)][prevSq]
<< bonus / 2;
}
// 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;
opponentWorsening = ss->staticEval + (ss - 1)->staticEval > 2;
// 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.
// Adjust razor margin according to cutoffCnt. (~1 Elo)
if (eval < alpha - 471 - (276 - 148 * ((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)
// The depth condition is important for mate finding.
if (!ss->ttPv && depth < 12
&& eval - futility_margin(depth, cutNode && !ss->ttHit, improving, opponentWorsening)
- (ss - 1)->statScore / 284
>= beta
&& eval >= beta && eval < VALUE_TB_WIN_IN_MAX_PLY && (!ttMove || ttCapture))
return beta > VALUE_TB_LOSS_IN_MAX_PLY ? (eval + beta) / 2 : eval;
// Step 9. Null move search with verification search (~35 Elo)
if (!PvNode && (ss - 1)->currentMove != Move::null() && (ss - 1)->statScore < 18001
&& eval >= beta && ss->staticEval >= beta - 21 * depth + 315 && !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) / 152, 6) + depth / 3 + 4;
ss->currentMove = Move::null();
ss->continuationHistory = &thisThread->continuationHistory[0][0][NO_PIECE][0];
pos.do_null_move(st, tt);
Value nullValue = -search<NonPV>(pos, ss + 1, -beta, -beta + 1, depth - R, !cutNode);
pos.undo_null_move();
// Do not return unproven mate or TB scores
if (nullValue >= beta && nullValue < VALUE_TB_WIN_IN_MAX_PLY)
{
if (thisThread->nmpMinPly || depth < 16)
return nullValue;
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;
Value v = search<NonPV>(pos, ss, beta - 1, beta, depth - R, false);
thisThread->nmpMinPly = 0;
if (v >= beta)
return nullValue;
}
}
// Step 10. Internal iterative reductions (~9 Elo)
// For PV nodes without a ttMove, we decrease depth by 3.
if (PvNode && !ttMove)
depth -= 3;
// Use qsearch if depth <= 0.
if (depth <= 0)
return qsearch<PV>(pos, ss, alpha, beta);
// For cutNodes without a ttMove, we decrease depth by 2 if depth is high enough.
if (cutNode && depth >= 8 && !ttMove)
depth -= 2;
// 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.
probCutBeta = beta + 169 - 63 * improving;
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 && probCutBeta > beta);
MovePicker mp(pos, ttMove, probCutBeta - ss->staticEval, &thisThread->captureHistory);
while ((move = mp.next_move()) != Move::none())
if (move != excludedMove && pos.legal(move))
{
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 =
&this
->continuationHistory[ss->inCheck][true][pos.moved_piece(move)][move.to_sq()];
thisThread->nodes.fetch_add(1, std::memory_order_relaxed);
pos.do_move(move, st);
// Perform a preliminary qsearch to verify that the move holds
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);
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, unadjustedStaticEval, tt.generation());
return std::abs(value) < VALUE_TB_WIN_IN_MAX_PLY ? value - (probCutBeta - beta)
: value;
}
}
Eval::NNUE::hint_common_parent_position(pos, networks);
}
moves_loop: // When in check, search starts here
// Step 12. A small Probcut idea, when we are in check (~4 Elo)
probCutBeta = beta + 436;
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,
(ss - 3)->continuationHistory,
(ss - 4)->continuationHistory,
nullptr,
(ss - 6)->continuationHistory};
Move countermove =
prevSq != SQ_NONE ? thisThread->counterMoves[pos.piece_on(prevSq)][prevSq] : Move::none();
MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory, &thisThread->captureHistory,
contHist, &thisThread->pawnHistory, countermove, ss->killers);
value = bestValue;
moveCountPruning = false;
// 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(move.is_ok());
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 && is_mainthread() && elapsed() > 3000)
{
main_manager()->updates.onIter(
{depth, UCIEngine::move(move, pos.is_chess960()), moveCount + thisThread->pvIdx});
}
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;
int delta = beta - alpha;
Depth r = reduction(improving, depth, moveCount, delta);
// 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(move.to_sq());
Value futilityValue =
ss->staticEval + 288 + 277 * lmrDepth + PieceValue[capturedPiece]
+ thisThread->captureHistory[movedPiece][move.to_sq()][type_of(capturedPiece)]
/ 7;
if (futilityValue <= alpha)
continue;
}
// SEE based pruning for captures and checks (~11 Elo)
if (!pos.see_ge(move, -199 * depth))
continue;
}
else
{
int history =
(*contHist[0])[movedPiece][move.to_sq()]
+ (*contHist[1])[movedPiece][move.to_sq()]
+ (*contHist[3])[movedPiece][move.to_sq()]
+ thisThread->pawnHistory[pawn_structure_index(pos)][movedPiece][move.to_sq()];
// Continuation history based pruning (~2 Elo)
if (lmrDepth < 6 && history < -4173 * depth)
continue;
history += 2 * thisThread->mainHistory[us][move.from_to()];
lmrDepth += history / 5285;
Value futilityValue =
ss->staticEval + (bestValue < ss->staticEval - 54 ? 128 : 58) + 131 * lmrDepth;
// Futility pruning: parent node (~13 Elo)
if (!ss->inCheck && lmrDepth < 15 && futilityValue <= alpha)
{
if (bestValue <= futilityValue && std::abs(bestValue) < VALUE_TB_WIN_IN_MAX_PLY