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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
Copyright (C) 2015-2019 Marco Costalba, Joona Kiiski, Gary Linscott, Tord Romstad
Stockfish is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.
Stockfish is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program. If not, see <http://www.gnu.org/licenses/>.
*/
#include <algorithm>
#include <cassert>
#include <cmath>
#include <cstring> // For std::memset
#include <iostream>
#include <sstream>
#include "evaluate.h"
#include "misc.h"
#include "movegen.h"
#include "movepick.h"
#include "position.h"
#include "search.h"
#include "thread.h"
#include "timeman.h"
#include "tt.h"
#include "uci.h"
#include "syzygy/tbprobe.h"
namespace Search {
LimitsType Limits;
}
namespace Tablebases {
int Cardinality;
bool RootInTB;
bool UseRule50;
Depth ProbeDepth;
}
namespace TB = Tablebases;
using std::string;
using Eval::evaluate;
using namespace Search;
namespace {
// Different node types, used as a template parameter
enum NodeType { NonPV, PV };
// Sizes and phases of the skip-blocks, used for distributing search depths across the threads
constexpr int SkipSize[] = { 1, 1, 2, 2, 2, 2, 3, 3, 3, 3, 3, 3, 4, 4, 4, 4, 4, 4, 4, 4 };
constexpr int SkipPhase[] = { 0, 1, 0, 1, 2, 3, 0, 1, 2, 3, 4, 5, 0, 1, 2, 3, 4, 5, 6, 7 };
// Razor and futility margins
constexpr int RazorMargin = 600;
Value futility_margin(Depth d, bool improving) {
return Value((175 - 50 * improving) * d / ONE_PLY);
}
// Futility and reductions lookup tables, initialized at startup
int FutilityMoveCounts[2][16]; // [improving][depth]
int Reductions[2][2][64][64]; // [pv][improving][depth][moveNumber]
template <bool PvNode> Depth reduction(bool i, Depth d, int mn) {
return Reductions[PvNode][i][std::min(d / ONE_PLY, 63)][std::min(mn, 63)] * ONE_PLY;
}
// History and stats update bonus, based on depth
int stat_bonus(Depth depth) {
int d = depth / ONE_PLY;
return d > 17 ? 0 : 29 * d * d + 138 * d - 134;
}
// Add a small random component to draw evaluations to keep search dynamic
// and to avoid 3fold-blindness.
Value value_draw(Depth depth, Thread* thisThread) {
return depth < 4 ? VALUE_DRAW
: VALUE_DRAW + Value(2 * (thisThread->nodes.load(std::memory_order_relaxed) % 2) - 1);
}
// Skill structure is used to implement strength limit
struct Skill {
explicit Skill(int l) : level(l) {}
bool enabled() const { return level < 20; }
bool time_to_pick(Depth depth) const { return depth / ONE_PLY == 1 + level; }
Move pick_best(size_t multiPV);
int level;
Move best = MOVE_NONE;
};
template <NodeType NT>
Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode);
template <NodeType NT>
Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth = DEPTH_ZERO);
Value value_to_tt(Value v, int ply);
Value value_from_tt(Value v, int ply);
void update_pv(Move* pv, Move move, Move* childPv);
void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus);
void update_quiet_stats(const Position& pos, Stack* ss, Move move, Move* quiets, int quietCount, int bonus);
void update_capture_stats(const Position& pos, Move move, Move* captures, int captureCount, int bonus);
inline bool gives_check(const Position& pos, Move move) {
Color us = pos.side_to_move();
return type_of(move) == NORMAL && !(pos.blockers_for_king(~us) & pos.pieces(us))
? pos.check_squares(type_of(pos.moved_piece(move))) & to_sq(move)
: pos.gives_check(move);
}
// perft() is our utility to verify move generation. All the leaf nodes up
// to the given depth are generated and counted, and the sum is returned.
template<bool Root>
uint64_t perft(Position& pos, Depth depth) {
StateInfo st;
uint64_t cnt, nodes = 0;
const bool leaf = (depth == 2 * ONE_PLY);
for (const auto& m : MoveList<LEGAL>(pos))
{
if (Root && depth <= ONE_PLY)
cnt = 1, nodes++;
else
{
pos.do_move(m, st);
cnt = leaf ? MoveList<LEGAL>(pos).size() : perft<false>(pos, depth - ONE_PLY);
nodes += cnt;
pos.undo_move(m);
}
if (Root)
sync_cout << UCI::move(m, pos.is_chess960()) << ": " << cnt << sync_endl;
}
return nodes;
}
} // namespace
/// Search::init() is called at startup to initialize various lookup tables
void Search::init() {
for (int imp = 0; imp <= 1; ++imp)
for (int d = 1; d < 64; ++d)
for (int mc = 1; mc < 64; ++mc)
{
double r = log(d) * log(mc) / 1.95;
Reductions[NonPV][imp][d][mc] = int(std::round(r));
Reductions[PV][imp][d][mc] = std::max(Reductions[NonPV][imp][d][mc] - 1, 0);
// Increase reduction for non-PV nodes when eval is not improving
if (!imp && r > 1.0)
Reductions[NonPV][imp][d][mc]++;
}
for (int d = 0; d < 16; ++d)
{
FutilityMoveCounts[0][d] = int(2.4 + 0.74 * pow(d, 1.78));
FutilityMoveCounts[1][d] = int(5.0 + 1.00 * pow(d, 2.00));
}
}
/// Search::clear() resets search state to its initial value
void Search::clear() {
Threads.main()->wait_for_search_finished();
Time.availableNodes = 0;
TT.clear();
Threads.clear();
Tablebases::init(Options["SyzygyPath"]); // Free up mapped files
}
/// MainThread::search() is called by the main thread when the program receives
/// the UCI 'go' command. It searches from the root position and outputs the "bestmove".
void MainThread::search() {
if (Limits.perft)
{
nodes = perft<true>(rootPos, Limits.perft * ONE_PLY);
sync_cout << "\nNodes searched: " << nodes << "\n" << sync_endl;
return;
}
Color us = rootPos.side_to_move();
Time.init(Limits, us, rootPos.game_ply());
TT.new_search();
if (rootMoves.empty())
{
rootMoves.emplace_back(MOVE_NONE);
sync_cout << "info depth 0 score "
<< UCI::value(rootPos.checkers() ? -VALUE_MATE : VALUE_DRAW)
<< sync_endl;
}
else
{
for (Thread* th : Threads)
if (th != this)
th->start_searching();
Thread::search(); // Let's start searching!
}
// When we reach the maximum depth, we can arrive here without a raise of
// Threads.stop. However, if we are pondering or in an infinite search,
// the UCI protocol states that we shouldn't print the best move before the
// GUI sends a "stop" or "ponderhit" command. We therefore simply wait here
// until the GUI sends one of those commands (which also raises Threads.stop).
Threads.stopOnPonderhit = true;
while (!Threads.stop && (Threads.ponder || Limits.infinite))
{} // Busy wait for a stop or a ponder reset
// Stop the threads if not already stopped (also raise the stop if
// "ponderhit" just reset Threads.ponder).
Threads.stop = true;
// Wait until all threads have finished
for (Thread* th : Threads)
if (th != this)
th->wait_for_search_finished();
// When playing in 'nodes as time' mode, subtract the searched nodes from
// the available ones before exiting.
if (Limits.npmsec)
Time.availableNodes += Limits.inc[us] - Threads.nodes_searched();
// Check if there are threads with a better score than main thread
Thread* bestThread = this;
if ( Options["MultiPV"] == 1
&& !Limits.depth
&& !Skill(Options["Skill Level"]).enabled()
&& rootMoves[0].pv[0] != MOVE_NONE)
{
std::map<Move, int64_t> votes;
Value minScore = this->rootMoves[0].score;
// Find out minimum score and reset votes for moves which can be voted
for (Thread* th: Threads)
{
minScore = std::min(minScore, th->rootMoves[0].score);
votes[th->rootMoves[0].pv[0]] = 0;
}
// Vote according to score and depth
auto square = [](int64_t x) { return x * x; };
for (Thread* th : Threads)
votes[th->rootMoves[0].pv[0]] += 200 + (square(th->rootMoves[0].score - minScore + 1)
* int64_t(th->completedDepth));
// Select best thread
int64_t bestVote = votes[this->rootMoves[0].pv[0]];
for (Thread* th : Threads)
{
if (votes[th->rootMoves[0].pv[0]] > bestVote)
{
bestVote = votes[th->rootMoves[0].pv[0]];
bestThread = th;
}
}
}
previousScore = bestThread->rootMoves[0].score;
// Send again PV info if we have a new best thread
if (bestThread != this)
sync_cout << UCI::pv(bestThread->rootPos, bestThread->completedDepth, -VALUE_INFINITE, VALUE_INFINITE) << sync_endl;
sync_cout << "bestmove " << UCI::move(bestThread->rootMoves[0].pv[0], rootPos.is_chess960());
if (bestThread->rootMoves[0].pv.size() > 1 || bestThread->rootMoves[0].extract_ponder_from_tt(rootPos))
std::cout << " ponder " << UCI::move(bestThread->rootMoves[0].pv[1], rootPos.is_chess960());
std::cout << sync_endl;
}
/// Thread::search() is the main iterative deepening loop. It calls search()
/// repeatedly with increasing depth until the allocated thinking time has been
/// consumed, the user stops the search, or the maximum search depth is reached.
void Thread::search() {
// To allow access to (ss-5) up to (ss+2), the stack must be oversized.
// The former is needed to allow update_continuation_histories(ss-1, ...),
// which accesses its argument at ss-4, also near the root.
// The latter is needed for statScores and killer initialization.
Stack stack[MAX_PLY+8], *ss = stack+5;
Move pv[MAX_PLY+1];
Value bestValue, alpha, beta, delta;
Move lastBestMove = MOVE_NONE;
Depth lastBestMoveDepth = DEPTH_ZERO;
MainThread* mainThread = (this == Threads.main() ? Threads.main() : nullptr);
double timeReduction = 1.0;
Color us = rootPos.side_to_move();
bool failedLow;
std::memset(ss-5, 0, 8 * sizeof(Stack));
for (int i = 5; i > 0; i--)
(ss-i)->continuationHistory = &this->continuationHistory[NO_PIECE][0]; // Use as sentinel
ss->pv = pv;
bestValue = delta = alpha = -VALUE_INFINITE;
beta = VALUE_INFINITE;
if (mainThread)
mainThread->bestMoveChanges = 0, failedLow = false;
size_t multiPV = Options["MultiPV"];
Skill skill(Options["Skill Level"]);
// When playing with strength handicap enable MultiPV search that we will
// use behind the scenes to retrieve a set of possible moves.
if (skill.enabled())
multiPV = std::max(multiPV, (size_t)4);
multiPV = std::min(multiPV, rootMoves.size());
int ct = int(Options["Contempt"]) * PawnValueEg / 100; // From centipawns
// In analysis mode, adjust contempt in accordance with user preference
if (Limits.infinite || Options["UCI_AnalyseMode"])
ct = Options["Analysis Contempt"] == "Off" ? 0
: Options["Analysis Contempt"] == "Both" ? ct
: Options["Analysis Contempt"] == "White" && us == BLACK ? -ct
: Options["Analysis Contempt"] == "Black" && us == WHITE ? -ct
: ct;
// In evaluate.cpp the evaluation is from the white point of view
contempt = (us == WHITE ? make_score(ct, ct / 2)
: -make_score(ct, ct / 2));
// Iterative deepening loop until requested to stop or the target depth is reached
while ( (rootDepth += ONE_PLY) < DEPTH_MAX
&& !Threads.stop
&& !(Limits.depth && mainThread && rootDepth / ONE_PLY > Limits.depth))
{
// Distribute search depths across the helper threads
if (idx > 0)
{
int i = (idx - 1) % 20;
if (((rootDepth / ONE_PLY + SkipPhase[i]) / SkipSize[i]) % 2)
continue; // Retry with an incremented rootDepth
}
// Age out PV variability metric
if (mainThread)
mainThread->bestMoveChanges *= 0.517, failedLow = false;
// Save the last iteration's scores before first PV line is searched and
// all the move scores except the (new) PV are set to -VALUE_INFINITE.
for (RootMove& rm : rootMoves)
rm.previousScore = rm.score;
size_t pvFirst = 0;
pvLast = 0;
// MultiPV loop. We perform a full root search for each PV line
for (pvIdx = 0; pvIdx < multiPV && !Threads.stop; ++pvIdx)
{
if (pvIdx == pvLast)
{
pvFirst = pvLast;
for (pvLast++; pvLast < rootMoves.size(); pvLast++)
if (rootMoves[pvLast].tbRank != rootMoves[pvFirst].tbRank)
break;
}
// Reset UCI info selDepth for each depth and each PV line
selDepth = 0;
// Reset aspiration window starting size
if (rootDepth >= 5 * ONE_PLY)
{
Value previousScore = rootMoves[pvIdx].previousScore;
delta = Value(20);
alpha = std::max(previousScore - delta,-VALUE_INFINITE);
beta = std::min(previousScore + delta, VALUE_INFINITE);
// Adjust contempt based on root move's previousScore (dynamic contempt)
int dct = ct + 88 * previousScore / (abs(previousScore) + 200);
contempt = (us == WHITE ? make_score(dct, dct / 2)
: -make_score(dct, dct / 2));
}
// Start with a small aspiration window and, in the case of a fail
// high/low, re-search with a bigger window until we don't fail
// high/low anymore.
int failedHighCnt = 0;
while (true)
{
Depth adjustedDepth = std::max(ONE_PLY, rootDepth - failedHighCnt * ONE_PLY);
bestValue = ::search<PV>(rootPos, ss, alpha, beta, adjustedDepth, false);
// Bring the best move to the front. It is critical that sorting
// is done with a stable algorithm because all the values but the
// first and eventually the new best one are set to -VALUE_INFINITE
// and we want to keep the same order for all the moves except the
// new PV that goes to the front. Note that in case of MultiPV
// search the already searched PV lines are preserved.
std::stable_sort(rootMoves.begin() + pvIdx, rootMoves.begin() + pvLast);
// If search has been stopped, we break immediately. Sorting is
// safe because RootMoves is still valid, although it refers to
// the previous iteration.
if (Threads.stop)
break;
// When failing high/low give some update (without cluttering
// the UI) before a re-search.
if ( mainThread
&& multiPV == 1
&& (bestValue <= alpha || bestValue >= beta)
&& Time.elapsed() > 3000)
sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
// In case of failing low/high increase aspiration window and
// re-search, otherwise exit the loop.
if (bestValue <= alpha)
{
beta = (alpha + beta) / 2;
alpha = std::max(bestValue - delta, -VALUE_INFINITE);
if (mainThread)
{
failedHighCnt = 0;
failedLow = true;
Threads.stopOnPonderhit = false;
}
}
else if (bestValue >= beta)
{
beta = std::min(bestValue + delta, VALUE_INFINITE);
if (mainThread)
++failedHighCnt;
}
else
break;
delta += delta / 4 + 5;
assert(alpha >= -VALUE_INFINITE && beta <= VALUE_INFINITE);
}
// Sort the PV lines searched so far and update the GUI
std::stable_sort(rootMoves.begin() + pvFirst, rootMoves.begin() + pvIdx + 1);
if ( mainThread
&& (Threads.stop || pvIdx + 1 == multiPV || Time.elapsed() > 3000))
sync_cout << UCI::pv(rootPos, rootDepth, alpha, beta) << sync_endl;
}
if (!Threads.stop)
completedDepth = rootDepth;
if (rootMoves[0].pv[0] != lastBestMove) {
lastBestMove = rootMoves[0].pv[0];
lastBestMoveDepth = rootDepth;
}
// Have we found a "mate in x"?
if ( Limits.mate
&& bestValue >= VALUE_MATE_IN_MAX_PLY
&& VALUE_MATE - bestValue <= 2 * Limits.mate)
Threads.stop = true;
if (!mainThread)
continue;
// If skill level is enabled and time is up, pick a sub-optimal best move
if (skill.enabled() && skill.time_to_pick(rootDepth))
skill.pick_best(multiPV);
// Do we have time for the next iteration? Can we stop searching now?
if ( Limits.use_time_management()
&& !Threads.stop
&& !Threads.stopOnPonderhit)
{
double fallingEval = (306 + 119 * failedLow + 6 * (mainThread->previousScore - bestValue)) / 581.0;
fallingEval = std::max(0.5, std::min(1.5, fallingEval));
// If the bestMove is stable over several iterations, reduce time accordingly
timeReduction = 1.0;
for (int i : {3, 4, 5})
if (lastBestMoveDepth * i < completedDepth)
timeReduction *= 1.25;
// Use part of the gained time from a previous stable move for the current move
double bestMoveInstability = 1.0 + mainThread->bestMoveChanges;
bestMoveInstability *= std::pow(mainThread->previousTimeReduction, 0.528) / timeReduction;
// Stop the search if we have only one legal move, or if available time elapsed
if ( rootMoves.size() == 1
|| Time.elapsed() > Time.optimum() * bestMoveInstability * fallingEval)
{
// If we are allowed to ponder do not stop the search now but
// keep pondering until the GUI sends "ponderhit" or "stop".
if (Threads.ponder)
Threads.stopOnPonderhit = true;
else
Threads.stop = true;
}
}
}
if (!mainThread)
return;
mainThread->previousTimeReduction = timeReduction;
// If skill level is enabled, swap best PV line with the sub-optimal one
if (skill.enabled())
std::swap(rootMoves[0], *std::find(rootMoves.begin(), rootMoves.end(),
skill.best ? skill.best : skill.pick_best(multiPV)));
}
namespace {
// search<>() is the main search function for both PV and non-PV nodes
template <NodeType NT>
Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, bool cutNode) {
constexpr bool PvNode = NT == PV;
const bool rootNode = PvNode && ss->ply == 0;
// Check if we have an upcoming move which draws by repetition, or
// if the opponent had an alternative move earlier to this position.
if ( pos.rule50_count() >= 3
&& alpha < VALUE_DRAW
&& !rootNode
&& pos.has_game_cycle(ss->ply))
{
alpha = value_draw(depth, pos.this_thread());
if (alpha >= beta)
return alpha;
}
// Dive into quiescence search when the depth reaches zero
if (depth < ONE_PLY)
return qsearch<NT>(pos, ss, alpha, beta);
assert(-VALUE_INFINITE <= alpha && alpha < beta && beta <= VALUE_INFINITE);
assert(PvNode || (alpha == beta - 1));
assert(DEPTH_ZERO < depth && depth < DEPTH_MAX);
assert(!(PvNode && cutNode));
assert(depth / ONE_PLY * ONE_PLY == depth);
Move pv[MAX_PLY+1], capturesSearched[32], quietsSearched[64];
StateInfo st;
TTEntry* tte;
Key posKey;
Move ttMove, move, excludedMove, bestMove;
Depth extension, newDepth;
Value bestValue, value, ttValue, eval, maxValue, pureStaticEval;
bool ttHit, inCheck, givesCheck, improving;
bool captureOrPromotion, doFullDepthSearch, moveCountPruning, skipQuiets, ttCapture, pvExact;
Piece movedPiece;
int moveCount, captureCount, quietCount;
// Step 1. Initialize node
Thread* thisThread = pos.this_thread();
inCheck = pos.checkers();
Color us = pos.side_to_move();
moveCount = captureCount = quietCount = ss->moveCount = 0;
bestValue = -VALUE_INFINITE;
maxValue = VALUE_INFINITE;
// Check for the available remaining time
if (thisThread == Threads.main())
static_cast<MainThread*>(thisThread)->check_time();
// Used to send selDepth info to GUI (selDepth counts from 1, ply from 0)
if (PvNode && thisThread->selDepth < ss->ply + 1)
thisThread->selDepth = ss->ply + 1;
if (!rootNode)
{
// Step 2. Check for aborted search and immediate draw
if ( Threads.stop.load(std::memory_order_relaxed)
|| pos.is_draw(ss->ply)
|| ss->ply >= MAX_PLY)
return (ss->ply >= MAX_PLY && !inCheck) ? evaluate(pos)
: value_draw(depth, pos.this_thread());
// Step 3. Mate distance pruning. Even if we mate at the next move our score
// would be at best mate_in(ss->ply+1), but if alpha is already bigger because
// a shorter mate was found upward in the tree then there is no need to search
// because we will never beat the current alpha. Same logic but with reversed
// signs applies also in the opposite condition of being mated instead of giving
// mate. In this case return a fail-high score.
alpha = std::max(mated_in(ss->ply), alpha);
beta = std::min(mate_in(ss->ply+1), beta);
if (alpha >= beta)
return alpha;
}
assert(0 <= ss->ply && ss->ply < MAX_PLY);
(ss+1)->ply = ss->ply + 1;
ss->currentMove = (ss+1)->excludedMove = bestMove = MOVE_NONE;
ss->continuationHistory = &thisThread->continuationHistory[NO_PIECE][0];
(ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
Square prevSq = to_sq((ss-1)->currentMove);
// Initialize statScore to zero for the grandchildren of the current position.
// So statScore is shared between all grandchildren and only the first grandchild
// starts with statScore = 0. Later grandchildren start with the last calculated
// statScore of the previous grandchild. This influences the reduction rules in
// LMR which are based on the statScore of parent position.
(ss+2)->statScore = 0;
// Step 4. Transposition table lookup. We don't want the score of a partial
// search to overwrite a previous full search TT value, so we use a different
// position key in case of an excluded move.
excludedMove = ss->excludedMove;
posKey = pos.key() ^ Key(excludedMove << 16); // Isn't a very good hash
tte = TT.probe(posKey, ttHit);
ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
ttMove = rootNode ? thisThread->rootMoves[thisThread->pvIdx].pv[0]
: ttHit ? tte->move() : MOVE_NONE;
// At non-PV nodes we check for an early TT cutoff
if ( !PvNode
&& ttHit
&& tte->depth() >= depth
&& ttValue != VALUE_NONE // Possible in case of TT access race
&& (ttValue >= beta ? (tte->bound() & BOUND_LOWER)
: (tte->bound() & BOUND_UPPER)))
{
// If ttMove is quiet, update move sorting heuristics on TT hit
if (ttMove)
{
if (ttValue >= beta)
{
if (!pos.capture_or_promotion(ttMove))
update_quiet_stats(pos, ss, ttMove, nullptr, 0, stat_bonus(depth));
// Extra penalty for a quiet TT or main killer move in previous ply when it gets refuted
if ( ((ss-1)->moveCount == 1 || (ss-1)->currentMove == (ss-1)->killers[0])
&& !pos.captured_piece())
update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + ONE_PLY));
}
// Penalty for a quiet ttMove that fails low
else if (!pos.capture_or_promotion(ttMove))
{
int penalty = -stat_bonus(depth);
thisThread->mainHistory[us][from_to(ttMove)] << penalty;
update_continuation_histories(ss, pos.moved_piece(ttMove), to_sq(ttMove), penalty);
}
}
return ttValue;
}
// Step 5. Tablebases probe
if (!rootNode && TB::Cardinality)
{
int piecesCount = pos.count<ALL_PIECES>();
if ( piecesCount <= TB::Cardinality
&& (piecesCount < TB::Cardinality || depth >= TB::ProbeDepth)
&& pos.rule50_count() == 0
&& !pos.can_castle(ANY_CASTLING))
{
TB::ProbeState err;
TB::WDLScore wdl = Tablebases::probe_wdl(pos, &err);
// Force check of time on the next occasion
if (thisThread == Threads.main())
static_cast<MainThread*>(thisThread)->callsCnt = 0;
if (err != TB::ProbeState::FAIL)
{
thisThread->tbHits.fetch_add(1, std::memory_order_relaxed);
int drawScore = TB::UseRule50 ? 1 : 0;
value = wdl < -drawScore ? -VALUE_MATE + MAX_PLY + ss->ply + 1
: wdl > drawScore ? VALUE_MATE - MAX_PLY - ss->ply - 1
: VALUE_DRAW + 2 * wdl * drawScore;
Bound b = wdl < -drawScore ? BOUND_UPPER
: wdl > drawScore ? BOUND_LOWER : BOUND_EXACT;
if ( b == BOUND_EXACT
|| (b == BOUND_LOWER ? value >= beta : value <= alpha))
{
tte->save(posKey, value_to_tt(value, ss->ply), b,
std::min(DEPTH_MAX - ONE_PLY, depth + 6 * ONE_PLY),
MOVE_NONE, VALUE_NONE);
return value;
}
if (PvNode)
{
if (b == BOUND_LOWER)
bestValue = value, alpha = std::max(alpha, bestValue);
else
maxValue = value;
}
}
}
}
// Step 6. Static evaluation of the position
if (inCheck)
{
ss->staticEval = eval = pureStaticEval = VALUE_NONE;
improving = false;
goto moves_loop; // Skip early pruning when in check
}
else if (ttHit)
{
// Never assume anything on values stored in TT
ss->staticEval = eval = pureStaticEval = tte->eval();
if (eval == VALUE_NONE)
ss->staticEval = eval = pureStaticEval = evaluate(pos);
// Can ttValue be used as a better position evaluation?
if ( ttValue != VALUE_NONE
&& (tte->bound() & (ttValue > eval ? BOUND_LOWER : BOUND_UPPER)))
eval = ttValue;
}
else
{
if ((ss-1)->currentMove != MOVE_NULL)
{
int p = (ss-1)->statScore;
int bonus = p > 0 ? (-p - 2500) / 512 :
p < 0 ? (-p + 2500) / 512 : 0;
pureStaticEval = evaluate(pos);
ss->staticEval = eval = pureStaticEval + bonus;
}
else
ss->staticEval = eval = pureStaticEval = -(ss-1)->staticEval + 2 * Eval::Tempo;
tte->save(posKey, VALUE_NONE, BOUND_NONE, DEPTH_NONE, MOVE_NONE, pureStaticEval);
}
// Step 7. Razoring (~2 Elo)
if ( !rootNode // The required rootNode PV handling is not available in qsearch
&& depth < 2 * ONE_PLY
&& eval <= alpha - RazorMargin)
return qsearch<NT>(pos, ss, alpha, beta);
improving = ss->staticEval >= (ss-2)->staticEval
|| (ss-2)->staticEval == VALUE_NONE;
// Step 8. Futility pruning: child node (~30 Elo)
if ( !PvNode
&& depth < 7 * ONE_PLY
&& eval - futility_margin(depth, improving) >= beta
&& eval < VALUE_KNOWN_WIN) // Do not return unproven wins
return eval;
// Step 9. Null move search with verification search (~40 Elo)
if ( !PvNode
&& (ss-1)->currentMove != MOVE_NULL
&& (ss-1)->statScore < 23200
&& eval >= beta
&& pureStaticEval >= beta - 36 * depth / ONE_PLY + 225
&& !excludedMove
&& pos.non_pawn_material(us)
&& (ss->ply >= thisThread->nmpMinPly || us != thisThread->nmpColor))
{
assert(eval - beta >= 0);
// Null move dynamic reduction based on depth and value
Depth R = ((823 + 67 * depth / ONE_PLY) / 256 + std::min(int(eval - beta) / 200, 3)) * ONE_PLY;
ss->currentMove = MOVE_NULL;
ss->continuationHistory = &thisThread->continuationHistory[NO_PIECE][0];
pos.do_null_move(st);
Value nullValue = -search<NonPV>(pos, ss+1, -beta, -beta+1, depth-R, !cutNode);
pos.undo_null_move();
if (nullValue >= beta)
{
// Do not return unproven mate scores
if (nullValue >= VALUE_MATE_IN_MAX_PLY)
nullValue = beta;
if (thisThread->nmpMinPly || (abs(beta) < VALUE_KNOWN_WIN && depth < 12 * ONE_PLY))
return nullValue;
assert(!thisThread->nmpMinPly); // Recursive verification is not allowed
// Do verification search at high depths, with null move pruning disabled
// for us, until ply exceeds nmpMinPly.
thisThread->nmpMinPly = ss->ply + 3 * (depth-R) / 4;
thisThread->nmpColor = us;
Value v = search<NonPV>(pos, ss, beta-1, beta, depth-R, false);
thisThread->nmpMinPly = 0;
if (v >= beta)
return nullValue;
}
}
// Step 10. ProbCut (~10 Elo)
// If we have a good enough capture and a reduced search returns a value
// much above beta, we can (almost) safely prune the previous move.
if ( !PvNode
&& depth >= 5 * ONE_PLY
&& abs(beta) < VALUE_MATE_IN_MAX_PLY)
{
Value raisedBeta = std::min(beta + 216 - 48 * improving, VALUE_INFINITE);
MovePicker mp(pos, ttMove, raisedBeta - ss->staticEval, &thisThread->captureHistory);
int probCutCount = 0;
while ( (move = mp.next_move()) != MOVE_NONE
&& probCutCount < 3)
if (move != excludedMove && pos.legal(move))
{
probCutCount++;
ss->currentMove = move;
ss->continuationHistory = &thisThread->continuationHistory[pos.moved_piece(move)][to_sq(move)];
assert(depth >= 5 * ONE_PLY);
pos.do_move(move, st);
// Perform a preliminary qsearch to verify that the move holds
value = -qsearch<NonPV>(pos, ss+1, -raisedBeta, -raisedBeta+1);
// If the qsearch held perform the regular search
if (value >= raisedBeta)
value = -search<NonPV>(pos, ss+1, -raisedBeta, -raisedBeta+1, depth - 4 * ONE_PLY, !cutNode);
pos.undo_move(move);
if (value >= raisedBeta)
return value;
}
}
// Step 11. Internal iterative deepening (~2 Elo)
if ( depth >= 8 * ONE_PLY
&& !ttMove)
{
search<NT>(pos, ss, alpha, beta, depth - 7 * ONE_PLY, cutNode);
tte = TT.probe(posKey, ttHit);
ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
ttMove = ttHit ? tte->move() : MOVE_NONE;
}
moves_loop: // When in check, search starts from here
const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory, nullptr, (ss-4)->continuationHistory };
Move countermove = thisThread->counterMoves[pos.piece_on(prevSq)][prevSq];
MovePicker mp(pos, ttMove, depth, &thisThread->mainHistory,
&thisThread->captureHistory,
contHist,
countermove,
ss->killers);
value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
skipQuiets = false;
ttCapture = ttMove && pos.capture_or_promotion(ttMove);
pvExact = PvNode && ttHit && tte->bound() == BOUND_EXACT;
// Step 12. Loop through all pseudo-legal moves until no moves remain
// or a beta cutoff occurs.
while ((move = mp.next_move(skipQuiets)) != MOVE_NONE)
{
assert(is_ok(move));
if (move == excludedMove)
continue;
// At root obey the "searchmoves" option and skip moves not listed in Root
// Move List. As a consequence any illegal move is also skipped. In MultiPV
// mode we also skip PV moves which have been already searched and those
// of lower "TB rank" if we are in a TB root position.
if (rootNode && !std::count(thisThread->rootMoves.begin() + thisThread->pvIdx,
thisThread->rootMoves.begin() + thisThread->pvLast, move))
continue;
ss->moveCount = ++moveCount;
if (rootNode && thisThread == Threads.main() && Time.elapsed() > 3000)
sync_cout << "info depth " << depth / ONE_PLY
<< " currmove " << UCI::move(move, pos.is_chess960())
<< " currmovenumber " << moveCount + thisThread->pvIdx << sync_endl;
if (PvNode)
(ss+1)->pv = nullptr;
extension = DEPTH_ZERO;
captureOrPromotion = pos.capture_or_promotion(move);
movedPiece = pos.moved_piece(move);
givesCheck = gives_check(pos, move);
moveCountPruning = depth < 16 * ONE_PLY
&& moveCount >= FutilityMoveCounts[improving][depth / ONE_PLY];
// Step 13. Extensions (~70 Elo)
// Singular extension search (~60 Elo). If all moves but one fail low on a
// search of (alpha-s, beta-s), and just one fails high on (alpha, beta),
// then that move is singular and should be extended. To verify this we do
// a reduced search on all the other moves but the ttMove and if the
// result is lower than ttValue minus a margin then we will extend the ttMove.
if ( depth >= 8 * ONE_PLY
&& move == ttMove
&& !rootNode
&& !excludedMove // Avoid recursive singular search
&& ttValue != VALUE_NONE
&& (tte->bound() & BOUND_LOWER)
&& tte->depth() >= depth - 3 * ONE_PLY
&& pos.legal(move))
{
Value reducedBeta = std::max(ttValue - 2 * depth / ONE_PLY, -VALUE_MATE);
ss->excludedMove = move;
value = search<NonPV>(pos, ss, reducedBeta - 1, reducedBeta, depth / 2, cutNode);
ss->excludedMove = MOVE_NONE;
if (value < reducedBeta)
extension = ONE_PLY;
}
else if ( givesCheck // Check extension (~2 Elo)
&& pos.see_ge(move))
extension = ONE_PLY;
// Extension if castling
else if (type_of(move) == CASTLING)
extension = ONE_PLY;
// Calculate new depth for this move
newDepth = depth - ONE_PLY + extension;
// Step 14. Pruning at shallow depth (~170 Elo)
if ( !rootNode
&& pos.non_pawn_material(us)
&& bestValue > VALUE_MATED_IN_MAX_PLY)
{
if ( !captureOrPromotion
&& !givesCheck
&& !pos.advanced_pawn_push(move))
{
// Move count based pruning (~30 Elo)
if (moveCountPruning)
{
skipQuiets = true;
continue;
}
// Reduced depth of the next LMR search
int lmrDepth = std::max(newDepth - reduction<PvNode>(improving, depth, moveCount), DEPTH_ZERO) / ONE_PLY;
// Countermoves based pruning (~20 Elo)
if ( lmrDepth < 3 + ((ss-1)->statScore > 0 || (ss-1)->moveCount == 1)
&& (*contHist[0])[movedPiece][to_sq(move)] < CounterMovePruneThreshold
&& (*contHist[1])[movedPiece][to_sq(move)] < CounterMovePruneThreshold)
continue;
// Futility pruning: parent node (~2 Elo)
if ( lmrDepth < 7
&& !inCheck
&& ss->staticEval + 256 + 200 * lmrDepth <= alpha)
continue;
// Prune moves with negative SEE (~10 Elo)
if (!pos.see_ge(move, Value(-29 * lmrDepth * lmrDepth)))
continue;
}
else if ( !extension // (~20 Elo)
&& !pos.see_ge(move, -PawnValueEg * (depth / ONE_PLY)))
continue;
}
// Speculative prefetch as early as possible
prefetch(TT.first_entry(pos.key_after(move)));
// Check for legality just before making the move
if (!rootNode && !pos.legal(move))
{
ss->moveCount = --moveCount;
continue;
}
// Update the current move (this must be done after singular extension search)
ss->currentMove = move;
ss->continuationHistory = &thisThread->continuationHistory[movedPiece][to_sq(move)];
// Step 15. Make the move
pos.do_move(move, st, givesCheck);
// Step 16. Reduced depth search (LMR). If the move fails high it will be
// re-searched at full depth.
if ( depth >= 3 * ONE_PLY
&& moveCount > 1
&& (!captureOrPromotion || moveCountPruning))
{
Depth r = reduction<PvNode>(improving, depth, moveCount);
// Decrease reduction if opponent's move count is high (~10 Elo)
if ((ss-1)->moveCount > 15)
r -= ONE_PLY;
if (!captureOrPromotion)
{
// Decrease reduction for exact PV nodes (~0 Elo)
if (pvExact)
r -= ONE_PLY;
// Increase reduction if ttMove is a capture (~0 Elo)
if (ttCapture)
r += ONE_PLY;
// Increase reduction for cut nodes (~5 Elo)
if (cutNode)
r += 2 * ONE_PLY;
// Decrease reduction for moves that escape a capture. Filter out
// castling moves, because they are coded as "king captures rook" and
// hence break make_move(). (~5 Elo)
else if ( type_of(move) == NORMAL
&& !pos.see_ge(make_move(to_sq(move), from_sq(move))))
r -= 2 * ONE_PLY;
ss->statScore = thisThread->mainHistory[us][from_to(move)]
+ (*contHist[0])[movedPiece][to_sq(move)]
+ (*contHist[1])[movedPiece][to_sq(move)]
+ (*contHist[3])[movedPiece][to_sq(move)]
- 4000;
// Decrease/increase reduction by comparing opponent's stat score (~10 Elo)
if (ss->statScore >= 0 && (ss-1)->statScore < 0)
r -= ONE_PLY;
else if ((ss-1)->statScore >= 0 && ss->statScore < 0)
r += ONE_PLY;
// Decrease/increase reduction for moves with a good/bad history (~30 Elo)
r -= ss->statScore / 20000 * ONE_PLY;
}
Depth d = std::max(newDepth - std::max(r, DEPTH_ZERO), ONE_PLY);
value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true);
doFullDepthSearch = (value > alpha && d != newDepth);
}
else
doFullDepthSearch = !PvNode || moveCount > 1;
// Step 17. Full depth search when LMR is skipped or fails high
if (doFullDepthSearch)
value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode);
// For PV nodes only, do a full PV search on the first move or after a fail
// high (in the latter case search only if value < beta), otherwise let the
// parent node fail low with value <= alpha and try another move.
if (PvNode && (moveCount == 1 || (value > alpha && (rootNode || value < beta))))
{
(ss+1)->pv = pv;
(ss+1)->pv[0] = MOVE_NONE;
value = -search<PV>(pos, ss+1, -beta, -alpha, newDepth, false);
}
// Step 18. Undo move
pos.undo_move(move);
assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
// Step 19. Check for a new best move
// Finished searching the move. If a stop occurred, the return value of
// the search cannot be trusted, and we return immediately without
// updating best move, PV and TT.
if (Threads.stop.load(std::memory_order_relaxed))
return VALUE_ZERO;
if (rootNode)
{
RootMove& rm = *std::find(thisThread->rootMoves.begin(),
thisThread->rootMoves.end(), move);
// PV move or new best move?
if (moveCount == 1 || value > alpha)
{
rm.score = value;
rm.selDepth = thisThread->selDepth;
rm.pv.resize(1);
assert((ss+1)->pv);
for (Move* m = (ss+1)->pv; *m != MOVE_NONE; ++m)
rm.pv.push_back(*m);
// We record how often the best move has been changed in each
// iteration. This information is used for time management: When
// the best move changes frequently, we allocate some more time.
if (moveCount > 1 && thisThread == Threads.main())
++static_cast<MainThread*>(thisThread)->bestMoveChanges;
}
else
// All other moves but the PV are set to the lowest value: this
// is not a problem when sorting because the sort is stable and the
// move position in the list is preserved - just the PV is pushed up.
rm.score = -VALUE_INFINITE;
}
if (value > bestValue)
{
bestValue = value;
if (value > alpha)
{
bestMove = move;
if (PvNode && !rootNode) // Update pv even in fail-high case
update_pv(ss->pv, move, (ss+1)->pv);
if (PvNode && value < beta) // Update alpha! Always alpha < beta
alpha = value;
else
{
assert(value >= beta); // Fail high
ss->statScore = 0;
break;
}
}
}
if (move != bestMove)
{
if (captureOrPromotion && captureCount < 32)
capturesSearched[captureCount++] = move;
else if (!captureOrPromotion && quietCount < 64)
quietsSearched[quietCount++] = move;
}
}
// The following condition would detect a stop only after move loop has been
// completed. But in this case bestValue is valid because we have fully
// searched our subtree, and we can anyhow save the result in TT.
/*
if (Threads.stop)
return VALUE_DRAW;
*/
// Step 20. Check for mate and stalemate
// All legal moves have been searched and if there are no legal moves, it
// must be a mate or a stalemate. If we are in a singular extension search then
// return a fail low score.
assert(moveCount || !inCheck || excludedMove || !MoveList<LEGAL>(pos).size());
if (!moveCount)
bestValue = excludedMove ? alpha
: inCheck ? mated_in(ss->ply) : VALUE_DRAW;
else if (bestMove)
{
// Quiet best move: update move sorting heuristics
if (!pos.capture_or_promotion(bestMove))
update_quiet_stats(pos, ss, bestMove, quietsSearched, quietCount,
stat_bonus(depth + (bestValue > beta + PawnValueMg ? ONE_PLY : DEPTH_ZERO)));
update_capture_stats(pos, bestMove, capturesSearched, captureCount, stat_bonus(depth + ONE_PLY));
// Extra penalty for a quiet TT or main killer move in previous ply when it gets refuted
if ( ((ss-1)->moveCount == 1 || ((ss-1)->currentMove == (ss-1)->killers[0]))
&& !pos.captured_piece())
update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, -stat_bonus(depth + ONE_PLY));
}
// Bonus for prior countermove that caused the fail low
else if ( (depth >= 3 * ONE_PLY || PvNode)
&& !pos.captured_piece())
update_continuation_histories(ss-1, pos.piece_on(prevSq), prevSq, stat_bonus(depth));
if (PvNode)
bestValue = std::min(bestValue, maxValue);
if (!excludedMove)
tte->save(posKey, value_to_tt(bestValue, ss->ply),
bestValue >= beta ? BOUND_LOWER :
PvNode && bestMove ? BOUND_EXACT : BOUND_UPPER,
depth, bestMove, pureStaticEval);
assert(bestValue > -VALUE_INFINITE && bestValue < VALUE_INFINITE);
return bestValue;
}
// qsearch() is the quiescence search function, which is called by the main
// search function with depth zero, or recursively with depth less than ONE_PLY.
template <NodeType NT>
Value qsearch(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth) {
constexpr bool PvNode = NT == PV;
assert(alpha >= -VALUE_INFINITE && alpha < beta && beta <= VALUE_INFINITE);
assert(PvNode || (alpha == beta - 1));
assert(depth <= DEPTH_ZERO);
assert(depth / ONE_PLY * ONE_PLY == depth);
Move pv[MAX_PLY+1];
StateInfo st;
TTEntry* tte;
Key posKey;
Move ttMove, move, bestMove;
Depth ttDepth;
Value bestValue, value, ttValue, futilityValue, futilityBase, oldAlpha;
bool ttHit, inCheck, givesCheck, evasionPrunable;
int moveCount;
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;
}
Thread* thisThread = pos.this_thread();
(ss+1)->ply = ss->ply + 1;
ss->currentMove = bestMove = MOVE_NONE;
ss->continuationHistory = &thisThread->continuationHistory[NO_PIECE][0];
inCheck = pos.checkers();
moveCount = 0;
// Check for an immediate draw or maximum ply reached
if ( pos.is_draw(ss->ply)
|| ss->ply >= MAX_PLY)
return (ss->ply >= MAX_PLY && !inCheck) ? evaluate(pos) : VALUE_DRAW;
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);
ttValue = ttHit ? value_from_tt(tte->value(), ss->ply) : VALUE_NONE;
ttMove = ttHit ? tte->move() : MOVE_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)))
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
&& (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(posKey, value_to_tt(bestValue, ss->ply), BOUND_LOWER,
DEPTH_NONE, MOVE_NONE, ss->staticEval);
return bestValue;
}
if (PvNode && bestValue > alpha)
alpha = bestValue;
futilityBase = bestValue + 128;
}
const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory, nullptr, (ss-4)->continuationHistory };
// 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, &thisThread->mainHistory,
&thisThread->captureHistory,
contHist,
to_sq((ss-1)->currentMove));
// Loop through the moves until no moves remain or a beta cutoff occurs
while ((move = mp.next_move()) != MOVE_NONE)
{
assert(is_ok(move));
givesCheck = gives_check(pos, move);
moveCount++;
// 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_ge(move, VALUE_ZERO + 1))
{
bestValue = std::max(bestValue, futilityBase);
continue;
}
}
// Detect non-capture evasions that are candidates to be pruned
evasionPrunable = inCheck
&& (depth != DEPTH_ZERO || moveCount > 2)
&& bestValue > VALUE_MATED_IN_MAX_PLY
&& !pos.capture(move);
// Don't search moves with negative SEE values
if ( (!inCheck || evasionPrunable)
&& !pos.see_ge(move))
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))
{
moveCount--;
continue;
}
ss->currentMove = move;
ss->continuationHistory = &thisThread->continuationHistory[pos.moved_piece(move)][to_sq(move)];
// Make and search the move
pos.do_move(move, st, givesCheck);
value = -qsearch<NT>(pos, ss+1, -beta, -alpha, depth - ONE_PLY);
pos.undo_move(move);
assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
// Check for a new best move
if (value > bestValue)
{
bestValue = value;
if (value > alpha)
{
bestMove = move;
if (PvNode) // Update pv even in fail-high case
update_pv(ss->pv, move, (ss+1)->pv);
if (PvNode && value < beta) // Update alpha here!
alpha = value;
else
break; // Fail high
}
}
}
// 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),
bestValue >= beta ? BOUND_LOWER :
PvNode && bestValue > oldAlpha ? BOUND_EXACT : BOUND_UPPER,
ttDepth, bestMove, ss->staticEval);
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_continuation_histories() updates histories of the move pairs formed
// by moves at ply -1, -2, and -4 with current move.
void update_continuation_histories(Stack* ss, Piece pc, Square to, int bonus) {
for (int i : {1, 2, 4})
if (is_ok((ss-i)->currentMove))
(*(ss-i)->continuationHistory)[pc][to] << bonus;
}
// update_capture_stats() updates move sorting heuristics when a new capture best move is found
void update_capture_stats(const Position& pos, Move move,
Move* captures, int captureCount, int bonus) {
CapturePieceToHistory& captureHistory = pos.this_thread()->captureHistory;
Piece moved_piece = pos.moved_piece(move);
PieceType captured = type_of(pos.piece_on(to_sq(move)));
if (pos.capture_or_promotion(move))
captureHistory[moved_piece][to_sq(move)][captured] << bonus;
// Decrease all the other played capture moves
for (int i = 0; i < captureCount; ++i)
{
moved_piece = pos.moved_piece(captures[i]);
captured = type_of(pos.piece_on(to_sq(captures[i])));
captureHistory[moved_piece][to_sq(captures[i])][captured] << -bonus;
}
}
// update_quiet_stats() updates move sorting heuristics when a new quiet best move is found
void update_quiet_stats(const Position& pos, Stack* ss, Move move,
Move* quiets, int quietCount, int bonus) {
if (ss->killers[0] != move)
{
ss->killers[1] = ss->killers[0];
ss->killers[0] = move;
}
Color us = pos.side_to_move();
Thread* thisThread = pos.this_thread();
thisThread->mainHistory[us][from_to(move)] << bonus;
update_continuation_histories(ss, pos.moved_piece(move), to_sq(move), bonus);
if (is_ok((ss-1)->currentMove))
{
Square prevSq = to_sq((ss-1)->currentMove);
thisThread->counterMoves[pos.piece_on(prevSq)][prevSq] = move;
}
// Decrease all the other played quiet moves
for (int i = 0; i < quietCount; ++i)
{
thisThread->mainHistory[us][from_to(quiets[i])] << -bonus;
update_continuation_histories(ss, pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
}
}
// 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) {
const RootMoves& rootMoves = Threads.main()->rootMoves;
static PRNG rng(now()); // PRNG sequence should be non-deterministic
// RootMoves are already sorted by score in descending order
Value topScore = rootMoves[0].score;
int delta = std::min(topScore - 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 is deterministic and bigger for weaker levels, and one is
// 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(topScore - rootMoves[i].score)
+ delta * (rng.rand<unsigned>() % weakness)) / 128;
if (rootMoves[i].score + push >= maxScore)
{
maxScore = rootMoves[i].score + push;
best = rootMoves[i].pv[0];
}
}
return best;
}
} // namespace
/// MainThread::check_time() is used to print debug info and, more importantly,
/// to detect when we are out of available time and thus stop the search.
void MainThread::check_time() {
if (--callsCnt > 0)
return;
// When using nodes, ensure checking rate is not lower than 0.1% of nodes
callsCnt = Limits.nodes ? std::min(1024, int(Limits.nodes / 1024)) : 1024;
static TimePoint lastInfoTime = now();
TimePoint elapsed = Time.elapsed();
TimePoint tick = Limits.startTime + elapsed;
if (tick - lastInfoTime >= 1000)
{
lastInfoTime = tick;
dbg_print();
}
// We should not stop pondering until told so by the GUI
if (Threads.ponder)
return;
if ( (Limits.use_time_management() && elapsed > Time.maximum() - 10)
|| (Limits.movetime && elapsed >= Limits.movetime)
|| (Limits.nodes && Threads.nodes_searched() >= (uint64_t)Limits.nodes))
Threads.stop = true;
}
/// 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 = Time.elapsed() + 1;
const RootMoves& rootMoves = pos.this_thread()->rootMoves;
size_t pvIdx = pos.this_thread()->pvIdx;
size_t multiPV = std::min((size_t)Options["MultiPV"], rootMoves.size());
uint64_t nodesSearched = Threads.nodes_searched();
uint64_t tbHits = Threads.tb_hits() + (TB::RootInTB ? rootMoves.size() : 0);
for (size_t i = 0; i < multiPV; ++i)
{
bool updated = (i <= pvIdx && rootMoves[i].score != -VALUE_INFINITE);
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 ? rootMoves[i].tbScore : v;
if (ss.rdbuf()->in_avail()) // Not at first line
ss << "\n";
ss << "info"
<< " depth " << d / ONE_PLY
<< " seldepth " << rootMoves[i].selDepth
<< " multipv " << i + 1
<< " score " << UCI::value(v);
if (!tb && i == pvIdx)
ss << (v >= beta ? " lowerbound" : v <= alpha ? " upperbound" : "");
ss << " nodes " << nodesSearched
<< " nps " << nodesSearched * 1000 / elapsed;
if (elapsed > 1000) // Earlier makes little sense
ss << " hashfull " << TT.hashfull();
ss << " tbhits " << tbHits
<< " time " << elapsed
<< " pv";
for (Move m : rootMoves[i].pv)
ss << " " << UCI::move(m, pos.is_chess960());
}
return ss.str();
}
/// 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);
if (pv[0] == MOVE_NONE)
return false;
pos.do_move(pv[0], st);
TTEntry* tte = TT.probe(pos.key(), ttHit);
if (ttHit)
{
Move m = tte->move(); // Local copy to be SMP safe
if (MoveList<LEGAL>(pos).contains(m))
pv.push_back(m);
}
pos.undo_move(pv[0]);
return pv.size() > 1;
}
void Tablebases::rank_root_moves(Position& pos, Search::RootMoves& rootMoves) {
RootInTB = false;
UseRule50 = bool(Options["Syzygy50MoveRule"]);
ProbeDepth = int(Options["SyzygyProbeDepth"]) * ONE_PLY;
Cardinality = int(Options["SyzygyProbeLimit"]);
bool dtz_available = true;
// Tables with fewer pieces than SyzygyProbeLimit are searched with
// ProbeDepth == DEPTH_ZERO
if (Cardinality > MaxCardinality)
{
Cardinality = MaxCardinality;
ProbeDepth = DEPTH_ZERO;
}
if (Cardinality >= popcount(pos.pieces()) && !pos.can_castle(ANY_CASTLING))
{
// Rank moves using DTZ tables
RootInTB = root_probe(pos, rootMoves);
if (!RootInTB)
{
// DTZ tables are missing; try to rank moves using WDL tables
dtz_available = false;
RootInTB = root_probe_wdl(pos, rootMoves);
}
}
if (RootInTB)
{
// Sort moves according to TB rank
std::sort(rootMoves.begin(), rootMoves.end(),
[](const RootMove &a, const RootMove &b) { return a.tbRank > b.tbRank; } );
// Probe during search only if DTZ is not available and we are winning
if (dtz_available || rootMoves[0].tbScore <= VALUE_DRAW)
Cardinality = 0;
}
else
{
// Assign the same rank to all moves
for (auto& m : rootMoves)
m.tbRank = 0;
}
}
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