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search.cpp
1475 lines (1163 loc) · 52.1 KB
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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) 2015-2018 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 "timeman.h"
#include "thread.h"
#include "tt.h"
#include "uci.h"
namespace Search {
LimitsType Limits;
}
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
const int skipSize[] = { 1, 1, 2, 2, 2, 2, 3, 3, 3, 3, 3, 3, 4, 4, 4, 4, 4, 4, 4, 4 };
const int skipPhase[] = { 0, 1, 0, 1, 2, 3, 0, 1, 2, 3, 4, 5, 0, 1, 2, 3, 4, 5, 6, 7 };
// Razoring and futility margin based on depth
const int razor_margin = 600;
//Value futility_margin(Depth d) { return Value(150 * d / ONE_PLY); }
Value futility_margin(Depth d) { return Value(150 * d / ONE_PLY); }
// Futility and reductions lookup tables, initialized at startup
int FutilityMoveCounts[2][16]; // [improving][depth]
int Reductions[2][2][64][64]; // [pv][improving][depth][moveNumber]
template <bool PvNode> Depth reduction(bool i, Depth d, int mn) {
return Reductions[PvNode][i][std::min(d / ONE_PLY, 63)][std::min(mn, 63)] * ONE_PLY;
}
// History and stats update bonus, based on depth
int stat_bonus(Depth depth) {
int d = depth / ONE_PLY;
return d > 17 ? 0 : d * d + 2 * d - 2;
}
template <NodeType NT>
Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode);
template <NodeType NT, bool InCheck>
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_stats(const Position& pos, Stack* ss, Move move, Move* quiets, int quietsCnt, int bonus);
void update_capture_stats(const Position& pos, Move move, Move* captures, int captureCnt, int bonus);
bool pv_is_draw(Position& pos);
// 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 during 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[NonPV][imp][d][mc] = int(std::round(r));
Reductions[PV][imp][d][mc] = std::max(Reductions[NonPV][imp][d][mc] - 1, 0);
// Increase reduction for non-PV nodes when eval is not improving
if (!imp && Reductions[NonPV][imp][d][mc] >= 2)
Reductions[NonPV][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();
}
/// 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();
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
{
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 (which also raises Threads.stop).
Threads.stopOnPonderhit = true;
while (!Threads.stop && (Threads.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 && rootMoves[0].pv[0] != MOVE_NONE)
{
for (Thread* th : Threads)
{
Depth depthDiff = th->completedDepth - bestThread->completedDepth;
Value scoreDiff = th->rootMoves[0].score - bestThread->rootMoves[0].score;
// Select the thread with the best score, always if it is a mate
if ( scoreDiff > 0
&& (depthDiff >= 0 || th->rootMoves[0].score >= VALUE_MATE_IN_MAX_PLY))
bestThread = th;
}
}
previousScore = bestThread->rootMoves[0].score;
// Send new PV when needed
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() {
Stack stack[MAX_PLY+7], *ss = stack+4; // To reference from (ss-4) to (ss+2)
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;
std::memset(ss-4, 0, 7 * sizeof(Stack));
for (int i = 4; i > 0; i--)
(ss-i)->contHistory = &this->contHistory[NO_PIECE][0]; // Use as sentinel
bestValue = delta = alpha = -VALUE_INFINITE;
beta = VALUE_INFINITE;
if (mainThread)
{
mainThread->failedLow = false;
mainThread->bestMoveChanges = 0;
}
size_t multiPV = Options["MultiPV"];
multiPV = std::min(multiPV, rootMoves.size());
// 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 threads
if (idx)
{
int i = (idx - 1) % 20;
if (((rootDepth / ONE_PLY + rootPos.game_ply() + skipPhase[i]) / skipSize[i]) % 2)
continue;
}
// Age out PV variability metric
if (mainThread)
mainThread->bestMoveChanges *= 0.505, mainThread->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;
// MultiPV loop. We perform a full root search for each PV line
for (PVIdx = 0; PVIdx < multiPV && !Threads.stop; ++PVIdx)
{
// Reset UCI info selDepth for each depth and each PV line
selDepth = 0;
// Reset aspiration window starting size
if (rootDepth >= 5 * ONE_PLY)
{
delta = Value(18);
alpha = std::max(rootMoves[PVIdx].previousScore - delta,-VALUE_INFINITE);
beta = std::min(rootMoves[PVIdx].previousScore + delta, VALUE_INFINITE);
}
// Start with a small aspiration window and, in the case of a fail
// high/low, re-search with a bigger window until we're not failing
// high/low anymore.
while (true)
{
bestValue = ::search<PV>(rootPos, ss, alpha, beta, rootDepth, 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.end());
// If search has been stopped, we break immediately. Sorting and
// writing PV back to TT 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)
{
mainThread->failedLow = true;
Threads.stopOnPonderhit = false;
}
}
else if (bestValue >= beta)
beta = std::min(bestValue + delta, VALUE_INFINITE);
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(), 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;
// Do we have time for the next iteration? Can we stop searching now?
if (Limits.use_time_management())
{
if (!Threads.stop && !Threads.stopOnPonderhit)
{
// Stop the search if only one legal move is available, or if all
// of the available time has been used
const int F[] = { mainThread->failedLow,
bestValue - mainThread->previousScore };
int improvingFactor = std::max(229, std::min(715, 357 + 119 * F[0] - 6 * F[1]));
Color us = rootPos.side_to_move();
bool thinkHard = bestValue == VALUE_DRAW
&& Limits.time[us] - Time.elapsed() > Limits.time[~us]
&& ::pv_is_draw(rootPos);
double unstablePvFactor = 1 + mainThread->bestMoveChanges + thinkHard;
// if the bestMove is stable over several iterations, reduce time for this move,
// the longer the move has been stable, the more.
// Use part of the gained time from a previous stable move for the current move.
timeReduction = 1;
for (int i : {3, 4, 5})
if (lastBestMoveDepth * i < completedDepth && !thinkHard)
timeReduction *= 1.3;
unstablePvFactor *= std::pow(mainThread->previousTimeReduction, 0.51) / timeReduction;
if ( rootMoves.size() == 1
|| Time.elapsed() > Time.optimum() * unstablePvFactor * improvingFactor / 628)
{
// If we are allowed to ponder do not stop the search now but
// keep pondering until the GUI sends "ponderhit" or "stop".
if (Threads.ponder)
Threads.stopOnPonderhit = true;
else
Threads.stop = true;
}
}
}
}
if (!mainThread)
return;
mainThread->previousTimeReduction = timeReduction;
}
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) {
const bool PvNode = NT == PV;
const bool rootNode = PvNode && ss->ply == 0;
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;
bool ttHit, inCheck, givesCheck, singularExtensionNode, improving;
bool captureOrPromotion, doFullDepthSearch, moveCountPruning, skipQuiets, ttCapture, pvExact;
Piece movedPiece;
int moveCount, captureCount, quietCount;
// Step 1. Initialize node
Thread* thisThread = pos.this_thread();
inCheck = pos.checkers();
moveCount = captureCount = quietCount = ss->moveCount = 0;
ss->statScore = 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;
// 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->contHistory = &thisThread->contHistory[NO_PIECE][0];
(ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
Square prevSq = to_sq((ss-1)->currentMove);
// 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;
// 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_stats(pos, ss, ttMove, nullptr, 0, stat_bonus(depth));
// Extra penalty for a quiet TT move in previous ply when it gets refuted
if ((ss-1)->moveCount == 1 && !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.update(pos.side_to_move(), ttMove, penalty);
update_continuation_histories(ss, pos.moved_piece(ttMove), to_sq(ttMove), penalty);
}
}
return ttValue;
}
// Step 5. Evaluate the position statically
if (inCheck)
{
ss->staticEval = eval = VALUE_NONE;
improving = true;
goto moves_loop;
}
else if (ttHit)
{
// Never assume anything on values stored in TT
if ((ss->staticEval = eval = tte->eval()) == VALUE_NONE)
eval = ss->staticEval = 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
{
eval = ss->staticEval =
(ss-1)->currentMove != MOVE_NULL ? evaluate(pos)
: -(ss-1)->staticEval + 2 * Eval::Tempo;
tte->save(posKey, VALUE_NONE, BOUND_NONE, DEPTH_NONE, MOVE_NONE,
ss->staticEval, TT.generation());
}
if (ss->excludedMove || !pos.non_pawn_material(pos.side_to_move()))
goto moves_loop;
// Step 6. Razoring (skipped when in check)
if ( !PvNode
&& depth < 4 * ONE_PLY
&& eval + razor_margin <= alpha)
{
if (depth <= ONE_PLY)
return qsearch<NonPV, false>(pos, ss, alpha, alpha+1);
Value ralpha = alpha - razor_margin;
Value v = qsearch<NonPV, false>(pos, ss, ralpha, ralpha+1);
if (v <= ralpha)
return v;
}
// Step 7. Futility pruning: child node (skipped when in check)
if ( !rootNode
&& depth < 7 * ONE_PLY
&& eval - futility_margin(depth) >= beta
&& eval < VALUE_KNOWN_WIN) // Do not return unproven wins
return eval;
// Step 8. Null move search with verification search (is omitted in PV nodes)
if ( !PvNode
&& (ss-1)->currentMove != MOVE_NULL
&& eval >= beta
&& ss->staticEval >= beta - 36 * depth / ONE_PLY + 225
&& (ss->ply >= thisThread->nmp_ply || ss->ply % 2 != thisThread->nmp_odd))
{
assert(eval - beta >= 0);
// Null move dynamic reduction based on depth and value
Depth R = ((823 + 67 * depth / ONE_PLY) / 256 + std::min((eval - beta) / PawnValueMg, 3)) * ONE_PLY;
ss->currentMove = MOVE_NULL;
ss->contHistory = &thisThread->contHistory[NO_PIECE][0];
pos.do_null_move(st);
Value nullValue = depth-R < ONE_PLY ? -qsearch<NonPV, false>(pos, ss+1, -beta, -beta+1)
: - 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 (abs(beta) < VALUE_KNOWN_WIN && (depth < 12 * ONE_PLY || thisThread->nmp_ply))
return nullValue;
// Do verification search at high depths
// disable null move pruning for side to move for the first part of the remaining search tree
thisThread->nmp_ply = ss->ply + 3 * (depth-R) / 4;
thisThread->nmp_odd = ss->ply % 2;
Value v = depth-R < ONE_PLY ? qsearch<NonPV, false>(pos, ss, beta-1, beta)
: search<NonPV>(pos, ss, beta-1, beta, depth-R, false);
thisThread->nmp_odd = thisThread->nmp_ply = 0;
if (v >= beta)
return nullValue;
}
}
// Step 9. ProbCut (skipped when in check)
// 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 rbeta = std::min(beta + 200, VALUE_INFINITE);
MovePicker mp(pos, ttMove, rbeta - ss->staticEval, &thisThread->captureHistory);
while ((move = mp.next_move()) != MOVE_NONE)
if (pos.legal(move))
{
ss->currentMove = move;
ss->contHistory = &thisThread->contHistory[pos.moved_piece(move)][to_sq(move)];
assert(depth >= 5 * ONE_PLY);
pos.do_move(move, st);
value = -search<NonPV>(pos, ss+1, -rbeta, -rbeta+1, depth - 4 * ONE_PLY, !cutNode);
pos.undo_move(move);
if (value >= rbeta)
return value;
}
}
// Step 10. Internal iterative deepening (skipped when in check)
if ( depth >= 6 * ONE_PLY
&& !ttMove
&& (PvNode || ss->staticEval + 256 >= beta))
{
Depth d = (3 * depth / (4 * ONE_PLY) - 2) * ONE_PLY;
search<NT>(pos, ss, alpha, beta, d, cutNode);
tte = TT.probe(posKey, ttHit);
ttMove = ttHit ? tte->move() : MOVE_NONE;
}
moves_loop: // When in check search starts from here
const PieceToHistory* contHist[] = { (ss-1)->contHistory, (ss-2)->contHistory, nullptr, (ss-4)->contHistory };
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
improving = ss->staticEval >= (ss-2)->staticEval
/* || ss->staticEval == VALUE_NONE Already implicit in the previous condition */
||(ss-2)->staticEval == VALUE_NONE;
singularExtensionNode = !rootNode
&& depth >= 8 * ONE_PLY
&& ttMove != MOVE_NONE
&& ttValue != VALUE_NONE
&& !excludedMove // Recursive singular search is not allowed
&& (tte->bound() & BOUND_LOWER)
&& tte->depth() >= depth - 3 * ONE_PLY;
skipQuiets = false;
ttCapture = false;
pvExact = PvNode && ttHit && tte->bound() == BOUND_EXACT;
// Step 11. Loop through moves
// Loop through all pseudo-legal moves until no moves remain or a beta cutoff occurs
while ((move = mp.next_move(skipQuiets)) != 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.
if (rootNode && !std::count(thisThread->rootMoves.begin() + thisThread->PVIdx,
thisThread->rootMoves.end(), 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 = type_of(move) == NORMAL && !pos.discovered_check_candidates()
? pos.check_squares(type_of(movedPiece)) & to_sq(move)
: pos.gives_check(move);
moveCountPruning = depth < 16 * ONE_PLY
&& moveCount >= FutilityMoveCounts[improving][depth / ONE_PLY];
// Step 12. Singular and Gives Check Extensions
// Singular extension search. 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 ( singularExtensionNode
&& move == ttMove
&& pos.legal(move))
{
Value rBeta = std::max(ttValue - 2 * depth / ONE_PLY, -VALUE_MATE);
Depth d = (depth / (2 * ONE_PLY)) * ONE_PLY;
ss->excludedMove = move;
value = search<NonPV>(pos, ss, rBeta - 1, rBeta, d, cutNode);
ss->excludedMove = MOVE_NONE;
if (value < rBeta)
extension = ONE_PLY;
}
else if ( givesCheck
&& !moveCountPruning
&& pos.see_ge(move))
extension = ONE_PLY;
// Calculate new depth for this move
newDepth = depth - ONE_PLY + extension;
// Step 13. Pruning at shallow depth
if ( !rootNode
&& pos.non_pawn_material(pos.side_to_move())
&& bestValue > VALUE_MATED_IN_MAX_PLY)
{
if ( !captureOrPromotion
&& !givesCheck
&& (!pos.advanced_pawn_push(move) || pos.non_pawn_material() >= Value(5000)))
{
// Move count based pruning
if (moveCountPruning)
{
skipQuiets = true;
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
if ( lmrDepth < 3
&& (*contHist[0])[movedPiece][to_sq(move)] < CounterMovePruneThreshold
&& (*contHist[1])[movedPiece][to_sq(move)] < CounterMovePruneThreshold)
continue;
// Futility pruning: parent node
if ( lmrDepth < 7
&& !inCheck
&& ss->staticEval + 256 + 200 * lmrDepth <= alpha)
continue;
// Prune moves with negative SEE
if ( lmrDepth < 8
&& !pos.see_ge(move, Value(-35 * lmrDepth * lmrDepth)))
continue;
}
else if ( depth < 7 * ONE_PLY
&& !extension
&& !pos.see_ge(move, -PawnValueEg * (depth / ONE_PLY)))
continue;
}
// Speculative prefetch as early as possible
prefetch(TT.first_entry(pos.key_after(move)));
// Check for legality just before making the move
if (!rootNode && !pos.legal(move))
{
ss->moveCount = --moveCount;
continue;
}
if (move == ttMove && captureOrPromotion)
ttCapture = true;
// Update the current move (this must be done after singular extension search)
ss->currentMove = move;
ss->contHistory = &thisThread->contHistory[movedPiece][to_sq(move)];
// Step 14. Make the move
pos.do_move(move, st, givesCheck);
// Step 15. Reduced depth search (LMR). If the move fails high it will be
// re-searched at full depth.
if ( depth >= 3 * ONE_PLY
&& moveCount > 1
&& (!captureOrPromotion || moveCountPruning))
{
Depth r = reduction<PvNode>(improving, depth, moveCount);
if (captureOrPromotion)
r -= r ? ONE_PLY : DEPTH_ZERO;
else
{
// Decrease reduction if opponent's move count is high
if ((ss-1)->moveCount > 15)
r -= ONE_PLY;
// Decrease reduction for exact PV nodes
if (pvExact)
r -= ONE_PLY;
// Increase reduction if ttMove is a capture
if (ttCapture)
r += ONE_PLY;
// Increase reduction for cut nodes
if (cutNode)
r += 2 * ONE_PLY;
// Decrease reduction for moves that escape a capture. Filter out
// castling moves, because they are coded as "king captures rook" and
// hence break make_move().
else if ( type_of(move) == NORMAL
&& !pos.see_ge(make_move(to_sq(move), from_sq(move))))
r -= 2 * ONE_PLY;
ss->statScore = thisThread->mainHistory[~pos.side_to_move()][from_to(move)]
+ (*contHist[0])[movedPiece][to_sq(move)]
+ (*contHist[1])[movedPiece][to_sq(move)]
+ (*contHist[3])[movedPiece][to_sq(move)]
- 4000;
// Decrease/increase reduction by comparing opponent's stat score
if (ss->statScore >= 0 && (ss-1)->statScore < 0)
r -= ONE_PLY;
else if ((ss-1)->statScore >= 0 && ss->statScore < 0)
r += ONE_PLY;
// Decrease/increase reduction for moves with a good/bad history
r = std::max(DEPTH_ZERO, (r / ONE_PLY - ss->statScore / 20000) * ONE_PLY);
}
Depth d = std::max(newDepth - r, ONE_PLY);
value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true);
doFullDepthSearch = (value > alpha && d != newDepth);
}
else
doFullDepthSearch = !PvNode || moveCount > 1;
// Step 16. Full depth search when LMR is skipped or fails high
if (doFullDepthSearch)
value = newDepth < ONE_PLY ?
givesCheck ? -qsearch<NonPV, true>(pos, ss+1, -(alpha+1), -alpha)
: -qsearch<NonPV, false>(pos, ss+1, -(alpha+1), -alpha)
: - search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode);
// For PV nodes only, do a full PV search on the first move or after a fail
// high (in the latter case search only if value < beta), otherwise let the
// parent node fail low with value <= alpha and try another move.
if (PvNode && (moveCount == 1 || (value > alpha && (rootNode || value < beta))))
{
(ss+1)->pv = pv;
(ss+1)->pv[0] = MOVE_NONE;
value = newDepth < ONE_PLY ?
givesCheck ? -qsearch<PV, true>(pos, ss+1, -beta, -alpha)
: -qsearch<PV, false>(pos, ss+1, -beta, -alpha)
: - search<PV>(pos, ss+1, -beta, -alpha, newDepth, false);
}
// Step 17. Undo move
pos.undo_move(move);
assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
// Step 18. Check for a new best move
// Finished searching the move. If a stop occurred, the return value of
// the search cannot be trusted, and we return immediately without
// updating best move, PV and TT.
if (Threads.stop.load(std::memory_order_relaxed))
return VALUE_ZERO;
if (rootNode)
{
RootMove& rm = *std::find(thisThread->rootMoves.begin(),
thisThread->rootMoves.end(), move);
// PV move or new best move ?
if (moveCount == 1 || value > alpha)
{
rm.score = value;
rm.selDepth = thisThread->selDepth;
rm.pv.resize(1);
assert((ss+1)->pv);
for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
rm.pv.push_back(*m);
// We record how often the best move has been changed in each
// iteration. This information is used for time management: When
// the best move changes frequently, we allocate some more time.
if (moveCount > 1 && thisThread == Threads.main())
++static_cast<MainThread*>(thisThread)->bestMoveChanges;
}
else
// All other moves but the PV are set to the lowest value: this
// is not a problem when sorting because the sort is stable and the
// move position in the list is preserved - just the PV is pushed up.
rm.score = -VALUE_INFINITE;
}
if (value > bestValue)
{
bestValue = value;
if (value > alpha)
{
bestMove = move;
if (PvNode && !rootNode) // Update pv even in fail-high case
update_pv(ss->pv, move, (ss+1)->pv);
if (PvNode && value < beta) // Update alpha! Always alpha < beta
alpha = value;
else
{
assert(value >= beta); // Fail high
break;
}
}
}
if (!captureOrPromotion && move != bestMove && quietCount < 64)
quietsSearched[quietCount++] = move;
else if (captureOrPromotion && move != bestMove && captureCount < 32)
capturesSearched[captureCount++] = move;
}
// The following condition would detect a stop only after move loop has been
// completed. But in this case bestValue is valid because we have fully
// searched our subtree, and we can anyhow save the result in TT.
/*
if (Threads.stop)
return VALUE_DRAW;
*/
// Step 20. Check for mate and stalemate
// All legal moves have been searched and if there are no legal moves, it
// must be a mate or a stalemate. If we are in a singular extension search then
// return a fail low score.
assert(moveCount || !inCheck || excludedMove || !MoveList<LEGAL>(pos).size());