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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 <cassert>
   22 
   23 #include "movepick.h"
   24 
   25 namespace {
   26 
   27   enum Stages {
   28     MAIN_TT, CAPTURE_INIT, GOOD_CAPTURE, REFUTATION, QUIET_INIT, QUIET, BAD_CAPTURE,
   29     EVASION_TT, EVASION_INIT, EVASION,
   30     PROBCUT_TT, PROBCUT_INIT, PROBCUT,
   31     QSEARCH_TT, QCAPTURE_INIT, QCAPTURE, QCHECK_INIT, QCHECK
   32   };
   33 
   34   // partial_insertion_sort() sorts moves in descending order up to and including
   35   // a given limit. The order of moves smaller than the limit is left unspecified.
   36   void partial_insertion_sort(ExtMove* begin, ExtMove* end, int limit) {
   37 
   38     for (ExtMove *sortedEnd = begin, *p = begin + 1; p < end; ++p)
   39         if (p->value >= limit)
   40         {
   41             ExtMove tmp = *p, *q;
   42             *p = *++sortedEnd;
   43             for (q = sortedEnd; q != begin && *(q - 1) < tmp; --q)
   44                 *q = *(q - 1);
   45             *q = tmp;
   46         }
   47   }
   48 
   49 } // namespace
   50 
   51 
   52 /// Constructors of the MovePicker class. As arguments we pass information
   53 /// to help it to return the (presumably) good moves first, to decide which
   54 /// moves to return (in the quiescence search, for instance, we only want to
   55 /// search captures, promotions, and some checks) and how important good move
   56 /// ordering is at the current node.
   57 
   58 /// MovePicker constructor for the main search
   59 MovePicker::MovePicker(const Position& p, Move ttm, Depth d, const ButterflyHistory* mh,
   60                        const CapturePieceToHistory* cph, const PieceToHistory** ch, Move cm, Move* killers)
   61            : pos(p), mainHistory(mh), captureHistory(cph), continuationHistory(ch),
   62              refutations{{killers[0], 0}, {killers[1], 0}, {cm, 0}}, depth(d) {
   63 
   64   assert(d > 0);
   65 
   66   stage = pos.checkers() ? EVASION_TT : MAIN_TT;
   67   ttMove = ttm && pos.pseudo_legal(ttm) ? ttm : MOVE_NONE;
   68   stage += (ttMove == MOVE_NONE);
   69 }
   70 
   71 /// MovePicker constructor for quiescence search
   72 MovePicker::MovePicker(const Position& p, Move ttm, Depth d, const ButterflyHistory* mh,
   73                        const CapturePieceToHistory* cph, const PieceToHistory** ch, Square rs)
   74            : pos(p), mainHistory(mh), captureHistory(cph), continuationHistory(ch), recaptureSquare(rs), depth(d) {
   75 
   76   assert(d <= 0);
   77 
   78   stage = pos.checkers() ? EVASION_TT : QSEARCH_TT;
   79   ttMove =   ttm
   80           && (depth > DEPTH_QS_RECAPTURES || to_sq(ttm) == recaptureSquare)
   81           && pos.pseudo_legal(ttm) ? ttm : MOVE_NONE;
   82   stage += (ttMove == MOVE_NONE);
   83 }
   84 
   85 /// MovePicker constructor for ProbCut: we generate captures with SEE greater
   86 /// than or equal to the given threshold.
   87 MovePicker::MovePicker(const Position& p, Move ttm, Value th, const CapturePieceToHistory* cph)
   88            : pos(p), captureHistory(cph), threshold(th) {
   89 
   90   assert(!pos.checkers());
   91 
   92   stage = PROBCUT_TT;
   93   ttMove =   ttm
   94           && pos.capture(ttm)
   95           && pos.pseudo_legal(ttm)
   96           && pos.see_ge(ttm, threshold) ? ttm : MOVE_NONE;
   97   stage += (ttMove == MOVE_NONE);
   98 }
   99 
  100 /// MovePicker::score() assigns a numerical value to each move in a list, used
  101 /// for sorting. Captures are ordered by Most Valuable Victim (MVV), preferring
  102 /// captures with a good history. Quiets moves are ordered using the histories.
  103 template<GenType Type>
  104 void MovePicker::score() {
  105 
  106   static_assert(Type == CAPTURES || Type == QUIETS || Type == EVASIONS, "Wrong type");
  107 
  108   for (auto& m : *this)
  109       if (Type == CAPTURES)
  110           m.value =  int(PieceValue[MG][pos.piece_on(to_sq(m))]) * 6
  111                    + (*captureHistory)[pos.moved_piece(m)][to_sq(m)][type_of(pos.piece_on(to_sq(m)))];
  112 
  113       else if (Type == QUIETS)
  114           m.value =      (*mainHistory)[pos.side_to_move()][from_to(m)]
  115                    + 2 * (*continuationHistory[0])[pos.moved_piece(m)][to_sq(m)]
  116                    + 2 * (*continuationHistory[1])[pos.moved_piece(m)][to_sq(m)]
  117                    + 2 * (*continuationHistory[3])[pos.moved_piece(m)][to_sq(m)]
  118                    +     (*continuationHistory[5])[pos.moved_piece(m)][to_sq(m)];
  119 
  120       else // Type == EVASIONS
  121       {
  122           if (pos.capture(m))
  123               m.value =  PieceValue[MG][pos.piece_on(to_sq(m))]
  124                        - Value(type_of(pos.moved_piece(m)));
  125           else
  126               m.value =  (*mainHistory)[pos.side_to_move()][from_to(m)]
  127                        + (*continuationHistory[0])[pos.moved_piece(m)][to_sq(m)]
  128                        - (1 << 28);
  129       }
  130 }
  131 
  132 /// MovePicker::select() returns the next move satisfying a predicate function.
  133 /// It never returns the TT move.
  134 template<MovePicker::PickType T, typename Pred>
  135 Move MovePicker::select(Pred filter) {
  136 
  137   while (cur < endMoves)
  138   {
  139       if (T == Best)
  140           std::swap(*cur, *std::max_element(cur, endMoves));
  141 
  142       if (*cur != ttMove && filter())
  143           return *cur++;
  144 
  145       cur++;
  146   }
  147   return MOVE_NONE;
  148 }
  149 
  150 /// MovePicker::next_move() is the most important method of the MovePicker class. It
  151 /// returns a new pseudo legal move every time it is called until there are no more
  152 /// moves left, picking the move with the highest score from a list of generated moves.
  153 Move MovePicker::next_move(bool skipQuiets) {
  154 
  155 top:
  156   switch (stage) {
  157 
  158   case MAIN_TT:
  159   case EVASION_TT:
  160   case QSEARCH_TT:
  161   case PROBCUT_TT:
  162       ++stage;
  163       return ttMove;
  164 
  165   case CAPTURE_INIT:
  166   case PROBCUT_INIT:
  167   case QCAPTURE_INIT:
  168       cur = endBadCaptures = moves;
  169       endMoves = generate<CAPTURES>(pos, cur);
  170 
  171       score<CAPTURES>();
  172       ++stage;
  173       goto top;
  174 
  175   case GOOD_CAPTURE:
  176       if (select<Best>([&](){
  177                        return pos.see_ge(*cur, Value(-55 * cur->value / 1024)) ?
  178                               // Move losing capture to endBadCaptures to be tried later
  179                               true : (*endBadCaptures++ = *cur, false); }))
  180           return *(cur - 1);
  181 
  182       // Prepare the pointers to loop over the refutations array
  183       cur = std::begin(refutations);
  184       endMoves = std::end(refutations);
  185 
  186       // If the countermove is the same as a killer, skip it
  187       if (   refutations[0].move == refutations[2].move
  188           || refutations[1].move == refutations[2].move)
  189           --endMoves;
  190 
  191       ++stage;
  192       /* fallthrough */
  193 
  194   case REFUTATION:
  195       if (select<Next>([&](){ return    *cur != MOVE_NONE
  196                                     && !pos.capture(*cur)
  197                                     &&  pos.pseudo_legal(*cur); }))
  198           return *(cur - 1);
  199       ++stage;
  200       /* fallthrough */
  201 
  202   case QUIET_INIT:
  203       if (!skipQuiets)
  204       {
  205           cur = endBadCaptures;
  206           endMoves = generate<QUIETS>(pos, cur);
  207 
  208           score<QUIETS>();
  209           partial_insertion_sort(cur, endMoves, -3000 * depth);
  210       }
  211 
  212       ++stage;
  213       /* fallthrough */
  214 
  215   case QUIET:
  216       if (   !skipQuiets
  217           && select<Next>([&](){return   *cur != refutations[0].move
  218                                       && *cur != refutations[1].move
  219                                       && *cur != refutations[2].move;}))
  220           return *(cur - 1);
  221 
  222       // Prepare the pointers to loop over the bad captures
  223       cur = moves;
  224       endMoves = endBadCaptures;
  225 
  226       ++stage;
  227       /* fallthrough */
  228 
  229   case BAD_CAPTURE:
  230       return select<Next>([](){ return true; });
  231 
  232   case EVASION_INIT:
  233       cur = moves;
  234       endMoves = generate<EVASIONS>(pos, cur);
  235 
  236       score<EVASIONS>();
  237       ++stage;
  238       /* fallthrough */
  239 
  240   case EVASION:
  241       return select<Best>([](){ return true; });
  242 
  243   case PROBCUT:
  244       return select<Best>([&](){ return pos.see_ge(*cur, threshold); });
  245 
  246   case QCAPTURE:
  247       if (select<Best>([&](){ return   depth > DEPTH_QS_RECAPTURES
  248                                     || to_sq(*cur) == recaptureSquare; }))
  249           return *(cur - 1);
  250 
  251       // If we did not find any move and we do not try checks, we have finished
  252       if (depth != DEPTH_QS_CHECKS)
  253           return MOVE_NONE;
  254 
  255       ++stage;
  256       /* fallthrough */
  257 
  258   case QCHECK_INIT:
  259       cur = moves;
  260       endMoves = generate<QUIET_CHECKS>(pos, cur);
  261 
  262       ++stage;
  263       /* fallthrough */
  264 
  265   case QCHECK:
  266       return select<Next>([](){ return true; });
  267   }
  268 
  269   assert(false);
  270   return MOVE_NONE; // Silence warning
  271 }