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288 lines
10 KiB
C++
288 lines
10 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 V Value
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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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// Weakness of our pawn shelter in front of the king indexed by [king pawn][rank]
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const Value ShelterWeakness[2][8] =
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{ { V(141), V(0), V(38), V(102), V(128), V(141), V(141) },
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{ V( 61), V(0), V(16), V( 44), V( 56), V( 61), V( 61) } };
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// Danger of enemy pawns moving toward our king indexed by [pawn blocked][rank]
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const Value StormDanger[2][8] =
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{ { V(26), V(0), V(128), V(51), V(26) },
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{ V(13), V(0), V( 64), V(25), V(13) } };
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// Max bonus for king safety. Corresponds to start position with all the pawns
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// in front of the king and no enemy pawn on the horizont.
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const Value MaxSafetyBonus = V(263);
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#undef S
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#undef V
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}
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/// PawnTable::probe() takes a position object as input, computes a PawnEntry
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/// object, and returns a pointer to it. The result is also stored in a hash
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/// table, so we don't have to recompute everything when the same pawn structure
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/// occurs again.
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PawnEntry* PawnTable::probe(const Position& pos) {
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Key key = pos.pawn_key();
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PawnEntry* e = entries[key];
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// If e->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 (e->key == key)
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return e;
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e->key = key;
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e->passedPawns[WHITE] = e->passedPawns[BLACK] = 0;
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e->kingSquares[WHITE] = e->kingSquares[BLACK] = SQ_NONE;
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e->halfOpenFiles[WHITE] = e->halfOpenFiles[BLACK] = 0xFF;
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Bitboard wPawns = pos.pieces(WHITE, PAWN);
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Bitboard bPawns = pos.pieces(BLACK, PAWN);
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e->pawnAttacks[WHITE] = ((wPawns & ~FileHBB) << 9) | ((wPawns & ~FileABB) << 7);
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e->pawnAttacks[BLACK] = ((bPawns & ~FileHBB) >> 7) | ((bPawns & ~FileABB) >> 9);
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e->value = evaluate_pawns<WHITE>(pos, wPawns, bPawns, e)
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- evaluate_pawns<BLACK>(pos, bPawns, wPawns, e);
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e->value = apply_weight(e->value, PawnStructureWeight);
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return e;
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}
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/// PawnTable::evaluate_pawns() evaluates each pawn of the given color
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template<Color Us>
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Score PawnTable::evaluate_pawns(const Position& pos, Bitboard ourPawns,
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Bitboard theirPawns, PawnEntry* e) {
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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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e->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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chain = ourPawns & adjacent_files_bb(f) & b;
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isolated = !(ourPawns & adjacent_files_bb(f));
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doubled = ourPawns & forward_bb(Us, s);
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opposed = theirPawns & forward_bb(Us, s);
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passed = !(theirPawns & passed_pawn_mask(Us, s));
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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 adjacent 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 adjacent files. We now check whether the pawn is
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// backward by looking in the forward direction on the adjacent
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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 adjacent files is higher or equal than the number of
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// enemy pawns in the forward direction on the adjacent 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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e->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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/// PawnEntry::shelter_storm() calculates shelter and storm penalties for the file
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/// the king is on, as well as the two adjacent files.
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template<Color Us>
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Value PawnEntry::shelter_storm(const Position& pos, Square ksq) {
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const Color Them = (Us == WHITE ? BLACK : WHITE);
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Value safety = MaxSafetyBonus;
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Bitboard b = pos.pieces(PAWN) & (in_front_bb(Us, ksq) | rank_bb(ksq));
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Bitboard ourPawns = b & pos.pieces(Us) & ~rank_bb(ksq);
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Bitboard theirPawns = b & pos.pieces(Them);
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Rank rkUs, rkThem;
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File kf = file_of(ksq);
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kf = (kf == FILE_A) ? kf++ : (kf == FILE_H) ? kf-- : kf;
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for (int f = kf - 1; f <= kf + 1; f++)
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{
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// Shelter penalty is higher for the pawn in front of the king
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b = ourPawns & FileBB[f];
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rkUs = b ? rank_of(Us == WHITE ? lsb(b) : ~msb(b)) : RANK_1;
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safety -= ShelterWeakness[f != kf][rkUs];
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// Storm danger is smaller if enemy pawn is blocked
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b = theirPawns & FileBB[f];
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rkThem = b ? rank_of(Us == WHITE ? lsb(b) : ~msb(b)) : RANK_1;
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safety -= StormDanger[rkThem == rkUs + 1][rkThem];
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}
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return safety;
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}
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/// PawnEntry::update_safety() calculates and caches a bonus for king safety. It is
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/// called only when king square changes, about 20% of total king_safety() calls.
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template<Color Us>
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Score PawnEntry::update_safety(const Position& pos, Square ksq) {
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kingSquares[Us] = ksq;
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castleRights[Us] = pos.can_castle(Us);
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minKPdistance[Us] = 0;
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Bitboard pawns = pos.pieces(Us, PAWN);
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if (pawns)
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while (!(DistanceRingsBB[ksq][minKPdistance[Us]++] & pawns)) {}
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if (relative_rank(Us, ksq) > RANK_4)
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return kingSafety[Us] = make_score(0, -16 * minKPdistance[Us]);
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Value bonus = shelter_storm<Us>(pos, ksq);
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// If we can castle use the bonus after the castle if is bigger
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if (pos.can_castle(make_castle_right(Us, KING_SIDE)))
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bonus = std::max(bonus, shelter_storm<Us>(pos, relative_square(Us, SQ_G1)));
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if (pos.can_castle(make_castle_right(Us, QUEEN_SIDE)))
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bonus = std::max(bonus, shelter_storm<Us>(pos, relative_square(Us, SQ_C1)));
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return kingSafety[Us] = make_score(bonus, -16 * minKPdistance[Us]);
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}
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// Explicit template instantiation
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template Score PawnEntry::update_safety<WHITE>(const Position& pos, Square ksq);
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template Score PawnEntry::update_safety<BLACK>(const Position& pos, Square ksq);
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