Stockfish is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
Stockfish is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
-// Macro to embed the default NNUE file data in the engine binary (using incbin.h, by Dale Weiler).
+// Macro to embed the default efficiently updatable neural network (NNUE) file
+// data in the engine binary (using incbin.h, by Dale Weiler).
// This macro invocation will declare the following three variables
// const unsigned char gEmbeddedNNUEData[]; // a pointer to the embedded data
// const unsigned char *const gEmbeddedNNUEEnd; // a marker to the end
// const unsigned int gEmbeddedNNUESize; // the size of the embedded file
// This macro invocation will declare the following three variables
// const unsigned char gEmbeddedNNUEData[]; // a pointer to the embedded data
// const unsigned char *const gEmbeddedNNUEEnd; // a marker to the end
// const unsigned int gEmbeddedNNUESize; // the size of the embedded file
#if !defined(_MSC_VER) && !defined(NNUE_EMBEDDING_OFF)
INCBIN(EmbeddedNNUE, EvalFileDefaultName);
#else
#if !defined(_MSC_VER) && !defined(NNUE_EMBEDDING_OFF)
INCBIN(EmbeddedNNUE, EvalFileDefaultName);
#else
/// We search the given network in three locations: internally (the default
/// network may be embedded in the binary), in the active working directory and
/// in the engine directory. Distro packagers may define the DEFAULT_NNUE_DIRECTORY
/// We search the given network in three locations: internally (the default
/// network may be embedded in the binary), in the active working directory and
/// in the engine directory. Distro packagers may define the DEFAULT_NNUE_DIRECTORY
+ /// NNUE::export_net() exports the currently loaded network to a file
+ void NNUE::export_net(const std::optional<std::string>& filename) {
+ std::string actualFilename;
+
+ if (filename.has_value())
+ actualFilename = filename.value();
+ else
+ {
+ if (eval_file_loaded != EvalFileDefaultName)
+ {
+ sync_cout << "Failed to export a net. A non-embedded net can only be saved if the filename is specified." << sync_endl;
+ return;
+ }
+ actualFilename = EvalFileDefaultName;
+ }
+
+ ofstream stream(actualFilename, std::ios_base::binary);
+
+ if (save_eval(stream))
+ sync_cout << "Network saved successfully to " << actualFilename << "." << sync_endl;
+ else
+ sync_cout << "Failed to export a net." << sync_endl;
+ }
+
- constexpr Value LazyThreshold1 = Value(1400);
- constexpr Value LazyThreshold2 = Value(1300);
- constexpr Value SpaceThreshold = Value(12222);
- constexpr Value NNUEThreshold1 = Value(550);
- constexpr Value NNUEThreshold2 = Value(150);
+ constexpr Value LazyThreshold1 = Value(1565);
+ constexpr Value LazyThreshold2 = Value(1102);
+ constexpr Value LazyThresholdNNUE = Value(1400);
+ constexpr Value SpaceThreshold = Value(11551);
+ constexpr Value NNUEThreshold1 = Value(682);
+ constexpr Value NNUEThreshold2 = Value(176);
// KingAttackWeights[PieceType] contains king attack weights by piece type
constexpr int KingAttackWeights[PIECE_TYPE_NB] = { 0, 0, 81, 52, 44, 10 };
// KingAttackWeights[PieceType] contains king attack weights by piece type
constexpr int KingAttackWeights[PIECE_TYPE_NB] = { 0, 0, 81, 52, 44, 10 };
{ S(-47,-59), S(-20,-25), S( 14, -8), S( 29, 12), S( 39, 21), S( 53, 40), // Bishop
S( 53, 56), S( 60, 58), S( 62, 65), S( 69, 72), S( 78, 78), S( 83, 87),
S( 91, 88), S( 96, 98) },
{ S(-47,-59), S(-20,-25), S( 14, -8), S( 29, 12), S( 39, 21), S( 53, 40), // Bishop
S( 53, 56), S( 60, 58), S( 62, 65), S( 69, 72), S( 78, 78), S( 83, 87),
S( 91, 88), S( 96, 98) },
- { S(-61,-82), S(-20,-17), S( 2, 23) ,S( 3, 40), S( 4, 72), S( 11,100), // Rook
- S( 22,104), S( 31,120), S( 39,134), S(40 ,138), S( 41,158), S( 47,163),
- S( 59,168), S( 60,169), S( 64,173) },
+ { S(-60,-82), S(-24,-15), S( 0, 17) ,S( 3, 43), S( 4, 72), S( 14,100), // Rook
+ S( 20,102), S( 30,122), S( 41,133), S(41 ,139), S( 41,153), S( 45,160),
+ S( 57,165), S( 58,170), S( 67,175) },
{ S(-29,-49), S(-16,-29), S( -8, -8), S( -8, 17), S( 18, 39), S( 25, 54), // Queen
S( 23, 59), S( 37, 73), S( 41, 76), S( 54, 95), S( 65, 95) ,S( 68,101),
S( 69,124), S( 70,128), S( 70,132), S( 70,133) ,S( 71,136), S( 72,140),
{ S(-29,-49), S(-16,-29), S( -8, -8), S( -8, 17), S( 18, 39), S( 25, 54), // Queen
S( 23, 59), S( 37, 73), S( 41, 76), S( 54, 95), S( 65, 95) ,S( 68,101),
S( 69,124), S( 70,128), S( 70,132), S( 70,133) ,S( 71,136), S( 72,140),
S(112,178), S(114,185), S(114,187), S(119,221) }
};
S(112,178), S(114,185), S(114,187), S(119,221) }
};
+ // BishopPawns[distance from edge] contains a file-dependent penalty for pawns on
+ // squares of the same color as our bishop.
+ constexpr Score BishopPawns[int(FILE_NB) / 2] = {
+ S(3, 8), S(3, 9), S(2, 8), S(3, 8)
+ };
+
// KingProtector[knight/bishop] contains penalty for each distance unit to own king
constexpr Score KingProtector[] = { S(8, 9), S(6, 9) };
// Outpost[knight/bishop] contains bonuses for each knight or bishop occupying a
// pawn protected square on rank 4 to 6 which is also safe from a pawn attack.
// KingProtector[knight/bishop] contains penalty for each distance unit to own king
constexpr Score KingProtector[] = { S(8, 9), S(6, 9) };
// Outpost[knight/bishop] contains bonuses for each knight or bishop occupying a
// pawn protected square on rank 4 to 6 which is also safe from a pawn attack.
- constexpr Score Outpost[] = { S(56, 34), S(31, 23) };
+ constexpr Score Outpost[] = { S(57, 38), S(31, 24) };
// PassedRank[Rank] contains a bonus according to the rank of a passed pawn
constexpr Score PassedRank[RANK_NB] = {
// PassedRank[Rank] contains a bonus according to the rank of a passed pawn
constexpr Score PassedRank[RANK_NB] = {
- S(0, 0), S(9, 28), S(15, 31), S(17, 39), S(64, 70), S(171, 177), S(277, 260)
+ S(0, 0), S(7, 27), S(16, 32), S(17, 40), S(64, 71), S(170, 174), S(278, 262)
- // RookOnFile[semiopen/open] contains bonuses for each rook when there is
- // no (friendly) pawn on the rook file.
- constexpr Score RookOnFile[] = { S(19, 7), S(48, 27) };
+ constexpr Score RookOnClosedFile = S(10, 5);
+ constexpr Score RookOnOpenFile[] = { S(19, 6), S(47, 26) };
// ThreatByMinor/ByRook[attacked PieceType] contains bonuses according to
// which piece type attacks which one. Attacks on lesser pieces which are
// ThreatByMinor/ByRook[attacked PieceType] contains bonuses according to
// which piece type attacks which one. Attacks on lesser pieces which are
S(0, 0), S(3, 44), S(37, 68), S(42, 60), S(0, 39), S(58, 43)
};
S(0, 0), S(3, 44), S(37, 68), S(42, 60), S(0, 39), S(58, 43)
};
constexpr Score FlankAttacks = S( 8, 0);
constexpr Score Hanging = S( 69, 36);
constexpr Score KnightOnQueen = S( 16, 11);
constexpr Score FlankAttacks = S( 8, 0);
constexpr Score Hanging = S( 69, 36);
constexpr Score KnightOnQueen = S( 16, 11);
constexpr Score ReachableOutpost = S( 31, 22);
constexpr Score RestrictedPiece = S( 7, 7);
constexpr Score RookOnKingRing = S( 16, 0);
constexpr Score ReachableOutpost = S( 31, 22);
constexpr Score RestrictedPiece = S( 7, 7);
constexpr Score RookOnKingRing = S( 16, 0);
constexpr Score SliderOnQueen = S( 60, 18);
constexpr Score ThreatByKing = S( 24, 89);
constexpr Score ThreatByPawnPush = S( 48, 39);
constexpr Score SliderOnQueen = S( 60, 18);
constexpr Score ThreatByKing = S( 24, 89);
constexpr Score ThreatByPawnPush = S( 48, 39);
constexpr Direction Down = -pawn_push(Us);
constexpr Bitboard OutpostRanks = (Us == WHITE ? Rank4BB | Rank5BB | Rank6BB
: Rank5BB | Rank4BB | Rank3BB);
constexpr Direction Down = -pawn_push(Us);
constexpr Bitboard OutpostRanks = (Us == WHITE ? Rank4BB | Rank5BB | Rank6BB
: Rank5BB | Rank4BB | Rank3BB);
// Find attacked squares, including x-ray attacks for bishops and rooks
b = Pt == BISHOP ? attacks_bb<BISHOP>(s, pos.pieces() ^ pos.pieces(QUEEN))
: Pt == ROOK ? attacks_bb< ROOK>(s, pos.pieces() ^ pos.pieces(QUEEN) ^ pos.pieces(Us, ROOK))
// Find attacked squares, including x-ray attacks for bishops and rooks
b = Pt == BISHOP ? attacks_bb<BISHOP>(s, pos.pieces() ^ pos.pieces(QUEEN))
: Pt == ROOK ? attacks_bb< ROOK>(s, pos.pieces() ^ pos.pieces(QUEEN) ^ pos.pieces(Us, ROOK))
mobility[Us] += MobilityBonus[Pt - 2][mob];
if (Pt == BISHOP || Pt == KNIGHT)
{
// Bonus if the piece is on an outpost square or can reach one
mobility[Us] += MobilityBonus[Pt - 2][mob];
if (Pt == BISHOP || Pt == KNIGHT)
{
// Bonus if the piece is on an outpost square or can reach one
bb = OutpostRanks & (attackedBy[Us][PAWN] | shift<Down>(pos.pieces(PAWN)))
& ~pe->pawn_attacks_span(Them);
Bitboard targets = pos.pieces(Them) & ~pos.pieces(PAWN);
bb = OutpostRanks & (attackedBy[Us][PAWN] | shift<Down>(pos.pieces(PAWN)))
& ~pe->pawn_attacks_span(Them);
Bitboard targets = pos.pieces(Them) & ~pos.pieces(PAWN);
&& bb & s & ~CenterFiles // on a side outpost
&& !(b & targets) // no relevant attacks
&& (!more_than_one(targets & (s & QueenSide ? QueenSide : KingSide))))
&& bb & s & ~CenterFiles // on a side outpost
&& !(b & targets) // no relevant attacks
&& (!more_than_one(targets & (s & QueenSide ? QueenSide : KingSide))))
// Penalty if the piece is far from the king
score -= KingProtector[Pt == BISHOP] * distance(pos.square<KING>(Us), s);
// Penalty if the piece is far from the king
score -= KingProtector[Pt == BISHOP] * distance(pos.square<KING>(Us), s);
{
// Penalty according to the number of our pawns on the same color square as the
// bishop, bigger when the center files are blocked with pawns and smaller
// when the bishop is outside the pawn chain.
Bitboard blocked = pos.pieces(Us, PAWN) & shift<Down>(pos.pieces());
{
// Penalty according to the number of our pawns on the same color square as the
// bishop, bigger when the center files are blocked with pawns and smaller
// when the bishop is outside the pawn chain.
Bitboard blocked = pos.pieces(Us, PAWN) & shift<Down>(pos.pieces());
* (!(attackedBy[Us][PAWN] & s) + popcount(blocked & CenterFiles));
// Penalty for all enemy pawns x-rayed
* (!(attackedBy[Us][PAWN] & s) + popcount(blocked & CenterFiles));
// Penalty for all enemy pawns x-rayed
{
Direction d = pawn_push(Us) + (file_of(s) == FILE_A ? EAST : WEST);
if (pos.piece_on(s + d) == make_piece(Us, PAWN))
{
Direction d = pawn_push(Us) + (file_of(s) == FILE_A ? EAST : WEST);
if (pos.piece_on(s + d) == make_piece(Us, PAWN))
- score -= !pos.empty(s + d + pawn_push(Us)) ? CorneredBishop * 4
- : pos.piece_on(s + d + d) == make_piece(Us, PAWN) ? CorneredBishop * 2
- : CorneredBishop;
+ score -= !pos.empty(s + d + pawn_push(Us)) ? 4 * make_score(CorneredBishop, CorneredBishop)
+ : 3 * make_score(CorneredBishop, CorneredBishop);
- // Bonus for rook on the same file as a queen
- if (file_bb(s) & pos.pieces(QUEEN))
- score += RookOnQueenFile;
-
- // Bonus for rook on an open or semi-open file
+ // Bonuses for rook on a (semi-)open or closed file
- File kf = file_of(pos.square<KING>(Us));
- if ((kf < FILE_E) == (file_of(s) < kf))
- score -= TrappedRook * (1 + !pos.castling_rights(Us));
+ score += RookOnOpenFile[pos.is_on_semiopen_file(Them, s)];
+ }
+ else
+ {
+ // If our pawn on this file is blocked, increase penalty
+ if ( pos.pieces(Us, PAWN)
+ & shift<Down>(pos.pieces())
+ & file_bb(s))
+ {
+ score -= RookOnClosedFile;
+ }
+
+ // Penalty when trapped by the king, even more if the king cannot castle
+ if (mob <= 3)
+ {
+ File kf = file_of(pos.square<KING>(Us));
+ if ((kf < FILE_E) == (file_of(s) < kf))
+ score -= TrappedRook * (1 + !pos.castling_rights(Us));
+ }
- kingDanger += kingAttackersCount[Them] * kingAttackersWeight[Them]
- + 185 * popcount(kingRing[Us] & weak)
- + 148 * popcount(unsafeChecks)
- + 98 * popcount(pos.blockers_for_king(Us))
- + 69 * kingAttacksCount[Them]
- + 3 * kingFlankAttack * kingFlankAttack / 8
- + mg_value(mobility[Them] - mobility[Us])
- - 873 * !pos.count<QUEEN>(Them)
- - 100 * bool(attackedBy[Us][KNIGHT] & attackedBy[Us][KING])
- - 6 * mg_value(score) / 8
- - 4 * kingFlankDefense
- + 37;
+ kingDanger += kingAttackersCount[Them] * kingAttackersWeight[Them] // (~10 Elo)
+ + 183 * popcount(kingRing[Us] & weak) // (~15 Elo)
+ + 148 * popcount(unsafeChecks) // (~4 Elo)
+ + 98 * popcount(pos.blockers_for_king(Us)) // (~2 Elo)
+ + 69 * kingAttacksCount[Them] // (~0.5 Elo)
+ + 3 * kingFlankAttack * kingFlankAttack / 8 // (~0.5 Elo)
+ + mg_value(mobility[Them] - mobility[Us]) // (~0.5 Elo)
+ - 873 * !pos.count<QUEEN>(Them) // (~24 Elo)
+ - 100 * bool(attackedBy[Us][KNIGHT] & attackedBy[Us][KING]) // (~5 Elo)
+ - 6 * mg_value(score) / 8 // (~8 Elo)
+ - 4 * kingFlankDefense // (~5 Elo)
+ + 37; // (~0.5 Elo)
// Penalty if king flank is under attack, potentially moving toward the king
score -= FlankAttacks * kingFlankAttack;
// Penalty if king flank is under attack, potentially moving toward the king
score -= FlankAttacks * kingFlankAttack;
bb = forward_file_bb(Them, s) & pos.pieces(ROOK, QUEEN);
if (!(pos.pieces(Them) & bb))
bb = forward_file_bb(Them, s) & pos.pieces(ROOK, QUEEN);
if (!(pos.pieces(Them) & bb))
- // If there are no enemy attacks on passed pawn span, assign a big bonus.
+ // If there are no enemy pieces or attacks on passed pawn span, assign a big bonus.
+ // Or if there is some, but they are all attacked by our pawns, assign a bit smaller bonus.
// Otherwise assign a smaller bonus if the path to queen is not attacked
// and even smaller bonus if it is attacked but block square is not.
// Otherwise assign a smaller bonus if the path to queen is not attacked
// and even smaller bonus if it is attacked but block square is not.
- int k = !unsafeSquares ? 35 :
- !(unsafeSquares & squaresToQueen) ? 20 :
- !(unsafeSquares & blockSq) ? 9 :
+ int k = !unsafeSquares ? 36 :
+ !(unsafeSquares & ~attackedBy[Us][PAWN]) ? 30 :
+ !(unsafeSquares & squaresToQueen) ? 17 :
+ !(unsafeSquares & blockSq) ? 7 :
int bonus = popcount(safe) + popcount(behind & safe & ~attackedBy[Them][ALL_PIECES]);
int weight = pos.count<ALL_PIECES>(Us) - 3 + std::min(pe->blocked_count(), 9);
Score score = make_score(bonus * weight * weight / 16, 0);
int bonus = popcount(safe) + popcount(behind & safe & ~attackedBy[Them][ALL_PIECES]);
int weight = pos.count<ALL_PIECES>(Us) - 3 + std::min(pe->blocked_count(), 9);
Score score = make_score(bonus * weight * weight / 16, 0);
Value Evaluation<T>::winnable(Score score) const {
int outflanking = distance<File>(pos.square<KING>(WHITE), pos.square<KING>(BLACK))
Value Evaluation<T>::winnable(Score score) const {
int outflanking = distance<File>(pos.square<KING>(WHITE), pos.square<KING>(BLACK))
Color strongSide = eg > VALUE_DRAW ? WHITE : BLACK;
int sf = me->scale_factor(pos, strongSide);
Color strongSide = eg > VALUE_DRAW ? WHITE : BLACK;
int sf = me->scale_factor(pos, strongSide);
if ( pos.non_pawn_material(WHITE) == BishopValueMg
&& pos.non_pawn_material(BLACK) == BishopValueMg)
sf = 18 + 4 * popcount(pe->passed_pawns(strongSide));
if ( pos.non_pawn_material(WHITE) == BishopValueMg
&& pos.non_pawn_material(BLACK) == BishopValueMg)
sf = 18 + 4 * popcount(pe->passed_pawns(strongSide));
else if ( pos.non_pawn_material(WHITE) == RookValueMg
&& pos.non_pawn_material(BLACK) == RookValueMg
&& pos.count<PAWN>(strongSide) - pos.count<PAWN>(~strongSide) <= 1
&& bool(KingSide & pos.pieces(strongSide, PAWN)) != bool(QueenSide & pos.pieces(strongSide, PAWN))
&& (attacks_bb<KING>(pos.square<KING>(~strongSide)) & pos.pieces(~strongSide, PAWN)))
sf = 36;
else if ( pos.non_pawn_material(WHITE) == RookValueMg
&& pos.non_pawn_material(BLACK) == RookValueMg
&& pos.count<PAWN>(strongSide) - pos.count<PAWN>(~strongSide) <= 1
&& bool(KingSide & pos.pieces(strongSide, PAWN)) != bool(QueenSide & pos.pieces(strongSide, PAWN))
&& (attacks_bb<KING>(pos.square<KING>(~strongSide)) & pos.pieces(~strongSide, PAWN)))
sf = 36;
else if (pos.count<QUEEN>() == 1)
sf = 37 + 3 * (pos.count<QUEEN>(WHITE) == 1 ? pos.count<BISHOP>(BLACK) + pos.count<KNIGHT>(BLACK)
: pos.count<BISHOP>(WHITE) + pos.count<KNIGHT>(WHITE));
else if (pos.count<QUEEN>() == 1)
sf = 37 + 3 * (pos.count<QUEEN>(WHITE) == 1 ? pos.count<BISHOP>(BLACK) + pos.count<KNIGHT>(BLACK)
: pos.count<BISHOP>(WHITE) + pos.count<KNIGHT>(WHITE));
- sf = std::min(sf, 36 + 7 * pos.count<PAWN>(strongSide));
+ sf = std::min(sf, 36 + 7 * pos.count<PAWN>(strongSide)) - 4 * !pawnsOnBothFlanks;
+
+ // Reduce scale factor in case of pawns being on a single flank
+ sf -= 4 * !pawnsOnBothFlanks;
{
Trace::add(WINNABLE, make_score(u, eg * ScaleFactor(sf) / SCALE_FACTOR_NORMAL - eg_value(score)));
Trace::add(TOTAL, make_score(mg, eg * ScaleFactor(sf) / SCALE_FACTOR_NORMAL));
{
Trace::add(WINNABLE, make_score(u, eg * ScaleFactor(sf) / SCALE_FACTOR_NORMAL - eg_value(score)));
Trace::add(TOTAL, make_score(mg, eg * ScaleFactor(sf) / SCALE_FACTOR_NORMAL));
{
Trace::add(MATERIAL, pos.psq_score());
Trace::add(IMBALANCE, me->imbalance());
{
Trace::add(MATERIAL, pos.psq_score());
Trace::add(IMBALANCE, me->imbalance());
+
+ /// Fisher Random Chess: correction for cornered bishops, to fix chess960 play with NNUE
+
+ Value fix_FRC(const Position& pos) {
+
+ constexpr Bitboard Corners = 1ULL << SQ_A1 | 1ULL << SQ_H1 | 1ULL << SQ_A8 | 1ULL << SQ_H8;
+
+ if (!(pos.pieces(BISHOP) & Corners))
+ return VALUE_ZERO;
+
+ int correction = 0;
+
+ if ( pos.piece_on(SQ_A1) == W_BISHOP
+ && pos.piece_on(SQ_B2) == W_PAWN)
+ correction += !pos.empty(SQ_B3) ? -CorneredBishop * 4
+ : -CorneredBishop * 3;
+
+ if ( pos.piece_on(SQ_H1) == W_BISHOP
+ && pos.piece_on(SQ_G2) == W_PAWN)
+ correction += !pos.empty(SQ_G3) ? -CorneredBishop * 4
+ : -CorneredBishop * 3;
+
+ if ( pos.piece_on(SQ_A8) == B_BISHOP
+ && pos.piece_on(SQ_B7) == B_PAWN)
+ correction += !pos.empty(SQ_B6) ? CorneredBishop * 4
+ : CorneredBishop * 3;
+
+ if ( pos.piece_on(SQ_H8) == B_BISHOP
+ && pos.piece_on(SQ_G7) == B_PAWN)
+ correction += !pos.empty(SQ_G6) ? CorneredBishop * 4
+ : CorneredBishop * 3;
+
+ return pos.side_to_move() == WHITE ? Value(correction)
+ : -Value(correction);
+ }
+
+} // namespace Eval
- auto adjusted_NNUE = [&](){
- int mat = pos.non_pawn_material();
- return NNUE::evaluate(pos) * (1024 + mat / 32) / 1024 + Tempo;
+ auto adjusted_NNUE = [&]()
+ {
+
+ int scale = 903 + 28 * pos.count<PAWN>() + 28 * pos.non_pawn_material() / 1024;
+
+ Value nnue = NNUE::evaluate(pos, true, LazyThresholdNNUE) * scale / 1024;
+
+ if (pos.is_chess960())
+ nnue += fix_FRC(pos);
+
+ return nnue;
Value psq = Value(abs(eg_value(pos.psq_score())));
int r50 = 16 + pos.rule50_count();
bool largePsq = psq * 16 > (NNUEThreshold1 + pos.non_pawn_material() / 64) * r50;
Value psq = Value(abs(eg_value(pos.psq_score())));
int r50 = 16 + pos.rule50_count();
bool largePsq = psq * 16 > (NNUEThreshold1 + pos.non_pawn_material() / 64) * r50;
- v = classical ? Evaluation<NO_TRACE>(pos).value() : adjusted_NNUE();
+ // Use classical evaluation for really low piece endgames.
+ // One critical case is the draw for bishop + A/H file pawn vs naked king.
+ bool lowPieceEndgame = pos.non_pawn_material() == BishopValueMg
+ || (pos.non_pawn_material() < 2 * RookValueMg && pos.count<PAWN>() < 2);
+
+ v = classical || lowPieceEndgame ? Evaluation<NO_TRACE>(pos).value()
+ : adjusted_NNUE();
- // For the case of opposite colored bishops, switch to NNUE eval with
- // small probability if the classical eval is less than the threshold.
- if ( largePsq
- && (abs(v) * 16 < NNUEThreshold2 * r50
- || ( pos.opposite_bishops()
- && abs(v) * 16 < (NNUEThreshold1 + pos.non_pawn_material() / 64) * r50
- && !(pos.this_thread()->nodes & 0xB))))
+ // For the case of opposite colored bishops, switch to NNUE eval with small
+ // probability if the classical eval is less than the threshold.
+ if ( largePsq
+ && !lowPieceEndgame
+ && ( abs(v) * 16 < NNUEThreshold2 * r50
+ || ( pos.opposite_bishops()
+ && abs(v) * 16 < (NNUEThreshold1 + pos.non_pawn_material() / 64) * r50)))