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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) 2004-2008 Tord Romstad (Glaurung author)
Copyright (C) 2008-2015 Marco Costalba, Joona Kiiski, Tord Romstad
Copyright (C) 2015-2019 Marco Costalba, Joona Kiiski, Gary Linscott, Tord Romstad
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 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 };
// Sizes and phases of the skip-blocks, used for distributing search depths across the threads
constexpr int SkipSize[] = { 1, 1, 2, 2, 2, 2, 3, 3, 3, 3, 3, 3, 4, 4, 4, 4, 4, 4, 4, 4 };
constexpr int SkipPhase[] = { 0, 1, 0, 1, 2, 3, 0, 1, 2, 3, 4, 5, 0, 1, 2, 3, 4, 5, 6, 7 };
// Razor and futility margins
constexpr int RazorMargin = 600;
Value futility_margin(Depth d, bool improving) {
return Value((175 - 50 * improving) * d / ONE_PLY);
}
// Futility and reductions lookup tables, initialized at startup
int FutilityMoveCounts[2][16]; // [improving][depth]
int Reductions[2][64][64]; // [improving][depth][moveNumber]
template <bool PvNode> Depth reduction(bool i, Depth d, int mn) {
return (Reductions[i][std::min(d / ONE_PLY, 63)][std::min(mn, 63)] - PvNode) * ONE_PLY;
}
// History and stats update bonus, based on depth
int stat_bonus(Depth depth) {
int d = depth / ONE_PLY;
return d > 17 ? 0 : 29 * d * d + 138 * d - 134;
}
// Add a small random component to draw evaluations to keep search dynamic
// and to avoid 3fold-blindness.
Value value_draw(Depth depth, Thread* thisThread) {
return depth < 4 ? VALUE_DRAW
: VALUE_DRAW + Value(2 * (thisThread->nodes.load(std::memory_order_relaxed) % 2) - 1);
}
// Skill structure is used to implement strength limit
struct Skill {
explicit Skill(int l) : level(l) {}
bool enabled() const { return level < 20; }
bool time_to_pick(Depth depth) const { return depth / ONE_PLY == 1 + level; }
Move pick_best(size_t multiPV);
int level;
Move best = MOVE_NONE;
};
template <NodeType NT>
Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode);
template <NodeType NT>
Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth = DEPTH_ZERO);
Value value_to_tt(Value v, int ply);
Value value_from_tt(Value v, int ply);
void update_pv(Move* pv, Move move, 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, Move* quiets, int quietCount, int bonus);
void update_capture_stats(const Position& pos, Move move, Move* captures, int captureCount, int bonus);
inline bool gives_check(const Position& pos, Move move) {
Color us = pos.side_to_move();
return type_of(move) == NORMAL && !(pos.blockers_for_king(~us) & pos.pieces(us))
? pos.check_squares(type_of(pos.moved_piece(move))) & to_sq(move)
: pos.gives_check(move);
}
// 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;
uint64_t cnt, nodes = 0;
const bool leaf = (depth == 2 * ONE_PLY);
for (const auto& m : MoveList<LEGAL>(pos))
{
if (Root && depth <= ONE_PLY)
cnt = 1, nodes++;
else
{
pos.do_move(m, st);
cnt = leaf ? MoveList<LEGAL>(pos).size() : perft<false>(pos, depth - ONE_PLY);
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 imp = 0; imp <= 1; ++imp)
for (int d = 1; d < 64; ++d)
for (int mc = 1; mc < 64; ++mc)
{
double r = log(d) * log(mc) / 1.95;
Reductions[imp][d][mc] = std::round(r);
// Increase reduction for non-PV nodes when eval is not improving
if (!imp && r > 1.0)
Reductions[imp][d][mc]++;
}
for (int d = 0; d < 16; ++d)
{
FutilityMoveCounts[0][d] = int(2.4 + 0.74 * pow(d, 1.78));
FutilityMoveCounts[1][d] = int(5.0 + 1.00 * pow(d, 2.00));
}
}
/// 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 up mapped files
}
/// MainThread::search() is called by the main thread 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 * ONE_PLY);
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();
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
{
for (Thread* th : Threads)
if (th != this)
th->start_searching();
Thread::search(); // Let's 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
for (Thread* th : Threads)
if (th != this)
th->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();
// Check if there are threads with a better score than main thread
Thread* bestThread = this;
if ( Options["MultiPV"] == 1
&& !Limits.depth
&& !Skill(Options["Skill Level"]).enabled()
&& rootMoves[0].pv[0] != MOVE_NONE)
{
std::map<Move, int64_t> votes;
Value minScore = this->rootMoves[0].score;
// Find out minimum score and reset votes for moves which can be voted
for (Thread* th: Threads)
minScore = std::min(minScore, th->rootMoves[0].score);
// Vote according to score and depth
for (Thread* th : Threads)
{
int64_t s = th->rootMoves[0].score - minScore + 1;
votes[th->rootMoves[0].pv[0]] += 200 + s * s * int(th->completedDepth);
}
// Select best thread
auto bestVote = votes[this->rootMoves[0].pv[0]];
for (Thread* th : Threads)
if (votes[th->rootMoves[0].pv[0]] > bestVote)
{
bestVote = votes[th->rootMoves[0].pv[0]];
bestThread = th;
}
}
previousScore = bestThread->rootMoves[0].score;
// 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-5) 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-4, also near the root.
// The latter is needed for statScores and killer initialization.
Stack stack[MAX_PLY+8], *ss = stack+5;
Move pv[MAX_PLY+1];
Value bestValue, alpha, beta, delta;
Move lastBestMove = MOVE_NONE;
Depth lastBestMoveDepth = DEPTH_ZERO;
MainThread* mainThread = (this == Threads.main() ? Threads.main() : nullptr);
double timeReduction = 1.0;
Color us = rootPos.side_to_move();
bool failedLow;
std::memset(ss-5, 0, 8 * sizeof(Stack));
for (int i = 5; i > 0; i--)
(ss-i)->continuationHistory = &this->continuationHistory[NO_PIECE][0]; // Use as sentinel
ss->pv = pv;
bestValue = delta = alpha = -VALUE_INFINITE;
beta = VALUE_INFINITE;
if (mainThread)
mainThread->bestMoveChanges = 0, failedLow = false;
size_t multiPV = Options["MultiPV"];
Skill skill(Options["Skill Level"]);
// 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 ct = int(Options["Contempt"]) * PawnValueEg / 100; // From centipawns
// In analysis mode, adjust contempt in accordance with user preference
if (Limits.infinite || Options["UCI_AnalyseMode"])
ct = Options["Analysis Contempt"] == "Off" ? 0
: Options["Analysis Contempt"] == "Both" ? ct
: Options["Analysis Contempt"] == "White" && us == BLACK ? -ct
: Options["Analysis Contempt"] == "Black" && us == WHITE ? -ct
: ct;
// In evaluate.cpp the evaluation is from the white point of view
contempt = (us == WHITE ? make_score(ct, ct / 2)
: -make_score(ct, ct / 2));
// Iterative deepening loop until requested to stop or the target depth is reached
while ( (rootDepth += ONE_PLY) < DEPTH_MAX
&& !Threads.stop
&& !(Limits.depth && mainThread && rootDepth / ONE_PLY > Limits.depth))
{
// Distribute search depths across the helper threads
if (idx > 0)
{
int i = (idx - 1) % 20;
if (((rootDepth / ONE_PLY + SkipPhase[i]) / SkipSize[i]) % 2)
continue; // Retry with an incremented rootDepth
}
// Age out PV variability metric
if (mainThread)
mainThread->bestMoveChanges *= 0.517, failedLow = false;
// 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;
// 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 >= 5 * ONE_PLY)
{
Value previousScore = rootMoves[pvIdx].previousScore;
delta = Value(20);
alpha = std::max(previousScore - delta,-VALUE_INFINITE);
beta = std::min(previousScore + delta, VALUE_INFINITE);
// Adjust contempt based on root move's previousScore (dynamic contempt)
int dct = ct + 88 * previousScore / (abs(previousScore) + 200);
contempt = (us == WHITE ? make_score(dct, dct / 2)
: -make_score(dct, dct / 2));
}
// 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)
{
Depth adjustedDepth = std::max(ONE_PLY, rootDepth - failedHighCnt * ONE_PLY);
bestValue = ::search<PV>(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);
if (mainThread)
{
failedHighCnt = 0;
failedLow = true;
mainThread->stopOnPonderhit = false;
}
}
else if (bestValue >= beta)
{
beta = std::min(bestValue + delta, VALUE_INFINITE);
if (mainThread)
++failedHighCnt;
}
else
break;
delta += delta / 4 + 5;
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);
// Do we have time for the next iteration? Can we stop searching now?
if ( Limits.use_time_management()
&& !Threads.stop
&& !mainThread->stopOnPonderhit)
{
double fallingEval = (306 + 119 * failedLow + 6 * (mainThread->previousScore - bestValue)) / 581.0;
fallingEval = std::max(0.5, std::min(1.5, fallingEval));
// If the bestMove is stable over several iterations, reduce time accordingly
timeReduction = lastBestMoveDepth + 10 * ONE_PLY < completedDepth ? 1.95 : 1.0;
// Use part of the gained time from a previous stable move for the current move
double bestMoveInstability = 1.0 + mainThread->bestMoveChanges;
bestMoveInstability *= std::pow(mainThread->previousTimeReduction, 0.528) / timeReduction;
// Stop the search if we have only one legal move, or if available time elapsed
if ( rootMoves.size() == 1
|| Time.elapsed() > Time.optimum() * bestMoveInstability * fallingEval)
{
// 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;
}
}
}
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 NT>
Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
constexpr bool PvNode = NT == PV;
const bool rootNode = PvNode && ss->ply == 0;
// Check if we have an upcoming move which draws by repetition, or
// if the opponent had an alternative move earlier to this position.
if ( pos.rule50_count() >= 3
&& alpha < VALUE_DRAW
&& !rootNode
&& pos.has_game_cycle(ss->ply))
{
alpha = value_draw(depth, pos.this_thread());
if (alpha >= beta)
return alpha;
}
// Dive into quiescence search when the depth reaches zero
if (depth < ONE_PLY)
return qsearch<NT>(pos, ss, alpha, beta);
assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
assert(PvNode || (alpha == beta - 1));
assert(DEPTH_ZERO < depth && depth < DEPTH_MAX);
assert(!(PvNode && cutNode));
assert(depth / ONE_PLY * ONE_PLY == depth);
Move pv[MAX_PLY+1], capturesSearched[32], quietsSearched[64];
StateInfo st;
TTEntry* tte;
Key posKey;
Move ttMove, move, excludedMove, bestMove;
Depth extension, newDepth;
Value bestValue, value, ttValue, eval, maxValue, pureStaticEval;
bool ttHit, ttPv, inCheck, givesCheck, improving;
bool captureOrPromotion, doFullDepthSearch, moveCountPruning, ttCapture;
Piece movedPiece;
int moveCount, captureCount, quietCount;
// Step 1. Initialize node
Thread* thisThread = pos.this_thread();
inCheck = pos.checkers();
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 && !inCheck) ? evaluate(pos)
: value_draw(depth, 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;
}
assert(0 <= ss->ply && ss->ply < MAX_PLY);
(ss+1)->ply = ss->ply + 1;
ss->currentMove = (ss+1)->excludedMove = bestMove = MOVE_NONE;
ss->continuationHistory = &thisThread->continuationHistory[NO_PIECE][0];
(ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
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.
(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 = pos.key() ^ Key(excludedMove << 16); // Isn't a very good hash
tte = TT.probe(posKey, ttHit);
ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
ttMove = rootNode ? thisThread->rootMoves[thisThread->pvIdx].pv[0]
: ttHit ? tte->move() : MOVE_NONE;
ttPv = (ttHit && tte->is_pv()) || (PvNode && depth > 4 * ONE_PLY);
// At non-PV nodes we check for an early TT cutoff
if ( !PvNode
&& ttHit
&& tte->depth() >= depth
&& ttValue != VALUE_NONE // Possible in case of TT access race
&& (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
: (tte->bound() & BOUND_UPPER)))
{
// If ttMove is quiet, update move sorting heuristics on TT hit
if (ttMove)
{
if (ttValue >= beta)
{
if (!pos.capture_or_promotion(ttMove))
update_quiet_stats(pos, ss, ttMove, nullptr, 0, stat_bonus(depth));
// Extra penalty for a quiet TT or main killer move in previous ply when it gets refuted
if ( ((ss-1)->moveCount == 1 || (ss-1)->currentMove == (ss-1)->killers[0])
&& !pos.captured_piece())
update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + ONE_PLY));
}
// Penalty for a quiet ttMove that fails low
else if (!pos.capture_or_promotion(ttMove))
{
int penalty = -stat_bonus(depth);
thisThread->mainHistory[us][from_to(ttMove)] << penalty;
update_continuation_histories(ss, pos.moved_piece(ttMove), to_sq(ttMove), penalty);
}
}
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;
value = wdl < -drawScore ? -VALUE_MATE + MAX_PLY + ss->ply + 1
: wdl > drawScore ? VALUE_MATE - 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), ttPv, b,
std::min(DEPTH_MAX - ONE_PLY, depth + 6 * ONE_PLY),
MOVE_NONE, VALUE_NONE);
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
if (inCheck)
{
ss->staticEval = eval = pureStaticEval = VALUE_NONE;
improving = false;
goto moves_loop; // Skip early pruning when in check
}
else if (ttHit)
{
// Never assume anything on values stored in TT
ss->staticEval = eval = pureStaticEval = tte->eval();
if (eval == VALUE_NONE)
ss->staticEval = eval = pureStaticEval = evaluate(pos);
// Can ttValue be used as a better position evaluation?
if ( ttValue != VALUE_NONE
&& (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER)))
eval = ttValue;
}
else
{
if ((ss-1)->currentMove != MOVE_NULL)
{
int bonus = -(ss-1)->statScore / 512;
pureStaticEval = evaluate(pos);
ss->staticEval = eval = pureStaticEval + bonus;
}
else
ss->staticEval = eval = pureStaticEval = -(ss-1)->staticEval + 2 * Eval::Tempo;
tte->save(posKey, VALUE_NONE, ttPv, BOUND_NONE, DEPTH_NONE, MOVE_NONE, pureStaticEval);
}
// Step 7. Razoring (~2 Elo)
if ( !rootNode // The required rootNode PV handling is not available in qsearch
&& depth < 2 * ONE_PLY
&& eval <= alpha - RazorMargin)
return qsearch<NT>(pos, ss, alpha, beta);
improving = ss->staticEval >= (ss-2)->staticEval
|| (ss-2)->staticEval == VALUE_NONE;
// Step 8. Futility pruning: child node (~30 Elo)
if ( !PvNode
&& depth < 7 * ONE_PLY
&& eval - futility_margin(depth, improving) >= beta
&& eval < VALUE_KNOWN_WIN) // Do not return unproven wins
return eval;
// Step 9. Null move search with verification search (~40 Elo)
if ( !PvNode
&& (ss-1)->currentMove != MOVE_NULL
&& (ss-1)->statScore < 23200
&& eval >= beta
&& pureStaticEval >= beta - 36 * depth / ONE_PLY + 225
&& !excludedMove
&& pos.non_pawn_material(us)
&& (ss->ply >= thisThread->nmpMinPly || us != thisThread->nmpColor))
{
assert(eval - beta >= 0);
// Null move dynamic reduction based on depth and value
Depth R = ((823 + 67 * depth / ONE_PLY) / 256 + std::min(int(eval - beta) / 200, 3)) * ONE_PLY;
ss->currentMove = MOVE_NULL;
ss->continuationHistory = &thisThread->continuationHistory[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 scores
if (nullValue >= VALUE_MATE_IN_MAX_PLY)
nullValue = beta;
if (thisThread->nmpMinPly || (abs(beta) < VALUE_KNOWN_WIN && depth < 12 * ONE_PLY))
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;
}
}
// Step 10. ProbCut (~10 Elo)
// If we have a good enough capture and a reduced search returns a value
// much above beta, we can (almost) safely prune the previous move.
if ( !PvNode
&& depth >= 5 * ONE_PLY
&& abs(beta) < VALUE_MATE_IN_MAX_PLY)
{
Value raisedBeta = std::min(beta + 216 - 48 * improving, VALUE_INFINITE);
MovePicker mp(pos, ttMove, raisedBeta - ss->staticEval, &thisThread->captureHistory);
int probCutCount = 0;
while ( (move = mp.next_move()) != MOVE_NONE
&& probCutCount < 2 + 2 * cutNode)
if (move != excludedMove && pos.legal(move))
{
probCutCount++;
ss->currentMove = move;
ss->continuationHistory = &thisThread->continuationHistory[pos.moved_piece(move)][to_sq(move)];
assert(depth >= 5 * ONE_PLY);
pos.do_move(move, st);
// Perform a preliminary qsearch to verify that the move holds
value = -qsearch<NonPV>(pos, ss+1, -raisedBeta, -raisedBeta+1);
// If the qsearch held perform the regular search
if (value >= raisedBeta)
value = -search<NonPV>(pos, ss+1, -raisedBeta, -raisedBeta+1, depth - 4 * ONE_PLY, !cutNode);
pos.undo_move(move);
if (value >= raisedBeta)
return value;
}
}
// Step 11. Internal iterative deepening (~2 Elo)
if ( depth >= 8 * ONE_PLY
&& !ttMove)
{
search<NT>(pos, ss, alpha, beta, depth - 7 * ONE_PLY, cutNode);
tte = TT.probe(posKey, ttHit);
ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
ttMove = ttHit ? tte->move() : MOVE_NONE;
}
moves_loop: // When in check, search starts from here
const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory, nullptr, (ss-4)->continuationHistory };
Move countermove = thisThread->counterMoves[pos.piece_on(prevSq)][prevSq];
MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
&thisThread->captureHistory,
contHist,
countermove,
ss->killers);
value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
moveCountPruning = false;
ttCapture = ttMove && pos.capture_or_promotion(ttMove);
// Step 12. 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;
ss->moveCount = ++moveCount;
if (rootNode && thisThread == Threads.main() && Time.elapsed() > 3000)
sync_cout << "info depth " << depth / ONE_PLY
<< " currmove " << UCI::move(move, pos.is_chess960())
<< " currmovenumber " << moveCount + thisThread->pvIdx << sync_endl;
if (PvNode)
(ss+1)->pv = nullptr;
extension = DEPTH_ZERO;
captureOrPromotion = pos.capture_or_promotion(move);
movedPiece = pos.moved_piece(move);
givesCheck = gives_check(pos, move);
// Step 13. Extensions (~70 Elo)
// Singular extension search (~60 Elo). If all moves but one fail low on a
// search of (alpha-s, beta-s), and just one fails high on (alpha, beta),
// then that move is singular and should be extended. To verify this we do
// a reduced search on all the other moves but the ttMove and if the
// result is lower than ttValue minus a margin then we will extend the ttMove.
if ( depth >= 8 * ONE_PLY
&& move == ttMove
&& !rootNode
&& !excludedMove // Avoid recursive singular search
&& ttValue != VALUE_NONE
&& (tte->bound() & BOUND_LOWER)
&& tte->depth() >= depth - 3 * ONE_PLY
&& pos.legal(move))
{
Value singularBeta = std::max(ttValue - 2 * depth / ONE_PLY, -VALUE_MATE);
ss->excludedMove = move;
value = search<NonPV>(pos, ss, singularBeta - 1, singularBeta, depth / 2, cutNode);
ss->excludedMove = MOVE_NONE;
if (value < singularBeta)
extension = ONE_PLY;
// 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 is multiple moves fail high, and we can prune the whole subtree by returning
// the hard beta bound.
else if (cutNode && singularBeta > beta)
return beta;
}
// Check extension (~2 Elo)
else if ( givesCheck
&& (pos.blockers_for_king(~us) & from_sq(move) || pos.see_ge(move)))
extension = ONE_PLY;
// Castling extension
else if (type_of(move) == CASTLING)
extension = ONE_PLY;
// Calculate new depth for this move
newDepth = depth - ONE_PLY + extension;
// Step 14. Pruning at shallow depth (~170 Elo)
if ( !rootNode
&& pos.non_pawn_material(us)
&& bestValue > VALUE_MATED_IN_MAX_PLY)
{
// Skip quiet moves if movecount exceeds our FutilityMoveCount threshold
moveCountPruning = depth < 16 * ONE_PLY
&& moveCount >= FutilityMoveCounts[improving][depth / ONE_PLY];
if ( !captureOrPromotion
&& !givesCheck
&& !pos.advanced_pawn_push(move))
{
// Move count based pruning (~30 Elo)
if (moveCountPruning)
continue;
// Reduced depth of the next LMR search
int lmrDepth = std::max(newDepth - reduction<PvNode>(improving, depth, moveCount), DEPTH_ZERO) / ONE_PLY;
// Countermoves based pruning (~20 Elo)
if ( lmrDepth < 3 + ((ss-1)->statScore > 0 || (ss-1)->moveCount == 1)
&& (*contHist[0])[movedPiece][to_sq(move)] < CounterMovePruneThreshold
&& (*contHist[1])[movedPiece][to_sq(move)] < CounterMovePruneThreshold)
continue;
// Futility pruning: parent node (~2 Elo)
if ( lmrDepth < 7
&& !inCheck
&& ss->staticEval + 256 + 200 * lmrDepth <= alpha)
continue;
// Prune moves with negative SEE (~10 Elo)
if (!pos.see_ge(move, Value(-29 * lmrDepth * lmrDepth)))
continue;
}
else if ( !extension // (~20 Elo)
&& !pos.see_ge(move, -PawnValueEg * (depth / ONE_PLY)))
continue;
}