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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) 2015-2017 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)
// Isolated pawn penalty by opposed flag
const Score Isolated[] = { S(45, 40), S(30, 27) };
// Backward pawn penalty by opposed flag
const Score Backward[] = { S(56, 33), S(41, 19) };
// Unsupported pawn penalty for pawns which are neither isolated or backward
const Score Unsupported = S(17, 8);
// Connected pawn bonus by opposed, phalanx, twice supported and rank
Score Connected[2][2][2][RANK_NB];
// Doubled pawn penalty
const Score Doubled = S(18, 38);
// Lever bonus by rank
const Score Lever[RANK_NB] = {
S( 0, 0), S( 0, 0), S(0, 0), S(0, 0),
S(17, 16), S(33, 32), S(0, 0), S(0, 0)
};
// Weakness of our pawn shelter in front of the king by [distance from edge][rank].
// RANK_1 = 0 is used for files where we have no pawns or our pawn is behind our king.
const Value ShelterWeakness[][RANK_NB] = {
{ V(100), V(20), V(10), V(46), V(82), V( 86), V( 98) },
{ V(116), V( 4), V(28), V(87), V(94), V(108), V(104) },
{ V(109), V( 1), V(59), V(87), V(62), V( 91), V(116) },
{ V( 75), V(12), V(43), V(59), V(90), V( 84), V(112) }
};
// Danger of enemy pawns moving toward our king by [type][distance from edge][rank].
// For the unopposed and unblocked cases, RANK_1 = 0 is used when opponent has no pawn
// on the given file, or their pawn is behind our king.
const Value StormDanger[][4][RANK_NB] = {
{ { V( 0), V(-290), V(-274), V(57), V(41) }, //BlockedByKing
{ V( 0), V( 60), V( 144), V(39), V(13) },
{ V( 0), V( 65), V( 141), V(41), V(34) },
{ V( 0), V( 53), V( 127), V(56), V(14) } },
{ { V( 4), V( 73), V( 132), V(46), V(31) }, //Unopposed
{ V( 1), V( 64), V( 143), V(26), V(13) },
{ V( 1), V( 47), V( 110), V(44), V(24) },
{ V( 0), V( 72), V( 127), V(50), V(31) } },
{ { V( 0), V( 0), V( 79), V(23), V( 1) }, //BlockedByPawn
{ V( 0), V( 0), V( 148), V(27), V( 2) },
{ V( 0), V( 0), V( 161), V(16), V( 1) },
{ V( 0), V( 0), V( 171), V(22), V(15) } },
{ { V(22), V( 45), V( 104), V(62), V( 6) }, //Unblocked
{ V(31), V( 30), V( 99), V(39), V(19) },
{ V(23), V( 29), V( 96), V(41), V(15) },
{ V(21), V( 23), V( 116), V(41), V(15) } }
};
// Max bonus for king safety. Corresponds to start position with all the pawns
// in front of the king and no enemy pawn on the horizon.
const Value MaxSafetyBonus = V(258);
#undef S
#undef V
template<Color Us>
Score evaluate(const Position& pos, Pawns::Entry* e) {
const Color Them = (Us == WHITE ? BLACK : WHITE);
const Square Up = (Us == WHITE ? NORTH : SOUTH);
const Square Right = (Us == WHITE ? NORTH_EAST : SOUTH_WEST);
const Square Left = (Us == WHITE ? NORTH_WEST : SOUTH_EAST);
Bitboard b, neighbours, stoppers, doubled, supported, phalanx;
Bitboard lever, leverPush, connected;
Square s;
bool opposed, backward;
Score score = SCORE_ZERO;
const Square* pl = pos.squares<PAWN>(Us);
Bitboard ourPawns = pos.pieces( Us, PAWN);
Bitboard theirPawns = pos.pieces(Them, PAWN);
e->passedPawns[Us] = e->pawnAttacksSpan[Us] = 0;
e->semiopenFiles[Us] = 0xFF;
e->kingSquares[Us] = SQ_NONE;
e->pawnAttacks[Us] = shift<Right>(ourPawns) | shift<Left>(ourPawns);
e->pawnsOnSquares[Us][BLACK] = popcount(ourPawns & DarkSquares);
e->pawnsOnSquares[Us][WHITE] = pos.count<PAWN>(Us) - e->pawnsOnSquares[Us][BLACK];
// 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));
File f = file_of(s);
e->semiopenFiles[Us] &= ~(1 << f);
e->pawnAttacksSpan[Us] |= pawn_attack_span(Us, s);
// Flag the pawn
opposed = theirPawns & forward_file_bb(Us, s);
stoppers = theirPawns & passed_pawn_mask(Us, s);
lever = theirPawns & PawnAttacks[Us][s];
leverPush = theirPawns & PawnAttacks[Us][s + Up];
doubled = ourPawns & (s - Up);
neighbours = ourPawns & adjacent_files_bb(f);
phalanx = neighbours & rank_bb(s);
supported = neighbours & rank_bb(s - Up);
connected = supported | phalanx;
// A pawn is backward when it is behind all pawns of the same color on the
// adjacent files and cannot be safely advanced.
if (!neighbours || lever || relative_rank(Us, s) >= RANK_5)
backward = false;
else
{
// Find the backmost rank with neighbours or stoppers
b = rank_bb(backmost_sq(Us, neighbours | stoppers));
// The pawn is backward when it cannot safely progress to that rank:
// either there is a stopper in the way on this rank, or there is a
// stopper on adjacent file which controls the way to that rank.
backward = (b | shift<Up>(b & adjacent_files_bb(f))) & stoppers;
assert(!backward || !(pawn_attack_span(Them, s + Up) & neighbours));
}
// Passed pawns will be properly scored in evaluation because we need
// full attack info to evaluate them. Include also not passed pawns
// which could become passed after one or two pawn pushes when are
// not attacked more times than defended.
if ( !(stoppers ^ lever ^ leverPush)
&& !(ourPawns & forward_file_bb(Us, s))
&& popcount(supported) >= popcount(lever)
&& popcount(phalanx) >= popcount(leverPush))
e->passedPawns[Us] |= s;
else if ( stoppers == SquareBB[s + Up]
&& relative_rank(Us, s) >= RANK_5)
{
b = shift<Up>(supported) & ~theirPawns;
while (b)
if (!more_than_one(theirPawns & PawnAttacks[Us][pop_lsb(&b)]))
e->passedPawns[Us] |= s;
}
// Score this pawn
if (!neighbours)
score -= Isolated[opposed];
else if (backward)
score -= Backward[opposed];
else if (!supported)
score -= Unsupported;
if (connected)
score += Connected[opposed][!!phalanx][more_than_one(supported)][relative_rank(Us, s)];
if (doubled && !supported)
score -= Doubled;
if (lever)
score += Lever[relative_rank(Us, s)];
}
return score;
}
} // namespace
namespace Pawns {
/// Pawns::init() initializes some tables needed by evaluation. Instead of using
/// hard-coded tables, when makes sense, we prefer to calculate them with a formula
/// to reduce independent parameters and to allow easier tuning and better insight.
void init() {
static const int Seed[RANK_NB] = { 0, 8, 19, 13, 71, 94, 169, 324 };
for (int opposed = 0; opposed <= 1; ++opposed)
for (int phalanx = 0; phalanx <= 1; ++phalanx)
for (int apex = 0; apex <= 1; ++apex)
for (Rank r = RANK_2; r < RANK_8; ++r)
{
int v = (Seed[r] + (phalanx ? (Seed[r + 1] - Seed[r]) / 2 : 0)) >> opposed;
v += (apex ? v / 2 : 0);
Connected[opposed][phalanx][apex][r] = make_score(v, v * (r-2) / 4);
}
}
/// 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->score = evaluate<WHITE>(pos, e) - evaluate<BLACK>(pos, e);
e->asymmetry = popcount(e->semiopenFiles[WHITE] ^ e->semiopenFiles[BLACK]);
e->openFiles = popcount(e->semiopenFiles[WHITE] & e->semiopenFiles[BLACK]);
return e;
}
/// Entry::shelter_storm() calculates shelter and storm penalties for the file
/// the king is on, as well as the two closest files.
template<Color Us>
Value Entry::shelter_storm(const Position& pos, Square ksq) {
const Color Them = (Us == WHITE ? BLACK : WHITE);
enum { BlockedByKing, Unopposed, BlockedByPawn, Unblocked };
Bitboard b = pos.pieces(PAWN) & (forward_ranks_bb(Us, ksq) | rank_bb(ksq));
Bitboard ourPawns = b & pos.pieces(Us);
Bitboard theirPawns = b & pos.pieces(Them);
Value safety = MaxSafetyBonus;
File center = std::max(FILE_B, std::min(FILE_G, file_of(ksq)));
for (File f = center - File(1); f <= center + File(1); ++f)
{
b = ourPawns & file_bb(f);
Rank rkUs = b ? relative_rank(Us, backmost_sq(Us, b)) : RANK_1;
b = theirPawns & file_bb(f);
Rank rkThem = b ? relative_rank(Us, frontmost_sq(Them, b)) : RANK_1;
int d = std::min(f, FILE_H - f);
safety -= ShelterWeakness[d][rkUs]
+ StormDanger
[f == file_of(ksq) && rkThem == relative_rank(Us, ksq) + 1 ? BlockedByKing :
rkUs == RANK_1 ? Unopposed :
rkThem == rkUs + 1 ? BlockedByPawn : Unblocked]
[d][rkThem];
}
return safety;
}
/// 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) {
kingSquares[Us] = ksq;
castlingRights[Us] = pos.can_castle(Us);
int minKingPawnDistance = 0;
Bitboard pawns = pos.pieces(Us, PAWN);
if (pawns)
while (!(DistanceRingBB[ksq][minKingPawnDistance++] & pawns)) {}
Value bonus = shelter_storm<Us>(pos, ksq);
// If we can castle use the bonus after the castling if it is bigger
if (pos.can_castle(MakeCastling<Us, KING_SIDE>::right))
bonus = std::max(bonus, shelter_storm<Us>(pos, relative_square(Us, SQ_G1)));
if (pos.can_castle(MakeCastling<Us, QUEEN_SIDE>::right))
bonus = std::max(bonus, shelter_storm<Us>(pos, relative_square(Us, SQ_C1)));
return make_score(bonus, -16 * minKingPawnDistance);
}
// Explicit template instantiation
template Score Entry::do_king_safety<WHITE>(const Position& pos, Square ksq);
template Score Entry::do_king_safety<BLACK>(const Position& pos, Square ksq);
} // namespace Pawns