/*

Stockfish, a UCI chess playing engine derived from Glaurung 2.1

Copyright (C) 2004-2022 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 <algorithm>

#include <cassert>

#include <cmath>

#include <cstring> // For std::memset

#include <iostream>

#include <sstream>

#include "evaluate.h"

#include "misc.h"

#include "movegen.h"

#include "movepick.h"

#include "position.h"

#include "search.h"

#include "thread.h"

#include "timeman.h"

#include "tt.h"

#include "uci.h"

#include "syzygy/tbprobe.h"

namespace Stockfish {

namespace Search {

LimitsType Limits;

}

namespace Tablebases {

int Cardinality;

bool RootInTB;

bool UseRule50;

Depth ProbeDepth;

}

namespace TB = Tablebases;

using std::string;

using Eval::evaluate;

using namespace Search;

namespace {

// Different node types, used as a template parameter

enum NodeType { NonPV, PV, Root };

// Futility margin

Value futility_margin(Depth d, bool improving) {

return Value(168 * (d - improving));

}

// Reductions lookup table, initialized at startup

int Reductions[MAX_MOVES]; // [depth or moveNumber]

Depth reduction(bool i, Depth d, int mn, Value delta, Value rootDelta) {

int r = Reductions[d] * Reductions[mn];

return (r + 1463 - int(delta) * 1024 / int(rootDelta)) / 1024 + (!i && r > 1010);

}

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((9 * d + 270) * d - 311 , 2145);

}

// 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 match (TC 60+0.6)

// results spanning a wide range of k values.

struct Skill {

Skill(int skill_level, int uci_elo) {

if (uci_elo)

level = std::clamp(std::pow((uci_elo - 1346.6) / 143.4, 1 / 0.806), 0.0, 20.0);

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(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) {

StateInfo st;

ASSERT_ALIGNED(&st, Eval::NNUE::CacheLineSize);

uint64_t cnt, nodes = 0;

const bool leaf = (depth == 2);

for (const auto& m : MoveList<LEGAL>(pos))

{

if (Root && depth <= 1)

cnt = 1, nodes++;

else

{

pos.do_move(m, st);

cnt = leaf ? MoveList<LEGAL>(pos).size() : perft<false>(pos, depth - 1);

nodes += cnt;

pos.undo_move(m);

}

if (Root)

sync_cout << UCI::move(m, pos.is_chess960()) << ": " << cnt << sync_endl;

}

return nodes;

}

} // namespace

/// Search::init() is called at startup to initialize various lookup tables

void Search::init() {

for (int i = 1; i < MAX_MOVES; ++i)

Reductions[i] = int((20.81 + std::log(Threads.size()) / 2) * std::log(i));

}

/// Search::clear() resets search state to its initial value

void Search::clear() {

Threads.main()->wait_for_search_finished();

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".

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;

for (Thread* th : Threads)

th->previousDepth = bestThread->completedDepth;

// Send again PV info if we have a new best thread

if (bestThread != this)

sync_cout << UCI::pv(bestThread->rootPos, bestThread->completedDepth, -VALUE_INFINITE, VALUE_INFINITE) << sync_endl;

sync_cout << "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;

}

/// 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.

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

for (int i = 0; i <= MAX_PLY + 2; ++i)

(ss+i)->ply = i;

ss->pv = pv;

bestValue = delta = alpha = -VALUE_INFINITE;

beta = 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());

complexityAverage.set(174, 1);

trend = SCORE_ZERO;

optimism[ us] = Value(39);

optimism[~us] = -optimism[us];

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 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

if (rootDepth >= 4)

{

Value prev = rootMoves[pvIdx].averageScore;

delta = Value(16) + int(prev) * prev / 19178;

alpha = std::max(prev - delta,-VALUE_INFINITE);

beta = std::min(prev + delta, VALUE_INFINITE);

// Adjust trend and optimism based on root move's previousScore

int tr = sigmoid(prev, 3, 8, 90, 125, 1);

trend = (us == WHITE ? make_score(tr, tr / 2)

: -make_score(tr, tr / 2));

int opt = sigmoid(prev, 8, 17, 144, 13966, 183);

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 ensuring at least one effective increment for every

// four searchAgain steps (see issue #2717).

Depth adjustedDepth = std::max(1, rootDepth - failedHighCnt - 3 * (searchAgainCounter + 1) / 4);

bestValue = Stockfish::search<Root>(rootPos, ss, alpha, beta, adjustedDepth, false);

// Bring the best move to the front. It is critical that sorting

// is done with a stable algorithm because all the values but the

// first and eventually the new best one are set to -VALUE_INFINITE

// and we want to keep the same order for all the moves except the

// new PV that goes to the front. Note that in case of MultiPV

// 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, alpha, beta) << 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 / 4 + 2;

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, alpha, beta) << 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 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 + 12 * (mainThread->bestPreviousAverageScore - bestValue)

+ 6 * (mainThread->iterValue[iterIdx] - bestValue)) / 781.4;

fallingEval = std::clamp(fallingEval, 0.5, 1.5);

// If the bestMove is stable over several iterations, reduce time accordingly

timeReduction = lastBestMoveDepth + 10 < completedDepth ? 1.63 : 0.73;

double reduction = (1.56 + mainThread->previousTimeReduction) / (2.20 * timeReduction);

double bestMoveInstability = 1 + 1.7 * totBestMoveChanges / Threads.size();

int complexity = mainThread->complexityAverage.value();

double complexPosition = std::min(1.0 + (complexity - 277) / 1819.1, 1.5);

double totalTime = Time.optimum() * fallingEval * reduction * bestMoveInstability * complexPosition;

// 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 ( Threads.increaseDepth

&& !mainThread->ponder

&& Time.elapsed() > totalTime * 0.43)

Threads.increaseDepth = false;

else

Threads.increaseDepth = true;

}

mainThread->iterValue[iterIdx] = bestValue;

iterIdx = (iterIdx + 1) & 3;

}

if (!mainThread)

return;

mainThread->previousTimeReduction = timeReduction;

// If skill level is enabled, swap 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

template <NodeType nodeType>

Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {

constexpr bool PvNode = nodeType != NonPV;

constexpr bool rootNode = nodeType == Root;

const Depth maxNextDepth = rootNode ? depth : depth + 1;

// Check if we have an upcoming move which draws by repetition, or

// 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))

{

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];

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, didLMR, priorCapture;

bool capture, doFullDepthSearch, moveCountPruning, ttCapture;

Piece movedPiece;

int moveCount, captureCount, quietCount, improvement, complexity;

// Step 1. Initialize node

Thread* thisThread = pos.this_thread();

thisThread->depth = depth;

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 (thisThread == Threads.main())

static_cast<MainThread*>(thisThread)->check_time();

// 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(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

// a shorter mate was found upward in the tree then there is no need to search

// because we will never beat the current alpha. Same logic but with reversed

// signs applies also in the opposite condition of being mated instead of giving

// mate. In this case return a fail-high score.

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);

(ss+1)->ttPv = false;

(ss+1)->excludedMove = bestMove = MOVE_NONE;

(ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;

(ss+2)->cutoffCnt = 0;

ss->doubleExtensions = (ss-1)->doubleExtensions;

Square prevSq = to_sq((ss-1)->currentMove);

// Initialize statScore to zero for the grandchildren of the current position.

// So statScore is shared between all grandchildren and only the first grandchild

// starts with statScore = 0. Later grandchildren start with the last calculated

// statScore of the previous grandchild. This influences the reduction rules in

// LMR which are based on the statScore of parent position.

if (!rootNode)

(ss+2)->statScore = 0;

// Step 4. Transposition table lookup. We don't want the score of a partial

// search to overwrite a previous full search TT value, so we use a different

// position key in case of an excluded move.

excludedMove = ss->excludedMove;

posKey = excludedMove == MOVE_NONE ? pos.key() : pos.key() ^ make_key(excludedMove);

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(ttMove);

if (!excludedMove)

ss->ttPv = PvNode || (ss->ttHit && tte->is_pv());

// At non-PV nodes we check for an early TT cutoff

if ( !PvNode

&& ss->ttHit

&& tte->depth() > depth - ((int)thisThread->id() & 0x1)

&& ttValue != VALUE_NONE // Possible in case of TT access race

&& (tte->bound() & (ttValue >= beta ? BOUND_LOWER : BOUND_UPPER)))

{

// If ttMove is quiet, update move sorting heuristics on TT hit (~1 Elo)

if (ttMove)

{

if (ttValue >= beta)

{

// Bonus for a quiet ttMove that fails high (~3 Elo)

if (!ttCapture)

update_quiet_stats(pos, ss, ttMove, stat_bonus(depth));

// Extra penalty for early quiet moves of the previous ply (~0 Elo)

if ((ss-1)->moveCount <= 2 && !priorCapture)

update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + 1));

}

// Penalty for a quiet ttMove that fails low (~1 Elo)

else if (!ttCapture)

{

int penalty = -stat_bonus(depth);

thisThread->mainHistory[us][from_to(ttMove)] << penalty;

update_continuation_histories(ss, pos.moved_piece(ttMove), to_sq(ttMove), penalty);

}

}

// 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;

}

// Step 5. Tablebases probe

if (!rootNode && TB::Cardinality)

{

int piecesCount = pos.count<ALL_PIECES>();

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);

// Force check of time on the next occasion

if (thisThread == Threads.main())

static_cast<MainThread*>(thisThread)->callsCnt = 0;

if (err != TB::ProbeState::FAIL)

{

thisThread->tbHits.fetch_add(1, std::memory_order_relaxed);

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;

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);

return value;

}

if (PvNode)

{

if (b == BOUND_LOWER)

bestValue = value, alpha = std::max(alpha, bestValue);

else

maxValue = value;

}

}

}

}

CapturePieceToHistory& captureHistory = thisThread->captureHistory;

// Step 6. Static evaluation of the position

if (ss->inCheck)

{

// Skip early pruning when in check

ss->staticEval = eval = VALUE_NONE;

improving = false;

improvement = 0;

complexity = 0;

goto moves_loop;

}

else if (ss->ttHit)

{

// Never assume anything about values stored in TT

ss->staticEval = eval = tte->eval();

if (eval == VALUE_NONE)

ss->staticEval = eval = evaluate(pos, &complexity);

else // Fall back to (semi)classical complexity for TT hits, the NNUE complexity is lost

complexity = abs(ss->staticEval - pos.psq_eg_stm());

// Randomize draw evaluation

if (eval == VALUE_DRAW)

eval = value_draw(thisThread);

// ttValue can be used as a better position evaluation (~4 Elo)

if ( ttValue != VALUE_NONE

&& (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER)))

eval = ttValue;

}

else

{

ss->staticEval = eval = evaluate(pos, &complexity);

// Save static evaluation into transposition table

if (!excludedMove)

tte->save(posKey, VALUE_NONE, ss->ttPv, BOUND_NONE, DEPTH_NONE, MOVE_NONE, eval);

}

thisThread->complexityAverage.update(complexity);

// Use static evaluation difference to improve quiet move ordering (~3 Elo)

if (is_ok((ss-1)->currentMove) && !(ss-1)->inCheck && !priorCapture)

{

int bonus = std::clamp(-16 * int((ss-1)->staticEval + ss->staticEval), -2000, 2000);

thisThread->mainHistory[~us][from_to((ss-1)->currentMove)] << bonus;

}

// Set up the improvement variable, which is the difference between the current

// static evaluation and the previous static evaluation at our turn (if we were

// in check at our previous move we look at the move prior to it). The improvement

// margin and the improving flag are used in various pruning heuristics.

improvement = (ss-2)->staticEval != VALUE_NONE ? ss->staticEval - (ss-2)->staticEval

: (ss-4)->staticEval != VALUE_NONE ? ss->staticEval - (ss-4)->staticEval

: 175;

improving = improvement > 0;

// Step 7. Razoring.

// If eval is really low check with qsearch if it can exceed alpha, if it can't,

// return a fail low.

if ( !PvNode

&& depth <= 7

&& eval < alpha - 348 - 258 * depth * depth)

{

value = qsearch<NonPV>(pos, ss, alpha - 1, alpha);

if (value < alpha)

return value;

}

// Step 8. Futility pruning: child node (~25 Elo).

// The depth condition is important for mate finding.

if ( !ss->ttPv

&& depth < 8

&& eval - futility_margin(depth, improving) - (ss-1)->statScore / 256 >= beta

&& eval >= beta

&& eval < 26305) // larger than VALUE_KNOWN_WIN, but smaller than TB wins.

return eval;

// Step 9. Null move search with verification search (~22 Elo)

if ( !PvNode

&& (ss-1)->currentMove != MOVE_NULL

&& (ss-1)->statScore < 14695

&& eval >= beta

&& eval >= ss->staticEval

&& ss->staticEval >= beta - 15 * depth - improvement / 15 + 201 + complexity / 24

&& !excludedMove

&& pos.non_pawn_material(us)

&& (ss->ply >= thisThread->nmpMinPly || us != thisThread->nmpColor))

{

assert(eval - beta >= 0);

// Null move dynamic reduction based on depth, eval and complexity of position

Depth R = std::min(int(eval - beta) / 147, 5) + depth / 3 + 4 - (complexity > 650);

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);

pos.undo_null_move();

if (nullValue >= beta)

{

// Do not return unproven mate or TB scores

if (nullValue >= VALUE_TB_WIN_IN_MAX_PLY)

nullValue = beta;

if (thisThread->nmpMinPly || (abs(beta) < VALUE_KNOWN_WIN && depth < 14))

return nullValue;

assert(!thisThread->nmpMinPly); // Recursive verification is not allowed

// Do verification search at high depths, with null move pruning disabled

// for us, until ply exceeds nmpMinPly.

thisThread->nmpMinPly = ss->ply + 3 * (depth-R) / 4;

thisThread->nmpColor = us;

Value v = search<NonPV>(pos, ss, beta-1, beta, depth-R, false);

thisThread->nmpMinPly = 0;

if (v >= beta)

return nullValue;

}

}

probCutBeta = beta + 179 - 46 * improving;

// Step 10. ProbCut (~4 Elo)

// If we have a good enough capture and a reduced search returns a value

// much above beta, we can (almost) safely prune the previous move.

if ( !PvNode

&& depth > 4

&& 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

&& !( ss->ttHit

&& tte->depth() >= depth - 3

&& ttValue != VALUE_NONE

&& ttValue < probCutBeta))

{

assert(probCutBeta < VALUE_INFINITE);

MovePicker mp(pos, ttMove, probCutBeta - ss->staticEval, depth - 3, &captureHistory);

while ((move = mp.next_move()) != MOVE_NONE)

if (move != excludedMove && pos.legal(move))

{

assert(pos.capture(move) || promotion_type(move) == QUEEN);

ss->currentMove = 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);

// 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, ss->staticEval);

return value;

}

}

}

// Step 11. If the position is not in TT, decrease depth by 3.

// Use qsearch if depth is equal or below zero (~4 Elo)

if ( PvNode

&& !ttMove)

depth -= 3;

if (depth <= 0)

return qsearch<PV>(pos, ss, alpha, beta);

if ( cutNode

&& depth >= 8

&& !ttMove)

depth--;

moves_loop: // When in check, search starts here

// Step 12. A small Probcut idea, when we are in check (~0 Elo)

probCutBeta = beta + 481;

if ( ss->inCheck

&& !PvNode

&& depth >= 2

&& ttCapture

&& (tte->bound() & BOUND_LOWER)

&& tte->depth() >= depth - 3

&& ttValue >= probCutBeta

&& abs(ttValue) <= VALUE_KNOWN_WIN

&& abs(beta) <= VALUE_KNOWN_WIN

)

return probCutBeta;

const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,

nullptr , (ss-4)->continuationHistory,

nullptr , (ss-6)->continuationHistory };

Move countermove = thisThread->counterMoves[pos.piece_on(prevSq)][prevSq];

MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,

&captureHistory,

contHist,

countermove,

ss->killers);

value = bestValue;

moveCountPruning = false;

// Indicate PvNodes that will probably fail low if the node was searched

// at a depth equal or greater than the current depth, and the result of this search was a fail low.

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 which 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(move);

movedPiece = pos.moved_piece(move);

givesCheck = pos.gives_check(move);

// Calculate new depth for this move

newDepth = depth - 1;

Value delta = beta - alpha;

// Step 14. Pruning at shallow depth (~98 Elo). Depth conditions are important for mate finding.

if ( !rootNode

&& pos.non_pawn_material(us)

&& bestValue > VALUE_TB_LOSS_IN_MAX_PLY)