2 Stockfish, a UCI chess playing engine derived from Glaurung 2.1
3 Copyright (C) 2004-2008 Tord Romstad (Glaurung author)
4 Copyright (C) 2008-2009 Marco Costalba
6 Stockfish is free software: you can redistribute it and/or modify
7 it under the terms of the GNU General Public License as published by
8 the Free Software Foundation, either version 3 of the License, or
9 (at your option) any later version.
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.
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/>.
36 //// Local definitions
42 PH_TT_MOVES, // Transposition table move and mate killer
43 PH_GOOD_CAPTURES, // Queen promotions and captures with SEE values >= 0
44 PH_KILLERS, // Killer moves from the current ply
45 PH_NONCAPTURES, // Non-captures and underpromotions
46 PH_BAD_CAPTURES, // Queen promotions and captures with SEE values < 0
47 PH_EVASIONS, // Check evasions
48 PH_QCAPTURES, // Captures in quiescence search
49 PH_QCHECKS, // Non-capture checks in quiescence search
54 const uint8_t MainSearchPhaseTable[] = { PH_TT_MOVES, PH_GOOD_CAPTURES, PH_KILLERS, PH_NONCAPTURES, PH_BAD_CAPTURES, PH_STOP};
55 const uint8_t EvasionsPhaseTable[] = { PH_TT_MOVES, PH_EVASIONS, PH_STOP};
56 const uint8_t QsearchWithChecksPhaseTable[] = { PH_TT_MOVES, PH_QCAPTURES, PH_QCHECKS, PH_STOP};
57 const uint8_t QsearchWithoutChecksPhaseTable[] = { PH_TT_MOVES, PH_QCAPTURES, PH_STOP};
66 /// Constructor for the MovePicker class. Apart from the position for which
67 /// it is asked to pick legal moves, MovePicker also wants some information
68 /// to help it to return the presumably good moves first, to decide which
69 /// moves to return (in the quiescence search, for instance, we only want to
70 /// search captures, promotions and some checks) and about how important good
71 /// move ordering is at the current node.
73 MovePicker::MovePicker(const Position& p, Move ttm, Depth d,
74 const History& h, SearchStack* ss) : pos(p), H(h) {
76 ttMoves[0].move = ttm;
77 lastBadCapture = badCaptures;
79 pinned = p.pinned_pieces(pos.side_to_move());
81 if (ss && !p.is_check())
83 ttMoves[1].move = (ss->mateKiller == ttm)? MOVE_NONE : ss->mateKiller;
84 searchTT |= ttMoves[1].move;
85 killers[0].move = ss->killers[0];
86 killers[1].move = ss->killers[1];
88 ttMoves[1].move = killers[0].move = killers[1].move = MOVE_NONE;
91 phasePtr = EvasionsPhaseTable;
92 else if (d > Depth(0))
93 phasePtr = MainSearchPhaseTable;
94 else if (d == Depth(0))
95 phasePtr = QsearchWithChecksPhaseTable;
98 phasePtr = QsearchWithoutChecksPhaseTable;
100 // Skip TT move if is not a capture or a promotion, this avoids
101 // qsearch tree explosion due to a possible perpetual check or
102 // similar rare cases when TT table is full.
103 if (ttm != MOVE_NONE && !pos.move_is_capture_or_promotion(ttm))
104 searchTT = ttMoves[0].move = MOVE_NONE;
107 phasePtr += !searchTT - 1;
112 /// MovePicker::go_next_phase() generates, scores and sorts the next bunch
113 /// of moves when there are no more moves to try for the current phase.
115 void MovePicker::go_next_phase() {
118 phase = *(++phasePtr);
123 lastMove = curMove + 2;
126 case PH_GOOD_CAPTURES:
127 lastMove = generate_captures(pos, moves);
133 lastMove = curMove + 2;
137 lastMove = generate_noncaptures(pos, moves);
139 sort_moves(moves, lastMove, &lastGoodNonCapture);
142 case PH_BAD_CAPTURES:
143 // Bad captures SEE value is already calculated so just sort them
144 // to get SEE move ordering.
145 curMove = badCaptures;
146 lastMove = lastBadCapture;
150 assert(pos.is_check());
151 lastMove = generate_evasions(pos, moves);
152 score_evasions_or_checks();
156 lastMove = generate_captures(pos, moves);
161 lastMove = generate_non_capture_checks(pos, moves);
162 score_evasions_or_checks();
166 lastMove = curMove + 1; // Avoids another go_next_phase() call
176 /// MovePicker::score_captures(), MovePicker::score_noncaptures() and
177 /// MovePicker::score_evasions() assign a numerical move ordering score
178 /// to each move in a move list. The moves with highest scores will be
179 /// picked first by get_next_move().
181 void MovePicker::score_captures() {
182 // Winning and equal captures in the main search are ordered by MVV/LVA.
183 // Suprisingly, this appears to perform slightly better than SEE based
184 // move ordering. The reason is probably that in a position with a winning
185 // capture, capturing a more valuable (but sufficiently defended) piece
186 // first usually doesn't hurt. The opponent will have to recapture, and
187 // the hanging piece will still be hanging (except in the unusual cases
188 // where it is possible to recapture with the hanging piece). Exchanging
189 // big pieces before capturing a hanging piece probably helps to reduce
191 // In main search we want to push captures with negative SEE values to
192 // badCaptures[] array, but instead of doing it now we delay till when
193 // the move has been picked up in pick_move_from_list(), this way we save
194 // some SEE calls in case we get a cutoff (idea from Pablo Vazquez).
197 // Use MVV/LVA ordering
198 for (MoveStack* cur = moves; cur != lastMove; cur++)
201 if (move_is_promotion(m))
202 cur->score = QueenValueMidgame;
204 cur->score = pos.midgame_value_of_piece_on(move_to(m))
205 - pos.type_of_piece_on(move_from(m));
209 void MovePicker::score_noncaptures() {
210 // First score by history, when no history is available then use
211 // piece/square tables values. This seems to be better then a
212 // random choice when we don't have an history for any move.
218 for (MoveStack* cur = moves; cur != lastMove; cur++)
223 piece = pos.piece_on(from);
224 hs = H.move_ordering_score(piece, to);
226 // Ensure history has always highest priority
230 // Gain table based scoring
231 cur->score = hs + 16 * H.gain(piece, to);
235 void MovePicker::score_evasions_or_checks() {
236 // Try good captures ordered by MVV/LVA, then non-captures if
237 // destination square is not under attack, ordered by history
238 // value, and at the end bad-captures and non-captures with a
239 // negative SEE. This last group is ordered by the SEE score.
243 // Skip if we don't have at least two moves to order
244 if (lastMove < moves + 2)
247 for (MoveStack* cur = moves; cur != lastMove; cur++)
250 if ((seeScore = pos.see_sign(m)) < 0)
251 cur->score = seeScore - HistoryMax; // Be sure are at the bottom
252 else if (pos.move_is_capture(m))
253 cur->score = pos.midgame_value_of_piece_on(move_to(m))
254 - pos.type_of_piece_on(move_from(m)) + HistoryMax;
256 cur->score = H.move_ordering_score(pos.piece_on(move_from(m)), move_to(m));
260 /// MovePicker::get_next_move() is the most important method of the MovePicker
261 /// class. It returns a new legal move every time it is called, until there
262 /// are no more moves left.
263 /// It picks the move with the biggest score from a list of generated moves taking
264 /// care not to return the tt move if has already been searched previously.
265 /// Note that this function is not thread safe so should be lock protected by
266 /// caller when accessed through a shared MovePicker object.
268 Move MovePicker::get_next_move() {
274 while (curMove != lastMove)
279 move = (curMove++)->move;
280 if ( move != MOVE_NONE
281 && move_is_legal(pos, move, pinned))
285 case PH_GOOD_CAPTURES:
286 move = pick_best(curMove++, lastMove).move;
287 if ( move != ttMoves[0].move
288 && move != ttMoves[1].move
289 && pos.pl_move_is_legal(move, pinned))
291 // Check for a non negative SEE now
292 int seeValue = pos.see_sign(move);
296 // Losing capture, move it to the badCaptures[] array, note
297 // that move has now been already checked for legality.
298 assert(int(lastBadCapture - badCaptures) < 63);
299 lastBadCapture->move = move;
300 lastBadCapture->score = seeValue;
306 move = (curMove++)->move;
307 if ( move != MOVE_NONE
308 && move != ttMoves[0].move
309 && move != ttMoves[1].move
310 && move_is_legal(pos, move, pinned)
311 && !pos.move_is_capture(move))
317 // Sort negative scored moves only when we get there
318 if (curMove == lastGoodNonCapture)
319 insertion_sort(lastGoodNonCapture, lastMove);
321 move = (curMove++)->move;
322 if ( move != ttMoves[0].move
323 && move != ttMoves[1].move
324 && move != killers[0].move
325 && move != killers[1].move
326 && pos.pl_move_is_legal(move, pinned))
330 case PH_BAD_CAPTURES:
331 move = pick_best(curMove++, lastMove).move;
336 move = pick_best(curMove++, lastMove).move;
337 if ( move != ttMoves[0].move
338 && pos.pl_move_is_legal(move, pinned))
343 move = (curMove++)->move;
344 if ( move != ttMoves[0].move
345 && pos.pl_move_is_legal(move, pinned))