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droidfish/DroidFish/jni/stockfish/search.cpp
Peter Osterlund 34b9fd139f DroidFish: Updated stockfish engine to version 3.
2013-05-03 17:03:42 +00:00

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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) 2008-2013 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>
#include <iostream>
#include <sstream>
#include "book.h"
#include "evaluate.h"
#include "movegen.h"
#include "movepick.h"
#include "notation.h"
#include "search.h"
#include "timeman.h"
#include "thread.h"
#include "tt.h"
#include "ucioption.h"
namespace Search {
volatile SignalsType Signals;
LimitsType Limits;
std::vector<RootMove> RootMoves;
Position RootPos;
Color RootColor;
Time::point SearchTime;
StateStackPtr SetupStates;
}
using std::string;
using Eval::evaluate;
using namespace Search;
namespace {
// Set to true to force running with one thread. Used for debugging
const bool FakeSplit = false;
// This is the minimum interval in msec between two check_time() calls
const int TimerResolution = 5;
// Different node types, used as template parameter
enum NodeType { Root, PV, NonPV, SplitPointRoot, SplitPointPV, SplitPointNonPV };
// Dynamic razoring margin based on depth
inline Value razor_margin(Depth d) { return Value(512 + 16 * int(d)); }
// Futility lookup tables (initialized at startup) and their access functions
Value FutilityMargins[16][64]; // [depth][moveNumber]
int FutilityMoveCounts[32]; // [depth]
inline Value futility_margin(Depth d, int mn) {
return d < 7 * ONE_PLY ? FutilityMargins[std::max(int(d), 1)][std::min(mn, 63)]
: 2 * VALUE_INFINITE;
}
// Reduction lookup tables (initialized at startup) and their access function
int8_t Reductions[2][64][64]; // [pv][depth][moveNumber]
template <bool PvNode> inline Depth reduction(Depth d, int mn) {
return (Depth) Reductions[PvNode][std::min(int(d) / ONE_PLY, 63)][std::min(mn, 63)];
}
size_t PVSize, PVIdx;
TimeManager TimeMgr;
int BestMoveChanges;
Value DrawValue[COLOR_NB];
History Hist;
Gains Gain;
template <NodeType NT>
Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth);
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);
bool check_is_dangerous(const Position& pos, Move move, Value futilityBase, Value beta);
bool allows(const Position& pos, Move first, Move second);
bool refutes(const Position& pos, Move first, Move second);
string uci_pv(const Position& pos, int depth, Value alpha, Value beta);
struct Skill {
Skill(int l) : level(l), best(MOVE_NONE) {}
~Skill() {
if (enabled()) // Swap best PV line with the sub-optimal one
std::swap(RootMoves[0], *std::find(RootMoves.begin(),
RootMoves.end(), best ? best : pick_move()));
}
bool enabled() const { return level < 20; }
bool time_to_pick(int depth) const { return depth == 1 + level; }
Move pick_move();
int level;
Move best;
};
} // namespace
/// Search::init() is called during startup to initialize various lookup tables
void Search::init() {
int d; // depth (ONE_PLY == 2)
int hd; // half depth (ONE_PLY == 1)
int mc; // moveCount
// Init reductions array
for (hd = 1; hd < 64; hd++) for (mc = 1; mc < 64; mc++)
{
double pvRed = log(double(hd)) * log(double(mc)) / 3.0;
double nonPVRed = 0.33 + log(double(hd)) * log(double(mc)) / 2.25;
Reductions[1][hd][mc] = (int8_t) ( pvRed >= 1.0 ? floor( pvRed * int(ONE_PLY)) : 0);
Reductions[0][hd][mc] = (int8_t) (nonPVRed >= 1.0 ? floor(nonPVRed * int(ONE_PLY)) : 0);
}
// Init futility margins array
for (d = 1; d < 16; d++) for (mc = 0; mc < 64; mc++)
FutilityMargins[d][mc] = Value(112 * int(log(double(d * d) / 2) / log(2.0) + 1.001) - 8 * mc + 45);
// Init futility move count array
for (d = 0; d < 32; d++)
FutilityMoveCounts[d] = int(3.001 + 0.25 * pow(double(d), 2.0));
}
/// 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.
size_t Search::perft(Position& pos, Depth depth) {
// At the last ply just return the number of legal moves (leaf nodes)
if (depth == ONE_PLY)
return MoveList<LEGAL>(pos).size();
StateInfo st;
size_t cnt = 0;
CheckInfo ci(pos);
for (MoveList<LEGAL> ml(pos); !ml.end(); ++ml)
{
pos.do_move(ml.move(), st, ci, pos.move_gives_check(ml.move(), ci));
cnt += perft(pos, depth - ONE_PLY);
pos.undo_move(ml.move());
}
return cnt;
}
/// 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() {
static PolyglotBook book; // Defined static to initialize the PRNG only once
RootColor = RootPos.side_to_move();
TimeMgr.init(Limits, RootPos.game_ply(), RootColor);
if (RootMoves.empty())
{
RootMoves.push_back(MOVE_NONE);
sync_cout << "info depth 0 score "
<< score_to_uci(RootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
<< sync_endl;
goto finalize;
}
if (Options["OwnBook"] && !Limits.infinite && !Limits.mate)
{
Move bookMove = book.probe(RootPos, Options["Book File"], Options["Best Book Move"]);
if (bookMove && std::count(RootMoves.begin(), RootMoves.end(), bookMove))
{
std::swap(RootMoves[0], *std::find(RootMoves.begin(), RootMoves.end(), bookMove));
goto finalize;
}
}
if (Options["Contempt Factor"] && !Options["UCI_AnalyseMode"])
{
int cf = Options["Contempt Factor"] * PawnValueMg / 100; // From centipawns
cf = cf * Material::game_phase(RootPos) / PHASE_MIDGAME; // Scale down with phase
DrawValue[ RootColor] = VALUE_DRAW - Value(cf);
DrawValue[~RootColor] = VALUE_DRAW + Value(cf);
}
else
DrawValue[WHITE] = DrawValue[BLACK] = VALUE_DRAW;
if (Options["Use Search Log"])
{
Log log(Options["Search Log Filename"]);
log << "\nSearching: " << RootPos.fen()
<< "\ninfinite: " << Limits.infinite
<< " ponder: " << Limits.ponder
<< " time: " << Limits.time[RootColor]
<< " increment: " << Limits.inc[RootColor]
<< " moves to go: " << Limits.movestogo
<< std::endl;
}
// Reset the threads, still sleeping: will be wake up at split time
for (size_t i = 0; i < Threads.size(); i++)
Threads[i]->maxPly = 0;
Threads.sleepWhileIdle = Options["Use Sleeping Threads"];
// Set best timer interval to avoid lagging under time pressure. Timer is
// used to check for remaining available thinking time.
Threads.timer->msec =
Limits.use_time_management() ? std::min(100, std::max(TimeMgr.available_time() / 16, TimerResolution)) :
Limits.nodes ? 2 * TimerResolution
: 100;
Threads.timer->notify_one(); // Wake up the recurring timer
id_loop(RootPos); // Let's start searching !
Threads.timer->msec = 0; // Stop the timer
Threads.sleepWhileIdle = true; // Send idle threads to sleep
if (Options["Use Search Log"])
{
Time::point elapsed = Time::now() - SearchTime + 1;
Log log(Options["Search Log Filename"]);
log << "Nodes: " << RootPos.nodes_searched()
<< "\nNodes/second: " << RootPos.nodes_searched() * 1000 / elapsed
<< "\nBest move: " << move_to_san(RootPos, RootMoves[0].pv[0]);
StateInfo st;
RootPos.do_move(RootMoves[0].pv[0], st);
log << "\nPonder move: " << move_to_san(RootPos, RootMoves[0].pv[1]) << std::endl;
RootPos.undo_move(RootMoves[0].pv[0]);
}
finalize:
// When we reach max depth we arrive here even without Signals.stop is raised,
// but if we are pondering or in infinite search, according to UCI protocol,
// we shouldn't print the best move before the GUI sends a "stop" or "ponderhit"
// command. We simply wait here until GUI sends one of those commands (that
// raise Signals.stop).
if (!Signals.stop && (Limits.ponder || Limits.infinite))
{
Signals.stopOnPonderhit = true;
RootPos.this_thread()->wait_for(Signals.stop);
}
// Best move could be MOVE_NONE when searching on a stalemate position
sync_cout << "bestmove " << move_to_uci(RootMoves[0].pv[0], RootPos.is_chess960())
<< " ponder " << move_to_uci(RootMoves[0].pv[1], RootPos.is_chess960())
<< 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 ss[MAX_PLY_PLUS_2];
int depth, prevBestMoveChanges;
Value bestValue, alpha, beta, delta;
memset(ss, 0, 4 * sizeof(Stack));
depth = BestMoveChanges = 0;
bestValue = delta = -VALUE_INFINITE;
ss->currentMove = MOVE_NULL; // Hack to skip update gains
TT.new_search();
Hist.clear();
Gain.clear();
PVSize = Options["MultiPV"];
Skill skill(Options["Skill Level"]);
// Do we have to play with skill handicap? In this case enable MultiPV search
// that we will use behind the scenes to retrieve a set of possible moves.
if (skill.enabled() && PVSize < 4)
PVSize = 4;
PVSize = std::min(PVSize, RootMoves.size());
// Iterative deepening loop until requested to stop or target depth reached
while (++depth <= MAX_PLY && !Signals.stop && (!Limits.depth || depth <= Limits.depth))
{
// Save last iteration's scores before first PV line is searched and all
// the move scores but the (new) PV are set to -VALUE_INFINITE.
for (size_t i = 0; i < RootMoves.size(); i++)
RootMoves[i].prevScore = RootMoves[i].score;
prevBestMoveChanges = BestMoveChanges; // Only sensible when PVSize == 1
BestMoveChanges = 0;
// MultiPV loop. We perform a full root search for each PV line
for (PVIdx = 0; PVIdx < PVSize; PVIdx++)
{
// Set aspiration window default width
if (depth >= 5 && abs(RootMoves[PVIdx].prevScore) < VALUE_KNOWN_WIN)
{
delta = Value(16);
alpha = RootMoves[PVIdx].prevScore - delta;
beta = RootMoves[PVIdx].prevScore + delta;
}
else
{
alpha = -VALUE_INFINITE;
beta = VALUE_INFINITE;
}
// Start with a small aspiration window and, in case of fail high/low,
// research with bigger window until not failing high/low anymore.
while (true)
{
// Search starts from ss+1 to allow referencing (ss-1). This is
// needed by update gains and ss copy when splitting at Root.
bestValue = search<Root>(pos, ss+1, alpha, beta, depth * ONE_PLY);
// Bring to front the best move. 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 but 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 return immediately. Sorting and
// writing PV back to TT is safe becuase RootMoves is still
// valid, although refers to previous iteration.
if (Signals.stop)
return;
// In case of failing high/low increase aspiration window and
// research, otherwise exit the loop.
if (bestValue > alpha && bestValue < beta)
break;
// Give some update (without cluttering the UI) before to research
if (Time::now() - SearchTime > 3000)
sync_cout << uci_pv(pos, depth, alpha, beta) << sync_endl;
if (abs(bestValue) >= VALUE_KNOWN_WIN)
{
alpha = -VALUE_INFINITE;
beta = VALUE_INFINITE;
}
else if (bestValue >= beta)
{
beta += delta;
delta += delta / 2;
}
else
{
Signals.failedLowAtRoot = true;
Signals.stopOnPonderhit = false;
alpha -= delta;
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 (PVIdx + 1 == PVSize || Time::now() - SearchTime > 3000)
sync_cout << uci_pv(pos, depth, alpha, beta) << sync_endl;
}
// Do we need to pick now the sub-optimal best move ?
if (skill.enabled() && skill.time_to_pick(depth))
skill.pick_move();
if (Options["Use Search Log"])
{
Log log(Options["Search Log Filename"]);
log << pretty_pv(pos, depth, bestValue, Time::now() - SearchTime, &RootMoves[0].pv[0])
<< std::endl;
}
// Do we have 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.stopOnPonderhit)
{
bool stop = false; // Local variable, not the volatile Signals.stop
// Take in account some extra time if the best move has changed
if (depth > 4 && depth < 50 && PVSize == 1)
TimeMgr.pv_instability(BestMoveChanges, prevBestMoveChanges);
// Stop search if most of available time is already consumed. We
// probably don't have enough time to search the first move at the
// next iteration anyway.
if (Time::now() - SearchTime > (TimeMgr.available_time() * 62) / 100)
stop = true;
// Stop search early if one move seems to be much better than others
if ( depth >= 12
&& !stop
&& PVSize == 1
&& bestValue > VALUE_MATED_IN_MAX_PLY
&& ( RootMoves.size() == 1
|| Time::now() - SearchTime > (TimeMgr.available_time() * 20) / 100))
{
Value rBeta = bestValue - 2 * PawnValueMg;
(ss+1)->excludedMove = RootMoves[0].pv[0];
(ss+1)->skipNullMove = true;
Value v = search<NonPV>(pos, ss+1, rBeta - 1, rBeta, (depth - 3) * ONE_PLY);
(ss+1)->skipNullMove = false;
(ss+1)->excludedMove = MOVE_NONE;
if (v < rBeta)
stop = true;
}
if (stop)
{
// If we are allowed to ponder do not stop the search now but
// keep pondering until GUI sends "ponderhit" or "stop".
if (Limits.ponder)
Signals.stopOnPonderhit = true;
else
Signals.stop = true;
}
}
}
}
// 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, 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>
Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
const bool PvNode = (NT == PV || NT == Root || NT == SplitPointPV || NT == SplitPointRoot);
const bool SpNode = (NT == SplitPointPV || NT == SplitPointNonPV || NT == SplitPointRoot);
const bool RootNode = (NT == Root || NT == SplitPointRoot);
assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
assert(PvNode || (alpha == beta - 1));
assert(depth > DEPTH_ZERO);
Move movesSearched[64];
StateInfo st;
const TTEntry *tte;
SplitPoint* splitPoint;
Key posKey;
Move ttMove, move, excludedMove, bestMove, threatMove;
Depth ext, newDepth;
Value bestValue, value, ttValue;
Value eval, nullValue, futilityValue;
bool inCheck, givesCheck, pvMove, singularExtensionNode;
bool captureOrPromotion, dangerous, doFullDepthSearch;
int moveCount, playedMoveCount;
// Step 1. Initialize node
Thread* thisThread = pos.this_thread();
moveCount = playedMoveCount = 0;
inCheck = pos.checkers();
if (SpNode)
{
splitPoint = ss->splitPoint;
bestMove = splitPoint->bestMove;
threatMove = splitPoint->threatMove;
bestValue = splitPoint->bestValue;
tte = NULL;
ttMove = excludedMove = MOVE_NONE;
ttValue = VALUE_NONE;
assert(splitPoint->bestValue > -VALUE_INFINITE && splitPoint->moveCount > 0);
goto split_point_start;
}
bestValue = -VALUE_INFINITE;
ss->currentMove = threatMove = (ss+1)->excludedMove = bestMove = MOVE_NONE;
ss->ply = (ss-1)->ply + 1;
ss->futilityMoveCount = 0;
(ss+1)->skipNullMove = false; (ss+1)->reduction = DEPTH_ZERO;
(ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
// 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 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
// further, we will never beat 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;
}
// 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);
ttMove = RootNode ? RootMoves[PVIdx].pv[0] : tte ? tte->move() : MOVE_NONE;
ttValue = tte ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
// At PV nodes we check for exact scores, while at non-PV nodes we check for
// a fail high/low. Biggest advantage at probing at PV nodes is to have a
// smooth experience in analysis mode. We don't probe at Root nodes otherwise
// we should also update RootMoveList to avoid bogus output.
if ( !RootNode
&& tte
&& tte->depth() >= depth
&& ttValue != VALUE_NONE // Only in case of TT access race
&& ( PvNode ? tte->type() == BOUND_EXACT
: ttValue >= beta ? (tte->type() & BOUND_LOWER)
: (tte->type() & BOUND_UPPER)))
{
TT.refresh(tte);
ss->currentMove = ttMove; // Can be MOVE_NONE
if ( ttValue >= beta
&& ttMove
&& !pos.is_capture_or_promotion(ttMove)
&& ttMove != ss->killers[0])
{
ss->killers[1] = ss->killers[0];
ss->killers[0] = ttMove;
}
return ttValue;
}
// Step 5. Evaluate the position statically and update parent's gain statistics
if (inCheck)
ss->staticEval = ss->evalMargin = eval = VALUE_NONE;
else if (tte)
{
// Never assume anything on values stored in TT
if ( (ss->staticEval = eval = tte->eval_value()) == VALUE_NONE
||(ss->evalMargin = tte->eval_margin()) == VALUE_NONE)
eval = ss->staticEval = evaluate(pos, ss->evalMargin);
// Can ttValue be used as a better position evaluation?
if (ttValue != VALUE_NONE)
if ( ((tte->type() & BOUND_LOWER) && ttValue > eval)
|| ((tte->type() & BOUND_UPPER) && ttValue < eval))
eval = ttValue;
}
else
{
eval = ss->staticEval = evaluate(pos, ss->evalMargin);
TT.store(posKey, VALUE_NONE, BOUND_NONE, DEPTH_NONE, MOVE_NONE,
ss->staticEval, ss->evalMargin);
}
// Update gain for the parent non-capture move given the static position
// evaluation before and after the move.
if ( (move = (ss-1)->currentMove) != MOVE_NULL
&& (ss-1)->staticEval != VALUE_NONE
&& ss->staticEval != VALUE_NONE
&& !pos.captured_piece_type()
&& type_of(move) == NORMAL)
{
Square to = to_sq(move);
Gain.update(pos.piece_on(to), to, -(ss-1)->staticEval - ss->staticEval);
}
// Step 6. Razoring (is omitted in PV nodes)
if ( !PvNode
&& depth < 4 * ONE_PLY
&& !inCheck
&& eval + razor_margin(depth) < beta
&& ttMove == MOVE_NONE
&& abs(beta) < VALUE_MATE_IN_MAX_PLY
&& !pos.pawn_on_7th(pos.side_to_move()))
{
Value rbeta = beta - razor_margin(depth);
Value v = qsearch<NonPV, false>(pos, ss, rbeta-1, rbeta, DEPTH_ZERO);
if (v < rbeta)
// Logically we should return (v + razor_margin(depth)), but
// surprisingly this did slightly weaker in tests.
return v;
}
// Step 7. Static null move pruning (is omitted in PV nodes)
// We're betting that the opponent doesn't have a move that will reduce
// the score by more than futility_margin(depth) if we do a null move.
if ( !PvNode
&& !ss->skipNullMove
&& depth < 4 * ONE_PLY
&& !inCheck
&& eval - futility_margin(depth, (ss-1)->futilityMoveCount) >= beta
&& abs(beta) < VALUE_MATE_IN_MAX_PLY
&& abs(eval) < VALUE_KNOWN_WIN
&& pos.non_pawn_material(pos.side_to_move()))
return eval - futility_margin(depth, (ss-1)->futilityMoveCount);
// Step 8. Null move search with verification search (is omitted in PV nodes)
if ( !PvNode
&& !ss->skipNullMove
&& depth > ONE_PLY
&& !inCheck
&& eval >= beta
&& abs(beta) < VALUE_MATE_IN_MAX_PLY
&& pos.non_pawn_material(pos.side_to_move()))
{
ss->currentMove = MOVE_NULL;
// Null move dynamic reduction based on depth
Depth R = 3 * ONE_PLY + depth / 4;
// Null move dynamic reduction based on value
if (eval - PawnValueMg > beta)
R += ONE_PLY;
pos.do_null_move(st);
(ss+1)->skipNullMove = true;
nullValue = depth-R < ONE_PLY ? -qsearch<NonPV, false>(pos, ss+1, -beta, -alpha, DEPTH_ZERO)
: - search<NonPV>(pos, ss+1, -beta, -alpha, depth-R);
(ss+1)->skipNullMove = 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)
return nullValue;
// Do verification search at high depths
ss->skipNullMove = true;
Value v = search<NonPV>(pos, ss, alpha, beta, depth-R);
ss->skipNullMove = false;
if (v >= beta)
return nullValue;
}
else
{
// The null move failed low, which means that we may be faced with
// some kind of threat. If the previous move was reduced, check if
// the move that refuted the null move was somehow connected to the
// move which was reduced. If a connection is found, return a fail
// low score (which will cause the reduced move to fail high in the
// parent node, which will trigger a re-search with full depth).
threatMove = (ss+1)->currentMove;
if ( depth < 5 * ONE_PLY
&& (ss-1)->reduction
&& threatMove != MOVE_NONE
&& allows(pos, (ss-1)->currentMove, threatMove))
return beta - 1;
}
}
// Step 9. ProbCut (is omitted in PV nodes)
// 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
&& !inCheck
&& !ss->skipNullMove
&& excludedMove == MOVE_NONE
&& abs(beta) < VALUE_MATE_IN_MAX_PLY)
{
Value rbeta = beta + 200;
Depth rdepth = depth - ONE_PLY - 3 * ONE_PLY;
assert(rdepth >= ONE_PLY);
assert((ss-1)->currentMove != MOVE_NONE);
assert((ss-1)->currentMove != MOVE_NULL);
MovePicker mp(pos, ttMove, Hist, pos.captured_piece_type());
CheckInfo ci(pos);
while ((move = mp.next_move<false>()) != MOVE_NONE)
if (pos.pl_move_is_legal(move, ci.pinned))
{
ss->currentMove = move;
pos.do_move(move, st, ci, pos.move_gives_check(move, ci));
value = -search<NonPV>(pos, ss+1, -rbeta, -rbeta+1, rdepth);
pos.undo_move(move);
if (value >= rbeta)
return value;
}
}
// Step 10. Internal iterative deepening
if ( depth >= (PvNode ? 5 * ONE_PLY : 8 * ONE_PLY)
&& ttMove == MOVE_NONE
&& (PvNode || (!inCheck && ss->staticEval + Value(256) >= beta)))
{
Depth d = depth - 2 * ONE_PLY - (PvNode ? DEPTH_ZERO : depth / 4);
ss->skipNullMove = true;
search<PvNode ? PV : NonPV>(pos, ss, alpha, beta, d);
ss->skipNullMove = false;
tte = TT.probe(posKey);
ttMove = tte ? tte->move() : MOVE_NONE;
}
split_point_start: // At split points actual search starts from here
MovePicker mp(pos, ttMove, depth, Hist, ss, PvNode ? -VALUE_INFINITE : beta);
CheckInfo ci(pos);
value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
singularExtensionNode = !RootNode
&& !SpNode
&& depth >= (PvNode ? 6 * ONE_PLY : 8 * ONE_PLY)
&& ttMove != MOVE_NONE
&& !excludedMove // Recursive singular search is not allowed
&& (tte->type() & 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 move turns out to be illegal
if (!pos.pl_move_is_legal(move, ci.pinned))
continue;
moveCount = ++splitPoint->moveCount;
splitPoint->mutex.unlock();
}
else
moveCount++;
if (RootNode)
{
Signals.firstRootMove = (moveCount == 1);
if (thisThread == Threads.main_thread() && Time::now() - SearchTime > 3000)
sync_cout << "info depth " << depth / ONE_PLY
<< " currmove " << move_to_uci(move, pos.is_chess960())
<< " currmovenumber " << moveCount + PVIdx << sync_endl;
}
ext = DEPTH_ZERO;
captureOrPromotion = pos.is_capture_or_promotion(move);
givesCheck = pos.move_gives_check(move, ci);
dangerous = givesCheck
|| pos.is_passed_pawn_push(move)
|| type_of(move) == CASTLE
|| ( captureOrPromotion // Entering a pawn endgame?
&& type_of(pos.piece_on(to_sq(move))) != PAWN
&& type_of(move) == NORMAL
&& ( pos.non_pawn_material(WHITE) + pos.non_pawn_material(BLACK)
- PieceValue[MG][pos.piece_on(to_sq(move))] == VALUE_ZERO));
// Step 12. Extend checks and, in PV nodes, also dangerous moves
if (PvNode && dangerous)
ext = ONE_PLY;
else if (givesCheck && pos.see_sign(move) >= 0)
ext = ONE_PLY / 2;
// 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, if result is lower than ttValue minus
// a margin then we extend ttMove.
if ( singularExtensionNode
&& move == ttMove
&& !ext
&& pos.pl_move_is_legal(move, ci.pinned)
&& abs(ttValue) < VALUE_KNOWN_WIN)
{
assert(ttValue != VALUE_NONE);
Value rBeta = ttValue - int(depth);
ss->excludedMove = move;
ss->skipNullMove = true;
value = search<NonPV>(pos, ss, rBeta - 1, rBeta, depth / 2);
ss->skipNullMove = false;
ss->excludedMove = MOVE_NONE;
if (value < rBeta)
ext = ONE_PLY;
}
// Update current move (this must be done after singular extension search)
newDepth = depth - ONE_PLY + ext;
// Step 13. Futility pruning (is omitted in PV nodes)
if ( !PvNode
&& !captureOrPromotion
&& !inCheck
&& !dangerous
/* && move != ttMove Already implicit in the next condition */
&& bestValue > VALUE_MATED_IN_MAX_PLY)
{
// Move count based pruning
if ( depth < 16 * ONE_PLY
&& moveCount >= FutilityMoveCounts[depth]
&& (!threatMove || !refutes(pos, move, threatMove)))
{
if (SpNode)
splitPoint->mutex.lock();
continue;
}
// Value based pruning
// We illogically ignore reduction condition depth >= 3*ONE_PLY for predicted depth,
// but fixing this made program slightly weaker.
Depth predictedDepth = newDepth - reduction<PvNode>(depth, moveCount);
futilityValue = ss->staticEval + ss->evalMargin + futility_margin(predictedDepth, moveCount)
+ Gain[pos.piece_moved(move)][to_sq(move)];
if (futilityValue < beta)
{
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) < 0)
{
if (SpNode)
splitPoint->mutex.lock();
continue;
}
// We have not pruned the move that will be searched, but remember how
// far in the move list we are to be more aggressive in the child node.
ss->futilityMoveCount = moveCount;
}
else
ss->futilityMoveCount = 0;
// Check for legality only before to do the move
if (!RootNode && !SpNode && !pos.pl_move_is_legal(move, ci.pinned))
{
moveCount--;
continue;
}
pvMove = PvNode && moveCount == 1;
ss->currentMove = move;
if (!SpNode && !captureOrPromotion && playedMoveCount < 64)
movesSearched[playedMoveCount++] = move;
// Step 14. Make the move
pos.do_move(move, st, ci, givesCheck);
// Step 15. Reduced depth search (LMR). If the move fails high will be
// re-searched at full depth.
if ( depth > 3 * ONE_PLY
&& !pvMove
&& !captureOrPromotion
&& !dangerous
&& move != ttMove
&& move != ss->killers[0]
&& move != ss->killers[1])
{
ss->reduction = reduction<PvNode>(depth, moveCount);
Depth d = std::max(newDepth - ss->reduction, ONE_PLY);
if (SpNode)
alpha = splitPoint->alpha;
value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d);
doFullDepthSearch = (value > alpha && ss->reduction != DEPTH_ZERO);
ss->reduction = DEPTH_ZERO;
}
else
doFullDepthSearch = !pvMove;
// 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>(pos, ss+1, -(alpha+1), -alpha, newDepth);
}
// Only for PV nodes 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 to fail low with value <= alpha and to try another move.
if (PvNode && (pvMove || (value > alpha && (RootNode || value < beta))))
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>(pos, ss+1, -beta, -alpha, newDepth);
// 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 Signals.stop is true, the search
// was aborted because the user interrupted the search or because we
// ran out of time. In this case, the return value of the search cannot
// be trusted, and we don't update the best move and/or PV.
if (Signals.stop || thisThread->cutoff_occurred())
return value; // To avoid returning VALUE_INFINITE
if (RootNode)
{
RootMove& rm = *std::find(RootMoves.begin(), RootMoves.end(), move);
// PV move or new best move ?
if (pvMove || value > alpha)
{
rm.score = value;
rm.extract_pv_from_tt(pos);
// 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 (!pvMove)
BestMoveChanges++;
}
else
// All other moves but the PV are set to the lowest value, this
// is not a problem when sorting becuase sort is stable and 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 && 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
&& depth >= Threads.minimumSplitDepth
&& Threads.available_slave(thisThread)
&& thisThread->splitPointsSize < MAX_SPLITPOINTS_PER_THREAD)
{
assert(bestValue < beta);
thisThread->split<FakeSplit>(pos, ss, alpha, beta, &bestValue, &bestMove,
depth, threatMove, moveCount, &mp, NT);
if (bestValue >= beta)
break;
}
}
if (SpNode)
return bestValue;
// 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. Note that we can have a false positive in
// case of Signals.stop or thread.cutoff_occurred() are set, but this is
// harmless because return value is discarded anyhow in the parent nodes.
// If we are in a singular extension search then return a fail low score.
// A split node has at least one move, the one tried before to be splitted.
if (!moveCount)
return excludedMove ? alpha
: inCheck ? mated_in(ss->ply) : DrawValue[pos.side_to_move()];
// If we have pruned all the moves without searching return a fail-low score
if (bestValue == -VALUE_INFINITE)
{
assert(!playedMoveCount);
bestValue = alpha;
}
if (bestValue >= beta) // Failed high
{
TT.store(posKey, value_to_tt(bestValue, ss->ply), BOUND_LOWER, depth,
bestMove, ss->staticEval, ss->evalMargin);
if (!pos.is_capture_or_promotion(bestMove) && !inCheck)
{
if (bestMove != ss->killers[0])
{
ss->killers[1] = ss->killers[0];
ss->killers[0] = bestMove;
}
// Increase history value of the cut-off move
Value bonus = Value(int(depth) * int(depth));
Hist.update(pos.piece_moved(bestMove), to_sq(bestMove), bonus);
// Decrease history of all the other played non-capture moves
for (int i = 0; i < playedMoveCount - 1; i++)
{
Move m = movesSearched[i];
Hist.update(pos.piece_moved(m), to_sq(m), -bonus);
}
}
}
else // Failed low or PV search
TT.store(posKey, value_to_tt(bestValue, ss->ply),
PvNode && bestMove != MOVE_NONE ? BOUND_EXACT : BOUND_UPPER,
depth, bestMove, ss->staticEval, ss->evalMargin);
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);
StateInfo st;
const TTEntry* tte;
Key posKey;
Move ttMove, move, bestMove;
Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
bool givesCheck, enoughMaterial, evasionPrunable;
Depth ttDepth;
// To flag BOUND_EXACT a node with eval above alpha and no available moves
if (PvNode)
oldAlpha = alpha;
ss->currentMove = bestMove = MOVE_NONE;
ss->ply = (ss-1)->ply + 1;
// Check for an instant draw or maximum ply reached
if (pos.is_draw() || ss->ply > MAX_PLY)
return DrawValue[pos.side_to_move()];
// 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. At PV nodes, we don't use the TT for
// pruning, but only for move ordering.
posKey = pos.key();
tte = TT.probe(posKey);
ttMove = tte ? tte->move() : MOVE_NONE;
ttValue = tte ? value_from_tt(tte->value(),ss->ply) : VALUE_NONE;
if ( tte
&& tte->depth() >= ttDepth
&& ttValue != VALUE_NONE // Only in case of TT access race
&& ( PvNode ? tte->type() == BOUND_EXACT
: ttValue >= beta ? (tte->type() & BOUND_LOWER)
: (tte->type() & BOUND_UPPER)))
{
ss->currentMove = ttMove; // Can be MOVE_NONE
return ttValue;
}
// Evaluate the position statically
if (InCheck)
{
ss->staticEval = ss->evalMargin = VALUE_NONE;
bestValue = futilityBase = -VALUE_INFINITE;
enoughMaterial = false;
}
else
{
if (tte)
{
// Never assume anything on values stored in TT
if ( (ss->staticEval = bestValue = tte->eval_value()) == VALUE_NONE
||(ss->evalMargin = tte->eval_margin()) == VALUE_NONE)
ss->staticEval = bestValue = evaluate(pos, ss->evalMargin);
}
else
ss->staticEval = bestValue = evaluate(pos, ss->evalMargin);
// Stand pat. Return immediately if static value is at least beta
if (bestValue >= beta)
{
if (!tte)
TT.store(pos.key(), value_to_tt(bestValue, ss->ply), BOUND_LOWER,
DEPTH_NONE, MOVE_NONE, ss->staticEval, ss->evalMargin);
return bestValue;
}
if (PvNode && bestValue > alpha)
alpha = bestValue;
futilityBase = ss->staticEval + ss->evalMargin + Value(128);
enoughMaterial = pos.non_pawn_material(pos.side_to_move()) > RookValueMg;
}
// 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, Hist, 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 = pos.move_gives_check(move, ci);
// Futility pruning
if ( !PvNode
&& !InCheck
&& !givesCheck
&& move != ttMove
&& enoughMaterial
&& type_of(move) != PROMOTION
&& !pos.is_passed_pawn_push(move))
{
futilityValue = futilityBase
+ PieceValue[EG][pos.piece_on(to_sq(move))]
+ (type_of(move) == ENPASSANT ? PawnValueEg : VALUE_ZERO);
if (futilityValue < beta)
{
bestValue = std::max(bestValue, futilityValue);
continue;
}
// Prune moves with negative or equal SEE and also moves with positive
// SEE where capturing piece loses a tempo and SEE < beta - futilityBase.
if ( futilityBase < beta
&& depth < DEPTH_ZERO
&& pos.see(move, beta - futilityBase) <= 0)
{
bestValue = std::max(bestValue, futilityBase);
continue;
}
}
// Detect non-capture evasions that are candidate to be pruned
evasionPrunable = !PvNode
&& InCheck
&& bestValue > VALUE_MATED_IN_MAX_PLY
&& !pos.is_capture(move)
&& !pos.can_castle(pos.side_to_move());
// Don't search moves with negative SEE values
if ( !PvNode
&& (!InCheck || evasionPrunable)
&& move != ttMove
&& type_of(move) != PROMOTION
&& pos.see_sign(move) < 0)
continue;
// Don't search useless checks
if ( !PvNode
&& !InCheck
&& givesCheck
&& move != ttMove
&& !pos.is_capture_or_promotion(move)
&& ss->staticEval + PawnValueMg / 4 < beta
&& !check_is_dangerous(pos, move, futilityBase, beta))
continue;
// Check for legality only before to do the move
if (!pos.pl_move_is_legal(move, ci.pinned))
continue;
ss->currentMove = move;
// Make and search the move
pos.do_move(move, st, ci, 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 && value < beta) // Update alpha here! Always alpha < beta
{
alpha = value;
bestMove = move;
}
else // Fail high
{
TT.store(posKey, value_to_tt(value, ss->ply), BOUND_LOWER,
ttDepth, move, ss->staticEval, ss->evalMargin);
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
TT.store(posKey, value_to_tt(bestValue, ss->ply),
PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
ttDepth, bestMove, ss->staticEval, ss->evalMargin);
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 to 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 (where 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;
}
// check_is_dangerous() tests if a checking move can be pruned in qsearch()
bool check_is_dangerous(const Position& pos, Move move, Value futilityBase, Value beta)
{
Piece pc = pos.piece_moved(move);
Square from = from_sq(move);
Square to = to_sq(move);
Color them = ~pos.side_to_move();
Square ksq = pos.king_square(them);
Bitboard enemies = pos.pieces(them);
Bitboard kingAtt = pos.attacks_from<KING>(ksq);
Bitboard occ = pos.pieces() ^ from ^ ksq;
Bitboard oldAtt = pos.attacks_from(pc, from, occ);
Bitboard newAtt = pos.attacks_from(pc, to, occ);
// Checks which give opponent's king at most one escape square are dangerous
if (!more_than_one(kingAtt & ~(enemies | newAtt | to)))
return true;
// Queen contact check is very dangerous
if (type_of(pc) == QUEEN && (kingAtt & to))
return true;
// Creating new double threats with checks is dangerous
Bitboard b = (enemies ^ ksq) & newAtt & ~oldAtt;
while (b)
{
// Note that here we generate illegal "double move"!
if (futilityBase + PieceValue[EG][pos.piece_on(pop_lsb(&b))] >= beta)
return true;
}
return false;
}
// allows() tests whether the 'first' move at previous ply somehow makes the
// 'second' move possible, for instance if the moving piece is the same in
// both moves. Normally the second move is the threat (the best move returned
// from a null search that fails low).
bool allows(const Position& pos, Move first, Move second) {
assert(is_ok(first));
assert(is_ok(second));
assert(color_of(pos.piece_on(from_sq(second))) == ~pos.side_to_move());
assert(color_of(pos.piece_on(to_sq(first))) == ~pos.side_to_move());
Square m1from = from_sq(first);
Square m2from = from_sq(second);
Square m1to = to_sq(first);
Square m2to = to_sq(second);
// The piece is the same or second's destination was vacated by the first move
if (m1to == m2from || m2to == m1from)
return true;
// Second one moves through the square vacated by first one
if (between_bb(m2from, m2to) & m1from)
return true;
// Second's destination is defended by the first move's piece
Bitboard m1att = pos.attacks_from(pos.piece_on(m1to), m1to, pos.pieces() ^ m2from);
if (m1att & m2to)
return true;
// Second move gives a discovered check through the first's checking piece
if (m1att & pos.king_square(pos.side_to_move()))
{
assert(between_bb(m1to, pos.king_square(pos.side_to_move())) & m2from);
return true;
}
return false;
}
// refutes() tests whether a 'first' move is able to defend against a 'second'
// opponent's move. In this case will not be pruned. Normally the second move
// is the threat (the best move returned from a null search that fails low).
bool refutes(const Position& pos, Move first, Move second) {
assert(is_ok(first));
assert(is_ok(second));
Square m1from = from_sq(first);
Square m2from = from_sq(second);
Square m1to = to_sq(first);
Square m2to = to_sq(second);
// Don't prune moves of the threatened piece
if (m1from == m2to)
return true;
// If the threatened piece has value less than or equal to the value of the
// threat piece, don't prune moves which defend it.
if ( pos.is_capture(second)
&& ( PieceValue[MG][pos.piece_on(m2from)] >= PieceValue[MG][pos.piece_on(m2to)]
|| type_of(pos.piece_on(m2from)) == KING))
{
// Update occupancy as if the piece and the threat are moving
Bitboard occ = pos.pieces() ^ m1from ^ m1to ^ m2from;
Piece piece = pos.piece_on(m1from);
// The moved piece attacks the square 'tto' ?
if (pos.attacks_from(piece, m1to, occ) & m2to)
return true;
// Scan for possible X-ray attackers behind the moved piece
Bitboard xray = (attacks_bb< ROOK>(m2to, occ) & pos.pieces(color_of(piece), QUEEN, ROOK))
| (attacks_bb<BISHOP>(m2to, occ) & pos.pieces(color_of(piece), QUEEN, BISHOP));
// Verify attackers are triggered by our move and not already existing
if (xray && (xray ^ (xray & pos.attacks_from<QUEEN>(m2to))))
return true;
}
// Don't prune safe moves which block the threat path
if ((between_bb(m2from, m2to) & m1to) && pos.see_sign(first) >= 0)
return true;
return false;
}
// When playing with strength handicap choose best move among the MultiPV set
// using a statistical rule dependent on 'level'. Idea by Heinz van Saanen.
Move Skill::pick_move() {
static RKISS rk;
// PRNG sequence should be not deterministic
for (int i = Time::now() % 50; i > 0; i--)
rk.rand<unsigned>();
// RootMoves are already sorted by score in descending order
int variance = std::min(RootMoves[0].score - RootMoves[PVSize - 1].score, PawnValueMg);
int weakness = 120 - 2 * level;
int max_s = -VALUE_INFINITE;
best = MOVE_NONE;
// Choose best move. For each move score we add two terms both dependent on
// weakness, one deterministic and bigger for weaker moves, and one random,
// then we choose the move with the resulting highest score.
for (size_t i = 0; i < PVSize; i++)
{
int s = RootMoves[i].score;
// Don't allow crazy blunders even at very low skills
if (i > 0 && RootMoves[i-1].score > s + 2 * PawnValueMg)
break;
// This is our magic formula
s += ( weakness * int(RootMoves[0].score - s)
+ variance * (rk.rand<unsigned>() % weakness)) / 128;
if (s > max_s)
{
max_s = s;
best = RootMoves[i].pv[0];
}
}
return best;
}
// uci_pv() formats PV information according to UCI protocol. UCI requires
// to send all the PV lines also if are still to be searched and so refer to
// the previous search score.
string uci_pv(const Position& pos, int depth, Value alpha, Value beta) {
std::stringstream s;
Time::point elaspsed = Time::now() - SearchTime + 1;
size_t uciPVSize = std::min((size_t)Options["MultiPV"], RootMoves.size());
int selDepth = 0;
for (size_t i = 0; i < Threads.size(); i++)
if (Threads[i]->maxPly > selDepth)
selDepth = Threads[i]->maxPly;
for (size_t i = 0; i < uciPVSize; i++)
{
bool updated = (i <= PVIdx);
if (depth == 1 && !updated)
continue;
int d = updated ? depth : depth - 1;
Value v = updated ? RootMoves[i].score : RootMoves[i].prevScore;
if (s.rdbuf()->in_avail()) // Not at first line
s << "\n";
s << "info depth " << d
<< " seldepth " << selDepth
<< " score " << (i == PVIdx ? score_to_uci(v, alpha, beta) : score_to_uci(v))
<< " nodes " << pos.nodes_searched()
<< " nps " << pos.nodes_searched() * 1000 / elaspsed
<< " time " << elaspsed
<< " multipv " << i + 1
<< " pv";
for (size_t j = 0; RootMoves[i].pv[j] != MOVE_NONE; j++)
s << " " << move_to_uci(RootMoves[i].pv[j], pos.is_chess960());
}
return s.str();
}
} // namespace
/// RootMove::extract_pv_from_tt() builds a PV by adding moves from the TT table.
/// We consider also failing high nodes and not only BOUND_EXACT nodes so to
/// allow to always have a ponder move even when we fail high at root, and a
/// long PV to print that is important for position analysis.
void RootMove::extract_pv_from_tt(Position& pos) {
StateInfo state[MAX_PLY_PLUS_2], *st = state;
TTEntry* tte;
int ply = 0;
Move m = pv[0];
pv.clear();
do {
pv.push_back(m);
assert(MoveList<LEGAL>(pos).contains(pv[ply]));
pos.do_move(pv[ply++], *st++);
tte = TT.probe(pos.key());
} while ( tte
&& pos.is_pseudo_legal(m = tte->move()) // Local copy, TT could change
&& pos.pl_move_is_legal(m, pos.pinned_pieces())
&& ply < MAX_PLY
&& (!pos.is_draw() || ply < 2));
pv.push_back(MOVE_NONE); // Must be zero-terminating
while (ply) pos.undo_move(pv[--ply]);
}
/// 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_PLUS_2], *st = state;
TTEntry* tte;
int ply = 0;
do {
tte = TT.probe(pos.key());
if (!tte || tte->move() != pv[ply]) // Don't overwrite correct entries
TT.store(pos.key(), VALUE_NONE, BOUND_NONE, DEPTH_NONE, pv[ply], VALUE_NONE, VALUE_NONE);
assert(MoveList<LEGAL>(pos).contains(pv[ply]));
pos.do_move(pv[ply++], *st++);
} while (pv[ply] != MOVE_NONE);
while (ply) pos.undo_move(pv[--ply]);
}
/// 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. So it means we are the split point's master.
SplitPoint* this_sp = splitPointsSize ? activeSplitPoint : NULL;
assert(!this_sp || (this_sp->masterThread == this && searching));
while (true)
{
// If we are not searching, wait for a condition to be signaled instead of
// wasting CPU time polling for work.
while ((!searching && Threads.sleepWhileIdle) || exit)
{
if (exit)
{
assert(!this_sp);
return;
}
// Grab the lock to avoid races with Thread::notify_one()
mutex.lock();
// If we are master and all slaves have finished then exit idle_loop
if (this_sp && !this_sp->slavesMask)
{
mutex.unlock();
break;
}
// Do sleep after retesting sleep conditions under lock protection, in
// particular we need to avoid a deadlock in case a master thread has,
// in the meanwhile, allocated us and sent the notify_one() call before
// we had the chance to grab the lock.
if (!searching && !exit)
sleepCondition.wait(mutex);
mutex.unlock();
}
// If this thread has been assigned work, launch a search
if (searching)
{
assert(!exit);
Threads.mutex.lock();
assert(searching);
SplitPoint* sp = activeSplitPoint;
Threads.mutex.unlock();
Stack ss[MAX_PLY_PLUS_2];
Position pos(*sp->pos, this);
memcpy(ss, sp->ss - 1, 4 * sizeof(Stack));
(ss+1)->splitPoint = sp;
sp->mutex.lock();
assert(activePosition == NULL);
activePosition = &pos;
switch (sp->nodeType) {
case Root:
search<SplitPointRoot>(pos, ss+1, sp->alpha, sp->beta, sp->depth);
break;
case PV:
search<SplitPointPV>(pos, ss+1, sp->alpha, sp->beta, sp->depth);
break;
case NonPV:
search<SplitPointNonPV>(pos, ss+1, sp->alpha, sp->beta, sp->depth);
break;
default:
assert(false);
}
assert(searching);
searching = false;
activePosition = NULL;
sp->slavesMask &= ~(1ULL << idx);
sp->nodes += pos.nodes_searched();
// Wake up master thread so to allow it to return from the idle loop
// in case we are the last slave of the split point.
if ( Threads.sleepWhileIdle
&& this != sp->masterThread
&& !sp->slavesMask)
{
assert(!sp->masterThread->searching);
sp->masterThread->notify_one();
}
// After releasing the lock we cannot access anymore any SplitPoint
// related data in a safe way becuase it could have been released under
// our feet by the sp master. Also accessing other Thread objects is
// unsafe because if we are exiting there is a chance are already freed.
sp->mutex.unlock();
}
// If this thread is the master of a split point and all slaves have finished
// their work at this split point, return from the idle loop.
if (this_sp && !this_sp->slavesMask)
{
this_sp->mutex.lock();
bool finished = !this_sp->slavesMask; // Retest under lock protection
this_sp->mutex.unlock();
if (finished)
return;
}
}
}
/// check_time() is called by the timer thread when the timer triggers. It is
/// used to print debug info and, more important, to detect when we are out of
/// available time and so stop the search.
void check_time() {
static Time::point lastInfoTime = Time::now();
int64_t nodes = 0; // Workaround silly 'uninitialized' gcc warning
if (Time::now() - lastInfoTime >= 1000)
{
lastInfoTime = Time::now();
dbg_print();
}
if (Limits.ponder)
return;
if (Limits.nodes)
{
Threads.mutex.lock();
nodes = RootPos.nodes_searched();
// Loop across all split points and sum accumulated SplitPoint nodes plus
// all the currently active positions nodes.
for (size_t i = 0; i < Threads.size(); i++)
for (int j = 0; j < Threads[i]->splitPointsSize; j++)
{
SplitPoint& sp = Threads[i]->splitPoints[j];
sp.mutex.lock();
nodes += sp.nodes;
Bitboard sm = sp.slavesMask;
while (sm)
{
Position* pos = Threads[pop_lsb(&sm)]->activePosition;
if (pos)
nodes += pos->nodes_searched();
}
sp.mutex.unlock();
}
Threads.mutex.unlock();
}
Time::point elapsed = Time::now() - SearchTime;
bool stillAtFirstMove = Signals.firstRootMove
&& !Signals.failedLowAtRoot
&& elapsed > TimeMgr.available_time();
bool noMoreTime = elapsed > TimeMgr.maximum_time() - 2 * TimerResolution
|| stillAtFirstMove;
if ( (Limits.use_time_management() && noMoreTime)
|| (Limits.movetime && elapsed >= Limits.movetime)
|| (Limits.nodes && nodes >= Limits.nodes))
Signals.stop = true;
}
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