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search.cpp
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search.cpp
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/*
Stockfish, a UCI chess playing engine derived from Glaurung 2.1
Copyright (C) 2004-2008 Tord Romstad (Glaurung author)
Copyright (C) 2008-2012 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.startpos_ply_counter(), 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;
bool bestMoveNeverChanged = true;
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.
sort<RootMove>(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
sort<RootMove>(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;
}
// Filter out startup noise when monitoring best move stability
if (depth > 2 && BestMoveChanges)
bestMoveNeverChanged = false;
// 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
&& ( (bestMoveNeverChanged && pos.captured_piece_type())
|| Time::now() - SearchTime > (TimeMgr.available_time() * 40) / 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* sp;
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)
{
sp = ss->sp;
bestMove = sp->bestMove;
threatMove = sp->threatMove;
bestValue = sp->bestValue;
tte = NULL;
ttMove = excludedMove = MOVE_NONE;
ttValue = VALUE_NONE;
assert(sp->bestValue > -VALUE_INFINITE && sp->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+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<false>() || 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->static_value()) == VALUE_NONE
||(ss->evalMargin = tte->static_value_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 - FutilityMargins[depth][0] >= beta
&& abs(beta) < VALUE_MATE_IN_MAX_PLY
&& pos.non_pawn_material(pos.side_to_move()))
return eval - FutilityMargins[depth][0];
// 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 < 6 * 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 = (PvNode ? depth - 2 * ONE_PLY : depth / 2);
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 = ++sp->moveCount;
sp->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
&& (bestValue > VALUE_MATED_IN_MAX_PLY || ( bestValue == -VALUE_INFINITE
&& alpha > VALUE_MATED_IN_MAX_PLY)))
{
// Move count based pruning
if ( depth < 16 * ONE_PLY
&& moveCount >= FutilityMoveCounts[depth]
&& (!threatMove || !refutes(pos, move, threatMove)))
{
if (SpNode)
sp->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)
{
if (SpNode)
sp->mutex.lock();
continue;
}
// Prune moves with negative SEE at low depths
if ( predictedDepth < 2 * ONE_PLY
&& pos.see_sign(move) < 0)
{
if (SpNode)
sp->mutex.lock();
continue;
}
}
// 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);
alpha = SpNode ? sp->alpha : 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)
{
alpha = SpNode ? sp->alpha : 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)
{
sp->mutex.lock();
bestValue = sp->bestValue;
alpha = sp->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;