2 Stockfish, a UCI chess playing engine derived from Glaurung 2.1
3 Copyright (C) 2004-2008 Tord Romstad (Glaurung author)
4 Copyright (C) 2008-2010 Marco Costalba, Joona Kiiski, Tord Romstad
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/>.
32 PH_TT_MOVES, // Transposition table move and mate killer
33 PH_GOOD_CAPTURES, // Queen promotions and captures with SEE values >= 0
34 PH_KILLERS, // Killer moves from the current ply
35 PH_NONCAPTURES, // Non-captures and underpromotions
36 PH_BAD_CAPTURES, // Queen promotions and captures with SEE values < 0
37 PH_EVASIONS, // Check evasions
38 PH_QCAPTURES, // Captures in quiescence search
39 PH_QCHECKS, // Non-capture checks in quiescence search
44 const uint8_t MainSearchPhaseTable[] = { PH_TT_MOVES, PH_GOOD_CAPTURES, PH_KILLERS, PH_NONCAPTURES, PH_BAD_CAPTURES, PH_STOP};
45 const uint8_t EvasionsPhaseTable[] = { PH_TT_MOVES, PH_EVASIONS, PH_STOP};
46 const uint8_t QsearchWithChecksPhaseTable[] = { PH_TT_MOVES, PH_QCAPTURES, PH_QCHECKS, PH_STOP};
47 const uint8_t QsearchWithoutChecksPhaseTable[] = { PH_TT_MOVES, PH_QCAPTURES, PH_STOP};
51 /// Constructor for the MovePicker class. Apart from the position for which
52 /// it is asked to pick legal moves, MovePicker also wants some 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 about how important good
56 /// move ordering is at the current node.
58 MovePicker::MovePicker(const Position& p, Move ttm, Depth d, const History& h,
59 SearchStack* ss, Value beta) : pos(p), H(h) {
61 ttMoves[0].move = ttm;
62 badCaptureThreshold = 0;
63 badCaptures = moves + MOVES_MAX;
65 assert(d > DEPTH_ZERO);
67 pinned = p.pinned_pieces(pos.side_to_move());
71 ttMoves[1].move = killers[0].move = killers[1].move = MOVE_NONE;
72 phasePtr = EvasionsPhaseTable;
76 ttMoves[1].move = (ss->mateKiller == ttm) ? MOVE_NONE : ss->mateKiller;
77 searchTT |= ttMoves[1].move;
78 killers[0].move = ss->killers[0];
79 killers[1].move = ss->killers[1];
81 // Consider sligtly negative captures as good if at low
82 // depth and far from beta.
83 if (ss && ss->eval < beta - PawnValueMidgame && d < 3 * ONE_PLY)
84 badCaptureThreshold = -PawnValueMidgame;
86 phasePtr = MainSearchPhaseTable;
89 phasePtr += int(!searchTT) - 1;
93 MovePicker::MovePicker(const Position& p, Move ttm, Depth d, const History& h)
96 ttMoves[0].move = ttm;
97 ttMoves[1].move = MOVE_NONE;
99 assert(d <= DEPTH_ZERO);
101 pinned = p.pinned_pieces(pos.side_to_move());
104 phasePtr = EvasionsPhaseTable;
105 else if (d >= DEPTH_QS_CHECKS)
106 phasePtr = QsearchWithChecksPhaseTable;
109 phasePtr = QsearchWithoutChecksPhaseTable;
111 // Skip TT move if is not a capture or a promotion, this avoids
112 // qsearch tree explosion due to a possible perpetual check or
113 // similar rare cases when TT table is full.
114 if (ttm != MOVE_NONE && !pos.move_is_capture_or_promotion(ttm))
115 searchTT = ttMoves[0].move = MOVE_NONE;
118 phasePtr += int(!searchTT) - 1;
123 /// MovePicker::go_next_phase() generates, scores and sorts the next bunch
124 /// of moves when there are no more moves to try for the current phase.
126 void MovePicker::go_next_phase() {
129 phase = *(++phasePtr);
134 lastMove = curMove + 2;
137 case PH_GOOD_CAPTURES:
138 lastMove = generate<MV_CAPTURE>(pos, moves);
144 lastMove = curMove + 2;
148 lastMove = generate<MV_NON_CAPTURE>(pos, moves);
150 sort_moves(moves, lastMove, &lastGoodNonCapture);
153 case PH_BAD_CAPTURES:
154 // Bad captures SEE value is already calculated so just pick
155 // them in order to get SEE move ordering.
156 curMove = badCaptures;
157 lastMove = moves + MOVES_MAX;
161 assert(pos.is_check());
162 lastMove = generate<MV_EVASION>(pos, moves);
167 lastMove = generate<MV_CAPTURE>(pos, moves);
172 lastMove = generate<MV_NON_CAPTURE_CHECK>(pos, moves);
176 lastMove = curMove + 1; // Avoids another go_next_phase() call
186 /// MovePicker::score_captures(), MovePicker::score_noncaptures() and
187 /// MovePicker::score_evasions() assign a numerical move ordering score
188 /// to each move in a move list. The moves with highest scores will be
189 /// picked first by get_next_move().
191 void MovePicker::score_captures() {
192 // Winning and equal captures in the main search are ordered by MVV/LVA.
193 // Suprisingly, this appears to perform slightly better than SEE based
194 // move ordering. The reason is probably that in a position with a winning
195 // capture, capturing a more valuable (but sufficiently defended) piece
196 // first usually doesn't hurt. The opponent will have to recapture, and
197 // the hanging piece will still be hanging (except in the unusual cases
198 // where it is possible to recapture with the hanging piece). Exchanging
199 // big pieces before capturing a hanging piece probably helps to reduce
201 // In main search we want to push captures with negative SEE values to
202 // badCaptures[] array, but instead of doing it now we delay till when
203 // the move has been picked up in pick_move_from_list(), this way we save
204 // some SEE calls in case we get a cutoff (idea from Pablo Vazquez).
207 // Use MVV/LVA ordering
208 for (MoveStack* cur = moves; cur != lastMove; cur++)
211 if (move_is_promotion(m))
212 cur->score = QueenValueMidgame;
214 cur->score = pos.midgame_value_of_piece_on(move_to(m))
215 - pos.type_of_piece_on(move_from(m));
219 void MovePicker::score_noncaptures() {
220 // First score by history, when no history is available then use
221 // piece/square tables values. This seems to be better then a
222 // random choice when we don't have an history for any move.
227 for (MoveStack* cur = moves; cur != lastMove; cur++)
232 piece = pos.piece_on(from);
233 cur->score = H.value(piece, to) + H.gain(piece, to);
237 void MovePicker::score_evasions() {
238 // Try good captures ordered by MVV/LVA, then non-captures if
239 // destination square is not under attack, ordered by history
240 // value, and at the end bad-captures and non-captures with a
241 // negative SEE. This last group is ordered by the SEE score.
245 // Skip if we don't have at least two moves to order
246 if (lastMove < moves + 2)
249 for (MoveStack* cur = moves; cur != lastMove; cur++)
252 if ((seeScore = pos.see_sign(m)) < 0)
253 cur->score = seeScore - History::MaxValue; // Be sure we are at the bottom
254 else if (pos.move_is_capture(m))
255 cur->score = pos.midgame_value_of_piece_on(move_to(m))
256 - pos.type_of_piece_on(move_from(m)) + History::MaxValue;
258 cur->score = H.value(pos.piece_on(move_from(m)), move_to(m));
262 /// MovePicker::get_next_move() is the most important method of the MovePicker
263 /// class. It returns a new legal move every time it is called, until there
264 /// are no more moves left.
265 /// It picks the move with the biggest score from a list of generated moves taking
266 /// care not to return the tt move if has already been searched previously.
267 /// Note that this function is not thread safe so should be lock protected by
268 /// caller when accessed through a shared MovePicker object.
270 Move MovePicker::get_next_move() {
276 while (curMove != lastMove)
281 move = (curMove++)->move;
282 if ( move != MOVE_NONE
283 && move_is_legal(pos, move, pinned))
287 case PH_GOOD_CAPTURES:
288 move = pick_best(curMove++, lastMove).move;
289 if ( move != ttMoves[0].move
290 && move != ttMoves[1].move
291 && pos.pl_move_is_legal(move, pinned))
293 // Check for a non negative SEE now
294 int seeValue = pos.see_sign(move);
295 if (seeValue >= badCaptureThreshold)
298 // Losing capture, move it to the tail of the array, note
299 // that move has now been already checked for legality.
300 (--badCaptures)->move = move;
301 badCaptures->score = seeValue;
306 move = (curMove++)->move;
307 if ( move != MOVE_NONE
308 && move_is_legal(pos, move, pinned)
309 && move != ttMoves[0].move
310 && move != ttMoves[1].move
311 && !pos.move_is_capture(move))
317 // Sort negative scored moves only when we get there
318 if (curMove == lastGoodNonCapture)
319 insertion_sort<MoveStack>(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))