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stockfish/src/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) 2008-2015 Marco Costalba, Joona Kiiski, Tord Romstad
Copyright (C) 2015-2016 Marco Costalba, Joona Kiiski, Gary Linscott, Tord Romstad
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/>.
*/
#include <cassert>
#include "movepick.h"
#include "thread.h"
namespace {
enum Stages {
MAIN_SEARCH, GOOD_CAPTURES, KILLERS, QUIET, BAD_CAPTURES,
EVASION, ALL_EVASIONS,
QSEARCH_WITH_CHECKS, QCAPTURES_1, CHECKS,
QSEARCH_WITHOUT_CHECKS, QCAPTURES_2,
PROBCUT, PROBCUT_CAPTURES,
RECAPTURE, RECAPTURES,
STOP
};
// Our insertion sort, which is guaranteed to be stable, as it should be
void insertion_sort(ExtMove* begin, ExtMove* end)
{
ExtMove tmp, *p, *q;
for (p = begin + 1; p < end; ++p)
{
tmp = *p;
for (q = p; q != begin && *(q-1) < tmp; --q)
*q = *(q-1);
*q = tmp;
}
}
// pick_best() finds the best move in the range (begin, end) and moves it to
// the front. It's faster than sorting all the moves in advance when there
// are few moves, e.g., the possible captures.
Move pick_best(ExtMove* begin, ExtMove* end)
{
std::swap(*begin, *std::max_element(begin, end));
return *begin;
}
} // namespace
/// Constructors of the MovePicker class. As arguments we pass 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 how important good move
/// ordering is at the current node.
MovePicker::MovePicker(const Position& p, Move ttm, Depth d, Search::Stack* s)
: pos(p), ss(s), depth(d) {
assert(d > DEPTH_ZERO);
Square prevSq = to_sq((ss-1)->currentMove);
countermove = pos.this_thread()->counterMoves[pos.piece_on(prevSq)][prevSq];
stage = pos.checkers() ? EVASION : MAIN_SEARCH;
ttMove = ttm && pos.pseudo_legal(ttm) ? ttm : MOVE_NONE;
endMoves += (ttMove != MOVE_NONE);
}
MovePicker::MovePicker(const Position& p, Move ttm, Depth d, Square s)
: pos(p) {
assert(d <= DEPTH_ZERO);
if (pos.checkers())
stage = EVASION;
else if (d > DEPTH_QS_NO_CHECKS)
stage = QSEARCH_WITH_CHECKS;
else if (d > DEPTH_QS_RECAPTURES)
stage = QSEARCH_WITHOUT_CHECKS;
else
{
stage = RECAPTURE;
recaptureSquare = s;
ttm = MOVE_NONE;
}
ttMove = ttm && pos.pseudo_legal(ttm) ? ttm : MOVE_NONE;
endMoves += (ttMove != MOVE_NONE);
}
MovePicker::MovePicker(const Position& p, Move ttm, Value th)
: pos(p), threshold(th) {
assert(!pos.checkers());
stage = PROBCUT;
// In ProbCut we generate captures with SEE higher than the given threshold
ttMove = ttm
&& pos.pseudo_legal(ttm)
&& pos.capture(ttm)
&& pos.see(ttm) > threshold ? ttm : MOVE_NONE;
endMoves += (ttMove != MOVE_NONE);
}
/// score() assigns a numerical value to each move in a move list. The moves with
/// highest values will be picked first.
template<>
void MovePicker::score<CAPTURES>() {
// Winning and equal captures in the main search are ordered by MVV, preferring
// captures near our home rank. Surprisingly, this appears to perform slightly
// better than SEE-based move ordering: exchanging big pieces before capturing
// a hanging piece probably helps to reduce the subtree size.
// In the main search we want to push captures with negative SEE values to the
// badCaptures[] array, but instead of doing it now we delay until the move
// has been picked up, saving some SEE calls in case we get a cutoff.
for (auto& m : *this)
m.value = PieceValue[MG][pos.piece_on(to_sq(m))]
- Value(200 * relative_rank(pos.side_to_move(), to_sq(m)));
}
template<>
void MovePicker::score<QUIETS>() {
const HistoryStats& history = pos.this_thread()->history;
const CounterMoveStats* cm = (ss-1)->counterMoves;
const CounterMoveStats* fm = (ss-2)->counterMoves;
const CounterMoveStats* f2 = (ss-4)->counterMoves;
for (auto& m : *this)
m.value = history[pos.moved_piece(m)][to_sq(m)]
+ (cm ? (*cm)[pos.moved_piece(m)][to_sq(m)] : VALUE_ZERO)
+ (fm ? (*fm)[pos.moved_piece(m)][to_sq(m)] : VALUE_ZERO)
+ (f2 ? (*f2)[pos.moved_piece(m)][to_sq(m)] : VALUE_ZERO);
}
template<>
void MovePicker::score<EVASIONS>() {
// Try winning and equal captures ordered by MVV/LVA, then non-captures ordered
// by history value, then bad captures and quiet moves with a negative SEE ordered
// by SEE value.
const HistoryStats& history = pos.this_thread()->history;
Value see;
for (auto& m : *this)
if ((see = pos.see_sign(m)) < VALUE_ZERO)
m.value = see - HistoryStats::Max; // At the bottom
else if (pos.capture(m))
m.value = PieceValue[MG][pos.piece_on(to_sq(m))]
- Value(type_of(pos.moved_piece(m))) + HistoryStats::Max;
else
m.value = history[pos.moved_piece(m)][to_sq(m)];
}
/// generate_next_stage() generates, scores, and sorts the next bunch of moves
/// when there are no more moves to try for the current stage.
void MovePicker::generate_next_stage() {
assert(stage != STOP);
cur = moves;
switch (++stage) {
case GOOD_CAPTURES: case QCAPTURES_1: case QCAPTURES_2:
case PROBCUT_CAPTURES: case RECAPTURES:
endMoves = generate<CAPTURES>(pos, moves);
score<CAPTURES>();
break;
case KILLERS:
killers[0] = ss->killers[0];
killers[1] = ss->killers[1];
killers[2] = countermove;
cur = killers;
endMoves = cur + 2 + (countermove != killers[0] && countermove != killers[1]);
break;
case QUIET:
endMoves = generate<QUIETS>(pos, moves);
score<QUIETS>();
if (depth < 3 * ONE_PLY)
{
ExtMove* goodQuiet = std::partition(cur, endMoves, [](const ExtMove& m)
{ return m.value > VALUE_ZERO; });
insertion_sort(cur, goodQuiet);
} else
insertion_sort(cur, endMoves);
break;
case BAD_CAPTURES:
// Just pick them in reverse order to get correct ordering
cur = moves + MAX_MOVES - 1;
endMoves = endBadCaptures;
break;
case ALL_EVASIONS:
endMoves = generate<EVASIONS>(pos, moves);
if (endMoves - moves > 1)
score<EVASIONS>();
break;
case CHECKS:
endMoves = generate<QUIET_CHECKS>(pos, moves);
break;
case EVASION: case QSEARCH_WITH_CHECKS: case QSEARCH_WITHOUT_CHECKS:
case PROBCUT: case RECAPTURE: case STOP:
stage = STOP;
break;
default:
assert(false);
}
}
/// next_move() is the most important method of the MovePicker class. It returns
/// a new pseudo legal move every time it is called, until there are no more moves
/// left. It picks the move with the biggest value from a list of generated moves
/// taking care not to return the ttMove if it has already been searched.
Move MovePicker::next_move() {
Move move;
while (true)
{
while (cur == endMoves && stage != STOP)
generate_next_stage();
switch (stage) {
case MAIN_SEARCH: case EVASION: case QSEARCH_WITH_CHECKS:
case QSEARCH_WITHOUT_CHECKS: case PROBCUT:
++cur;
return ttMove;
case GOOD_CAPTURES:
move = pick_best(cur++, endMoves);
if (move != ttMove)
{
if (pos.see_sign(move) >= VALUE_ZERO)
return move;
// Losing capture, move it to the tail of the array
*endBadCaptures-- = move;
}
break;
case KILLERS:
move = *cur++;
if ( move != MOVE_NONE
&& move != ttMove
&& pos.pseudo_legal(move)
&& !pos.capture(move))
return move;
break;
case QUIET:
move = *cur++;
if ( move != ttMove
&& move != killers[0]
&& move != killers[1]
&& move != killers[2])
return move;
break;
case BAD_CAPTURES:
return *cur--;
case ALL_EVASIONS: case QCAPTURES_1: case QCAPTURES_2:
move = pick_best(cur++, endMoves);
if (move != ttMove)
return move;
break;
case PROBCUT_CAPTURES:
move = pick_best(cur++, endMoves);
if (move != ttMove && pos.see(move) > threshold)
return move;
break;
case RECAPTURES:
move = pick_best(cur++, endMoves);
if (to_sq(move) == recaptureSquare)
return move;
break;
case CHECKS:
move = *cur++;
if (move != ttMove)
return move;
break;
case STOP:
return MOVE_NONE;
default:
assert(false);
}
}
}
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