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stockfish/src/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
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 "search.h"
#include "timeman.h"
#include "thread.h"
#include "tt.h"
#include "uci.h"
#include "syzygy/tbprobe.h"
namespace Search {
volatile SignalsType Signals;
LimitsType Limits;
RootMoveVector RootMoves;
Position RootPos;
TimePoint SearchTime;
StateStackPtr SetupStates;
}
namespace Tablebases {
int Cardinality;
uint64_t Hits;
bool RootInTB;
bool UseRule50;
Depth ProbeDepth;
Value Score;
}
namespace TB = Tablebases;
using std::string;
using Eval::evaluate;
using namespace Search;
namespace {
// Different node types, used as template parameter
enum NodeType { Root, PV, NonPV };
// Razoring and futility margin based on depth
inline Value razor_margin(Depth d) { return Value(512 + 32 * d); }
inline Value futility_margin(Depth d) { return Value(200 * d); }
// Futility and reductions lookup tables, initialized at startup
int FutilityMoveCounts[2][16]; // [improving][depth]
Depth Reductions[2][2][64][64]; // [pv][improving][depth][moveNumber]
template <bool PvNode> inline Depth reduction(bool i, Depth d, int mn) {
return Reductions[PvNode][i][std::min(d, 63 * ONE_PLY)][std::min(mn, 63)];
}
// Skill struct is used to implement strength limiting
struct Skill {
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 best_move(size_t multiPV) { return best ? best : pick_best(multiPV); }
Move pick_best(size_t multiPV);
int level;
Move best = MOVE_NONE;
};
size_t PVIdx;
TimeManager TimeMgr;
double BestMoveChanges;
Value DrawValue[COLOR_NB];
HistoryStats History;
GainsStats Gains;
MovesStats Countermoves, Followupmoves;
template <NodeType NT, bool SpNode>
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);
void id_loop(Position& pos);
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_stats(const Position& pos, Stack* ss, Move move, Depth depth, Move* quiets, int quietsCnt);
} // namespace
/// Search::init() is called during startup to initialize various lookup tables
void Search::init() {
const double K[][2] = {{ 0.83, 2.25 }, { 0.50, 3.00 }};
for (int pv = 0; pv <= 1; ++pv)
for (int imp = 0; imp <= 1; ++imp)
for (int d = 1; d < 64; ++d)
for (int mc = 1; mc < 64; ++mc)
{
double r = K[pv][0] + log(d) * log(mc) / K[pv][1];
if (r >= 1.5)
Reductions[pv][imp][d][mc] = int(r) * ONE_PLY;
// Increase reduction when eval is not improving
if (!pv && !imp && Reductions[pv][imp][d][mc] >= 2 * ONE_PLY)
Reductions[pv][imp][d][mc] += ONE_PLY;
}
for (int d = 0; d < 16; ++d)
{
FutilityMoveCounts[0][d] = int(2.4 + 0.773 * pow(d + 0.00, 1.8));
FutilityMoveCounts[1][d] = int(2.9 + 1.045 * pow(d + 0.49, 1.8));
}
}
/// Search::perft() is our utility to verify move generation. All the leaf nodes
/// up to the given depth are generated and counted and the sum returned.
template<bool Root>
uint64_t Search::perft(Position& pos, Depth depth) {
StateInfo st;
uint64_t cnt, nodes = 0;
CheckInfo ci(pos);
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, pos.gives_check(m, ci));
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;
}
template uint64_t Search::perft<true>(Position& pos, Depth depth);
/// Search::think() is the external interface to Stockfish's search, and is
/// called by the main thread when the program receives the UCI 'go' command. It
/// searches from RootPos and at the end prints the "bestmove" to output.
void Search::think() {
TimeMgr.init(Limits, RootPos.side_to_move(), RootPos.game_ply());
int contempt = Options["Contempt"] * PawnValueEg / 100; // From centipawns
DrawValue[ RootPos.side_to_move()] = VALUE_DRAW - Value(contempt);
DrawValue[~RootPos.side_to_move()] = VALUE_DRAW + Value(contempt);
TB::Hits = 0;
TB::RootInTB = false;
TB::UseRule50 = Options["Syzygy50MoveRule"];
TB::ProbeDepth = Options["SyzygyProbeDepth"] * ONE_PLY;
TB::Cardinality = Options["SyzygyProbeLimit"];
// Skip TB probing when no TB found: !TBLargest -> !TB::Cardinality
if (TB::Cardinality > TB::MaxCardinality)
{
TB::Cardinality = TB::MaxCardinality;
TB::ProbeDepth = DEPTH_ZERO;
}
if (RootMoves.empty())
{
RootMoves.push_back(RootMove(MOVE_NONE));
sync_cout << "info depth 0 score "
<< UCI::value(RootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
<< sync_endl;
}
else
{
if (TB::Cardinality >= RootPos.count<ALL_PIECES>(WHITE)
+ RootPos.count<ALL_PIECES>(BLACK))
{
// If the current root position is in the tablebases then RootMoves
// contains only moves that preserve the draw or win.
TB::RootInTB = Tablebases::root_probe(RootPos, RootMoves, TB::Score);
if (TB::RootInTB)
TB::Cardinality = 0; // Do not probe tablebases during the search
else // If DTZ tables are missing, use WDL tables as a fallback
{
// Filter out moves that do not preserve a draw or win
TB::RootInTB = Tablebases::root_probe_wdl(RootPos, RootMoves, TB::Score);
// Only probe during search if winning
if (TB::Score <= VALUE_DRAW)
TB::Cardinality = 0;
}
if (TB::RootInTB)
{
TB::Hits = RootMoves.size();
if (!TB::UseRule50)
TB::Score = TB::Score > VALUE_DRAW ? VALUE_MATE - MAX_PLY - 1
: TB::Score < VALUE_DRAW ? -VALUE_MATE + MAX_PLY + 1
: VALUE_DRAW;
}
}
for (Thread* th : Threads)
th->maxPly = 0;
Threads.timer->run = true;
Threads.timer->notify_one(); // Wake up the recurring timer
id_loop(RootPos); // Let's start searching !
Threads.timer->run = false;
}
// When we reach the maximum depth, we can arrive here without a raise of
// Signals.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 Signals.stop).
if (!Signals.stop && (Limits.ponder || Limits.infinite))
{
Signals.stopOnPonderhit = true;
RootPos.this_thread()->wait_for(Signals.stop);
}
sync_cout << "bestmove " << UCI::move(RootMoves[0].pv[0], RootPos.is_chess960());
if (RootMoves[0].pv.size() > 1 || RootMoves[0].extract_ponder_from_tt(RootPos))
std::cout << " ponder " << UCI::move(RootMoves[0].pv[1], RootPos.is_chess960());
std::cout << sync_endl;
}
namespace {
// id_loop() is the main iterative deepening loop. It calls search() repeatedly
// with increasing depth until the allocated thinking time has been consumed,
// user stops the search, or the maximum search depth is reached.
void id_loop(Position& pos) {
Stack stack[MAX_PLY+4], *ss = stack+2; // To allow referencing (ss-2) and (ss+2)
Depth depth;
Value bestValue, alpha, beta, delta;
std::memset(ss-2, 0, 5 * sizeof(Stack));
depth = DEPTH_ZERO;
BestMoveChanges = 0;
bestValue = delta = alpha = -VALUE_INFINITE;
beta = VALUE_INFINITE;
TT.new_search();
History.clear();
Gains.clear();
Countermoves.clear();
Followupmoves.clear();
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());
// Iterative deepening loop until requested to stop or target depth reached
while (++depth < DEPTH_MAX && !Signals.stop && (!Limits.depth || depth <= Limits.depth))
{
// Age out PV variability metric
BestMoveChanges *= 0.5;
// 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 && !Signals.stop; ++PVIdx)
{
// Reset aspiration window starting size
if (depth >= 5 * ONE_PLY)
{
delta = Value(16);
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<Root, false>(pos, ss, alpha, beta, depth, 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());
// Write PV back to transposition table in case the relevant
// entries have been overwritten during the search.
for (size_t i = 0; i <= PVIdx; ++i)
RootMoves[i].insert_pv_in_tt(pos);
// If search has been stopped break immediately. Sorting and
// writing PV back to TT is safe because RootMoves is still
// valid, although it refers to previous iteration.
if (Signals.stop)
break;
// When failing high/low give some update (without cluttering
// the UI) before a re-search.
if ( multiPV == 1
&& (bestValue <= alpha || bestValue >= beta)
&& now() - SearchTime > 3000)
sync_cout << UCI::pv(pos, depth, 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);
Signals.failedLowAtRoot = true;
Signals.stopOnPonderhit = false;
}
else if (bestValue >= beta)
{
alpha = (alpha + beta) / 2;
beta = std::min(bestValue + delta, VALUE_INFINITE);
}
else
break;
delta += delta / 2;
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 (Signals.stop)
sync_cout << "info nodes " << RootPos.nodes_searched()
<< " time " << now() - SearchTime << sync_endl;
else if (PVIdx + 1 == multiPV || now() - SearchTime > 3000)
sync_cout << UCI::pv(pos, depth, alpha, beta) << sync_endl;
}
// If skill level is enabled and time is up, pick a sub-optimal best move
if (skill.enabled() && skill.time_to_pick(depth))
skill.pick_best(multiPV);
// Have we found a "mate in x"?
if ( Limits.mate
&& bestValue >= VALUE_MATE_IN_MAX_PLY
&& VALUE_MATE - bestValue <= 2 * Limits.mate)
Signals.stop = true;
// Do we have time for the next iteration? Can we stop searching now?
if (Limits.use_time_management() && !Signals.stop && !Signals.stopOnPonderhit)
{
// Take some extra time if the best move has changed
if (depth > 4 * ONE_PLY && multiPV == 1)
TimeMgr.pv_instability(BestMoveChanges);
// Stop the search if only one legal move is available or all
// of the available time has been used.
if ( RootMoves.size() == 1
|| now() - SearchTime > TimeMgr.available_time())
{
// If we are allowed to ponder do not stop the search now but
// keep pondering until the GUI sends "ponderhit" or "stop".
if (Limits.ponder)
Signals.stopOnPonderhit = true;
else
Signals.stop = true;
}
}
}
// 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_move(multiPV)));
}
// search<>() is the main search function for both PV and non-PV nodes and for
// normal and SplitPoint nodes. When called just after a split point the search
// is simpler because we have already probed the hash table, done a null move
// search, and searched the first move before splitting, so we don't have to
// repeat all this work again. We also don't need to store anything to the hash
// table here: This is taken care of after we return from the split point.
template <NodeType NT, bool SpNode>
Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
const bool RootNode = NT == Root;
const bool PvNode = NT == PV || NT == Root;
assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
assert(PvNode || (alpha == beta - 1));
assert(depth > DEPTH_ZERO);
Move pv[MAX_PLY+1], quietsSearched[64];
StateInfo st;
TTEntry* tte;
SplitPoint* splitPoint;
Key posKey;
Move ttMove, move, excludedMove, bestMove;
Depth extension, newDepth, predictedDepth;
Value bestValue, value, ttValue, eval, nullValue, futilityValue;
bool ttHit, inCheck, givesCheck, singularExtensionNode, improving;
bool captureOrPromotion, dangerous, doFullDepthSearch;
int moveCount, quietCount;
// Step 1. Initialize node
Thread* thisThread = pos.this_thread();
inCheck = pos.checkers();
if (SpNode)
{
splitPoint = ss->splitPoint;
bestMove = splitPoint->bestMove;
bestValue = splitPoint->bestValue;
tte = nullptr;
ttHit = false;
ttMove = excludedMove = MOVE_NONE;
ttValue = VALUE_NONE;
assert(splitPoint->bestValue > -VALUE_INFINITE && splitPoint->moveCount > 0);
goto moves_loop;
}
moveCount = quietCount = 0;
bestValue = -VALUE_INFINITE;
ss->ply = (ss-1)->ply + 1;
// Used to send selDepth info to GUI
if (PvNode && thisThread->maxPly < ss->ply)
thisThread->maxPly = ss->ply;
if (!RootNode)
{
// Step 2. Check for aborted search and immediate draw
if (Signals.stop || pos.is_draw() || ss->ply >= MAX_PLY)
return ss->ply >= MAX_PLY && !inCheck ? evaluate(pos) : DrawValue[pos.side_to_move()];
// 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->currentMove = ss->ttMove = (ss+1)->excludedMove = bestMove = MOVE_NONE;
(ss+1)->skipEarlyPruning = false; (ss+1)->reduction = DEPTH_ZERO;
(ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
// 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 ? pos.exclusion_key() : pos.key();
tte = TT.probe(posKey, ttHit);
ss->ttMove = ttMove = RootNode ? RootMoves[PVIdx].pv[0] : ttHit ? tte->move() : MOVE_NONE;
ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
// At non-PV nodes we check for a fail high/low. We don't probe at PV nodes
if ( !PvNode
&& ttHit
&& tte->depth() >= depth
&& ttValue != VALUE_NONE // Only in case of TT access race
&& (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
: (tte->bound() & BOUND_UPPER)))
{
ss->currentMove = ttMove; // Can be MOVE_NONE
// If ttMove is quiet, update killers, history, counter move and followup move on TT hit
if (ttValue >= beta && ttMove && !pos.capture_or_promotion(ttMove) && !inCheck)
update_stats(pos, ss, ttMove, depth, nullptr, 0);
return ttValue;
}
// Step 4a. Tablebase probe
if (!RootNode && TB::Cardinality)
{
int piecesCnt = pos.count<ALL_PIECES>(WHITE) + pos.count<ALL_PIECES>(BLACK);
if ( piecesCnt <= TB::Cardinality
&& (piecesCnt < TB::Cardinality || depth >= TB::ProbeDepth)
&& pos.rule50_count() == 0)
{
int found, v = Tablebases::probe_wdl(pos, &found);
if (found)
{
TB::Hits++;
int drawScore = TB::UseRule50 ? 1 : 0;
value = v < -drawScore ? -VALUE_MATE + MAX_PLY + ss->ply
: v > drawScore ? VALUE_MATE - MAX_PLY - ss->ply
: VALUE_DRAW + 2 * v * drawScore;
tte->save(posKey, value_to_tt(value, ss->ply), BOUND_EXACT,
std::min(DEPTH_MAX - ONE_PLY, depth + 6 * ONE_PLY),
MOVE_NONE, VALUE_NONE, TT.generation());
return value;
}
}
}
// Step 5. Evaluate the position statically and update parent's gain statistics
if (inCheck)
{
ss->staticEval = eval = VALUE_NONE;
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)
if (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->skipEarlyPruning)
goto moves_loop;
if ( !pos.captured_piece_type()
&& ss->staticEval != VALUE_NONE
&& (ss-1)->staticEval != VALUE_NONE
&& (move = (ss-1)->currentMove) != MOVE_NULL
&& move != MOVE_NONE
&& type_of(move) == NORMAL)
{
Square to = to_sq(move);
Gains.update(pos.piece_on(to), to, -(ss-1)->staticEval - ss->staticEval);
}
// Step 6. Razoring (skipped when in check)
if ( !PvNode
&& depth < 4 * ONE_PLY
&& eval + razor_margin(depth) <= alpha
&& ttMove == MOVE_NONE
&& !pos.pawn_on_7th(pos.side_to_move()))
{
if ( depth <= ONE_PLY
&& eval + razor_margin(3 * ONE_PLY) <= alpha)
return qsearch<NonPV, false>(pos, ss, alpha, beta, DEPTH_ZERO);
Value ralpha = alpha - razor_margin(depth);
Value v = qsearch<NonPV, false>(pos, ss, ralpha, ralpha+1, DEPTH_ZERO);
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
&& pos.non_pawn_material(pos.side_to_move()))
return eval - futility_margin(depth);
// Step 8. Null move search with verification search (is omitted in PV nodes)
if ( !PvNode
&& depth >= 2 * ONE_PLY
&& eval >= beta
&& pos.non_pawn_material(pos.side_to_move()))
{
ss->currentMove = MOVE_NULL;
assert(eval - beta >= 0);
// Null move dynamic reduction based on depth and value
Depth R = ((823 + 67 * depth) / 256 + std::min((eval - beta) / PawnValueMg, 3)) * ONE_PLY;
pos.do_null_move(st);
(ss+1)->skipEarlyPruning = true;
nullValue = depth-R < ONE_PLY ? -qsearch<NonPV, false>(pos, ss+1, -beta, -beta+1, DEPTH_ZERO)
: - search<NonPV, false>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode);
(ss+1)->skipEarlyPruning = false;
pos.undo_null_move();
if (nullValue >= beta)
{
// Do not return unproven mate scores
if (nullValue >= VALUE_MATE_IN_MAX_PLY)
nullValue = beta;
if (depth < 12 * ONE_PLY && abs(beta) < VALUE_KNOWN_WIN)
return nullValue;
// Do verification search at high depths
ss->skipEarlyPruning = true;
Value v = depth-R < ONE_PLY ? qsearch<NonPV, false>(pos, ss, beta-1, beta, DEPTH_ZERO)
: search<NonPV, false>(pos, ss, beta-1, beta, depth-R, false);
ss->skipEarlyPruning = false;
if (v >= beta)
return nullValue;
}
}
// Step 9. ProbCut (skipped when in check)
// If we have a very good capture (i.e. SEE > seeValues[captured_piece_type])
// 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);
Depth rdepth = depth - 4 * ONE_PLY;
assert(rdepth >= ONE_PLY);
assert((ss-1)->currentMove != MOVE_NONE);
assert((ss-1)->currentMove != MOVE_NULL);
MovePicker mp(pos, ttMove, History, pos.captured_piece_type());
CheckInfo ci(pos);
while ((move = mp.next_move<false>()) != MOVE_NONE)
if (pos.legal(move, ci.pinned))
{
ss->currentMove = move;
pos.do_move(move, st, pos.gives_check(move, ci));
value = -search<NonPV, false>(pos, ss+1, -rbeta, -rbeta+1, rdepth, !cutNode);
pos.undo_move(move);
if (value >= rbeta)
return value;
}
}
// Step 10. Internal iterative deepening (skipped when in check)
if ( depth >= (PvNode ? 5 * ONE_PLY : 8 * ONE_PLY)
&& !ttMove
&& (PvNode || ss->staticEval + 256 >= beta))
{
Depth d = 2 * (depth - 2 * ONE_PLY) - (PvNode ? DEPTH_ZERO : depth / 2);
ss->skipEarlyPruning = true;
search<PvNode ? PV : NonPV, false>(pos, ss, alpha, beta, d / 2, true);
ss->skipEarlyPruning = false;
tte = TT.probe(posKey, ttHit);
ttMove = ttHit ? tte->move() : MOVE_NONE;
}
moves_loop: // When in check and at SpNode search starts from here
Square prevMoveSq = to_sq((ss-1)->currentMove);
Move countermoves[] = { Countermoves[pos.piece_on(prevMoveSq)][prevMoveSq].first,
Countermoves[pos.piece_on(prevMoveSq)][prevMoveSq].second };
Square prevOwnMoveSq = to_sq((ss-2)->currentMove);
Move followupmoves[] = { Followupmoves[pos.piece_on(prevOwnMoveSq)][prevOwnMoveSq].first,
Followupmoves[pos.piece_on(prevOwnMoveSq)][prevOwnMoveSq].second };
MovePicker mp(pos, ttMove, depth, History, countermoves, followupmoves, ss);
CheckInfo ci(pos);
value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
improving = ss->staticEval >= (ss-2)->staticEval
|| ss->staticEval == VALUE_NONE
||(ss-2)->staticEval == VALUE_NONE;
singularExtensionNode = !RootNode
&& !SpNode
&& depth >= 8 * ONE_PLY
&& ttMove != MOVE_NONE
/* && ttValue != VALUE_NONE Already implicit in the next condition */
&& abs(ttValue) < VALUE_KNOWN_WIN
&& !excludedMove // Recursive singular search is not allowed
&& (tte->bound() & BOUND_LOWER)
&& tte->depth() >= depth - 3 * ONE_PLY;
// 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<SpNode>()) != 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(RootMoves.begin() + PVIdx, RootMoves.end(), move))
continue;
if (SpNode)
{
// Shared counter cannot be decremented later if the move turns out to be illegal
if (!pos.legal(move, ci.pinned))
continue;
moveCount = ++splitPoint->moveCount;
splitPoint->mutex.unlock();
}
else
++moveCount;
if (RootNode)
{
Signals.firstRootMove = (moveCount == 1);
if (thisThread == Threads.main() && now() - SearchTime > 3000)
sync_cout << "info depth " << depth / ONE_PLY
<< " currmove " << UCI::move(move, pos.is_chess960())
<< " currmovenumber " << moveCount + PVIdx << sync_endl;
}
if (PvNode)
(ss+1)->pv = nullptr;
extension = DEPTH_ZERO;
captureOrPromotion = pos.capture_or_promotion(move);
givesCheck = type_of(move) == NORMAL && !ci.dcCandidates
? ci.checkSq[type_of(pos.piece_on(from_sq(move)))] & to_sq(move)
: pos.gives_check(move, ci);
dangerous = givesCheck
|| type_of(move) != NORMAL
|| pos.advanced_pawn_push(move);
// Step 12. Extend checks
if (givesCheck && pos.see_sign(move) >= VALUE_ZERO)
extension = ONE_PLY;
// 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 extend the ttMove.
if ( singularExtensionNode
&& move == ttMove
&& !extension
&& pos.legal(move, ci.pinned))
{
Value rBeta = ttValue - 2 * depth / ONE_PLY;
ss->excludedMove = move;
ss->skipEarlyPruning = true;
value = search<NonPV, false>(pos, ss, rBeta - 1, rBeta, depth / 2, cutNode);
ss->skipEarlyPruning = false;
ss->excludedMove = MOVE_NONE;
if (value < rBeta)
extension = ONE_PLY;
}
// Update the current move (this must be done after singular extension search)
newDepth = depth - ONE_PLY + extension;
// Step 13. Pruning at shallow depth
if ( !RootNode
&& !captureOrPromotion
&& !inCheck
&& !dangerous
&& bestValue > VALUE_MATED_IN_MAX_PLY)
{
// Move count based pruning
if ( depth < 16 * ONE_PLY
&& moveCount >= FutilityMoveCounts[improving][depth])
{
if (SpNode)
splitPoint->mutex.lock();
continue;
}
predictedDepth = newDepth - reduction<PvNode>(improving, depth, moveCount);
// Futility pruning: parent node
if (predictedDepth < 7 * ONE_PLY)
{
futilityValue = ss->staticEval + futility_margin(predictedDepth)
+ 128 + Gains[pos.moved_piece(move)][to_sq(move)];
if (futilityValue <= alpha)
{
bestValue = std::max(bestValue, futilityValue);
if (SpNode)
{
splitPoint->mutex.lock();
if (bestValue > splitPoint->bestValue)
splitPoint->bestValue = bestValue;
}
continue;
}
}
// Prune moves with negative SEE at low depths
if (predictedDepth < 4 * ONE_PLY && pos.see_sign(move) < VALUE_ZERO)
{
if (SpNode)
splitPoint->mutex.lock();
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 && !SpNode && !pos.legal(move, ci.pinned))
{
moveCount--;
continue;
}
ss->currentMove = move;
if (!SpNode && !captureOrPromotion && quietCount < 64)
quietsSearched[quietCount++] = 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
&& move != ss->killers[0]
&& move != ss->killers[1])
{
ss->reduction = reduction<PvNode>(improving, depth, moveCount);
if ( (!PvNode && cutNode)
|| History[pos.piece_on(to_sq(move))][to_sq(move)] < VALUE_ZERO)
ss->reduction += ONE_PLY;
if (move == countermoves[0] || move == countermoves[1])
ss->reduction = std::max(DEPTH_ZERO, ss->reduction - ONE_PLY);
// Decrease reduction for moves that escape a capture
if ( ss->reduction
&& type_of(move) == NORMAL
&& type_of(pos.piece_on(to_sq(move))) != PAWN
&& pos.see(make_move(to_sq(move), from_sq(move))) < VALUE_ZERO)
ss->reduction = std::max(DEPTH_ZERO, ss->reduction - ONE_PLY);
Depth d = std::max(newDepth - ss->reduction, ONE_PLY);
if (SpNode)
alpha = splitPoint->alpha;
value = -search<NonPV, false>(pos, ss+1, -(alpha+1), -alpha, d, true);
// Re-search at intermediate depth if reduction is very high
if (value > alpha && ss->reduction >= 4 * ONE_PLY)
{
Depth d2 = std::max(newDepth - 2 * ONE_PLY, ONE_PLY);
value = -search<NonPV, false>(pos, ss+1, -(alpha+1), -alpha, d2, true);
}
doFullDepthSearch = (value > alpha && ss->reduction != DEPTH_ZERO);
ss->reduction = DEPTH_ZERO;
}
else
doFullDepthSearch = !PvNode || moveCount > 1;
// Step 16. Full depth search, when LMR is skipped or fails high
if (doFullDepthSearch)
{
if (SpNode)
alpha = splitPoint->alpha;
value = newDepth < ONE_PLY ?
givesCheck ? -qsearch<NonPV, true>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
: -qsearch<NonPV, false>(pos, ss+1, -(alpha+1), -alpha, DEPTH_ZERO)
: - search<NonPV, false>(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 to 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, DEPTH_ZERO)
: -qsearch<PV, false>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
: - search<PV, false>(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 new best move
if (SpNode)
{
splitPoint->mutex.lock();
bestValue = splitPoint->bestValue;
alpha = splitPoint->alpha;
}
// Finished searching the move. If a stop or a cutoff occurred, the return
// value of the search cannot be trusted, and we return immediately without
// updating best move, PV and TT.
if (Signals.stop || thisThread->cutoff_occurred())
return VALUE_ZERO;
if (RootNode)
{
RootMove& rm = *std::find(RootMoves.begin(), RootMoves.end(), move);
// PV move or new best move ?
if (moveCount == 1 || value > alpha)
{
rm.score = value;
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)
++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 = SpNode ? splitPoint->bestValue = value : value;
if (value > alpha)
{
bestMove = SpNode ? splitPoint->bestMove = move : move;
if (PvNode && !RootNode) // Update pv even in fail-high case
update_pv(SpNode ? splitPoint->ss->pv : ss->pv, move, (ss+1)->pv);
if (PvNode && value < beta) // Update alpha! Always alpha < beta
alpha = SpNode ? splitPoint->alpha = value : value;
else
{
assert(value >= beta); // Fail high
if (SpNode)
splitPoint->cutoff = true;
break;
}
}
}
// Step 19. Check for splitting the search
if ( !SpNode
&& Threads.size() >= 2
&& depth >= Threads.minimumSplitDepth
&& ( !thisThread->activeSplitPoint
|| !thisThread->activeSplitPoint->allSlavesSearching
|| ( Threads.size() > MAX_SLAVES_PER_SPLITPOINT
&& thisThread->activeSplitPoint->slavesMask.count() == MAX_SLAVES_PER_SPLITPOINT))
&& thisThread->splitPointsSize < MAX_SPLITPOINTS_PER_THREAD)
{
assert(bestValue > -VALUE_INFINITE && bestValue < beta);
thisThread->split(pos, ss, alpha, beta, &bestValue, &bestMove,
depth, moveCount, &mp, NT, cutNode);
if (Signals.stop || thisThread->cutoff_occurred())
return VALUE_ZERO;
if (bestValue >= beta)
break;
}
}
if (SpNode)
return bestValue;
// Following condition would detect a stop or a cutoff set 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 (Signals.stop || thisThread->cutoff_occurred())
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 mate or stalemate. If we are in a singular extension search then
// return a fail low score.
if (!moveCount)
bestValue = excludedMove ? alpha
: inCheck ? mated_in(ss->ply) : DrawValue[pos.side_to_move()];
// Quiet best move: update killers, history, countermoves and followupmoves
else if (bestValue >= beta && !pos.capture_or_promotion(bestMove) && !inCheck)
update_stats(pos, ss, bestMove, depth, quietsSearched, quietCount - 1);
tte->save(posKey, value_to_tt(bestValue, ss->ply),
bestValue >= beta ? BOUND_LOWER :
PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
depth, bestMove, ss->staticEval, TT.generation());
assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
return bestValue;
}
// qsearch() is the quiescence search function, which is called by the main
// search function when the remaining depth is zero (or, to be more precise,
// less than ONE_PLY).
template <NodeType NT, bool InCheck>
Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
const bool PvNode = NT == PV;
assert(NT == PV || NT == NonPV);
assert(InCheck == !!pos.checkers());
assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
assert(PvNode || (alpha == beta - 1));
assert(depth <= DEPTH_ZERO);
Move pv[MAX_PLY+1];
StateInfo st;
TTEntry* tte;
Key posKey;
Move ttMove, move, bestMove;
Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
bool ttHit, givesCheck, evasionPrunable;
Depth ttDepth;
if (PvNode)
{
oldAlpha = alpha; // To flag BOUND_EXACT when eval above alpha and no available moves
(ss+1)->pv = pv;
ss->pv[0] = MOVE_NONE;
}
ss->currentMove = bestMove = MOVE_NONE;
ss->ply = (ss-1)->ply + 1;
// Check for an instant draw or if the maximum ply has been reached
if (pos.is_draw() || ss->ply >= MAX_PLY)
return ss->ply >= MAX_PLY && !InCheck ? evaluate(pos) : DrawValue[pos.side_to_move()];
assert(0 <= ss->ply && ss->ply < MAX_PLY);
// Decide whether or not to include checks: this fixes also the type of
// TT entry depth that we are going to use. Note that in qsearch we use
// only two types of depth in TT: DEPTH_QS_CHECKS or DEPTH_QS_NO_CHECKS.
ttDepth = InCheck || depth >= DEPTH_QS_CHECKS ? DEPTH_QS_CHECKS
: DEPTH_QS_NO_CHECKS;
// Transposition table lookup
posKey = pos.key();
tte = TT.probe(posKey, ttHit);
ttMove = ttHit ? tte->move() : MOVE_NONE;
ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
if ( !PvNode
&& ttHit
&& tte->depth() >= ttDepth
&& ttValue != VALUE_NONE // Only in case of TT access race
&& (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
: (tte->bound() & BOUND_UPPER)))
{
ss->currentMove = ttMove; // Can be MOVE_NONE
return ttValue;
}
// Evaluate the position statically
if (InCheck)
{
ss->staticEval = VALUE_NONE;
bestValue = futilityBase = -VALUE_INFINITE;
}
else
{
if (ttHit)
{
// Never assume anything on values stored in TT
if ((ss->staticEval = bestValue = tte->eval()) == VALUE_NONE)
ss->staticEval = bestValue = evaluate(pos);
// Can ttValue be used as a better position evaluation?
if (ttValue != VALUE_NONE)
if (tte->bound() & (ttValue > bestValue ? BOUND_LOWER : BOUND_UPPER))
bestValue = ttValue;
}
else
ss->staticEval = bestValue =
(ss-1)->currentMove != MOVE_NULL ? evaluate(pos) : -(ss-1)->staticEval + 2 * Eval::Tempo;
// Stand pat. Return immediately if static value is at least beta
if (bestValue >= beta)
{
if (!ttHit)
tte->save(pos.key(), value_to_tt(bestValue, ss->ply), BOUND_LOWER,
DEPTH_NONE, MOVE_NONE, ss->staticEval, TT.generation());
return bestValue;
}
if (PvNode && bestValue > alpha)
alpha = bestValue;
futilityBase = bestValue + 128;
}
// Initialize a MovePicker object for the current position, and prepare
// to search the moves. Because the depth is <= 0 here, only captures,
// queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
// be generated.
MovePicker mp(pos, ttMove, depth, History, to_sq((ss-1)->currentMove));
CheckInfo ci(pos);
// Loop through the moves until no moves remain or a beta cutoff occurs
while ((move = mp.next_move<false>()) != MOVE_NONE)
{
assert(is_ok(move));
givesCheck = type_of(move) == NORMAL && !ci.dcCandidates
? ci.checkSq[type_of(pos.piece_on(from_sq(move)))] & to_sq(move)
: pos.gives_check(move, ci);
// Futility pruning
if ( !InCheck
&& !givesCheck
&& futilityBase > -VALUE_KNOWN_WIN
&& !pos.advanced_pawn_push(move))
{
assert(type_of(move) != ENPASSANT); // Due to !pos.advanced_pawn_push
futilityValue = futilityBase + PieceValue[EG][pos.piece_on(to_sq(move))];
if (futilityValue <= alpha)
{
bestValue = std::max(bestValue, futilityValue);
continue;
}
if (futilityBase <= alpha && pos.see(move) <= VALUE_ZERO)
{
bestValue = std::max(bestValue, futilityBase);
continue;
}
}
// Detect non-capture evasions that are candidates to be pruned
evasionPrunable = InCheck
&& bestValue > VALUE_MATED_IN_MAX_PLY
&& !pos.capture(move)
&& !pos.can_castle(pos.side_to_move());
// Don't search moves with negative SEE values
if ( (!InCheck || evasionPrunable)
&& type_of(move) != PROMOTION
&& pos.see_sign(move) < VALUE_ZERO)
continue;
// Speculative prefetch as early as possible
prefetch(TT.first_entry(pos.key_after(move)));
// Check for legality just before making the move
if (!pos.legal(move, ci.pinned))
continue;
ss->currentMove = move;
// Make and search the move
pos.do_move(move, st, givesCheck);
value = givesCheck ? -qsearch<NT, true>(pos, ss+1, -beta, -alpha, depth - ONE_PLY)
: -qsearch<NT, false>(pos, ss+1, -beta, -alpha, depth - ONE_PLY);
pos.undo_move(move);
assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
// Check for new best move
if (value > bestValue)
{
bestValue = value;
if (value > alpha)
{
if (PvNode) // Update pv even in fail-high case
update_pv(ss->pv, move, (ss+1)->pv);
if (PvNode && value < beta) // Update alpha here! Always alpha < beta
{
alpha = value;
bestMove = move;
}
else // Fail high
{
tte->save(posKey, value_to_tt(value, ss->ply), BOUND_LOWER,
ttDepth, move, ss->staticEval, TT.generation());
return value;
}
}
}
}
// All legal moves have been searched. A special case: If we're in check
// and no legal moves were found, it is checkmate.
if (InCheck && bestValue == -VALUE_INFINITE)
return mated_in(ss->ply); // Plies to mate from the root
tte->save(posKey, value_to_tt(bestValue, ss->ply),
PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
ttDepth, bestMove, ss->staticEval, TT.generation());
assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
return bestValue;
}
// value_to_tt() adjusts a mate score from "plies to mate from the root" to
// "plies to mate from the current position". Non-mate scores are unchanged.
// The function is called before storing a value in the transposition table.
Value value_to_tt(Value v, int ply) {
assert(v != VALUE_NONE);
return v >= VALUE_MATE_IN_MAX_PLY ? v + ply
: v <= VALUE_MATED_IN_MAX_PLY ? v - ply : v;
}
// value_from_tt() is the inverse of value_to_tt(): It adjusts a mate score
// from the transposition table (which refers to the plies to mate/be mated
// from current position) to "plies to mate/be mated from the root".
Value value_from_tt(Value v, int ply) {
return v == VALUE_NONE ? VALUE_NONE
: v >= VALUE_MATE_IN_MAX_PLY ? v - ply
: v <= VALUE_MATED_IN_MAX_PLY ? v + ply : v;
}
// update_pv() adds current move and appends child pv[]
void update_pv(Move* pv, Move move, Move* childPv) {
for (*pv++ = move; childPv && *childPv != MOVE_NONE; )
*pv++ = *childPv++;
*pv = MOVE_NONE;
}
// update_stats() updates killers, history, countermoves and followupmoves stats after a fail-high
// of a quiet move.
void update_stats(const Position& pos, Stack* ss, Move move, Depth depth, Move* quiets, int quietsCnt) {
if (ss->killers[0] != move)
{
ss->killers[1] = ss->killers[0];
ss->killers[0] = move;
}
// Increase history value of the cut-off move and decrease all the other
// played quiet moves.
Value bonus = Value((depth / ONE_PLY) * (depth / ONE_PLY));
History.update(pos.moved_piece(move), to_sq(move), bonus);
for (int i = 0; i < quietsCnt; ++i)
{
Move m = quiets[i];
History.update(pos.moved_piece(m), to_sq(m), -bonus);
}
if (is_ok((ss-1)->currentMove))
{
Square prevMoveSq = to_sq((ss-1)->currentMove);
Countermoves.update(pos.piece_on(prevMoveSq), prevMoveSq, move);
}
if (is_ok((ss-2)->currentMove) && (ss-1)->currentMove == (ss-1)->ttMove)
{
Square prevOwnMoveSq = to_sq((ss-2)->currentMove);
Followupmoves.update(pos.piece_on(prevOwnMoveSq), prevOwnMoveSq, move);
}
}
// When playing with strength handicap, choose best move among a set of RootMoves
// using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
Move Skill::pick_best(size_t multiPV) {
// PRNG sequence should be non-deterministic, so we seed it with the time at init
static PRNG rng(now());
// RootMoves are already sorted by score in descending order
int variance = std::min(RootMoves[0].score - RootMoves[multiPV - 1].score, PawnValueMg);
int weakness = 120 - 2 * level;
int maxScore = -VALUE_INFINITE;
// Choose best move. For each move score we add two terms both dependent on
// weakness. One deterministic and bigger for weaker levels, and one random,
// then we choose the move with the resulting highest score.
for (size_t i = 0; i < multiPV; ++i)
{
// This is our magic formula
int push = ( weakness * int(RootMoves[0].score - RootMoves[i].score)
+ variance * (rng.rand<unsigned>() % weakness)) / 128;
if (RootMoves[i].score + push > maxScore)
{
maxScore = RootMoves[i].score + push;
best = RootMoves[i].pv[0];
}
}
return best;
}
} // namespace
/// UCI::pv() formats PV information according to the UCI protocol. UCI requires
/// that all (if any) unsearched PV lines are sent using a previous search score.
string UCI::pv(const Position& pos, Depth depth, Value alpha, Value beta) {
std::stringstream ss;
TimePoint elapsed = now() - SearchTime + 1;
size_t multiPV = std::min((size_t)Options["MultiPV"], RootMoves.size());
int selDepth = 0;
for (Thread* th : Threads)
if (th->maxPly > selDepth)
selDepth = th->maxPly;
for (size_t i = 0; i < multiPV; ++i)
{
bool updated = (i <= PVIdx);
if (depth == ONE_PLY && !updated)
continue;
Depth d = updated ? depth : depth - ONE_PLY;
Value v = updated ? RootMoves[i].score : RootMoves[i].previousScore;
bool tb = TB::RootInTB && abs(v) < VALUE_MATE - MAX_PLY;
v = tb ? TB::Score : v;
if (ss.rdbuf()->in_avail()) // Not at first line
ss << "\n";
ss << "info"
<< " depth " << d / ONE_PLY
<< " seldepth " << selDepth
<< " multipv " << i + 1
<< " score " << UCI::value(v);
if (!tb && i == PVIdx)
ss << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
ss << " nodes " << pos.nodes_searched()
<< " nps " << pos.nodes_searched() * 1000 / elapsed;
if (elapsed > 1000) // Earlier makes little sense
ss << " hashfull " << TT.hashfull();
ss << " tbhits " << TB::Hits
<< " time " << elapsed
<< " pv";
for (Move m : RootMoves[i].pv)
ss << " " << UCI::move(m, pos.is_chess960());
}
return ss.str();
}
/// RootMove::insert_pv_in_tt() is called at the end of a search iteration, and
/// inserts the PV back into the TT. This makes sure the old PV moves are searched
/// first, even if the old TT entries have been overwritten.
void RootMove::insert_pv_in_tt(Position& pos) {
StateInfo state[MAX_PLY], *st = state;
bool ttHit;
for (Move m : pv)
{
assert(MoveList<LEGAL>(pos).contains(m));
TTEntry* tte = TT.probe(pos.key(), ttHit);
if (!ttHit || tte->move() != m) // Don't overwrite correct entries
tte->save(pos.key(), VALUE_NONE, BOUND_NONE, DEPTH_NONE, m, VALUE_NONE, TT.generation());
pos.do_move(m, *st++, pos.gives_check(m, CheckInfo(pos)));
}
for (size_t i = pv.size(); i > 0; )
pos.undo_move(pv[--i]);
}
/// RootMove::extract_ponder_from_tt() is called in case we have no ponder move before
/// exiting the search, for instance in case we stop the search during a fail high at
/// root. We try hard to have a ponder move to return to the GUI, otherwise in case of
/// 'ponder on' we have nothing to think on.
bool RootMove::extract_ponder_from_tt(Position& pos)
{
StateInfo st;
bool ttHit;
assert(pv.size() == 1);
pos.do_move(pv[0], st, pos.gives_check(pv[0], CheckInfo(pos)));
TTEntry* tte = TT.probe(pos.key(), ttHit);
pos.undo_move(pv[0]);
if (ttHit)
{
Move m = tte->move(); // Local copy to be SMP safe
if (MoveList<LEGAL>(pos).contains(m))
return pv.push_back(m), true;
}
return false;
}
/// Thread::idle_loop() is where the thread is parked when it has no work to do
void Thread::idle_loop() {
// Pointer 'this_sp' is not null only if we are called from split(), and not
// at the thread creation. This means we are the split point's master.
SplitPoint* this_sp = activeSplitPoint;
assert(!this_sp || (this_sp->master == this && searching));
while (!exit)
{
// If this thread has been assigned work, launch a search
while (searching)
{
Threads.mutex.lock();
assert(activeSplitPoint);
SplitPoint* sp = activeSplitPoint;
Threads.mutex.unlock();
Stack stack[MAX_PLY+4], *ss = stack+2; // To allow referencing (ss-2) and (ss+2)
Position pos(*sp->pos, this);
std::memcpy(ss-2, sp->ss-2, 5 * sizeof(Stack));
ss->splitPoint = sp;
sp->mutex.lock();
assert(activePosition == nullptr);
activePosition = &pos;
if (sp->nodeType == NonPV)
search<NonPV, true>(pos, ss, sp->alpha, sp->beta, sp->depth, sp->cutNode);
else if (sp->nodeType == PV)
search<PV, true>(pos, ss, sp->alpha, sp->beta, sp->depth, sp->cutNode);
else if (sp->nodeType == Root)
search<Root, true>(pos, ss, sp->alpha, sp->beta, sp->depth, sp->cutNode);
else
assert(false);
assert(searching);
searching = false;
activePosition = nullptr;
sp->slavesMask.reset(idx);
sp->allSlavesSearching = false;
sp->nodes += pos.nodes_searched();
// Wake up the master thread so to allow it to return from the idle
// loop in case we are the last slave of the split point.
if (this != sp->master && sp->slavesMask.none())
{
assert(!sp->master->searching);
sp->master->notify_one();
}
// After releasing the lock we can't access any SplitPoint related data
// in a safe way because it could have been released under our feet by
// the sp master.
sp->mutex.unlock();
// Try to late join to another split point if none of its slaves has
// already finished.
SplitPoint* bestSp = NULL;
int minLevel = INT_MAX;
for (Thread* th : Threads)
{
const size_t size = th->splitPointsSize; // Local copy
sp = size ? &th->splitPoints[size - 1] : nullptr;
if ( sp
&& sp->allSlavesSearching
&& sp->slavesMask.count() < MAX_SLAVES_PER_SPLITPOINT
&& can_join(sp))
{
assert(this != th);
assert(!(this_sp && this_sp->slavesMask.none()));
assert(Threads.size() > 2);
// Prefer to join to SP with few parents to reduce the probability
// that a cut-off occurs above us, and hence we waste our work.
int level = 0;
for (SplitPoint* p = th->activeSplitPoint; p; p = p->parentSplitPoint)
level++;
if (level < minLevel)
{
bestSp = sp;
minLevel = level;
}
}
}
if (bestSp)
{
sp = bestSp;
// Recheck the conditions under lock protection
Threads.mutex.lock();
sp->mutex.lock();
if ( sp->allSlavesSearching
&& sp->slavesMask.count() < MAX_SLAVES_PER_SPLITPOINT
&& can_join(sp))
{
sp->slavesMask.set(idx);
activeSplitPoint = sp;
searching = true;
}
sp->mutex.unlock();
Threads.mutex.unlock();
}
}
// Avoid races with notify_one() fired from last slave of the split point
std::unique_lock<Mutex> lk(mutex);
// If we are master and all slaves have finished then exit idle_loop
if (this_sp && this_sp->slavesMask.none())
{
assert(!searching);
break;
}
// If we are not searching, wait for a condition to be signaled instead of
// wasting CPU time polling for work.
if (!searching && !exit)
sleepCondition.wait(lk);
}
}
/// check_time() is called by the timer thread when the timer triggers. It is
/// used to print debug info and, more importantly, to detect when we are out of
/// available time and thus stop the search.
void check_time() {
static TimePoint lastInfoTime = now();
TimePoint elapsed = now() - SearchTime;
if (now() - lastInfoTime >= 1000)
{
lastInfoTime = now();
dbg_print();
}
// An engine may not stop pondering until told so by the GUI
if (Limits.ponder)
return;
if (Limits.use_time_management())
{
bool stillAtFirstMove = Signals.firstRootMove
&& !Signals.failedLowAtRoot
&& elapsed > TimeMgr.available_time() * 75 / 100;
if ( stillAtFirstMove
|| elapsed > TimeMgr.maximum_time() - 2 * TimerThread::Resolution)
Signals.stop = true;
}
else if (Limits.movetime && elapsed >= Limits.movetime)
Signals.stop = true;
else if (Limits.nodes)
{
Threads.mutex.lock();
int64_t nodes = RootPos.nodes_searched();
// Loop across all split points and sum accumulated SplitPoint nodes plus
// all the currently active positions nodes.
for (Thread* th : Threads)
for (size_t i = 0; i < th->splitPointsSize; ++i)
{
SplitPoint& sp = th->splitPoints[i];
sp.mutex.lock();
nodes += sp.nodes;
for (size_t idx = 0; idx < Threads.size(); ++idx)
if (sp.slavesMask.test(idx) && Threads[idx]->activePosition)
nodes += Threads[idx]->activePosition->nodes_searched();
sp.mutex.unlock();
}
Threads.mutex.unlock();
if (nodes >= Limits.nodes)
Signals.stop = true;
}
}
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