/*
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
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 .
*/
#include
#include "movepick.h"
#include "thread.h"
namespace {
enum Stages {
MAIN_SEARCH, CAPTURES_S1, KILLERS_S1, QUIETS_1_S1, QUIETS_2_S1, BAD_CAPTURES_S1,
EVASION, EVASIONS_S2,
QSEARCH_0, CAPTURES_S3, QUIET_CHECKS_S3,
QSEARCH_1, CAPTURES_S4,
PROBCUT, CAPTURES_S5,
RECAPTURE, CAPTURES_S6,
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.
inline 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, const HistoryStats& h,
Move* cm, Move* fm, Search::Stack* s) : pos(p), history(h), depth(d) {
assert(d > DEPTH_ZERO);
endBadCaptures = moves + MAX_MOVES - 1;
countermoves = cm;
followupmoves = fm;
ss = s;
if (pos.checkers())
stage = EVASION;
else
stage = MAIN_SEARCH;
ttMove = (ttm && pos.pseudo_legal(ttm) ? ttm : MOVE_NONE);
endMoves += (ttMove != MOVE_NONE);
}
MovePicker::MovePicker(const Position& p, Move ttm, Depth d, const HistoryStats& h,
Square s) : pos(p), history(h) {
assert(d <= DEPTH_ZERO);
if (pos.checkers())
stage = EVASION;
else if (d > DEPTH_QS_NO_CHECKS)
stage = QSEARCH_0;
else if (d > DEPTH_QS_RECAPTURES)
stage = QSEARCH_1;
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, const HistoryStats& h, PieceType pt)
: pos(p), history(h) {
assert(!pos.checkers());
stage = PROBCUT;
// In ProbCut we generate only captures that are better than the parent's
// captured piece.
captureThreshold = PieceValue[MG][pt];
ttMove = (ttm && pos.pseudo_legal(ttm) ? ttm : MOVE_NONE);
if (ttMove && (!pos.capture(ttMove) || pos.see(ttMove) <= captureThreshold))
ttMove = MOVE_NONE;
endMoves += (ttMove != MOVE_NONE);
}
/// score() assign a numerical value to each move in a move list. The moves with
/// highest values will be picked first.
template<>
void MovePicker::score() {
// 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 the
// badCaptures[] array, but instead of doing it now we delay until the move
// has been picked up in pick_move_from_list(). This way we save some SEE
// calls in case we get a cutoff.
for (auto& m : *this)
if (type_of(m) == ENPASSANT)
m.value = PieceValue[MG][PAWN] - Value(PAWN);
else if (type_of(m) == PROMOTION)
m.value = PieceValue[MG][pos.piece_on(to_sq(m))] - Value(PAWN)
+ PieceValue[MG][promotion_type(m)] - PieceValue[MG][PAWN];
else
m.value = PieceValue[MG][pos.piece_on(to_sq(m))]
- Value(type_of(pos.moved_piece(m)));
}
template<>
void MovePicker::score() {
for (auto& m : *this)
m.value = history[pos.moved_piece(m)][to_sq(m)];
}
template<>
void MovePicker::score() {
// Try good captures ordered by MVV/LVA, then non-captures if destination square
// is not under attack, ordered by history value, then bad-captures and quiet
// moves with a negative SEE. This last group is ordered by the SEE value.
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() {
cur = moves;
switch (++stage) {
case CAPTURES_S1: case CAPTURES_S3: case CAPTURES_S4: case CAPTURES_S5: case CAPTURES_S6:
endMoves = generate(pos, moves);
score();
break;
case KILLERS_S1:
cur = killers;
endMoves = cur + 6;
killers[0] = ss->killers[0];
killers[1] = ss->killers[1];
killers[2] = countermoves[0];
killers[3] = countermoves[1];
killers[4] = followupmoves[0];
killers[5] = followupmoves[1];
break;
case QUIETS_1_S1:
endQuiets = endMoves = generate(pos, moves);
score();
endMoves = std::partition(cur, endMoves, [](const ExtMove& m) { return m.value > VALUE_ZERO; });
insertion_sort(cur, endMoves);
break;
case QUIETS_2_S1:
cur = endMoves;
endMoves = endQuiets;
if (depth >= 3 * ONE_PLY)
insertion_sort(cur, endMoves);
break;
case BAD_CAPTURES_S1:
// Just pick them in reverse order to get MVV/LVA ordering
cur = moves + MAX_MOVES - 1;
endMoves = endBadCaptures;
break;
case EVASIONS_S2:
endMoves = generate(pos, moves);
if (endMoves - moves > 1)
score();
break;
case QUIET_CHECKS_S3:
endMoves = generate(pos, moves);
break;
case EVASION: case QSEARCH_0: case QSEARCH_1: case PROBCUT: case RECAPTURE:
stage = STOP;
/* Fall through */
case STOP:
endMoves = cur + 1; // Avoid another generate_next_stage() call
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.
template<>
Move MovePicker::next_move() {
Move move;
while (true)
{
while (cur == endMoves)
generate_next_stage();
switch (stage) {
case MAIN_SEARCH: case EVASION: case QSEARCH_0: case QSEARCH_1: case PROBCUT:
++cur;
return ttMove;
case CAPTURES_S1:
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_S1:
move = *cur++;
if ( move != MOVE_NONE
&& move != ttMove
&& pos.pseudo_legal(move)
&& !pos.capture(move))
{
for (int i = 0; i < cur - 1 - killers; i++) // Skip duplicated
if (move == killers[i])
goto skip;
return move;
}
skip:
break;
case QUIETS_1_S1: case QUIETS_2_S1:
move = *cur++;
if ( move != ttMove
&& move != killers[0]
&& move != killers[1]
&& move != killers[2]
&& move != killers[3]
&& move != killers[4]
&& move != killers[5])
return move;
break;
case BAD_CAPTURES_S1:
return *cur--;
case EVASIONS_S2: case CAPTURES_S3: case CAPTURES_S4:
move = pick_best(cur++, endMoves);
if (move != ttMove)
return move;
break;
case CAPTURES_S5:
move = pick_best(cur++, endMoves);
if (move != ttMove && pos.see(move) > captureThreshold)
return move;
break;
case CAPTURES_S6:
move = pick_best(cur++, endMoves);
if (to_sq(move) == recaptureSquare)
return move;
break;
case QUIET_CHECKS_S3:
move = *cur++;
if (move != ttMove)
return move;
break;
case STOP:
return MOVE_NONE;
default:
assert(false);
}
}
}
/// Version of next_move() to use at split point nodes where the move is grabbed
/// from the split point's shared MovePicker object. This function is not thread
/// safe so must be lock protected by the caller.
template<>
Move MovePicker::next_move() { return ss->splitPoint->movePicker->next_move(); }