src/movepick.cpp

   1 /*
   2   Glaurung, a UCI chess playing engine.
   3   Copyright (C) 2004-2008 Tord Romstad
   4
   5   Glaurung is free software: you can redistribute it and/or modify
   6   it under the terms of the GNU General Public License as published by
   7   the Free Software Foundation, either version 3 of the License, or
   8   (at your option) any later version.
   9
  10   Glaurung is distributed in the hope that it will be useful,
  11   but WITHOUT ANY WARRANTY; without even the implied warranty of
  12   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  13   GNU General Public License for more details.
  14
  15   You should have received a copy of the GNU General Public License
  16   along with this program.  If not, see <http://www.gnu.org/licenses/>.
  17 */
  18
  19
  20 ////
  21 //// Includes
  22 ////
  23
  24 #include <cassert>
  25
  26 #include "history.h"
  27 #include "movegen.h"
  28 #include "movepick.h"
  29 #include "search.h"
  30 #include "value.h"
  31
  32
  33 ////
  34 //// Local definitions
  35 ////
  36
  37 namespace {
  38
  39   /// Types
  40
  41   enum MovegenPhase {
  42     PH_TT_MOVE,        // Transposition table move
  43     PH_MATE_KILLER,    // Mate killer from the current ply
  44     PH_GOOD_CAPTURES,  // Queen promotions and captures with SEE values >= 0
  45     PH_BAD_CAPTURES,   // Queen promotions and captures with SEE valuse <= 0
  46     PH_KILLER_1,       // Killer move 1 from the current ply (not used yet).
  47     PH_KILLER_2,       // Killer move 2 from the current ply (not used yet).
  48     PH_NONCAPTURES,    // Non-captures and underpromotions
  49     PH_EVASIONS,       // Check evasions
  50     PH_QCAPTURES,      // Captures in quiescence search
  51     PH_QCHECKS,        // Checks in quiescence search
  52     PH_STOP
  53   };
  54
  55
  56   /// Variables
  57
  58   MovegenPhase PhaseTable[32];
  59   int MainSearchPhaseIndex;
  60   int EvasionsPhaseIndex;
  61   int QsearchWithChecksPhaseIndex;
  62   int QsearchWithoutChecksPhaseIndex;
  63
  64 }
  65
  66
  67 ////
  68 //// Functions
  69 ////
  70
  71
  72 /// Constructor for the MovePicker class.  Apart from the position for which
  73 /// it is asked to pick legal moves, MovePicker also wants some information
  74 /// to help it to return the presumably good moves first, to decide which
  75 /// moves to return (in the quiescence search, for instance, we only want to
  76 /// search captures, promotions and some checks) and about how important good
  77 /// move ordering is at the current node.
  78
  79 MovePicker::MovePicker(Position &p, bool pvnode, Move ttm, Move mk,
  80                        Move k1, Move k2, Depth dpth) {
  81   pos = &p;
  82   pvNode = pvnode;
  83   ttMove = ttm;
  84   mateKiller = (mk == ttm)? MOVE_NONE : mk;
  85   killer1 = k1;
  86   killer2 = k2;
  87   depth = dpth;
  88   movesPicked = 0;
  89   numOfMoves = 0;
  90   numOfBadCaptures = 0;
  91   dc = p.discovered_check_candidates(p.side_to_move());
  92
  93   if(p.is_check())
  94     phaseIndex = EvasionsPhaseIndex;
  95   else if(depth > Depth(0))
  96     phaseIndex = MainSearchPhaseIndex;
  97   else if(depth == Depth(0))
  98     phaseIndex = QsearchWithChecksPhaseIndex;
  99   else
 100     phaseIndex = QsearchWithoutChecksPhaseIndex;
 101
 102   pinned = p.pinned_pieces(p.side_to_move());
 103
 104   finished = false;
 105 }
 106
 107
 108 /// MovePicker::get_next_move() is the most important method of the MovePicker
 109 /// class.  It returns a new legal move every time it is called, until there
 110 /// are no more moves left of the types we are interested in.
 111
 112 Move MovePicker::get_next_move() {
 113   Move move;
 114
 115   while(true) {
 116     // If we already have a list of generated moves, pick the best move from
 117     // the list, and return it:
 118     move = this->pick_move_from_list();
 119     if(move != MOVE_NONE) {
 120       assert(move_is_ok(move));
 121       return move;
 122     }
 123
 124     // Next phase:
 125     phaseIndex++;
 126     switch(PhaseTable[phaseIndex]) {
 127
 128     case PH_TT_MOVE:
 129       if(ttMove != MOVE_NONE) {
 130         assert(move_is_ok(ttMove));
 131         Move m = generate_move_if_legal(*pos, ttMove, pinned);
 132         if(m != MOVE_NONE) {
 133           assert(m == ttMove);
 134           return m;
 135         }
 136       }
 137       break;
 138
 139     case PH_MATE_KILLER:
 140       if(mateKiller != MOVE_NONE) {
 141         assert(move_is_ok(mateKiller));
 142         Move m = generate_move_if_legal(*pos, mateKiller, pinned);
 143         if(m != MOVE_NONE) {
 144           assert(m == mateKiller);
 145           return m;
 146         }
 147       }
 148       break;
 149
 150     case PH_GOOD_CAPTURES:
 151       // pinned = pos->pinned_pieces(pos->side_to_move());
 152       numOfMoves = generate_captures(*pos, moves);
 153       this->score_captures();
 154       movesPicked = 0;
 155       break;
 156
 157     case PH_BAD_CAPTURES:
 158       badCapturesPicked = 0;
 159       break;
 160
 161     case PH_NONCAPTURES:
 162       numOfMoves = generate_noncaptures(*pos, moves);
 163       this->score_noncaptures();
 164       movesPicked = 0;
 165       break;
 166
 167     case PH_EVASIONS:
 168       assert(pos->is_check());
 169       // pinned = pos->pinned_pieces(pos->side_to_move());
 170       numOfMoves = generate_evasions(*pos, moves);
 171       this->score_evasions();
 172       movesPicked = 0;
 173       break;
 174
 175     case PH_QCAPTURES:
 176       // pinned = pos->pinned_pieces(pos->side_to_move());
 177       numOfMoves = generate_captures(*pos, moves);
 178       this->score_qcaptures();
 179       movesPicked = 0;
 180       break;
 181
 182     case PH_QCHECKS:
 183       numOfMoves = generate_checks(*pos, moves, dc);
 184       movesPicked = 0;
 185       break;
 186
 187     case PH_STOP:
 188       return MOVE_NONE;
 189
 190     default:
 191       assert(false);
 192       return MOVE_NONE;
 193     }
 194   }
 195
 196   assert(false);
 197
 198   return MOVE_NONE;
 199 }
 200
 201
 202 /// A variant of get_next_move() which takes a lock as a parameter, used to
 203 /// prevent multiple threads from picking the same move at a split point.
 204
 205 Move MovePicker::get_next_move(Lock &lock) {
 206    Move m;
 207
 208    lock_grab(&lock);
 209    if(finished) {
 210      lock_release(&lock);
 211      return MOVE_NONE;
 212    }
 213    m = this->get_next_move();
 214    if(m == MOVE_NONE)
 215      finished = true;
 216    lock_release(&lock);
 217
 218    return m;
 219 }
 220
 221
 222 /// MovePicker::number_of_moves() simply returns the numOfMoves member
 223 /// variable.  It is intended to be used in positions where the side to move
 224 /// is in check, for detecting checkmates or situations where there is only
 225 /// a single reply to check.
 226
 227 int MovePicker::number_of_moves() const {
 228   return numOfMoves;
 229 }
 230
 231
 232 /// MovePicker::score_captures(), MovePicker::score_noncaptures(),
 233 /// MovePicker::score_evasions() and MovePicker::score_qcaptures() assign a
 234 /// numerical move ordering score to each move in a move list.  The moves
 235 /// with highest scores will be picked first by
 236 /// MovePicker::pick_move_from_list().
 237
 238 void MovePicker::score_captures() {
 239   // Winning and equal captures in the main search are ordered by MVV/LVA.
 240   // Suprisingly, this appears to perform slightly better than SEE based
 241   // move ordering.  The reason is probably that in a position with a winning
 242   // capture, capturing a more valuable (but sufficiently defended) piece
 243   // first usually doesn't hurt.  The opponent will have to recapture, and
 244   // the hanging piece will still be hanging (except in the unusual cases
 245   // where it is possible to recapture with the hanging piece).  Exchanging
 246   // big pieces before capturing a hanging piece probably helps to reduce
 247   // the subtree size.
 248   for(int i = 0; i < numOfMoves; i++) {
 249     int seeValue = pos->see(moves[i].move);
 250     if(seeValue >= 0) {
 251       if(move_promotion(moves[i].move))
 252         moves[i].score = QueenValueMidgame;
 253       else
 254         moves[i].score =
 255           int(pos->midgame_value_of_piece_on(move_to(moves[i].move))) -
 256           int(pos->type_of_piece_on(move_from(moves[i].move)));
 257     }
 258     else
 259       moves[i].score = seeValue;
 260
 261   }
 262 }
 263
 264 void MovePicker::score_noncaptures() {
 265   for(int i = 0; i < numOfMoves; i++) {
 266     Move m = moves[i].move;
 267     if(m == killer1)
 268       moves[i].score = HistoryMax + 2;
 269     else if(m == killer2)
 270       moves[i].score = HistoryMax + 1;
 271     else
 272       moves[i].score = H.move_ordering_score(pos->piece_on(move_from(m)), m);
 273   }
 274 }
 275
 276 void MovePicker::score_evasions() {
 277   for(int i = 0; i < numOfMoves; i++) {
 278     Move m = moves[i].move;
 279     if(m == ttMove)
 280       moves[i].score = 2*HistoryMax;
 281     else if(!pos->square_is_empty(move_to(m))) {
 282       int seeScore = pos->see(m);
 283       moves[i].score = (seeScore >= 0)? seeScore + HistoryMax : seeScore;
 284     }
 285     else
 286       moves[i].score = H.move_ordering_score(pos->piece_on(move_from(m)), m);
 287   }
 288 }
 289
 290 void MovePicker::score_qcaptures() {
 291   // Use MVV/LVA ordering.
 292   for(int i = 0; i < numOfMoves; i++) {
 293     Move m = moves[i].move;
 294     if(move_promotion(m))
 295       moves[i].score = QueenValueMidgame;
 296     else
 297       moves[i].score =
 298         int(pos->midgame_value_of_piece_on(move_to(m))) -
 299         int(pos->midgame_value_of_piece_on(move_to(m))) / 64;
 300   }
 301 }
 302
 303
 304 /// MovePicker::pick_move_from_list() picks the move with the biggest score
 305 /// from a list of generated moves (moves[] or badCaptures[], depending on
 306 /// the current move generation phase).  It takes care not to return the
 307 /// transposition table move if that has already been serched previously.
 308 /// While picking captures in the PH_GOOD_CAPTURES phase (i.e. while picking
 309 /// non-losing captures in the main search), it moves all captures with
 310 /// negative SEE values to the badCaptures[] array.
 311
 312 Move MovePicker::pick_move_from_list() {
 313   int bestScore = -10000000;
 314   int bestIndex;
 315   Move move;
 316
 317   switch(PhaseTable[phaseIndex]) {
 318
 319   case PH_GOOD_CAPTURES:
 320     assert(!pos->is_check());
 321     assert(movesPicked >= 0);
 322     while(movesPicked < numOfMoves) {
 323       bestScore = -10000000;
 324       bestIndex = -1;
 325       for(int i = movesPicked; i < numOfMoves; i++) {
 326         if(moves[i].score < 0) {
 327           // Losing capture, move it to the badCaptures[] array
 328           assert(numOfBadCaptures < 63);
 329           badCaptures[numOfBadCaptures++] = moves[i];
 330           moves[i--] = moves[--numOfMoves];
 331         }
 332         else if(moves[i].score > bestScore) {
 333           bestIndex = i;
 334           bestScore = moves[i].score;
 335         }
 336       }
 337       if(bestIndex != -1) { // Found a good capture
 338         move = moves[bestIndex].move;
 339         moves[bestIndex] = moves[movesPicked++];
 340         if(move != ttMove && move != mateKiller &&
 341            pos->move_is_legal(move, pinned))
 342           return move;
 343       }
 344     }
 345     break;
 346
 347   case PH_NONCAPTURES:
 348     assert(!pos->is_check());
 349     assert(movesPicked >= 0);
 350     while(movesPicked < numOfMoves) {
 351       bestScore = -10000000;
 352
 353       // If this is a PV node or we have only picked a few moves, scan
 354       // the entire move list for the best move.  If many moves have already
 355       // been searched and it is not a PV node, we are probably failing low
 356       // anyway, so we just pick the first move from the list.
 357       if(pvNode || movesPicked < 12) {
 358         bestIndex = -1;
 359         for(int i = movesPicked; i < numOfMoves; i++)
 360           if(moves[i].score > bestScore) {
 361             bestIndex = i;
 362             bestScore = moves[i].score;
 363           }
 364       }
 365       else
 366         bestIndex = movesPicked;
 367
 368       if(bestIndex != -1) {
 369         move = moves[bestIndex].move;
 370         moves[bestIndex] = moves[movesPicked++];
 371         if(move != ttMove && move != mateKiller &&
 372            pos->move_is_legal(move, pinned))
 373           return move;
 374       }
 375     }
 376     break;
 377
 378   case PH_EVASIONS:
 379     assert(pos->is_check());
 380     assert(movesPicked >= 0);
 381     while(movesPicked < numOfMoves) {
 382       bestScore = -10000000;
 383       bestIndex = -1;
 384       for(int i = movesPicked; i < numOfMoves; i++)
 385         if(moves[i].score > bestScore) {
 386           bestIndex = i;
 387           bestScore = moves[i].score;
 388         }
 389
 390       if(bestIndex != -1) {
 391         move = moves[bestIndex].move;
 392         moves[bestIndex] = moves[movesPicked++];
 393         return move;
 394       }
 395     }
 396     break;
 397
 398   case PH_BAD_CAPTURES:
 399     assert(!pos->is_check());
 400     assert(badCapturesPicked >= 0);
 401     // It's probably a good idea to use SEE move ordering here, instead
 402     // of just picking the first move.  FIXME
 403     while(badCapturesPicked < numOfBadCaptures) {
 404       move = badCaptures[badCapturesPicked++].move;
 405       if(move != ttMove && move != mateKiller &&
 406          pos->move_is_legal(move, pinned))
 407         return move;
 408     }
 409     break;
 410
 411   case PH_QCAPTURES:
 412     assert(!pos->is_check());
 413     assert(movesPicked >= 0);
 414     while(movesPicked < numOfMoves) {
 415       bestScore = -10000000;
 416       if(movesPicked < 4) {
 417         bestIndex = -1;
 418         for(int i = movesPicked; i < numOfMoves; i++)
 419           if(moves[i].score > bestScore) {
 420             bestIndex = i;
 421             bestScore = moves[i].score;
 422           }
 423       }
 424       else
 425         bestIndex = movesPicked;
 426
 427       if(bestIndex != -1) {
 428         move = moves[bestIndex].move;
 429         moves[bestIndex] = moves[movesPicked++];
 430         // Remember to change the line below if we decide to hash the qsearch!
 431         // Maybe also postpone the legality check until after futility pruning?
 432         if(/* move != ttMove && */ pos->move_is_legal(move, pinned))
 433           return move;
 434       }
 435     }
 436     break;
 437
 438   case PH_QCHECKS:
 439     assert(!pos->is_check());
 440     assert(movesPicked >= 0);
 441     // Perhaps we should do something better than just picking the first
 442     // move here?  FIXME
 443     while(movesPicked < numOfMoves) {
 444       move = moves[movesPicked++].move;
 445       // Remember to change the line below if we decide to hash the qsearch!
 446       if(/* move != ttMove && */ pos->move_is_legal(move, pinned))
 447         return move;
 448     }
 449     break;
 450
 451   default:
 452     break;
 453   }
 454
 455   return MOVE_NONE;
 456 }
 457
 458
 459 /// MovePicker::init_phase_table() initializes the PhaseTable[],
 460 /// MainSearchPhaseIndex, EvasionPhaseIndex, QsearchWithChecksPhaseIndex
 461 /// and QsearchWithoutChecksPhaseIndex variables.  It is only called once
 462 /// during program startup, and never again while the program is running.
 463
 464 void MovePicker::init_phase_table() {
 465   int i = 0;
 466
 467   // Main search
 468   MainSearchPhaseIndex = i - 1;
 469   PhaseTable[i++] = PH_TT_MOVE;
 470   PhaseTable[i++] = PH_MATE_KILLER;
 471   PhaseTable[i++] = PH_GOOD_CAPTURES;
 472   // PH_KILLER_1 and PH_KILLER_2 are not yet used.
 473   // PhaseTable[i++] = PH_KILLER_1;
 474   // PhaseTable[i++] = PH_KILLER_2;
 475   PhaseTable[i++] = PH_NONCAPTURES;
 476   PhaseTable[i++] = PH_BAD_CAPTURES;
 477   PhaseTable[i++] = PH_STOP;
 478
 479   // Check evasions
 480   EvasionsPhaseIndex = i - 1;
 481   PhaseTable[i++] = PH_EVASIONS;
 482   PhaseTable[i++] = PH_STOP;
 483
 484   // Quiescence search with checks
 485   QsearchWithChecksPhaseIndex = i - 1;
 486   PhaseTable[i++] = PH_QCAPTURES;
 487   PhaseTable[i++] = PH_QCHECKS;
 488   PhaseTable[i++] = PH_STOP;
 489
 490   // Quiescence search without checks
 491   QsearchWithoutChecksPhaseIndex = i - 1;
 492   PhaseTable[i++] = PH_QCAPTURES;
 493   PhaseTable[i++] = PH_STOP;
 494 }