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pawns.cpp
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pawns.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) 2015-2019 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 <algorithm>
#include <cassert>
#include "bitboard.h"
#include "pawns.h"
#include "position.h"
#include "thread.h"
namespace {
#define V Value
#define S(mg, eg) make_score(mg, eg)
// Pawn penalties
constexpr Score Backward = S( 9, 24);
constexpr Score BlockedStorm = S(82, 82);
constexpr Score Doubled = S(11, 56);
constexpr Score Isolated = S( 5, 15);
constexpr Score WeakLever = S( 0, 56);
constexpr Score WeakUnopposed = S(13, 27);
// Connected pawn bonus
constexpr int Connected[RANK_NB] = { 0, 7, 8, 12, 29, 48, 86 };
// Strength of pawn shelter for our king by [distance from edge][rank].
// RANK_1 = 0 is used for files where we have no pawn, or pawn is behind our king.
constexpr Value ShelterStrength[int(FILE_NB) / 2][RANK_NB] = {
{ V( -6), V( 81), V( 93), V( 58), V( 39), V( 18), V( 25) },
{ V(-43), V( 61), V( 35), V(-49), V(-29), V(-11), V( -63) },
{ V(-10), V( 75), V( 23), V( -2), V( 32), V( 3), V( -45) },
{ V(-39), V(-13), V(-29), V(-52), V(-48), V(-67), V(-166) }
};
// Danger of enemy pawns moving toward our king by [distance from edge][rank].
// RANK_1 = 0 is used for files where the enemy has no pawn, or their pawn
// is behind our king. Note that UnblockedStorm[0][1-2] accommodate opponent pawn
// on edge, likely blocked by our king.
constexpr Value UnblockedStorm[int(FILE_NB) / 2][RANK_NB] = {
{ V( 89), V(-285), V(-185), V(93), V(57), V( 45), V( 51) },
{ V( 44), V( -18), V( 123), V(46), V(39), V( -7), V( 23) },
{ V( 4), V( 52), V( 162), V(37), V( 7), V(-14), V( -2) },
{ V(-10), V( -14), V( 90), V(15), V( 2), V( -7), V(-16) }
};
#undef S
#undef V
template<Color Us>
Score evaluate(const Position& pos, Pawns::Entry* e) {
constexpr Color Them = (Us == WHITE ? BLACK : WHITE);
constexpr Direction Up = (Us == WHITE ? NORTH : SOUTH);
Bitboard neighbours, stoppers, support, phalanx, opposed;
Bitboard lever, leverPush;
Square s;
bool backward, passed, doubled;
Score score = SCORE_ZERO;
const Square* pl = pos.squares<PAWN>(Us);
Bitboard ourPawns = pos.pieces( Us, PAWN);
Bitboard theirPawns = pos.pieces(Them, PAWN);
Bitboard doubleAttackThem = pawn_double_attacks_bb<Them>(theirPawns);
e->passedPawns[Us] = e->pawnAttacksSpan[Us] = 0;
e->kingSquares[Us] = SQ_NONE;
e->pawnAttacks[Us] = pawn_attacks_bb<Us>(ourPawns);
// Loop through all pawns of the current color and score each pawn
while ((s = *pl++) != SQ_NONE)
{
assert(pos.piece_on(s) == make_piece(Us, PAWN));
Rank r = relative_rank(Us, s);
// Flag the pawn
opposed = theirPawns & forward_file_bb(Us, s);
stoppers = theirPawns & passed_pawn_span(Us, s);
lever = theirPawns & PawnAttacks[Us][s];
leverPush = theirPawns & PawnAttacks[Us][s + Up];
doubled = ourPawns & (s - Up);
neighbours = ourPawns & adjacent_files_bb(s);
phalanx = neighbours & rank_bb(s);
support = neighbours & rank_bb(s - Up);
// A pawn is backward when it is behind all pawns of the same color on
// the adjacent files and cannot safely advance. Phalanx and isolated
// pawns will be excluded when the pawn is scored.
backward = !(neighbours & forward_ranks_bb(Them, s))
&& (stoppers & (leverPush | (s + Up)));
// Span of backward pawns and span behind opposing pawns are not included
// in the pawnAttacksSpan bitboard.
if (!backward || phalanx)
{
if (opposed)
e->pawnAttacksSpan[Us] |= pawn_attack_span(Us, s) &
~pawn_attack_span(Us, frontmost_sq(Them, opposed));
else
e->pawnAttacksSpan[Us] |= pawn_attack_span(Us, s);
}
// A pawn is passed if one of the three following conditions is true:
// (a) there is no stoppers except some levers
// (b) the only stoppers are the leverPush, but we outnumber them
// (c) there is only one front stopper which can be levered.
passed = !(stoppers ^ lever)
|| ( !(stoppers ^ leverPush)
&& popcount(phalanx) >= popcount(leverPush))
|| ( stoppers == square_bb(s + Up) && r >= RANK_5
&& (shift<Up>(support) & ~(theirPawns | doubleAttackThem)));
// Passed pawns will be properly scored later in evaluation when we have
// full attack info.
if (passed)
e->passedPawns[Us] |= s;
// Score this pawn
if (support | phalanx)
{
int v = Connected[r] * (2 + bool(phalanx) - bool(opposed))
+ 21 * popcount(support);
score += make_score(v, v * (r - 2) / 4);
}
else if (!neighbours)
score -= Isolated
+ WeakUnopposed * !opposed;
else if (backward)
score -= Backward
+ WeakUnopposed * !opposed;
if (!support)
score -= Doubled * doubled
+ WeakLever * more_than_one(lever);
}
return score;
}
} // namespace
namespace Pawns {
/// Pawns::probe() looks up the current position's pawns configuration in
/// the pawns hash table. It returns a pointer to the Entry if the position
/// is found. Otherwise a new Entry is computed and stored there, so we don't
/// have to recompute all when the same pawns configuration occurs again.
Entry* probe(const Position& pos) {
Key key = pos.pawn_key();
Entry* e = pos.this_thread()->pawnsTable[key];
if (e->key == key)
return e;
e->key = key;
e->scores[WHITE] = evaluate<WHITE>(pos, e);
e->scores[BLACK] = evaluate<BLACK>(pos, e);
return e;
}
/// Entry::evaluate_shelter() calculates the shelter bonus and the storm
/// penalty for a king, looking at the king file and the two closest files.
template<Color Us>
Score Entry::evaluate_shelter(const Position& pos, Square ksq) {
constexpr Color Them = (Us == WHITE ? BLACK : WHITE);
Bitboard b = pos.pieces(PAWN) & ~forward_ranks_bb(Them, ksq);
Bitboard ourPawns = b & pos.pieces(Us);
Bitboard theirPawns = b & pos.pieces(Them);
Score bonus = make_score(5, 5);
File center = clamp(file_of(ksq), FILE_B, FILE_G);
for (File f = File(center - 1); f <= File(center + 1); ++f)
{
b = ourPawns & file_bb(f);
int ourRank = b ? relative_rank(Us, frontmost_sq(Them, b)) : 0;
b = theirPawns & file_bb(f);
int theirRank = b ? relative_rank(Us, frontmost_sq(Them, b)) : 0;
File d = map_to_queenside(f);
bonus += make_score(ShelterStrength[d][ourRank], 0);
if (ourRank && (ourRank == theirRank - 1))
bonus -= BlockedStorm * int(theirRank == RANK_3);
else
bonus -= make_score(UnblockedStorm[d][theirRank], 0);
}
return bonus;
}
/// Entry::do_king_safety() calculates a bonus for king safety. It is called only
/// when king square changes, which is about 20% of total king_safety() calls.
template<Color Us>
Score Entry::do_king_safety(const Position& pos) {
Square ksq = pos.square<KING>(Us);
kingSquares[Us] = ksq;
castlingRights[Us] = pos.castling_rights(Us);
auto compare = [](Score a, Score b) { return mg_value(a) < mg_value(b); };
Score shelter = evaluate_shelter<Us>(pos, ksq);
// If we can castle use the bonus after castling if it is bigger
if (pos.can_castle(Us & KING_SIDE))
shelter = std::max(shelter, evaluate_shelter<Us>(pos, relative_square(Us, SQ_G1)), compare);
if (pos.can_castle(Us & QUEEN_SIDE))
shelter = std::max(shelter, evaluate_shelter<Us>(pos, relative_square(Us, SQ_C1)), compare);
// In endgame we like to bring our king near our closest pawn
Bitboard pawns = pos.pieces(Us, PAWN);
int minPawnDist = pawns ? 8 : 0;
if (pawns & PseudoAttacks[KING][ksq])
minPawnDist = 1;
else while (pawns)
minPawnDist = std::min(minPawnDist, distance(ksq, pop_lsb(&pawns)));
return shelter - make_score(0, 16 * minPawnDist);
}
// Explicit template instantiation
template Score Entry::do_king_safety<WHITE>(const Position& pos);
template Score Entry::do_king_safety<BLACK>(const Position& pos);
} // namespace Pawns