/*
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 .
*/
////
//// Includes
////
#include
#include
#include "history.h"
#include "evaluate.h"
#include "movegen.h"
#include "movepick.h"
#include "search.h"
#include "value.h"
////
//// Local definitions
////
namespace {
/// Variables
MovePicker::MovegenPhase PhaseTable[32];
int MainSearchPhaseIndex;
int EvasionsPhaseIndex;
int QsearchWithChecksPhaseIndex;
int QsearchWithoutChecksPhaseIndex;
}
////
//// 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) {
ttMove = ttm;
if (ss)
{
mateKiller = (ss->mateKiller == ttm)? MOVE_NONE : ss->mateKiller;
killer1 = ss->killers[0];
killer2 = ss->killers[1];
} else
mateKiller = killer1 = killer2 = MOVE_NONE;
movesPicked = numOfMoves = numOfBadCaptures = 0;
checkKillers = checkLegal = finished = false;
if (p.is_check())
phaseIndex = EvasionsPhaseIndex;
else if (d > Depth(0))
phaseIndex = MainSearchPhaseIndex;
else if (d == Depth(0))
phaseIndex = QsearchWithChecksPhaseIndex;
else
phaseIndex = QsearchWithoutChecksPhaseIndex;
Color us = pos.side_to_move();
dc = p.discovered_check_candidates(us);
pinned = p.pinned_pieces(us);
finished = false;
}
/// 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 of the types we are interested in.
Move MovePicker::get_next_move() {
Move move;
while (true)
{
// If we already have a list of generated moves, pick the best move from
// the list, and return it.
move = pick_move_from_list();
if (move != MOVE_NONE)
{
assert(move_is_ok(move));
return move;
}
// Next phase
phaseIndex++;
switch (PhaseTable[phaseIndex]) {
case PH_TT_MOVE:
if (ttMove != MOVE_NONE)
{
assert(move_is_ok(ttMove));
if (move_is_legal(pos, ttMove, pinned))
return ttMove;
}
break;
case PH_MATE_KILLER:
if (mateKiller != MOVE_NONE)
{
assert(move_is_ok(mateKiller));
if (move_is_legal(pos, mateKiller, pinned))
return mateKiller;
}
break;
case PH_GOOD_CAPTURES:
numOfMoves = generate_captures(pos, moves);
score_captures();
std::sort(moves, moves + numOfMoves);
movesPicked = 0;
break;
case PH_KILLERS:
movesPicked = numOfMoves = 0;
if (killer1 != MOVE_NONE && move_is_legal(pos, killer1, pinned) && !pos.move_is_capture(killer1))
moves[numOfMoves++].move = killer1;
if (killer2 != MOVE_NONE && move_is_legal(pos, killer2, pinned) && !pos.move_is_capture(killer2))
moves[numOfMoves++].move = killer2;
break;
case PH_NONCAPTURES:
checkKillers = (numOfMoves != 0); // previous phase is PH_KILLERS
checkLegal = true;
numOfMoves = generate_noncaptures(pos, moves);
score_noncaptures();
std::sort(moves, moves + numOfMoves);
movesPicked = 0;
break;
case PH_BAD_CAPTURES:
// Bad captures SEE value is already calculated so just sort them
// to get SEE move ordering.
std::sort(badCaptures, badCaptures + numOfBadCaptures);
movesPicked = 0;
break;
case PH_EVASIONS:
assert(pos.is_check());
numOfMoves = generate_evasions(pos, moves, pinned);
score_evasions();
std::sort(moves, moves + numOfMoves);
movesPicked = 0;
break;
case PH_QCAPTURES:
numOfMoves = generate_captures(pos, moves);
score_captures();
std::sort(moves, moves + numOfMoves);
movesPicked = 0;
break;
case PH_QCHECKS:
// Perhaps we should order moves move here? FIXME
numOfMoves = generate_non_capture_checks(pos, moves, dc);
movesPicked = 0;
break;
case PH_STOP:
return MOVE_NONE;
default:
assert(false);
return MOVE_NONE;
}
}
}
/// 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);
if (finished)
{
lock_release(&lock);
return MOVE_NONE;
}
Move m = get_next_move();
if (m == MOVE_NONE)
finished = true;
lock_release(&lock);
return m;
}
/// MovePicker::score_captures(), MovePicker::score_noncaptures(),
/// MovePicker::score_evasions() and MovePicker::score_qcaptures() assign a
/// numerical move ordering score to each move in a move list. The moves
/// with highest scores will be picked first by pick_move_from_list().
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 (int i = 0; i < numOfMoves; i++)
{
m = moves[i].move;
if (move_is_promotion(m))
moves[i].score = QueenValueMidgame;
else
moves[i].score = int(pos.midgame_value_of_piece_on(move_to(m)))
-int(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.
Piece piece;
Square from, to;
int hs;
for (int i = 0; i < numOfMoves; i++)
{
from = move_from(moves[i].move);
to = move_to(moves[i].move);
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
moves[i].score = hs + pos.pst_delta(piece, from, to);
}
}
void MovePicker::score_evasions() {
for (int i = 0; i < numOfMoves; i++)
{
Move m = moves[i].move;
if (m == ttMove)
moves[i].score = 2*HistoryMax;
else if (!pos.square_is_empty(move_to(m)))
{
int seeScore = pos.see(m);
moves[i].score = (seeScore >= 0)? seeScore + HistoryMax : seeScore;
} else
moves[i].score = H.move_ordering_score(pos.piece_on(move_from(m)), move_to(m));
}
}
/// MovePicker::pick_move_from_list() picks the move with the biggest score
/// from a list of generated moves (moves[] or badCaptures[], depending on
/// the current move generation phase). It takes care not to return the
/// transposition table move if that has already been serched previously.
Move MovePicker::pick_move_from_list() {
assert(movesPicked >= 0);
assert(!pos.is_check() || PhaseTable[phaseIndex] == PH_EVASIONS || PhaseTable[phaseIndex] == PH_STOP);
assert( pos.is_check() || PhaseTable[phaseIndex] != PH_EVASIONS);
switch (PhaseTable[phaseIndex]) {
case PH_GOOD_CAPTURES:
while (movesPicked < numOfMoves)
{
Move move = moves[movesPicked++].move;
if ( move != ttMove
&& move != mateKiller
&& 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(numOfBadCaptures < 63);
badCaptures[numOfBadCaptures].move = move;
badCaptures[numOfBadCaptures++].score = seeValue;
}
}
break;
case PH_KILLERS:
case PH_NONCAPTURES:
while (movesPicked < numOfMoves)
{
Move move = moves[movesPicked++].move;
if ( move != ttMove
&& move != mateKiller
&& (!checkKillers || (move != killer1 && move != killer2))
&& (!checkLegal || pos.pl_move_is_legal(move, pinned)))
return move;
}
break;
case PH_EVASIONS:
if (movesPicked < numOfMoves)
return moves[movesPicked++].move;
break;
case PH_BAD_CAPTURES:
if (movesPicked < numOfBadCaptures)
return badCaptures[movesPicked++].move;
break;
case PH_QCAPTURES:
case PH_QCHECKS:
while (movesPicked < numOfMoves)
{
Move move = moves[movesPicked++].move;
// Maybe postpone the legality check until after futility pruning?
if ( move != ttMove
&& pos.pl_move_is_legal(move, pinned))
return move;
}
break;
default:
break;
}
return MOVE_NONE;
}
/// MovePicker::init_phase_table() initializes the PhaseTable[],
/// MainSearchPhaseIndex, EvasionPhaseIndex, QsearchWithChecksPhaseIndex
/// and QsearchWithoutChecksPhaseIndex. It is only called once during
/// program startup, and never again while the program is running.
void MovePicker::init_phase_table() {
int i = 0;
// Main search
MainSearchPhaseIndex = i - 1;
PhaseTable[i++] = PH_TT_MOVE;
PhaseTable[i++] = PH_MATE_KILLER;
PhaseTable[i++] = PH_GOOD_CAPTURES;
PhaseTable[i++] = PH_KILLERS;
PhaseTable[i++] = PH_NONCAPTURES;
PhaseTable[i++] = PH_BAD_CAPTURES;
PhaseTable[i++] = PH_STOP;
// Check evasions
EvasionsPhaseIndex = i - 1;
PhaseTable[i++] = PH_EVASIONS;
PhaseTable[i++] = PH_STOP;
// Quiescence search with checks
QsearchWithChecksPhaseIndex = i - 1;
PhaseTable[i++] = PH_TT_MOVE;
PhaseTable[i++] = PH_QCAPTURES;
PhaseTable[i++] = PH_QCHECKS;
PhaseTable[i++] = PH_STOP;
// Quiescence search without checks
QsearchWithoutChecksPhaseIndex = i - 1;
PhaseTable[i++] = PH_TT_MOVE;
PhaseTable[i++] = PH_QCAPTURES;
PhaseTable[i++] = PH_STOP;
}