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/>.
31 PH_TT_MOVES, // Transposition table move and mate killer
32 PH_GOOD_CAPTURES, // Queen promotions and captures with SEE values >= 0
33 PH_KILLERS, // Killer moves from the current ply
34 PH_NONCAPTURES, // Non-captures and underpromotions
35 PH_BAD_CAPTURES, // Queen promotions and captures with SEE values < 0
36 PH_EVASIONS, // Check evasions
37 PH_QCAPTURES, // Captures in quiescence search
38 PH_QCHECKS, // Non-capture checks in quiescence search
43 const uint8_t MainSearchTable[] = { PH_TT_MOVES, PH_GOOD_CAPTURES, PH_KILLERS, PH_NONCAPTURES, PH_BAD_CAPTURES, PH_STOP };
44 const uint8_t EvasionTable[] = { PH_TT_MOVES, PH_EVASIONS, PH_STOP };
45 const uint8_t QsearchWithChecksTable[] = { PH_TT_MOVES, PH_QCAPTURES, PH_QCHECKS, PH_STOP };
46 const uint8_t QsearchWithoutChecksTable[] = { PH_TT_MOVES, PH_QCAPTURES, PH_STOP };
50 /// Constructor for the MovePicker class. Apart from the position for which
51 /// it is asked to pick legal moves, MovePicker also wants some information
52 /// to help it to return the presumably good moves first, to decide which
53 /// moves to return (in the quiescence search, for instance, we only want to
54 /// search captures, promotions and some checks) and about how important good
55 /// move ordering is at the current node.
57 MovePicker::MovePicker(const Position& p, Move ttm, Depth d, const History& h,
58 SearchStack* ss, Value beta) : pos(p), H(h) {
60 ttMoves[0].move = ttm;
61 badCaptureThreshold = 0;
62 badCaptures = moves + MAX_MOVES;
64 assert(d > DEPTH_ZERO);
66 pinned = p.pinned_pieces(pos.side_to_move());
70 ttMoves[1].move = killers[0].move = killers[1].move = MOVE_NONE;
71 phasePtr = EvasionTable;
75 ttMoves[1].move = (ss->mateKiller == ttm) ? MOVE_NONE : ss->mateKiller;
76 searchTT |= ttMoves[1].move;
77 killers[0].move = ss->killers[0];
78 killers[1].move = ss->killers[1];
80 // Consider sligtly negative captures as good if at low
81 // depth and far from beta.
82 if (ss && ss->eval < beta - PawnValueMidgame && d < 3 * ONE_PLY)
83 badCaptureThreshold = -PawnValueMidgame;
85 phasePtr = MainSearchTable;
88 phasePtr += int(!searchTT) - 1;
92 MovePicker::MovePicker(const Position& p, Move ttm, Depth d, const History& h)
95 ttMoves[0].move = ttm;
96 ttMoves[1].move = MOVE_NONE;
98 assert(d <= DEPTH_ZERO);
100 pinned = p.pinned_pieces(pos.side_to_move());
103 phasePtr = EvasionTable;
104 else if (d >= DEPTH_QS_CHECKS)
105 phasePtr = QsearchWithChecksTable;
108 phasePtr = QsearchWithoutChecksTable;
110 // Skip TT move if is not a capture or a promotion, this avoids
111 // qsearch tree explosion due to a possible perpetual check or
112 // similar rare cases when TT table is full.
113 if (ttm != MOVE_NONE && !pos.move_is_capture_or_promotion(ttm))
114 searchTT = ttMoves[0].move = MOVE_NONE;
117 phasePtr += int(!searchTT) - 1;
122 /// MovePicker::go_next_phase() generates, scores and sorts the next bunch
123 /// of moves when there are no more moves to try for the current phase.
125 void MovePicker::go_next_phase() {
128 phase = *(++phasePtr);
133 lastMove = curMove + 2;
136 case PH_GOOD_CAPTURES:
137 lastMove = generate<MV_CAPTURE>(pos, moves);
143 lastMove = curMove + 2;
147 lastMove = generate<MV_NON_CAPTURE>(pos, moves);
149 sort_moves(moves, lastMove, &lastGoodNonCapture);
152 case PH_BAD_CAPTURES:
153 // Bad captures SEE value is already calculated so just pick
154 // them in order to get SEE move ordering.
155 curMove = badCaptures;
156 lastMove = moves + MAX_MOVES;
160 assert(pos.is_check());
161 lastMove = generate<MV_EVASION>(pos, moves);
166 lastMove = generate<MV_CAPTURE>(pos, moves);
171 lastMove = generate<MV_NON_CAPTURE_CHECK>(pos, moves);
175 lastMove = curMove + 1; // Avoid another go_next_phase() call
185 /// MovePicker::score_captures(), MovePicker::score_noncaptures() and
186 /// MovePicker::score_evasions() assign a numerical move ordering score
187 /// to each move in a move list. The moves with highest scores will be
188 /// picked first by get_next_move().
190 void MovePicker::score_captures() {
191 // Winning and equal captures in the main search are ordered by MVV/LVA.
192 // Suprisingly, this appears to perform slightly better than SEE based
193 // move ordering. The reason is probably that in a position with a winning
194 // capture, capturing a more valuable (but sufficiently defended) piece
195 // first usually doesn't hurt. The opponent will have to recapture, and
196 // the hanging piece will still be hanging (except in the unusual cases
197 // where it is possible to recapture with the hanging piece). Exchanging
198 // big pieces before capturing a hanging piece probably helps to reduce
200 // In main search we want to push captures with negative SEE values to
201 // badCaptures[] array, but instead of doing it now we delay till when
202 // the move has been picked up in pick_move_from_list(), this way we save
203 // some SEE calls in case we get a cutoff (idea from Pablo Vazquez).
206 // Use MVV/LVA ordering
207 for (MoveStack* cur = moves; cur != lastMove; cur++)
210 if (move_is_promotion(m))
211 cur->score = QueenValueMidgame;
213 cur->score = pos.midgame_value_of_piece_on(move_to(m))
214 - pos.type_of_piece_on(move_from(m));
218 void MovePicker::score_noncaptures() {
223 for (MoveStack* cur = moves; cur != lastMove; cur++)
227 cur->score = H.value(pos.piece_on(from), move_to(m));
231 void MovePicker::score_evasions() {
232 // Try good captures ordered by MVV/LVA, then non-captures if
233 // destination square is not under attack, ordered by history
234 // value, and at the end bad-captures and non-captures with a
235 // negative SEE. This last group is ordered by the SEE score.
239 // Skip if we don't have at least two moves to order
240 if (lastMove < moves + 2)
243 for (MoveStack* cur = moves; cur != lastMove; cur++)
246 if ((seeScore = pos.see_sign(m)) < 0)
247 cur->score = seeScore - History::MaxValue; // Be sure we are at the bottom
248 else if (pos.move_is_capture(m))
249 cur->score = pos.midgame_value_of_piece_on(move_to(m))
250 - pos.type_of_piece_on(move_from(m)) + History::MaxValue;
252 cur->score = H.value(pos.piece_on(move_from(m)), move_to(m));
256 /// MovePicker::get_next_move() is the most important method of the MovePicker
257 /// class. It returns a new legal move every time it is called, until there
258 /// are no more moves left. It picks the move with the biggest score from a list
259 /// of generated moves taking care not to return the tt move if has already been
260 /// searched previously. Note that this function is not thread safe so should be
261 /// lock protected by caller when accessed through a shared MovePicker object.
263 Move MovePicker::get_next_move() {
269 while (curMove == lastMove)
275 move = (curMove++)->move;
276 if ( move != MOVE_NONE
277 && pos.move_is_legal(move, pinned))
281 case PH_GOOD_CAPTURES:
282 move = pick_best(curMove++, lastMove).move;
283 if ( move != ttMoves[0].move
284 && move != ttMoves[1].move
285 && pos.pl_move_is_legal(move, pinned))
287 // Check for a non negative SEE now
288 int seeValue = pos.see_sign(move);
289 if (seeValue >= badCaptureThreshold)
292 // Losing capture, move it to the tail of the array, note
293 // that move has now been already checked for legality.
294 (--badCaptures)->move = move;
295 badCaptures->score = seeValue;
300 move = (curMove++)->move;
301 if ( move != MOVE_NONE
302 && pos.move_is_legal(move, pinned)
303 && move != ttMoves[0].move
304 && move != ttMoves[1].move
305 && !pos.move_is_capture(move))
310 // Sort negative scored moves only when we get there
311 if (curMove == lastGoodNonCapture)
312 insertion_sort<MoveStack>(lastGoodNonCapture, lastMove);
314 move = (curMove++)->move;
315 if ( move != ttMoves[0].move
316 && move != ttMoves[1].move
317 && move != killers[0].move
318 && move != killers[1].move
319 && pos.pl_move_is_legal(move, pinned))
323 case PH_BAD_CAPTURES:
324 move = pick_best(curMove++, lastMove).move;
329 move = pick_best(curMove++, lastMove).move;
330 if ( move != ttMoves[0].move
331 && pos.pl_move_is_legal(move, pinned))
336 move = (curMove++)->move;
337 if ( move != ttMoves[0].move
338 && pos.pl_move_is_legal(move, pinned))