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_MOVE, // Transposition table move
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_MOVE, PH_GOOD_CAPTURES, PH_KILLERS, PH_NONCAPTURES, PH_BAD_CAPTURES, PH_STOP };
44 const uint8_t EvasionTable[] = { PH_TT_MOVE, PH_EVASIONS, PH_STOP };
45 const uint8_t QsearchWithChecksTable[] = { PH_TT_MOVE, PH_QCAPTURES, PH_QCHECKS, PH_STOP };
46 const uint8_t QsearchWithoutChecksTable[] = { PH_TT_MOVE, PH_QCAPTURES, PH_STOP };
49 bool MovePicker::isBadCapture() const { return phase == PH_BAD_CAPTURES; }
51 /// Constructor for the MovePicker class. As arguments we pass 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) {
59 badCaptureThreshold = 0;
60 badCaptures = moves + MAX_MOVES;
62 assert(d > DEPTH_ZERO);
66 killers[0].move = killers[1].move = MOVE_NONE;
67 phasePtr = EvasionTable;
71 killers[0].move = ss->killers[0];
72 killers[1].move = ss->killers[1];
74 // Consider sligtly negative captures as good if at low
75 // depth and far from beta.
76 if (ss && ss->eval < beta - PawnValueMidgame && d < 3 * ONE_PLY)
77 badCaptureThreshold = -PawnValueMidgame;
79 phasePtr = MainSearchTable;
82 ttMove = (ttm && pos.move_is_pl(ttm) ? ttm : MOVE_NONE);
83 phasePtr += int(ttMove == MOVE_NONE) - 1;
87 MovePicker::MovePicker(const Position& p, Move ttm, Depth d, const History& h)
90 assert(d <= DEPTH_ZERO);
93 phasePtr = EvasionTable;
94 else if (d >= DEPTH_QS_CHECKS)
95 phasePtr = QsearchWithChecksTable;
98 phasePtr = QsearchWithoutChecksTable;
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(ttm) && !move_is_promotion(ttm))
107 ttMove = (ttm && pos.move_is_pl(ttm) ? ttm : MOVE_NONE);
108 phasePtr += int(ttMove == MOVE_NONE) - 1;
113 /// MovePicker::go_next_phase() generates, scores and sorts the next bunch
114 /// of moves when there are no more moves to try for the current phase.
116 void MovePicker::go_next_phase() {
119 phase = *(++phasePtr);
123 lastMove = curMove + 1;
126 case PH_GOOD_CAPTURES:
127 lastMove = generate<MV_CAPTURE>(pos, moves);
133 lastMove = curMove + 2;
137 lastMove = generate<MV_NON_CAPTURE>(pos, moves);
139 sort_moves(moves, lastMove, &lastGoodNonCapture);
142 case PH_BAD_CAPTURES:
143 // Bad captures SEE value is already calculated so just pick
144 // them in order to get SEE move ordering.
145 curMove = badCaptures;
146 lastMove = moves + MAX_MOVES;
150 assert(pos.in_check());
151 lastMove = generate<MV_EVASION>(pos, moves);
156 lastMove = generate<MV_CAPTURE>(pos, moves);
161 lastMove = generate<MV_NON_CAPTURE_CHECK>(pos, moves);
165 lastMove = curMove + 1; // Avoid another go_next_phase() call
175 /// MovePicker::score_captures(), MovePicker::score_noncaptures() and
176 /// MovePicker::score_evasions() assign a numerical move ordering score
177 /// to each move in a move list. The moves with highest scores will be
178 /// picked first by get_next_move().
180 void MovePicker::score_captures() {
181 // Winning and equal captures in the main search are ordered by MVV/LVA.
182 // Suprisingly, this appears to perform slightly better than SEE based
183 // move ordering. The reason is probably that in a position with a winning
184 // capture, capturing a more valuable (but sufficiently defended) piece
185 // first usually doesn't hurt. The opponent will have to recapture, and
186 // the hanging piece will still be hanging (except in the unusual cases
187 // where it is possible to recapture with the hanging piece). Exchanging
188 // big pieces before capturing a hanging piece probably helps to reduce
190 // In main search we want to push captures with negative SEE values to
191 // badCaptures[] array, but instead of doing it now we delay till when
192 // the move has been picked up in pick_move_from_list(), this way we save
193 // some SEE calls in case we get a cutoff (idea from Pablo Vazquez).
196 // Use MVV/LVA ordering
197 for (MoveStack* cur = moves; cur != lastMove; cur++)
200 if (move_is_promotion(m))
201 cur->score = QueenValueMidgame;
203 cur->score = pos.midgame_value_of_piece_on(move_to(m))
204 - pos.type_of_piece_on(move_from(m));
208 void MovePicker::score_noncaptures() {
213 for (MoveStack* cur = moves; cur != lastMove; cur++)
217 cur->score = H.value(pos.piece_on(from), move_to(m));
221 void MovePicker::score_evasions() {
222 // Try good captures ordered by MVV/LVA, then non-captures if
223 // destination square is not under attack, ordered by history
224 // value, and at the end bad-captures and non-captures with a
225 // negative SEE. This last group is ordered by the SEE score.
229 // Skip if we don't have at least two moves to order
230 if (lastMove < moves + 2)
233 for (MoveStack* cur = moves; cur != lastMove; cur++)
236 if ((seeScore = pos.see_sign(m)) < 0)
237 cur->score = seeScore - History::MaxValue; // Be sure we are at the bottom
238 else if (pos.move_is_capture(m))
239 cur->score = pos.midgame_value_of_piece_on(move_to(m))
240 - pos.type_of_piece_on(move_from(m)) + History::MaxValue;
242 cur->score = H.value(pos.piece_on(move_from(m)), move_to(m));
246 /// MovePicker::get_next_move() is the most important method of the MovePicker
247 /// class. It returns a new pseudo legal move every time it is called, until there
248 /// are no more moves left. It picks the move with the biggest score from a list
249 /// of generated moves taking care not to return the tt move if has already been
250 /// searched previously. Note that this function is not thread safe so should be
251 /// lock protected by caller when accessed through a shared MovePicker object.
253 Move MovePicker::get_next_move() {
259 while (curMove == lastMove)
269 case PH_GOOD_CAPTURES:
270 move = pick_best(curMove++, lastMove).move;
273 // Check for a non negative SEE now
274 int seeValue = pos.see_sign(move);
275 if (seeValue >= badCaptureThreshold)
278 // Losing capture, move it to the tail of the array, note
279 // that move has now been already checked for pseudo legality.
280 (--badCaptures)->move = move;
281 badCaptures->score = seeValue;
286 move = (curMove++)->move;
287 if ( move != MOVE_NONE
288 && pos.move_is_pl(move)
290 && !pos.move_is_capture(move))
295 // Sort negative scored moves only when we get there
296 if (curMove == lastGoodNonCapture)
297 insertion_sort<MoveStack>(lastGoodNonCapture, lastMove);
299 move = (curMove++)->move;
301 && move != killers[0].move
302 && move != killers[1].move)
306 case PH_BAD_CAPTURES:
307 move = pick_best(curMove++, lastMove).move;
312 move = pick_best(curMove++, lastMove).move;
318 move = (curMove++)->move;