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movepick.cpp
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movepick.cpp
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/*
Stockfish, a UCI chess playing engine derived from Glaurung 2.1
Copyright (C) 2004-2008 Tord Romstad (Glaurung author)
Copyright (C) 2008-2009 Marco Costalba
Stockfish is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.
Stockfish is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program. If not, see <http://www.gnu.org/licenses/>.
*/
////
//// Includes
////
#include <cassert>
#include "history.h"
#include "movegen.h"
#include "movepick.h"
#include "search.h"
#include "value.h"
////
//// Local definitions
////
namespace {
enum MovegenPhase {
PH_TT_MOVES, // Transposition table move and mate killer
PH_GOOD_CAPTURES, // Queen promotions and captures with SEE values >= 0
PH_KILLERS, // Killer moves from the current ply
PH_NONCAPTURES, // Non-captures and underpromotions
PH_BAD_CAPTURES, // Queen promotions and captures with SEE values < 0
PH_EVASIONS, // Check evasions
PH_QCAPTURES, // Captures in quiescence search
PH_QCHECKS, // Non-capture checks in quiescence search
PH_STOP
};
CACHE_LINE_ALIGNMENT
const uint8_t MainSearchPhaseTable[] = { PH_TT_MOVES, PH_GOOD_CAPTURES, PH_KILLERS, PH_NONCAPTURES, PH_BAD_CAPTURES, PH_STOP};
const uint8_t EvasionsPhaseTable[] = { PH_TT_MOVES, PH_EVASIONS, PH_STOP};
const uint8_t QsearchWithChecksPhaseTable[] = { PH_TT_MOVES, PH_QCAPTURES, PH_QCHECKS, PH_STOP};
const uint8_t QsearchWithoutChecksPhaseTable[] = { PH_TT_MOVES, PH_QCAPTURES, PH_STOP};
}
////
//// Functions
////
/// Constructor for the MovePicker class. Apart from the position for which
/// it is asked to pick legal moves, MovePicker also wants some information
/// to help it to return the presumably good moves first, to decide which
/// moves to return (in the quiescence search, for instance, we only want to
/// search captures, promotions and some checks) and about how important good
/// move ordering is at the current node.
MovePicker::MovePicker(const Position& p, Move ttm, Depth d,
const History& h, SearchStack* ss) : pos(p), H(h) {
int searchTT = ttm;
ttMoves[0].move = ttm;
lastBadCapture = badCaptures;
pinned = p.pinned_pieces(pos.side_to_move());
if (ss && !p.is_check())
{
ttMoves[1].move = (ss->mateKiller == ttm)? MOVE_NONE : ss->mateKiller;
searchTT |= ttMoves[1].move;
killers[0].move = ss->killers[0];
killers[1].move = ss->killers[1];
} else
ttMoves[1].move = killers[0].move = killers[1].move = MOVE_NONE;
if (p.is_check())
phasePtr = EvasionsPhaseTable;
else if (d > Depth(0))
phasePtr = MainSearchPhaseTable;
else if (d == Depth(0))
phasePtr = QsearchWithChecksPhaseTable;
else
phasePtr = QsearchWithoutChecksPhaseTable;
phasePtr += !searchTT - 1;
go_next_phase();
}
/// MovePicker::go_next_phase() generates, scores and sorts the next bunch
/// of moves when there are no more moves to try for the current phase.
void MovePicker::go_next_phase() {
curMove = moves;
phase = *(++phasePtr);
switch (phase) {
case PH_TT_MOVES:
curMove = ttMoves;
lastMove = curMove + 2;
return;
case PH_GOOD_CAPTURES:
lastMove = generate_captures(pos, moves);
score_captures();
return;
case PH_KILLERS:
curMove = killers;
lastMove = curMove + 2;
return;
case PH_NONCAPTURES:
lastMove = generate_noncaptures(pos, moves);
score_noncaptures();
sort_moves(moves, lastMove);
return;
case PH_BAD_CAPTURES:
// Bad captures SEE value is already calculated so just sort them
// to get SEE move ordering.
curMove = badCaptures;
lastMove = lastBadCapture;
return;
case PH_EVASIONS:
assert(pos.is_check());
lastMove = generate_evasions(pos, moves);
score_evasions();
return;
case PH_QCAPTURES:
lastMove = generate_captures(pos, moves);
score_captures();
return;
case PH_QCHECKS:
// Perhaps we should order moves move here? FIXME
lastMove = generate_non_capture_checks(pos, moves);
return;
case PH_STOP:
lastMove = curMove + 1; // Avoids another go_next_phase() call
return;
default:
assert(false);
return;
}
}
/// MovePicker::score_captures(), MovePicker::score_noncaptures() and
/// MovePicker::score_evasions() assign a numerical move ordering score
/// to each move in a move list. The moves with highest scores will be
/// picked first by get_next_move().
void MovePicker::score_captures() {
// Winning and equal captures in the main search are ordered by MVV/LVA.
// Suprisingly, this appears to perform slightly better than SEE based
// move ordering. The reason is probably that in a position with a winning
// capture, capturing a more valuable (but sufficiently defended) piece
// first usually doesn't hurt. The opponent will have to recapture, and
// the hanging piece will still be hanging (except in the unusual cases
// where it is possible to recapture with the hanging piece). Exchanging
// big pieces before capturing a hanging piece probably helps to reduce
// the subtree size.
// In main search we want to push captures with negative SEE values to
// badCaptures[] array, but instead of doing it now we delay till when
// the move has been picked up in pick_move_from_list(), this way we save
// some SEE calls in case we get a cutoff (idea from Pablo Vazquez).
Move m;
// Use MVV/LVA ordering
for (MoveStack* cur = moves; cur != lastMove; cur++)
{
m = cur->move;
if (move_is_promotion(m))
cur->score = QueenValueMidgame;
else
cur->score = pos.midgame_value_of_piece_on(move_to(m))
- pos.type_of_piece_on(move_from(m));
}
}
void MovePicker::score_noncaptures() {
// First score by history, when no history is available then use
// piece/square tables values. This seems to be better then a
// random choice when we don't have an history for any move.
Move m;
Piece piece;
Square from, to;
int hs;
for (MoveStack* cur = moves; cur != lastMove; cur++)
{
m = cur->move;
from = move_from(m);
to = move_to(m);
piece = pos.piece_on(from);
hs = H.move_ordering_score(piece, to);
// Ensure history is always preferred to pst
if (hs > 0)
hs += 1000;
// pst based scoring
cur->score = hs + mg_value(pos.pst_delta(piece, from, to));
}
}
void MovePicker::score_evasions() {
// Try good captures ordered by MVV/LVA, then non-captures if
// destination square is not under attack, ordered by history
// value, and at the end bad-captures and non-captures with a
// negative SEE. This last group is ordered by the SEE score.
Move m;
int seeScore;
for (MoveStack* cur = moves; cur != lastMove; cur++)
{
m = cur->move;
if ((seeScore = pos.see_sign(m)) < 0)
cur->score = seeScore;
else if (pos.move_is_capture(m))
cur->score = pos.midgame_value_of_piece_on(move_to(m))
- pos.type_of_piece_on(move_from(m)) + HistoryMax;
else
cur->score = H.move_ordering_score(pos.piece_on(move_from(m)), move_to(m));
}
}
/// MovePicker::get_next_move() is the most important method of the MovePicker
/// class. It returns a new legal move every time it is called, until there
/// are no more moves left.
/// It picks the move with the biggest score from a list of generated moves taking
/// care not to return the tt move if has already been searched previously.
Move MovePicker::get_next_move() {
Move move;
while (true)
{
while (curMove != lastMove)
{
switch (phase) {
case PH_TT_MOVES:
move = (curMove++)->move;
if ( move != MOVE_NONE
&& move_is_legal(pos, move, pinned))
return move;
break;
case PH_GOOD_CAPTURES:
move = pick_best(curMove++, lastMove).move;
if ( move != ttMoves[0].move
&& move != ttMoves[1].move
&& pos.pl_move_is_legal(move, pinned))
{
// Check for a non negative SEE now
int seeValue = pos.see_sign(move);
if (seeValue >= 0)
return move;
// Losing capture, move it to the badCaptures[] array, note
// that move has now been already checked for legality.
assert(int(lastBadCapture - badCaptures) < 63);
lastBadCapture->move = move;
lastBadCapture->score = seeValue;
lastBadCapture++;
}
break;
case PH_KILLERS:
move = (curMove++)->move;
if ( move != MOVE_NONE
&& move != ttMoves[0].move
&& move != ttMoves[1].move
&& move_is_legal(pos, move, pinned)
&& !pos.move_is_capture(move))
return move;
break;
case PH_NONCAPTURES:
move = (curMove++)->move;
if ( move != ttMoves[0].move
&& move != ttMoves[1].move
&& move != killers[0].move
&& move != killers[1].move
&& pos.pl_move_is_legal(move, pinned))
return move;
break;
case PH_BAD_CAPTURES:
move = pick_best(curMove++, lastMove).move;
return move;
case PH_EVASIONS:
case PH_QCAPTURES:
move = pick_best(curMove++, lastMove).move;
if ( move != ttMoves[0].move
&& pos.pl_move_is_legal(move, pinned))
return move;
break;
case PH_QCHECKS:
move = (curMove++)->move;
if ( move != ttMoves[0].move
&& pos.pl_move_is_legal(move, pinned))
return move;
break;
case PH_STOP:
return MOVE_NONE;
default:
assert(false);
break;
}
}
go_next_phase();
}
}
/// A variant of get_next_move() which takes a lock as a parameter, used to
/// prevent multiple threads from picking the same move at a split point.
Move MovePicker::get_next_move(Lock &lock) {
lock_grab(&lock);
// Note that it is safe to call many times
// get_next_move() when phase == PH_STOP
Move m = get_next_move();
lock_release(&lock);
return m;
}