stockfish/src/evaluate.cpp

/*
  Stockfish, a UCI chess playing engine derived from Glaurung 2.1
  Copyright (C) 2004-2008 Tord Romstad (Glaurung author)
  Copyright (C) 2008-2010 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/>.
*/


////
//// Includes
////

#include <cassert>
#include <cstring>

#include "bitcount.h"
#include "evaluate.h"
#include "material.h"
#include "pawns.h"
#include "thread.h"
#include "ucioption.h"


////
//// Local definitions
////

namespace {

  const int Sign[2] = { 1, -1 };

  // Evaluation grain size, must be a power of 2
  const int GrainSize = 8;

  // Evaluation weights, initialized from UCI options
  enum { Mobility, PawnStructure, PassedPawns, Space, KingDangerUs, KingDangerThem };
  Score Weights[6];

  typedef Value V;
  #define S(mg, eg) make_score(mg, eg)

  // Internal evaluation weights. These are applied on top of the evaluation
  // weights read from UCI parameters. The purpose is to be able to change
  // the evaluation weights while keeping the default values of the UCI
  // parameters at 100, which looks prettier.
  //
  // Values modified by Joona Kiiski
  const Score WeightsInternal[] = {
      S(248, 271), S(233, 201), S(252, 259), S(46, 0), S(247, 0), S(259, 0)
  };

  // MobilityBonus[PieceType][attacked] contains mobility bonuses for middle and
  // end game, indexed by piece type and number of attacked squares not occupied
  // by friendly pieces.
  const Score MobilityBonus[][32] = {
     {}, {},
     { S(-38,-33), S(-25,-23), S(-12,-13), S( 0, -3), S(12,  7), S(25, 17), // Knights
       S( 31, 22), S( 38, 27), S( 38, 27) },
     { S(-25,-30), S(-11,-16), S(  3, -2), S(17, 12), S(31, 26), S(45, 40), // Bishops
       S( 57, 52), S( 65, 60), S( 71, 65), S(74, 69), S(76, 71), S(78, 73),
       S( 79, 74), S( 80, 75), S( 81, 76), S(81, 76) },
     { S(-20,-36), S(-14,-19), S( -8, -3), S(-2, 13), S( 4, 29), S(10, 46), // Rooks
       S( 14, 62), S( 19, 79), S( 23, 95), S(26,106), S(27,111), S(28,114),
       S( 29,116), S( 30,117), S( 31,118), S(32,118) },
     { S(-10,-18), S( -8,-13), S( -6, -7), S(-3, -2), S(-1,  3), S( 1,  8), // Queens
       S(  3, 13), S(  5, 19), S(  8, 23), S(10, 27), S(12, 32), S(15, 34),
       S( 16, 35), S( 17, 35), S( 18, 35), S(20, 35), S(20, 35), S(20, 35),
       S( 20, 35), S( 20, 35), S( 20, 35), S(20, 35), S(20, 35), S(20, 35),
       S( 20, 35), S( 20, 35), S( 20, 35), S(20, 35), S(20, 35), S(20, 35),
       S( 20, 35), S( 20, 35) }
  };

  // OutpostBonus[PieceType][Square] contains outpost bonuses of knights and
  // bishops, indexed by piece type and square (from white's point of view).
  const Value OutpostBonus[][64] = {
  {
  //  A     B     C     D     E     F     G     H
    V(0), V(0), V(0), V(0), V(0), V(0), V(0), V(0), // Knights
    V(0), V(0), V(0), V(0), V(0), V(0), V(0), V(0),
    V(0), V(0), V(4), V(8), V(8), V(4), V(0), V(0),
    V(0), V(4),V(17),V(26),V(26),V(17), V(4), V(0),
    V(0), V(8),V(26),V(35),V(35),V(26), V(8), V(0),
    V(0), V(4),V(17),V(17),V(17),V(17), V(4), V(0),
    V(0), V(0), V(0), V(0), V(0), V(0), V(0), V(0),
    V(0), V(0), V(0), V(0), V(0), V(0), V(0), V(0) },
  {
    V(0), V(0), V(0), V(0), V(0), V(0), V(0), V(0), // Bishops
    V(0), V(0), V(0), V(0), V(0), V(0), V(0), V(0),
    V(0), V(0), V(5), V(5), V(5), V(5), V(0), V(0),
    V(0), V(5),V(10),V(10),V(10),V(10), V(5), V(0),
    V(0),V(10),V(21),V(21),V(21),V(21),V(10), V(0),
    V(0), V(5), V(8), V(8), V(8), V(8), V(5), V(0),
    V(0), V(0), V(0), V(0), V(0), V(0), V(0), V(0),
    V(0), V(0), V(0), V(0), V(0), V(0), V(0), V(0) }
  };

  // ThreatBonus[attacking][attacked] contains threat bonuses according to
  // which piece type attacks which one.
  const Score ThreatBonus[][8] = {
    {}, {},
    { S(0, 0), S( 7, 39), S( 0,  0), S(24, 49), S(41,100), S(41,100) }, // KNIGHT
    { S(0, 0), S( 7, 39), S(24, 49), S( 0,  0), S(41,100), S(41,100) }, // BISHOP
    { S(0, 0), S(-1, 29), S(15, 49), S(15, 49), S( 0,  0), S(24, 49) }, // ROOK
    { S(0, 0), S(15, 39), S(15, 39), S(15, 39), S(15, 39), S( 0,  0) }  // QUEEN
  };

  // ThreatedByPawnPenalty[PieceType] contains a penalty according to which
  // piece type is attacked by an enemy pawn.
  const Score ThreatedByPawnPenalty[] = {
    S(0, 0), S(0, 0), S(56, 70), S(56, 70), S(76, 99), S(86, 118)
  };

  #undef S

  // Rooks and queens on the 7th rank (modified by Joona Kiiski)
  const Score RookOn7thBonus  = make_score(47, 98);
  const Score QueenOn7thBonus = make_score(27, 54);

  // Rooks on open files (modified by Joona Kiiski)
  const Score RookOpenFileBonus = make_score(43, 43);
  const Score RookHalfOpenFileBonus = make_score(19, 19);

  // Penalty for rooks trapped inside a friendly king which has lost the
  // right to castle.
  const Value TrappedRookPenalty = Value(180);

  // The SpaceMask[Color] contains the area of the board which is considered
  // by the space evaluation. In the middle game, each side is given a bonus
  // based on how many squares inside this area are safe and available for
  // friendly minor pieces.
  const Bitboard SpaceMask[2] = {
    (1ULL << SQ_C2) | (1ULL << SQ_D2) | (1ULL << SQ_E2) | (1ULL << SQ_F2) |
    (1ULL << SQ_C3) | (1ULL << SQ_D3) | (1ULL << SQ_E3) | (1ULL << SQ_F3) |
    (1ULL << SQ_C4) | (1ULL << SQ_D4) | (1ULL << SQ_E4) | (1ULL << SQ_F4),
    (1ULL << SQ_C7) | (1ULL << SQ_D7) | (1ULL << SQ_E7) | (1ULL << SQ_F7) |
    (1ULL << SQ_C6) | (1ULL << SQ_D6) | (1ULL << SQ_E6) | (1ULL << SQ_F6) |
    (1ULL << SQ_C5) | (1ULL << SQ_D5) | (1ULL << SQ_E5) | (1ULL << SQ_F5)
  };

  // King danger constants and variables. The king danger scores are taken
  // from the KingDangerTable[]. Various little "meta-bonuses" measuring
  // the strength of the enemy attack are added up into an integer, which
  // is used as an index to KingDangerTable[].
  //
  // KingAttackWeights[PieceType] contains king attack weights by piece type
  const int KingAttackWeights[] = { 0, 0, 2, 2, 3, 5 };

  // Bonuses for enemy's safe checks
  const int QueenContactCheckBonus = 3;
  const int QueenCheckBonus        = 2;
  const int RookCheckBonus         = 1;
  const int BishopCheckBonus       = 1;
  const int KnightCheckBonus       = 1;

  // InitKingDanger[Square] contains penalties based on the position of the
  // defending king, indexed by king's square (from white's point of view).
  const int InitKingDanger[] = {
     2,  0,  2,  5,  5,  2,  0,  2,
     2,  2,  4,  8,  8,  4,  2,  2,
     7, 10, 12, 12, 12, 12, 10,  7,
    15, 15, 15, 15, 15, 15, 15, 15,
    15, 15, 15, 15, 15, 15, 15, 15,
    15, 15, 15, 15, 15, 15, 15, 15,
    15, 15, 15, 15, 15, 15, 15, 15,
    15, 15, 15, 15, 15, 15, 15, 15
  };

  // KingDangerTable[Color][attackUnits] contains the actual king danger
  // weighted scores, indexed by color and by a calculated integer number.
  Score KingDangerTable[2][128];

  // Pawn and material hash tables, indexed by the current thread id.
  // Note that they will be initialized at 0 being global variables.
  MaterialInfoTable* MaterialTable[MAX_THREADS];
  PawnInfoTable* PawnTable[MAX_THREADS];

  // Function prototypes
  template<bool HasPopCnt>
  Value do_evaluate(const Position& pos, EvalInfo& ei);

  template<Color Us, bool HasPopCnt>
  void init_attack_tables(const Position& pos, EvalInfo& ei);

  template<Color Us, bool HasPopCnt>
  Score evaluate_pieces_of_color(const Position& pos, EvalInfo& ei);

  template<Color Us, bool HasPopCnt>
  void evaluate_king(const Position& pos, EvalInfo& ei);

  template<Color Us>
  void evaluate_threats(const Position& pos, EvalInfo& ei);

  template<Color Us, bool HasPopCnt>
  int evaluate_space(const Position& pos, EvalInfo& ei);

  template<Color Us>
  void evaluate_passed_pawns(const Position& pos, EvalInfo& ei);

  inline Score apply_weight(Score v, Score weight);
  Value scale_by_game_phase(const Score& v, Phase ph, const ScaleFactor sf[]);
  Score weight_option(const std::string& mgOpt, const std::string& egOpt, Score internalWeight);
  void init_safety();
}


////
//// Functions
////


/// Prefetches in pawn hash tables

void prefetchPawn(Key key, int threadID) {

    PawnTable[threadID]->prefetch(key);
}

/// evaluate() is the main evaluation function. It always computes two
/// values, an endgame score and a middle game score, and interpolates
/// between them based on the remaining material.
Value evaluate(const Position& pos, EvalInfo& ei) {

    return CpuHasPOPCNT ? do_evaluate<true>(pos, ei)
                        : do_evaluate<false>(pos, ei);
}

namespace {

template<bool HasPopCnt>
Value do_evaluate(const Position& pos, EvalInfo& ei) {

  ScaleFactor factor[2];
  Score mobility;

  assert(pos.is_ok());
  assert(pos.thread() >= 0 && pos.thread() < MAX_THREADS);
  assert(!pos.is_check());

  memset(&ei, 0, sizeof(EvalInfo));

  // Initialize by reading the incrementally updated scores included in the
  // position object (material + piece square tables).
  ei.value = pos.value();

  // Probe the material hash table
  ei.mi = MaterialTable[pos.thread()]->get_material_info(pos);
  ei.value += ei.mi->material_value();

  // If we have a specialized evaluation function for the current material
  // configuration, call it and return.
  if (ei.mi->specialized_eval_exists())
      return ei.mi->evaluate(pos);

  // After get_material_info() call that modifies them
  factor[WHITE] = ei.mi->scale_factor(pos, WHITE);
  factor[BLACK] = ei.mi->scale_factor(pos, BLACK);

  // Probe the pawn hash table
  ei.pi = PawnTable[pos.thread()]->get_pawn_info(pos);
  ei.value += apply_weight(ei.pi->pawns_value(), Weights[PawnStructure]);

  // Initialize attack bitboards with pawns evaluation
  init_attack_tables<WHITE, HasPopCnt>(pos, ei);
  init_attack_tables<BLACK, HasPopCnt>(pos, ei);

  // Evaluate pieces and mobility
  mobility =   evaluate_pieces_of_color<WHITE, HasPopCnt>(pos, ei)
             - evaluate_pieces_of_color<BLACK, HasPopCnt>(pos, ei);
  ei.value += apply_weight(mobility, Weights[Mobility]);

  // Kings. Kings are evaluated after all other pieces for both sides,
  // because we need complete attack information for all pieces when computing
  // the king safety evaluation.
  evaluate_king<WHITE, HasPopCnt>(pos, ei);
  evaluate_king<BLACK, HasPopCnt>(pos, ei);

  // Evaluate tactical threats, we need full attack info including king
  evaluate_threats<WHITE>(pos, ei);
  evaluate_threats<BLACK>(pos, ei);

  // Evaluate passed pawns, we need full attack info including king
  evaluate_passed_pawns<WHITE>(pos, ei);
  evaluate_passed_pawns<BLACK>(pos, ei);

  Phase phase = ei.mi->game_phase();

  // Middle-game specific evaluation terms
  if (phase > PHASE_ENDGAME)
  {
      // Evaluate pawn storms in positions with opposite castling
      if (   square_file(pos.king_square(WHITE)) >= FILE_E
          && square_file(pos.king_square(BLACK)) <= FILE_D)

          ei.value += make_score(ei.pi->queenside_storm_value(WHITE) - ei.pi->kingside_storm_value(BLACK), 0);

      else if (   square_file(pos.king_square(WHITE)) <= FILE_D
               && square_file(pos.king_square(BLACK)) >= FILE_E)

          ei.value += make_score(ei.pi->kingside_storm_value(WHITE) - ei.pi->queenside_storm_value(BLACK), 0);

      // Evaluate space for both sides
      if (ei.mi->space_weight() > 0)
      {
          int s = evaluate_space<WHITE, HasPopCnt>(pos, ei) - evaluate_space<BLACK, HasPopCnt>(pos, ei);
          ei.value += apply_weight(make_score(s * ei.mi->space_weight(), 0), Weights[Space]);
      }
  }

  // If we don't already have an unusual scale factor, check for opposite
  // colored bishop endgames, and use a lower scale for those
  if (   phase < PHASE_MIDGAME
      && pos.opposite_colored_bishops()
      && (   (factor[WHITE] == SCALE_FACTOR_NORMAL && eg_value(ei.value) > VALUE_ZERO)
          || (factor[BLACK] == SCALE_FACTOR_NORMAL && eg_value(ei.value) < VALUE_ZERO)))
  {
      ScaleFactor sf;

      // Only the two bishops ?
      if (   pos.non_pawn_material(WHITE) == BishopValueMidgame
          && pos.non_pawn_material(BLACK) == BishopValueMidgame)
      {
          // Check for KBP vs KB with only a single pawn that is almost
          // certainly a draw or at least two pawns.
          bool one_pawn = (pos.piece_count(WHITE, PAWN) + pos.piece_count(BLACK, PAWN) == 1);
          sf = one_pawn ? ScaleFactor(8) : ScaleFactor(32);
      }
      else
          // Endgame with opposite-colored bishops, but also other pieces. Still
          // a bit drawish, but not as drawish as with only the two bishops.
           sf = ScaleFactor(50);

      if (factor[WHITE] == SCALE_FACTOR_NORMAL)
          factor[WHITE] = sf;
      if (factor[BLACK] == SCALE_FACTOR_NORMAL)
          factor[BLACK] = sf;
  }

  // Interpolate between the middle game and the endgame score
  return Sign[pos.side_to_move()] * scale_by_game_phase(ei.value, phase, factor);
}

} // namespace

/// init_eval() initializes various tables used by the evaluation function

void init_eval(int threads) {

  assert(threads <= MAX_THREADS);

  for (int i = 0; i < MAX_THREADS; i++)
  {
    if (i >= threads)
    {
        delete PawnTable[i];
        delete MaterialTable[i];
        PawnTable[i] = NULL;
        MaterialTable[i] = NULL;
        continue;
    }
    if (!PawnTable[i])
        PawnTable[i] = new PawnInfoTable();
    if (!MaterialTable[i])
        MaterialTable[i] = new MaterialInfoTable();
  }
}


/// quit_eval() releases heap-allocated memory at program termination

void quit_eval() {

  for (int i = 0; i < MAX_THREADS; i++)
  {
      delete PawnTable[i];
      delete MaterialTable[i];
      PawnTable[i] = NULL;
      MaterialTable[i] = NULL;
  }
}


/// read_weights() reads evaluation weights from the corresponding UCI parameters

void read_weights(Color us) {

  // King safety is asymmetrical. Our king danger level is weighted by
  // "Cowardice" UCI parameter, instead the opponent one by "Aggressiveness".
  const int kingDangerUs   = (us == WHITE ? KingDangerUs   : KingDangerThem);
  const int kingDangerThem = (us == WHITE ? KingDangerThem : KingDangerUs);

  Weights[Mobility]       = weight_option("Mobility (Middle Game)", "Mobility (Endgame)", WeightsInternal[Mobility]);
  Weights[PawnStructure]  = weight_option("Pawn Structure (Middle Game)", "Pawn Structure (Endgame)", WeightsInternal[PawnStructure]);
  Weights[PassedPawns]    = weight_option("Passed Pawns (Middle Game)", "Passed Pawns (Endgame)", WeightsInternal[PassedPawns]);
  Weights[Space]          = weight_option("Space", "Space", WeightsInternal[Space]);
  Weights[kingDangerUs]   = weight_option("Cowardice", "Cowardice", WeightsInternal[KingDangerUs]);
  Weights[kingDangerThem] = weight_option("Aggressiveness", "Aggressiveness", WeightsInternal[KingDangerThem]);

  // If running in analysis mode, make sure we use symmetrical king safety. We do this
  // by replacing both Weights[kingDangerUs] and Weights[kingDangerThem] by their average.
  if (get_option_value_bool("UCI_AnalyseMode"))
      Weights[kingDangerUs] = Weights[kingDangerThem] = (Weights[kingDangerUs] + Weights[kingDangerThem]) / 2;

  init_safety();
}


namespace {

  // init_attack_tables() initializes king bitboards for both sides adding
  // pawn attacks. To be done before other evaluations.

  template<Color Us, bool HasPopCnt>
  void init_attack_tables(const Position& pos, EvalInfo& ei) {

    const Color Them = (Us == WHITE ? BLACK : WHITE);

    Bitboard b = ei.attackedBy[Them][KING] = pos.attacks_from<KING>(pos.king_square(Them));
    ei.kingZone[Us] = (b | (Us == WHITE ? b >> 8 : b << 8));
    ei.attackedBy[Us][PAWN] = ei.pi->pawn_attacks(Us);
    b &= ei.attackedBy[Us][PAWN];
    if (b)
        ei.kingAttackersCount[Us] = count_1s_max_15<HasPopCnt>(b) / 2;
  }


  // evaluate_outposts() evaluates bishop and knight outposts squares

  template<PieceType Piece, Color Us>
  void evaluate_outposts(const Position& pos, EvalInfo& ei, Square s) {

    const Color Them = (Us == WHITE ? BLACK : WHITE);

    assert (Piece == BISHOP || Piece == KNIGHT);

    // Initial bonus based on square
    Value bonus = OutpostBonus[Piece == BISHOP][relative_square(Us, s)];

    // Increase bonus if supported by pawn, especially if the opponent has
    // no minor piece which can exchange the outpost piece
    if (bonus && bit_is_set(ei.attackedBy[Us][PAWN], s))
    {
        if (    pos.pieces(KNIGHT, Them) == EmptyBoardBB
            && (SquaresByColorBB[square_color(s)] & pos.pieces(BISHOP, Them)) == EmptyBoardBB)
            bonus += bonus + bonus / 2;
        else
            bonus += bonus / 2;
    }
    ei.value += Sign[Us] * make_score(bonus, bonus);
  }


  // evaluate_pieces<>() assigns bonuses and penalties to the pieces of a given color

  template<PieceType Piece, Color Us, bool HasPopCnt>
  Score evaluate_pieces(const Position& pos, EvalInfo& ei, Bitboard no_mob_area) {

    Bitboard b;
    Square s, ksq;
    int mob;
    File f;
    Score mobility = SCORE_ZERO;

    const Color Them = (Us == WHITE ? BLACK : WHITE);
    const Square* ptr = pos.piece_list_begin(Us, Piece);

    while ((s = *ptr++) != SQ_NONE)
    {
        // Find attacked squares, including x-ray attacks for bishops and rooks
        if (Piece == KNIGHT || Piece == QUEEN)
            b = pos.attacks_from<Piece>(s);
        else if (Piece == BISHOP)
            b = bishop_attacks_bb(s, pos.occupied_squares() & ~pos.pieces(QUEEN, Us));
        else if (Piece == ROOK)
            b = rook_attacks_bb(s, pos.occupied_squares() & ~pos.pieces(ROOK, QUEEN, Us));
        else
            assert(false);

        // Update attack info
        ei.attackedBy[Us][Piece] |= b;

        // King attacks
        if (b & ei.kingZone[Us])
        {
            ei.kingAttackersCount[Us]++;
            ei.kingAttackersWeight[Us] += KingAttackWeights[Piece];
            Bitboard bb = (b & ei.attackedBy[Them][KING]);
            if (bb)
                ei.kingAdjacentZoneAttacksCount[Us] += count_1s_max_15<HasPopCnt>(bb);
        }

        // Mobility
        mob = (Piece != QUEEN ? count_1s_max_15<HasPopCnt>(b & no_mob_area)
                              : count_1s<HasPopCnt>(b & no_mob_area));

        mobility += MobilityBonus[Piece][mob];

        // Decrease score if we are attacked by an enemy pawn. Remaining part
        // of threat evaluation must be done later when we have full attack info.
        if (bit_is_set(ei.attackedBy[Them][PAWN], s))
            ei.value -= Sign[Us] * ThreatedByPawnPenalty[Piece];

        // Bishop and knight outposts squares
        if ((Piece == BISHOP || Piece == KNIGHT) && pos.square_is_weak(s, Us))
            evaluate_outposts<Piece, Us>(pos, ei, s);

        // Queen or rook on 7th rank
        if (  (Piece == ROOK || Piece == QUEEN)
            && relative_rank(Us, s) == RANK_7
            && relative_rank(Us, pos.king_square(Them)) == RANK_8)
        {
            ei.value += Sign[Us] * (Piece == ROOK ? RookOn7thBonus : QueenOn7thBonus);
        }

        // Special extra evaluation for rooks
        if (Piece == ROOK)
        {
            // Open and half-open files
            f = square_file(s);
            if (ei.pi->file_is_half_open(Us, f))
            {
                if (ei.pi->file_is_half_open(Them, f))
                    ei.value += Sign[Us] * RookOpenFileBonus;
                else
                    ei.value += Sign[Us] * RookHalfOpenFileBonus;
            }

            // Penalize rooks which are trapped inside a king. Penalize more if
            // king has lost right to castle.
            if (mob > 6 || ei.pi->file_is_half_open(Us, f))
                continue;

            ksq = pos.king_square(Us);

            if (    square_file(ksq) >= FILE_E
                &&  square_file(s) > square_file(ksq)
                && (relative_rank(Us, ksq) == RANK_1 || square_rank(ksq) == square_rank(s)))
            {
                // Is there a half-open file between the king and the edge of the board?
                if (!ei.pi->has_open_file_to_right(Us, square_file(ksq)))
                    ei.value -= Sign[Us] * make_score(pos.can_castle(Us) ? (TrappedRookPenalty - mob * 16) / 2
                                                                         : (TrappedRookPenalty - mob * 16), 0);
            }
            else if (    square_file(ksq) <= FILE_D
                     &&  square_file(s) < square_file(ksq)
                     && (relative_rank(Us, ksq) == RANK_1 || square_rank(ksq) == square_rank(s)))
            {
                // Is there a half-open file between the king and the edge of the board?
                if (!ei.pi->has_open_file_to_left(Us, square_file(ksq)))
                    ei.value -= Sign[Us] * make_score(pos.can_castle(Us) ? (TrappedRookPenalty - mob * 16) / 2
                                                                         : (TrappedRookPenalty - mob * 16), 0);
            }
        }
    }
    return mobility;
  }


  // evaluate_threats<>() assigns bonuses according to the type of attacking piece
  // and the type of attacked one.

  template<Color Us>
  void evaluate_threats(const Position& pos, EvalInfo& ei) {

    const Color Them = (Us == WHITE ? BLACK : WHITE);

    Bitboard b;
    Score bonus = SCORE_ZERO;

    // Enemy pieces not defended by a pawn and under our attack
    Bitboard weakEnemies =  pos.pieces_of_color(Them)
                          & ~ei.attackedBy[Them][PAWN]
                          & ei.attackedBy[Us][0];
    if (!weakEnemies)
        return;

    // Add bonus according to type of attacked enemy pieces and to the
    // type of attacking piece, from knights to queens. Kings are not
    // considered because are already special handled in king evaluation.
    for (PieceType pt1 = KNIGHT; pt1 < KING; pt1++)
    {
        b = ei.attackedBy[Us][pt1] & weakEnemies;
        if (b)
            for (PieceType pt2 = PAWN; pt2 < KING; pt2++)
                if (b & pos.pieces(pt2))
                    bonus += ThreatBonus[pt1][pt2];
    }
    ei.value += Sign[Us] * bonus;
  }


  // evaluate_pieces_of_color<>() assigns bonuses and penalties to all the
  // pieces of a given color.

  template<Color Us, bool HasPopCnt>
  Score evaluate_pieces_of_color(const Position& pos, EvalInfo& ei) {

    const Color Them = (Us == WHITE ? BLACK : WHITE);

    Score mobility = SCORE_ZERO;

    // Do not include in mobility squares protected by enemy pawns or occupied by our pieces
    const Bitboard no_mob_area = ~(ei.attackedBy[Them][PAWN] | pos.pieces_of_color(Us));

    mobility += evaluate_pieces<KNIGHT, Us, HasPopCnt>(pos, ei, no_mob_area);
    mobility += evaluate_pieces<BISHOP, Us, HasPopCnt>(pos, ei, no_mob_area);
    mobility += evaluate_pieces<ROOK,   Us, HasPopCnt>(pos, ei, no_mob_area);
    mobility += evaluate_pieces<QUEEN,  Us, HasPopCnt>(pos, ei, no_mob_area);

    // Sum up all attacked squares
    ei.attackedBy[Us][0] =   ei.attackedBy[Us][PAWN]   | ei.attackedBy[Us][KNIGHT]
                           | ei.attackedBy[Us][BISHOP] | ei.attackedBy[Us][ROOK]
                           | ei.attackedBy[Us][QUEEN]  | ei.attackedBy[Us][KING];
    return mobility;
  }


  // evaluate_king<>() assigns bonuses and penalties to a king of a given color

  template<Color Us, bool HasPopCnt>
  void evaluate_king(const Position& pos, EvalInfo& ei) {

    const Color Them = (Us == WHITE ? BLACK : WHITE);

    Bitboard undefended, b, b1, b2, safe;
    bool sente;
    int attackUnits;
    const Square ksq = pos.king_square(Us);

    // King shelter
    ei.value += Sign[Us] * ei.pi->king_shelter(pos, Us, ksq);

    // King safety. This is quite complicated, and is almost certainly far
    // from optimally tuned.
    if (   pos.piece_count(Them, QUEEN) >= 1
        && ei.kingAttackersCount[Them]  >= 2
        && pos.non_pawn_material(Them)  >= QueenValueMidgame + RookValueMidgame
        && ei.kingAdjacentZoneAttacksCount[Them])
    {
        // Is it the attackers turn to move?
        sente = (Them == pos.side_to_move());

        // Find the attacked squares around the king which has no defenders
        // apart from the king itself
        undefended = ei.attacked_by(Them) & ei.attacked_by(Us, KING);
        undefended &= ~(  ei.attacked_by(Us, PAWN)   | ei.attacked_by(Us, KNIGHT)
                        | ei.attacked_by(Us, BISHOP) | ei.attacked_by(Us, ROOK)
                        | ei.attacked_by(Us, QUEEN));

        // Initialize the 'attackUnits' variable, which is used later on as an
        // index to the KingDangerTable[] array. The initial value is based on
        // the number and types of the enemy's attacking pieces, the number of
        // attacked and undefended squares around our king, the square of the
        // king, and the quality of the pawn shelter.
        attackUnits =  Min(25, (ei.kingAttackersCount[Them] * ei.kingAttackersWeight[Them]) / 2)
                     + 3 * (ei.kingAdjacentZoneAttacksCount[Them] + count_1s_max_15<HasPopCnt>(undefended))
                     + InitKingDanger[relative_square(Us, ksq)]
                     - mg_value(ei.pi->king_shelter(pos, Us, ksq)) / 32;

        // Analyse enemy's safe queen contact checks. First find undefended
        // squares around the king attacked by enemy queen...
        b = undefended & ei.attacked_by(Them, QUEEN) & ~pos.pieces_of_color(Them);
        if (b)
        {
            // ...then remove squares not supported by another enemy piece
            b &= (  ei.attacked_by(Them, PAWN)   | ei.attacked_by(Them, KNIGHT)
                  | ei.attacked_by(Them, BISHOP) | ei.attacked_by(Them, ROOK));
            if (b)
                attackUnits += QueenContactCheckBonus * count_1s_max_15<HasPopCnt>(b) * (sente ? 2 : 1);
        }

        // Analyse enemy's safe distance checks for sliders and knights
        safe = ~(pos.pieces_of_color(Them) | ei.attacked_by(Us));

        b1 = pos.attacks_from<ROOK>(ksq) & safe;
        b2 = pos.attacks_from<BISHOP>(ksq) & safe;

        // Enemy queen safe checks
        b = (b1 | b2) & ei.attacked_by(Them, QUEEN);
        if (b)
            attackUnits += QueenCheckBonus * count_1s_max_15<HasPopCnt>(b);

        // Enemy rooks safe checks
        b = b1 & ei.attacked_by(Them, ROOK);
        if (b)
            attackUnits += RookCheckBonus * count_1s_max_15<HasPopCnt>(b);

        // Enemy bishops safe checks
        b = b2 & ei.attacked_by(Them, BISHOP);
        if (b)
            attackUnits += BishopCheckBonus * count_1s_max_15<HasPopCnt>(b);

        // Enemy knights safe checks
        b = pos.attacks_from<KNIGHT>(ksq) & ei.attacked_by(Them, KNIGHT) & safe;
        if (b)
            attackUnits += KnightCheckBonus * count_1s_max_15<HasPopCnt>(b);

        // To index KingDangerTable[] attackUnits must be in [0, 99] range
        attackUnits = Min(99, Max(0, attackUnits));

        // Finally, extract the king danger score from the KingDangerTable[]
        // array and subtract the score from evaluation. Set also ei.kingDanger[]
        // value that will be used for pruning because this value can sometimes
        // be very big, and so capturing a single attacking piece can therefore
        // result in a score change far bigger than the value of the captured piece.
        ei.value -= Sign[Us] * KingDangerTable[Us][attackUnits];
        ei.kingDanger[Us] = mg_value(KingDangerTable[Us][attackUnits]);
    }
  }


  // evaluate_passed_pawns<>() evaluates the passed pawns of the given color

  template<Color Us>
  void evaluate_passed_pawns(const Position& pos, EvalInfo& ei) {

    const Color Them = (Us == WHITE ? BLACK : WHITE);

    Bitboard squaresToQueen, defendedSquares, unsafeSquares, supportingPawns;
    Bitboard b = ei.pi->passed_pawns(Us);

    while (b)
    {
        Square s = pop_1st_bit(&b);

        assert(pos.pawn_is_passed(Us, s));

        int r = int(relative_rank(Us, s) - RANK_2);
        int tr = r * (r - 1);

        // Base bonus based on rank
        Value mbonus = Value(20 * tr);
        Value ebonus = Value(10 + r * r * 10);

        if (tr)
        {
            Square blockSq = s + pawn_push(Us);

            // Adjust bonus based on kings proximity
            ebonus -= Value(square_distance(pos.king_square(Us), blockSq) * 3 * tr);
            ebonus -= Value(square_distance(pos.king_square(Us), blockSq + pawn_push(Us)) * 1 * tr);
            ebonus += Value(square_distance(pos.king_square(Them), blockSq) * 6 * tr);

            // If the pawn is free to advance, increase bonus
            if (pos.square_is_empty(blockSq))
            {
                squaresToQueen = squares_in_front_of(Us, s);
                defendedSquares = squaresToQueen & ei.attacked_by(Us);

                // If there is an enemy rook or queen attacking the pawn from behind,
                // add all X-ray attacks by the rook or queen. Otherwise consider only
                // the squares in the pawn's path attacked or occupied by the enemy.
                if (   (squares_behind(Us, s) & pos.pieces(ROOK, QUEEN, Them))
                    && (squares_behind(Us, s) & pos.pieces(ROOK, QUEEN, Them) & pos.attacks_from<ROOK>(s)))
                    unsafeSquares = squaresToQueen;
                else
                    unsafeSquares = squaresToQueen & (ei.attacked_by(Them) | pos.pieces_of_color(Them));

                // If there aren't enemy attacks or pieces along the path to queen give
                // huge bonus. Even bigger if we protect the pawn's path.
                if (!unsafeSquares)
                    ebonus += Value(tr * (squaresToQueen == defendedSquares ? 17 : 15));
                else
                    // OK, there are enemy attacks or pieces (but not pawns). Are those
                    // squares which are attacked by the enemy also attacked by us ?
                    // If yes, big bonus (but smaller than when there are no enemy attacks),
                    // if no, somewhat smaller bonus.
                    ebonus += Value(tr * ((unsafeSquares & defendedSquares) == unsafeSquares ? 13 : 8));

                // At last, add a small bonus when there are no *friendly* pieces
                // in the pawn's path.
                if (!(squaresToQueen & pos.pieces_of_color(Us)))
                    ebonus += Value(tr);
            }
        } // tr != 0

        // Increase the bonus if the passed pawn is supported by a friendly pawn
        // on the same rank and a bit smaller if it's on the previous rank.
        supportingPawns = pos.pieces(PAWN, Us) & neighboring_files_bb(s);
        if (supportingPawns & rank_bb(s))
            ebonus += Value(r * 20);
        else if (supportingPawns & rank_bb(s - pawn_push(Us)))
            ebonus += Value(r * 12);

        // Rook pawns are a special case: They are sometimes worse, and
        // sometimes better than other passed pawns. It is difficult to find
        // good rules for determining whether they are good or bad. For now,
        // we try the following: Increase the value for rook pawns if the
        // other side has no pieces apart from a knight, and decrease the
        // value if the other side has a rook or queen.
        if (square_file(s) == FILE_A || square_file(s) == FILE_H)
        {
            if (pos.non_pawn_material(Them) <= KnightValueMidgame)
                ebonus += ebonus / 4;
            else if (pos.pieces(ROOK, QUEEN, Them))
                ebonus -= ebonus / 4;
        }

        // Add the scores for this pawn to the middle game and endgame eval
        ei.value += Sign[Us] * apply_weight(make_score(mbonus, ebonus), Weights[PassedPawns]);

    } // while
  }


  // evaluate_space() computes the space evaluation for a given side. The
  // space evaluation is a simple bonus based on the number of safe squares
  // available for minor pieces on the central four files on ranks 2--4. Safe
  // squares one, two or three squares behind a friendly pawn are counted
  // twice. Finally, the space bonus is scaled by a weight taken from the
  // material hash table.
  template<Color Us, bool HasPopCnt>
  int evaluate_space(const Position& pos, EvalInfo& ei) {

    const Color Them = (Us == WHITE ? BLACK : WHITE);

    // Find the safe squares for our pieces inside the area defined by
    // SpaceMask[us]. A square is unsafe if it is attacked by an enemy
    // pawn, or if it is undefended and attacked by an enemy piece.
    Bitboard safe =   SpaceMask[Us]
                   & ~pos.pieces(PAWN, Us)
                   & ~ei.attacked_by(Them, PAWN)
                   & (ei.attacked_by(Us) | ~ei.attacked_by(Them));

    // Find all squares which are at most three squares behind some friendly pawn
    Bitboard behind = pos.pieces(PAWN, Us);
    behind |= (Us == WHITE ? behind >>  8 : behind <<  8);
    behind |= (Us == WHITE ? behind >> 16 : behind << 16);

    return count_1s_max_15<HasPopCnt>(safe) + count_1s_max_15<HasPopCnt>(behind & safe);
  }


  // apply_weight() applies an evaluation weight to a value trying to prevent overflow

  inline Score apply_weight(Score v, Score w) {
      return make_score((int(mg_value(v)) * mg_value(w)) / 0x100, (int(eg_value(v)) * eg_value(w)) / 0x100);
  }


  // scale_by_game_phase() interpolates between a middle game and an endgame score,
  // based on game phase. It also scales the return value by a ScaleFactor array.

  Value scale_by_game_phase(const Score& v, Phase ph, const ScaleFactor sf[]) {

    assert(mg_value(v) > -VALUE_INFINITE && mg_value(v) < VALUE_INFINITE);
    assert(eg_value(v) > -VALUE_INFINITE && eg_value(v) < VALUE_INFINITE);
    assert(ph >= PHASE_ENDGAME && ph <= PHASE_MIDGAME);

    Value eg = eg_value(v);
    ScaleFactor f = sf[eg > VALUE_ZERO ? WHITE : BLACK];
    Value ev = Value((eg * int(f)) / SCALE_FACTOR_NORMAL);

    int result = (mg_value(v) * int(ph) + ev * int(128 - ph)) / 128;
    return Value(result & ~(GrainSize - 1));
  }


  // weight_option() computes the value of an evaluation weight, by combining
  // two UCI-configurable weights (midgame and endgame) with an internal weight.

  Score weight_option(const std::string& mgOpt, const std::string& egOpt, Score internalWeight) {

    // Scale option value from 100 to 256
    int mg = get_option_value_int(mgOpt) * 256 / 100;
    int eg = get_option_value_int(egOpt) * 256 / 100;

    return apply_weight(make_score(mg, eg), internalWeight);
  }

  // init_safety() initizes the king safety evaluation, based on UCI
  // parameters. It is called from read_weights().

  void init_safety() {

    const Value MaxSlope = Value(30);
    const Value Peak = Value(1280);
    Value t[100];

    // First setup the base table
    for (int i = 0; i < 100; i++)
    {
        t[i] = Value(int(0.4 * i * i));

        if (i > 0)
            t[i] = Min(t[i], t[i - 1] + MaxSlope);

        t[i] = Min(t[i], Peak);
    }

    // Then apply the weights and get the final KingDangerTable[] array
    for (Color c = WHITE; c <= BLACK; c++)
        for (int i = 0; i < 100; i++)
            KingDangerTable[c][i] = apply_weight(make_score(t[i], 0), Weights[KingDangerUs + c]);
  }
}