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241 lines
8.5 KiB
C++
241 lines
8.5 KiB
C++
/*
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Stockfish, a UCI chess playing engine derived from Glaurung 2.1
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Copyright (C) 2004-2008 Tord Romstad (Glaurung author)
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Copyright (C) 2008-2012 Marco Costalba, Joona Kiiski, Tord Romstad
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Stockfish is free software: you can redistribute it and/or modify
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it under the terms of the GNU General Public License as published by
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the Free Software Foundation, either version 3 of the License, or
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(at your option) any later version.
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Stockfish is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU General Public License for more details.
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You should have received a copy of the GNU General Public License
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along with this program. If not, see <http://www.gnu.org/licenses/>.
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*/
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#include <cassert>
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#include "bitboard.h"
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#include "bitcount.h"
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#include "pawns.h"
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#include "position.h"
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namespace {
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#define S(mg, eg) make_score(mg, eg)
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// Doubled pawn penalty by opposed flag and file
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const Score DoubledPawnPenalty[2][8] = {
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{ S(13, 43), S(20, 48), S(23, 48), S(23, 48),
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S(23, 48), S(23, 48), S(20, 48), S(13, 43) },
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{ S(13, 43), S(20, 48), S(23, 48), S(23, 48),
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S(23, 48), S(23, 48), S(20, 48), S(13, 43) }};
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// Isolated pawn penalty by opposed flag and file
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const Score IsolatedPawnPenalty[2][8] = {
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{ S(37, 45), S(54, 52), S(60, 52), S(60, 52),
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S(60, 52), S(60, 52), S(54, 52), S(37, 45) },
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{ S(25, 30), S(36, 35), S(40, 35), S(40, 35),
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S(40, 35), S(40, 35), S(36, 35), S(25, 30) }};
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// Backward pawn penalty by opposed flag and file
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const Score BackwardPawnPenalty[2][8] = {
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{ S(30, 42), S(43, 46), S(49, 46), S(49, 46),
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S(49, 46), S(49, 46), S(43, 46), S(30, 42) },
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{ S(20, 28), S(29, 31), S(33, 31), S(33, 31),
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S(33, 31), S(33, 31), S(29, 31), S(20, 28) }};
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// Pawn chain membership bonus by file
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const Score ChainBonus[8] = {
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S(11,-1), S(13,-1), S(13,-1), S(14,-1),
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S(14,-1), S(13,-1), S(13,-1), S(11,-1)
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};
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// Candidate passed pawn bonus by rank
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const Score CandidateBonus[8] = {
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S( 0, 0), S( 6, 13), S(6,13), S(14,29),
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S(34,68), S(83,166), S(0, 0), S( 0, 0)
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};
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const Score PawnStructureWeight = S(233, 201);
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#undef S
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inline Score apply_weight(Score v, Score w) {
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return make_score((int(mg_value(v)) * mg_value(w)) / 0x100,
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(int(eg_value(v)) * eg_value(w)) / 0x100);
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}
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}
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/// PawnInfoTable::pawn_info() takes a position object as input, computes
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/// a PawnInfo object, and returns a pointer to it. The result is also stored
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/// in an hash table, so we don't have to recompute everything when the same
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/// pawn structure occurs again.
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PawnInfo* PawnInfoTable::pawn_info(const Position& pos) const {
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Key key = pos.pawn_key();
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PawnInfo* pi = probe(key);
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// If pi->key matches the position's pawn hash key, it means that we
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// have analysed this pawn structure before, and we can simply return
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// the information we found the last time instead of recomputing it.
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if (pi->key == key)
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return pi;
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// Initialize PawnInfo entry
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pi->key = key;
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pi->passedPawns[WHITE] = pi->passedPawns[BLACK] = 0;
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pi->kingSquares[WHITE] = pi->kingSquares[BLACK] = SQ_NONE;
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pi->halfOpenFiles[WHITE] = pi->halfOpenFiles[BLACK] = 0xFF;
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// Calculate pawn attacks
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Bitboard wPawns = pos.pieces(PAWN, WHITE);
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Bitboard bPawns = pos.pieces(PAWN, BLACK);
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pi->pawnAttacks[WHITE] = ((wPawns << 9) & ~FileABB) | ((wPawns << 7) & ~FileHBB);
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pi->pawnAttacks[BLACK] = ((bPawns >> 7) & ~FileABB) | ((bPawns >> 9) & ~FileHBB);
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// Evaluate pawns for both colors and weight the result
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pi->value = evaluate_pawns<WHITE>(pos, wPawns, bPawns, pi)
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- evaluate_pawns<BLACK>(pos, bPawns, wPawns, pi);
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pi->value = apply_weight(pi->value, PawnStructureWeight);
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return pi;
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}
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/// PawnInfoTable::evaluate_pawns() evaluates each pawn of the given color
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template<Color Us>
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Score PawnInfoTable::evaluate_pawns(const Position& pos, Bitboard ourPawns,
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Bitboard theirPawns, PawnInfo* pi) {
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const Color Them = (Us == WHITE ? BLACK : WHITE);
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Bitboard b;
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Square s;
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File f;
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Rank r;
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bool passed, isolated, doubled, opposed, chain, backward, candidate;
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Score value = SCORE_ZERO;
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const Square* pl = pos.piece_list(Us, PAWN);
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// Loop through all pawns of the current color and score each pawn
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while ((s = *pl++) != SQ_NONE)
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{
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assert(pos.piece_on(s) == make_piece(Us, PAWN));
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f = file_of(s);
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r = rank_of(s);
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// This file cannot be half open
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pi->halfOpenFiles[Us] &= ~(1 << f);
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// Our rank plus previous one. Used for chain detection
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b = rank_bb(r) | rank_bb(Us == WHITE ? r - Rank(1) : r + Rank(1));
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// Flag the pawn as passed, isolated, doubled or member of a pawn
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// chain (but not the backward one).
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passed = !(theirPawns & passed_pawn_mask(Us, s));
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doubled = ourPawns & squares_in_front_of(Us, s);
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opposed = theirPawns & squares_in_front_of(Us, s);
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isolated = !(ourPawns & neighboring_files_bb(f));
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chain = ourPawns & neighboring_files_bb(f) & b;
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// Test for backward pawn
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backward = false;
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// If the pawn is passed, isolated, or member of a pawn chain it cannot
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// be backward. If there are friendly pawns behind on neighboring files
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// or if can capture an enemy pawn it cannot be backward either.
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if ( !(passed | isolated | chain)
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&& !(ourPawns & attack_span_mask(Them, s))
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&& !(pos.attacks_from<PAWN>(s, Us) & theirPawns))
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{
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// We now know that there are no friendly pawns beside or behind this
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// pawn on neighboring files. We now check whether the pawn is
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// backward by looking in the forward direction on the neighboring
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// files, and seeing whether we meet a friendly or an enemy pawn first.
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b = pos.attacks_from<PAWN>(s, Us);
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// Note that we are sure to find something because pawn is not passed
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// nor isolated, so loop is potentially infinite, but it isn't.
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while (!(b & (ourPawns | theirPawns)))
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Us == WHITE ? b <<= 8 : b >>= 8;
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// The friendly pawn needs to be at least two ranks closer than the
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// enemy pawn in order to help the potentially backward pawn advance.
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backward = (b | (Us == WHITE ? b << 8 : b >> 8)) & theirPawns;
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}
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assert(opposed | passed | (attack_span_mask(Us, s) & theirPawns));
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// A not passed pawn is a candidate to become passed if it is free to
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// advance and if the number of friendly pawns beside or behind this
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// pawn on neighboring files is higher or equal than the number of
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// enemy pawns in the forward direction on the neighboring files.
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candidate = !(opposed | passed | backward | isolated)
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&& (b = attack_span_mask(Them, s + pawn_push(Us)) & ourPawns) != 0
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&& popcount<Max15>(b) >= popcount<Max15>(attack_span_mask(Us, s) & theirPawns);
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// Passed pawns will be properly scored in evaluation because we need
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// full attack info to evaluate passed pawns. Only the frontmost passed
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// pawn on each file is considered a true passed pawn.
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if (passed && !doubled)
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set_bit(&(pi->passedPawns[Us]), s);
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// Score this pawn
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if (isolated)
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value -= IsolatedPawnPenalty[opposed][f];
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if (doubled)
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value -= DoubledPawnPenalty[opposed][f];
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if (backward)
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value -= BackwardPawnPenalty[opposed][f];
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if (chain)
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value += ChainBonus[f];
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if (candidate)
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value += CandidateBonus[relative_rank(Us, s)];
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}
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return value;
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}
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/// PawnInfo::updateShelter() calculates and caches king shelter. It is called
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/// only when king square changes, about 20% of total king_shelter() calls.
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template<Color Us>
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Score PawnInfo::updateShelter(const Position& pos, Square ksq) {
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const int Shift = (Us == WHITE ? 8 : -8);
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Bitboard pawns;
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int r, shelter = 0;
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if (relative_rank(Us, ksq) <= RANK_4)
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{
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pawns = pos.pieces(PAWN, Us) & this_and_neighboring_files_bb(file_of(ksq));
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r = ksq & (7 << 3);
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for (int i = 0; i < 3; i++)
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{
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r += Shift;
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shelter += BitCount8Bit[(pawns >> r) & 0xFF] << (6 - i);
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}
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}
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kingSquares[Us] = ksq;
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kingShelters[Us] = make_score(shelter, 0);
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return kingShelters[Us];
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}
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// Explicit template instantiation
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template Score PawnInfo::updateShelter<WHITE>(const Position& pos, Square ksq);
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template Score PawnInfo::updateShelter<BLACK>(const Position& pos, Square ksq);
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