/*
Stockfish, a UCI chess playing engine derived from Glaurung 2.1
- Copyright (C) 2004-2020 The Stockfish developers (see AUTHORS file)
+ Copyright (C) 2004-2021 The Stockfish developers (see AUTHORS file)
Stockfish is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
#include "uci.h"
#include "syzygy/tbprobe.h"
+namespace Stockfish {
+
namespace Search {
LimitsType Limits;
constexpr uint64_t TtHitAverageWindow = 4096;
constexpr uint64_t TtHitAverageResolution = 1024;
- // Razor and futility margins
- constexpr int RazorMargin = 510;
+ // Futility margin
Value futility_margin(Depth d, bool improving) {
return Value(234 * (d - improving));
}
// History and stats update bonus, based on depth
int stat_bonus(Depth d) {
- return d > 13 ? 29 : 17 * d * d + 134 * d - 134;
+ return d > 14 ? 66 : 6 * d * d + 231 * d - 206;
}
- // Add a small random component to draw evaluations to avoid 3fold-blindness
+ // Add a small random component to draw evaluations to avoid 3-fold blindness
Value value_draw(Thread* thisThread) {
return VALUE_DRAW + Value(2 * (thisThread->nodes & 1) - 1);
}
while (true)
{
Depth adjustedDepth = std::max(1, rootDepth - failedHighCnt - searchAgainCounter);
- bestValue = ::search<PV>(rootPos, ss, alpha, beta, adjustedDepth, false);
+ bestValue =
Stockfish::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
moveCount = captureCount = quietCount = ss->moveCount = 0;
bestValue = -VALUE_INFINITE;
maxValue = VALUE_INFINITE;
+ ss->distanceFromPv = (PvNode ? 0 : ss->distanceFromPv);
// Check for the available remaining time
if (thisThread == Threads.main())
tte->save(posKey, VALUE_NONE, ss->ttPv, BOUND_NONE, DEPTH_NONE, MOVE_NONE, eval);
}
- if ((ss-1)->moveCount > 1 && is_ok((ss-1)->currentMove) && !(ss-1)->inCheck && !priorCapture && depth < 7)
+ // Use static evaluation difference to improve quiet move ordering
+ if (is_ok((ss-1)->currentMove) && !(ss-1)->inCheck && !priorCapture)
{
- int bonus = std::clamp(- (depth+1) * 2 * int((ss-1)->staticEval + ss->staticEval - 2 * Tempo), -1000, 1000);
+ int bonus = std::clamp(-depth * 4 * int((ss-1)->staticEval + ss->staticEval - 2 * Tempo), -1000, 1000);
thisThread->mainHistory[~us][from_to((ss-1)->currentMove)] << bonus;
}
- // Step 7. Razoring (~1 Elo)
- if ( !rootNode // The required rootNode PV handling is not available in qsearch
- && depth == 1
- && eval <= alpha - RazorMargin)
- return qsearch<NT>(pos, ss, alpha, beta);
-
// Set up improving flag that is used in various pruning heuristics
// We define position as improving if static evaluation of position is better
// Than the previous static evaluation at our turn
? ss->staticEval > (ss-4)->staticEval || (ss-4)->staticEval == VALUE_NONE
: ss->staticEval > (ss-2)->staticEval;
- // Step 8. Futility pruning: child node (~50 Elo)
+ // Step 7. Futility pruning: child node (~50 Elo)
if ( !PvNode
- && depth < 8
+ && depth < 9
&& 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)
+ // Step 8. Null move search with verification search (~40 Elo)
if ( !PvNode
&& (ss-1)->currentMove != MOVE_NULL
- && (ss-1)->statScore < 22977
+ && (ss-1)->statScore < 24185
&& eval >= beta
&& eval >= ss->staticEval
- && ss->staticEval >= beta - 30 * depth - 28 * improving + 84 * ss->ttPv + 168
+ && ss->staticEval >= beta - 24 * depth - 34 * improving + 162 * ss->ttPv + 159
&& !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 = (1015 + 85 * depth) / 256 + std::min(int(eval - beta) / 191, 3);
+ Depth R = (1062 + 68 * depth) / 256 + std::min(int(eval - beta) / 190, 3);
ss->currentMove = MOVE_NULL;
ss->continuationHistory = &thisThread->continuationHistory[0][0][NO_PIECE][0];
}
}
- probCutBeta = beta + 183 - 49 * improving;
+ probCutBeta = beta + 209 - 44 * improving;
- // Step 10. ProbCut (~10 Elo)
+ // Step 9. 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
ss->ttPv = ttPv;
}
- // Step 11. If the position is not in TT, decrease depth by 2
+ // Step 10. If the position is not in TT, decrease depth by 2
if ( PvNode
&& depth >= 6
&& !ttMove)
moves_loop: // When in check, search starts from here
+ ttCapture = ttMove && pos.capture_or_promotion(ttMove);
+
+ // Step 11. A small Probcut idea, when we are in check
+ probCutBeta = beta + 400;
+ if ( ss->inCheck
+ && !PvNode
+ && depth >= 4
+ && ttCapture
+ && (tte->bound() & BOUND_LOWER)
+ && tte->depth() >= depth - 3
+ && ttValue >= probCutBeta
+ && abs(ttValue) <= VALUE_KNOWN_WIN
+ && abs(beta) <= VALUE_KNOWN_WIN
+ )
+ return probCutBeta;
+
+
const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,
nullptr , (ss-4)->continuationHistory,
nullptr , (ss-6)->continuationHistory };
value = bestValue;
singularQuietLMR = moveCountPruning = false;
- ttCapture = ttMove && pos.capture_or_promotion(ttMove);
// Mark this node as being searched
ThreadHolding th(thisThread, posKey, ss->ply);
movedPiece = pos.moved_piece(move);
givesCheck = pos.gives_check(move);
+ // Indicate PvNodes that will probably fail low if node was searched with non-PV search
+ // at depth equal or greater to current depth and result of this search was far below alpha
+ bool likelyFailLow = PvNode
+ && ttMove
+ && (tte->bound() & BOUND_UPPER)
+ && ttValue < alpha + 200 + 100 * depth
+ && tte->depth() >= depth;
+
// Calculate new depth for this move
newDepth = depth - 1;
// Reduced depth of the next LMR search
int lmrDepth = std::max(newDepth - reduction(improving, depth, moveCount), 0);
- if ( !captureOrPromotion
- && !givesCheck)
+ if ( captureOrPromotion
+ || givesCheck)
+ {
+ // Capture history based pruning when the move doesn't give check
+ if ( !givesCheck
+ && lmrDepth < 1
+ && captureHistory[movedPiece][to_sq(move)][type_of(pos.piece_on(to_sq(move)))] < 0)
+ continue;
+
+ // SEE based pruning
+ if (!pos.see_ge(move, Value(-218) * depth)) // (~25 Elo)
+ continue;
+ }
+ else
{
// Countermoves based pruning (~20 Elo)
if ( lmrDepth < 4 + ((ss-1)->statScore > 0 || (ss-1)->moveCount == 1)
// Futility pruning: parent node (~5 Elo)
if ( lmrDepth < 7
&& !ss->inCheck
- && ss->staticEval + 266 + 170 * lmrDepth <= alpha
+ && ss->staticEval + 174 + 157 * lmrDepth <= alpha
&& (*contHist[0])[movedPiece][to_sq(move)]
+ (*contHist[1])[movedPiece][to_sq(move)]
+ (*contHist[3])[movedPiece][to_sq(move)]
- + (*contHist[5])[movedPiece][to_sq(move)] / 2 < 27376)
+ + (*contHist[5])[movedPiece][to_sq(move)] / 3 < 28255)
continue;
// Prune moves with negative SEE (~20 Elo)
if (!pos.see_ge(move, Value(-(30 - std::min(lmrDepth, 18)) * lmrDepth * lmrDepth)))
continue;
}
- else
- {
- // Capture history based pruning when the move doesn't give check
- if ( !givesCheck
- && lmrDepth < 1
- && captureHistory[movedPiece][to_sq(move)][type_of(pos.piece_on(to_sq(move)))] < 0)
- continue;
-
- // SEE based pruning
- if (!pos.see_ge(move, Value(-213) * depth)) // (~25 Elo)
- continue;
- }
}
// Step 14. Extensions (~75 Elo)
// Check extension (~2 Elo)
else if ( givesCheck
- && (pos.is_discovery_check_on_king(~us, move) || pos.see_ge(move)))
+ && (pos.is_discovered_check_on_king(~us, move) || pos.see_ge(move)))
extension = 1;
// Last captures extension
&& pos.non_pawn_material() <= 2 * RookValueMg)
extension = 1;
- // Late irreversible move extension
- if ( move == ttMove
- && pos.rule50_count() > 80
- && (captureOrPromotion || type_of(movedPiece) == PAWN))
- extension = 2;
-
// Add extension to new depth
newDepth += extension;
// Step 15. Make the move
pos.do_move(move, st, givesCheck);
- // Step 16. Reduced depth search (LMR, ~200 Elo). If the move fails high it will be
- // re-searched at full depth.
+ (ss+1)->distanceFromPv = ss->distanceFromPv + moveCount - 1;
+
+ // Step 16. Late moves reduction / extension (LMR, ~200 Elo)
+ // We use various heuristics for the sons of a node after the first son has
+ // been searched. In general we would like to reduce them, but there are many
+ // cases where we extend a son if it has good chances to be "interesting".
if ( depth >= 3
&& moveCount > 1 + 2 * rootNode
&& ( !captureOrPromotion
|| moveCountPruning
|| ss->staticEval + PieceValue[EG][pos.captured_piece()] <= alpha
|| cutNode
+ || (!PvNode && !formerPv && captureHistory[movedPiece][to_sq(move)][type_of(pos.captured_piece())] < 3678)
|| thisThread->ttHitAverage < 432 * TtHitAverageResolution * TtHitAverageWindow / 1024))
{
Depth r = reduction(improving, depth, moveCount);
if (th.marked())
r++;
- // Decrease reduction if position is or has been on the PV (~10 Elo)
- if (ss->ttPv)
+ // Decrease reduction if position is or has been on the PV
+ // and node is not likely to fail low. (~10 Elo)
+ if (ss->ttPv && !likelyFailLow)
r -= 2;
// Increase reduction at root and non-PV nodes when the best move does not change frequently
if (singularQuietLMR)
r--;
- if (!captureOrPromotion)
+ if (captureOrPromotion)
+ {
+ // Unless giving check, this capture is likely bad
+ if ( !givesCheck
+ && ss->staticEval + PieceValue[EG][pos.captured_piece()] + 210 * depth <= alpha)
+ r++;
+ }
+ else
{
// Increase reduction if ttMove is a capture (~5 Elo)
if (ttCapture)
+ (*contHist[0])[movedPiece][to_sq(move)]
+ (*contHist[1])[movedPiece][to_sq(move)]
+ (*contHist[3])[movedPiece][to_sq(move)]
- - 5287;
+ - 4741;
// Decrease/increase reduction by comparing opponent's stat score (~10 Elo)
- if (ss->statScore >= -105 && (ss-1)->statScore < -103)
+ if (ss->statScore >= -89 && (ss-1)->statScore < -116)
r--;
- else if ((ss-1)->statScore >= -122 && ss->statScore < -129)
+ else if ((ss-1)->statScore >= -112 && ss->statScore < -100)
r++;
// Decrease/increase reduction for moves with a good/bad history (~30 Elo)
- r -= ss->statScore / 14884;
- }
- else
- {
- // Unless giving check, this capture is likely bad
- if ( !givesCheck
- && ss->staticEval + PieceValue[EG][pos.captured_piece()] + 210 * depth <= alpha)
- r++;
+ // If we are not in check use statScore, but if we are in check we use
+ // the sum of main history and first continuation history with an offset.
+ if (ss->inCheck)
+ r -= (thisThread->mainHistory[us][from_to(move)]
+ + (*contHist[0])[movedPiece][to_sq(move)] - 3833) / 16384;
+ else
+ r -= ss->statScore / 14790;
}
- Depth d = std::clamp(newDepth - r, 1, newDepth);
+ // In general we want to cap the LMR depth search at newDepth. But for nodes
+ // close to the principal variation the cap is at (newDepth + 1), which will
+ // allow these nodes to be searched deeper than the pv (up to 4 plies deeper).
+ Depth d = std::clamp(newDepth - r, 1, newDepth + ((ss+1)->distanceFromPv <= 4));
value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, d, true);
- doFullDepthSearch = value > alpha && d != newDepth;
-
+ // If the son is reduced and fails high it will be re-searched at full depth
+ doFullDepthSearch = value > alpha && d < newDepth;
didLMR = true;
}
else
{
doFullDepthSearch = !PvNode || moveCount > 1;
-
didLMR = false;
}
moveCount++;
- // Futility pruning
+ // Futility pruning and moveCount pruning
if ( bestValue > VALUE_TB_LOSS_IN_MAX_PLY
&& !givesCheck
&& futilityBase > -VALUE_KNOWN_WIN
&& !pos.advanced_pawn_push(move))
{
- assert(type_of(move) != ENPASSANT); // Due to !pos.advanced_pawn_push
- // moveCount pruning
if (moveCount > 2)
continue;
PieceType captured = type_of(pos.piece_on(to_sq(bestMove)));
bonus1 = stat_bonus(depth + 1);
- bonus2 = bestValue > beta + PawnValueMg ? bonus1 // larger bonus
- : stat_bonus(depth); // smaller bonus
+ bonus2 = bestValue > beta + PawnValueMg ? bonus1 // larger bonus
+ : std::min(bonus1, stat_bonus(depth)); // smaller bonus
if (!pos.capture_or_promotion(bestMove))
{
m.tbRank = 0;
}
}
+
+} // namespace Stockfish
projects / stockfish / blobdiff