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    1 /*
    2   Stockfish, a UCI chess playing engine derived from Glaurung 2.1
    3   Copyright (C) 2004-2008 Tord Romstad (Glaurung author)
    4   Copyright (C) 2008-2015 Marco Costalba, Joona Kiiski, Tord Romstad
    5   Copyright (C) 2015-2020 Marco Costalba, Joona Kiiski, Gary Linscott, Tord Romstad
    6 
    7   Stockfish is free software: you can redistribute it and/or modify
    8   it under the terms of the GNU General Public License as published by
    9   the Free Software Foundation, either version 3 of the License, or
   10   (at your option) any later version.
   11 
   12   Stockfish is distributed in the hope that it will be useful,
   13   but WITHOUT ANY WARRANTY; without even the implied warranty of
   14   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   15   GNU General Public License for more details.
   16 
   17   You should have received a copy of the GNU General Public License
   18   along with this program.  If not, see <http://www.gnu.org/licenses/>.
   19 */
   20 
   21 #include <algorithm>
   22 #include <cassert>
   23 
   24 #include "bitboard.h"
   25 #include "pawns.h"
   26 #include "position.h"
   27 #include "thread.h"
   28 
   29 namespace {
   30 
   31   #define V Value
   32   #define S(mg, eg) make_score(mg, eg)
   33 
   34   // Pawn penalties
   35   constexpr Score Backward      = S( 9, 24);
   36   constexpr Score BlockedStorm  = S(82, 82);
   37   constexpr Score Doubled       = S(11, 56);
   38   constexpr Score Isolated      = S( 5, 15);
   39   constexpr Score WeakLever     = S( 0, 56);
   40   constexpr Score WeakUnopposed = S(13, 27);
   41 
   42   // Connected pawn bonus
   43   constexpr int Connected[RANK_NB] = { 0, 7, 8, 12, 29, 48, 86 };
   44 
   45   // Strength of pawn shelter for our king by [distance from edge][rank].
   46   // RANK_1 = 0 is used for files where we have no pawn, or pawn is behind our king.
   47   constexpr Value ShelterStrength[int(FILE_NB) / 2][RANK_NB] = {
   48     { V( -6), V( 81), V( 93), V( 58), V( 39), V( 18), V(  25) },
   49     { V(-43), V( 61), V( 35), V(-49), V(-29), V(-11), V( -63) },
   50     { V(-10), V( 75), V( 23), V( -2), V( 32), V(  3), V( -45) },
   51     { V(-39), V(-13), V(-29), V(-52), V(-48), V(-67), V(-166) }
   52   };
   53 
   54   // Danger of enemy pawns moving toward our king by [distance from edge][rank].
   55   // RANK_1 = 0 is used for files where the enemy has no pawn, or their pawn
   56   // is behind our king. Note that UnblockedStorm[0][1-2] accommodate opponent pawn
   57   // on edge, likely blocked by our king.
   58   constexpr Value UnblockedStorm[int(FILE_NB) / 2][RANK_NB] = {
   59     { V( 85), V(-289), V(-166), V(97), V(50), V( 45), V( 50) },
   60     { V( 46), V( -25), V( 122), V(45), V(37), V(-10), V( 20) },
   61     { V( -6), V(  51), V( 168), V(34), V(-2), V(-22), V(-14) },
   62     { V(-15), V( -11), V( 101), V( 4), V(11), V(-15), V(-29) }
   63   };
   64 
   65   #undef S
   66   #undef V
   67 
   68   template<Color Us>
   69   Score evaluate(const Position& pos, Pawns::Entry* e) {
   70 
   71     constexpr Color     Them = (Us == WHITE ? BLACK : WHITE);
   72     constexpr Direction Up   = pawn_push(Us);
   73 
   74     Bitboard neighbours, stoppers, support, phalanx, opposed;
   75     Bitboard lever, leverPush, blocked;
   76     Square s;
   77     bool backward, passed, doubled;
   78     Score score = SCORE_ZERO;
   79     const Square* pl = pos.squares<PAWN>(Us);
   80 
   81     Bitboard ourPawns   = pos.pieces(  Us, PAWN);
   82     Bitboard theirPawns = pos.pieces(Them, PAWN);
   83 
   84     Bitboard doubleAttackThem = pawn_double_attacks_bb<Them>(theirPawns);
   85 
   86     e->passedPawns[Us] = 0;
   87     e->kingSquares[Us] = SQ_NONE;
   88     e->pawnAttacks[Us] = e->pawnAttacksSpan[Us] = pawn_attacks_bb<Us>(ourPawns);
   89 
   90     // Loop through all pawns of the current color and score each pawn
   91     while ((s = *pl++) != SQ_NONE)
   92     {
   93         assert(pos.piece_on(s) == make_piece(Us, PAWN));
   94 
   95         Rank r = relative_rank(Us, s);
   96 
   97         // Flag the pawn
   98         opposed    = theirPawns & forward_file_bb(Us, s);
   99         blocked    = theirPawns & (s + Up);
  100         stoppers   = theirPawns & passed_pawn_span(Us, s);
  101         lever      = theirPawns & PawnAttacks[Us][s];
  102         leverPush  = theirPawns & PawnAttacks[Us][s + Up];
  103         doubled    = ourPawns   & (s - Up);
  104         neighbours = ourPawns   & adjacent_files_bb(s);
  105         phalanx    = neighbours & rank_bb(s);
  106         support    = neighbours & rank_bb(s - Up);
  107 
  108         // A pawn is backward when it is behind all pawns of the same color on
  109         // the adjacent files and cannot safely advance.
  110         backward =  !(neighbours & forward_ranks_bb(Them, s + Up))
  111                   && (leverPush | blocked);
  112 
  113         // Compute additional span if pawn is not backward nor blocked
  114         if (!backward && !blocked)
  115             e->pawnAttacksSpan[Us] |= pawn_attack_span(Us, s);
  116 
  117         // A pawn is passed if one of the three following conditions is true:
  118         // (a) there is no stoppers except some levers
  119         // (b) the only stoppers are the leverPush, but we outnumber them
  120         // (c) there is only one front stopper which can be levered.
  121         passed =   !(stoppers ^ lever)
  122                 || (   !(stoppers ^ leverPush)
  123                     && popcount(phalanx) >= popcount(leverPush))
  124                 || (   stoppers == blocked && r >= RANK_5
  125                     && (shift<Up>(support) & ~(theirPawns | doubleAttackThem)));
  126 
  127         // Passed pawns will be properly scored later in evaluation when we have
  128         // full attack info.
  129         if (passed)
  130             e->passedPawns[Us] |= s;
  131 
  132         // Score this pawn
  133         if (support | phalanx)
  134         {
  135             int v =  Connected[r] * (2 + bool(phalanx) - bool(opposed))
  136                    + 21 * popcount(support);
  137 
  138             score += make_score(v, v * (r - 2) / 4);
  139         }
  140 
  141         else if (!neighbours)
  142             score -=   Isolated
  143                      + WeakUnopposed * !opposed;
  144 
  145         else if (backward)
  146             score -=   Backward
  147                      + WeakUnopposed * !opposed;
  148 
  149         if (!support)
  150             score -=   Doubled * doubled
  151                      + WeakLever * more_than_one(lever);
  152     }
  153 
  154     return score;
  155   }
  156 
  157 } // namespace
  158 
  159 namespace Pawns {
  160 
  161 /// Pawns::probe() looks up the current position's pawns configuration in
  162 /// the pawns hash table. It returns a pointer to the Entry if the position
  163 /// is found. Otherwise a new Entry is computed and stored there, so we don't
  164 /// have to recompute all when the same pawns configuration occurs again.
  165 
  166 Entry* probe(const Position& pos) {
  167 
  168   Key key = pos.pawn_key();
  169   Entry* e = pos.this_thread()->pawnsTable[key];
  170 
  171   if (e->key == key)
  172       return e;
  173 
  174   e->key = key;
  175   e->scores[WHITE] = evaluate<WHITE>(pos, e);
  176   e->scores[BLACK] = evaluate<BLACK>(pos, e);
  177 
  178   return e;
  179 }
  180 
  181 
  182 /// Entry::evaluate_shelter() calculates the shelter bonus and the storm
  183 /// penalty for a king, looking at the king file and the two closest files.
  184 
  185 template<Color Us>
  186 Score Entry::evaluate_shelter(const Position& pos, Square ksq) {
  187 
  188   constexpr Color Them = (Us == WHITE ? BLACK : WHITE);
  189 
  190   Bitboard b = pos.pieces(PAWN) & ~forward_ranks_bb(Them, ksq);
  191   Bitboard ourPawns = b & pos.pieces(Us);
  192   Bitboard theirPawns = b & pos.pieces(Them);
  193 
  194   Score bonus = make_score(5, 5);
  195 
  196   File center = clamp(file_of(ksq), FILE_B, FILE_G);
  197   for (File f = File(center - 1); f <= File(center + 1); ++f)
  198   {
  199       b = ourPawns & file_bb(f);
  200       int ourRank = b ? relative_rank(Us, frontmost_sq(Them, b)) : 0;
  201 
  202       b = theirPawns & file_bb(f);
  203       int theirRank = b ? relative_rank(Us, frontmost_sq(Them, b)) : 0;
  204 
  205       File d = map_to_queenside(f);
  206       bonus += make_score(ShelterStrength[d][ourRank], 0);
  207 
  208       if (ourRank && (ourRank == theirRank - 1))
  209           bonus -= BlockedStorm * int(theirRank == RANK_3);
  210       else
  211           bonus -= make_score(UnblockedStorm[d][theirRank], 0);
  212   }
  213 
  214   return bonus;
  215 }
  216 
  217 
  218 /// Entry::do_king_safety() calculates a bonus for king safety. It is called only
  219 /// when king square changes, which is about 20% of total king_safety() calls.
  220 
  221 template<Color Us>
  222 Score Entry::do_king_safety(const Position& pos) {
  223 
  224   Square ksq = pos.square<KING>(Us);
  225   kingSquares[Us] = ksq;
  226   castlingRights[Us] = pos.castling_rights(Us);
  227   auto compare = [](Score a, Score b) { return mg_value(a) < mg_value(b); };
  228 
  229   Score shelter = evaluate_shelter<Us>(pos, ksq);
  230 
  231   // If we can castle use the bonus after castling if it is bigger
  232 
  233   if (pos.can_castle(Us & KING_SIDE))
  234       shelter = std::max(shelter, evaluate_shelter<Us>(pos, relative_square(Us, SQ_G1)), compare);
  235 
  236   if (pos.can_castle(Us & QUEEN_SIDE))
  237       shelter = std::max(shelter, evaluate_shelter<Us>(pos, relative_square(Us, SQ_C1)), compare);
  238 
  239   // In endgame we like to bring our king near our closest pawn
  240   Bitboard pawns = pos.pieces(Us, PAWN);
  241   int minPawnDist = pawns ? 8 : 0;
  242 
  243   if (pawns & PseudoAttacks[KING][ksq])
  244       minPawnDist = 1;
  245   else while (pawns)
  246       minPawnDist = std::min(minPawnDist, distance(ksq, pop_lsb(&pawns)));
  247 
  248   return shelter - make_score(0, 16 * minPawnDist);
  249 }
  250 
  251 // Explicit template instantiation
  252 template Score Entry::do_king_safety<WHITE>(const Position& pos);
  253 template Score Entry::do_king_safety<BLACK>(const Position& pos);
  254 
  255 } // namespace Pawns