306 lines
10 KiB
C++
306 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) 2004-2021 The Stockfish developers (see AUTHORS file)
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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 <algorithm>
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#include <cassert>
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#include "bitboard.h"
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#include "pawns.h"
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#include "position.h"
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#include "thread.h"
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namespace Stockfish {
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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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// Pawn penalties
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constexpr Score Backward = S( 9, 22);
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constexpr Score Doubled = S(13, 51);
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constexpr Score DoubledEarly = S(20, 7);
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constexpr Score Isolated = S( 3, 15);
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constexpr Score WeakLever = S( 4, 58);
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constexpr Score WeakUnopposed = S(13, 24);
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// Bonus for blocked pawns at 5th or 6th rank
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constexpr Score BlockedPawn[2] = { S(-17, -6), S(-9, 2) };
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constexpr Score BlockedStorm[RANK_NB] = {
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S(0, 0), S(0, 0), S(75, 78), S(-8, 16), S(-6, 10), S(-6, 6), S(0, 2)
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};
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// Connected pawn bonus
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constexpr int Connected[RANK_NB] = { 0, 5, 7, 11, 23, 48, 87 };
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// Strength of pawn shelter for our king by [distance from edge][rank].
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// RANK_1 = 0 is used for files where we have no pawn, or pawn is behind our king.
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constexpr Value ShelterStrength[int(FILE_NB) / 2][RANK_NB] = {
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{ V( -5), V( 82), V( 92), V( 54), V( 36), V( 22), V( 28) },
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{ V(-44), V( 63), V( 33), V(-50), V(-30), V(-12), V( -62) },
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{ V(-11), V( 77), V( 22), V( -6), V( 31), V( 8), V( -45) },
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{ V(-39), V(-12), V(-29), V(-50), V(-43), V(-68), V(-164) }
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};
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// Danger of enemy pawns moving toward our king by [distance from edge][rank].
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// RANK_1 = 0 is used for files where the enemy has no pawn, or their pawn
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// is behind our king. Note that UnblockedStorm[0][1-2] accommodate opponent pawn
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// on edge, likely blocked by our king.
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constexpr Value UnblockedStorm[int(FILE_NB) / 2][RANK_NB] = {
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{ V( 87), V(-288), V(-168), V( 96), V( 47), V( 44), V( 46) },
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{ V( 42), V( -25), V( 120), V( 45), V( 34), V( -9), V( 24) },
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{ V( -8), V( 51), V( 167), V( 35), V( -4), V(-16), V(-12) },
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{ V(-17), V( -13), V( 100), V( 4), V( 9), V(-16), V(-31) }
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};
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// KingOnFile[semi-open Us][semi-open Them] contains bonuses/penalties
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// for king when the king is on a semi-open or open file.
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constexpr Score KingOnFile[2][2] = {{ S(-21,10), S(-7, 1) },
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{ S( 0,-3), S( 9,-4) }};
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#undef S
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#undef V
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/// evaluate() calculates a score for the static pawn structure of the given position.
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/// We cannot use the location of pieces or king in this function, as the evaluation
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/// of the pawn structure will be stored in a small cache for speed reasons, and will
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/// be re-used even when the pieces have moved.
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template<Color Us>
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Score evaluate(const Position& pos, Pawns::Entry* e) {
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constexpr Color Them = ~Us;
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constexpr Direction Up = pawn_push(Us);
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constexpr Direction Down = -Up;
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Bitboard neighbours, stoppers, support, phalanx, opposed;
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Bitboard lever, leverPush, blocked;
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Square s;
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bool backward, passed, doubled;
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Score score = SCORE_ZERO;
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Bitboard b = pos.pieces(Us, PAWN);
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Bitboard ourPawns = pos.pieces( Us, PAWN);
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Bitboard theirPawns = pos.pieces(Them, PAWN);
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Bitboard doubleAttackThem = pawn_double_attacks_bb<Them>(theirPawns);
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e->passedPawns[Us] = 0;
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e->kingSquares[Us] = SQ_NONE;
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e->pawnAttacks[Us] = e->pawnAttacksSpan[Us] = pawn_attacks_bb<Us>(ourPawns);
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e->blockedCount += popcount(shift<Up>(ourPawns) & (theirPawns | doubleAttackThem));
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// Loop through all pawns of the current color and score each pawn
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while (b)
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{
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s = pop_lsb(b);
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assert(pos.piece_on(s) == make_piece(Us, PAWN));
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Rank r = relative_rank(Us, s);
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// Flag the pawn
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opposed = theirPawns & forward_file_bb(Us, s);
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blocked = theirPawns & (s + Up);
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stoppers = theirPawns & passed_pawn_span(Us, s);
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lever = theirPawns & pawn_attacks_bb(Us, s);
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leverPush = theirPawns & pawn_attacks_bb(Us, s + Up);
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doubled = ourPawns & (s - Up);
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neighbours = ourPawns & adjacent_files_bb(s);
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phalanx = neighbours & rank_bb(s);
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support = neighbours & rank_bb(s - Up);
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if (doubled)
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{
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// Additional doubled penalty if none of their pawns is fixed
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if (!(ourPawns & shift<Down>(theirPawns | pawn_attacks_bb<Them>(theirPawns))))
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score -= DoubledEarly;
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}
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// A pawn is backward when it is behind all pawns of the same color on
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// the adjacent files and cannot safely advance.
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backward = !(neighbours & forward_ranks_bb(Them, s + Up))
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&& (leverPush | blocked);
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// Compute additional span if pawn is not backward nor blocked
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if (!backward && !blocked)
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e->pawnAttacksSpan[Us] |= pawn_attack_span(Us, s);
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// A pawn is passed if one of the three following conditions is true:
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// (a) there is no stoppers except some levers
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// (b) the only stoppers are the leverPush, but we outnumber them
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// (c) there is only one front stopper which can be levered.
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// (Refined in Evaluation::passed)
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passed = !(stoppers ^ lever)
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|| ( !(stoppers ^ leverPush)
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&& popcount(phalanx) >= popcount(leverPush))
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|| ( stoppers == blocked && r >= RANK_5
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&& (shift<Up>(support) & ~(theirPawns | doubleAttackThem)));
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passed &= !(forward_file_bb(Us, s) & ourPawns);
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// Passed pawns will be properly scored later in evaluation when we have
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// full attack info.
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if (passed)
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e->passedPawns[Us] |= s;
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// Score this pawn
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if (support | phalanx)
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{
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int v = Connected[r] * (2 + bool(phalanx) - bool(opposed))
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+ 22 * popcount(support);
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score += make_score(v, v * (r - 2) / 4);
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}
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else if (!neighbours)
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{
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if ( opposed
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&& (ourPawns & forward_file_bb(Them, s))
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&& !(theirPawns & adjacent_files_bb(s)))
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score -= Doubled;
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else
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score -= Isolated
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+ WeakUnopposed * !opposed;
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}
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else if (backward)
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score -= Backward
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+ WeakUnopposed * !opposed * bool(~(FileABB | FileHBB) & s);
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if (!support)
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score -= Doubled * doubled
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+ WeakLever * more_than_one(lever);
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if (blocked && r >= RANK_5)
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score += BlockedPawn[r - RANK_5];
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}
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return score;
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}
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} // namespace
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namespace Pawns {
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/// Pawns::probe() looks up the current position's pawns configuration in
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/// the pawns hash table. It returns a pointer to the Entry if the position
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/// is found. Otherwise a new Entry is computed and stored there, so we don't
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/// have to recompute all when the same pawns configuration occurs again.
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Entry* probe(const Position& pos) {
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Key key = pos.pawn_key();
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Entry* e = pos.this_thread()->pawnsTable[key];
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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->blockedCount = 0;
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e->scores[WHITE] = evaluate<WHITE>(pos, e);
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e->scores[BLACK] = evaluate<BLACK>(pos, e);
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return e;
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}
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/// Entry::evaluate_shelter() calculates the shelter bonus and the storm
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/// penalty for a king, looking at the king file and the two closest files.
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template<Color Us>
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Score Entry::evaluate_shelter(const Position& pos, Square ksq) const {
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constexpr Color Them = ~Us;
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Bitboard b = pos.pieces(PAWN) & ~forward_ranks_bb(Them, ksq);
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Bitboard ourPawns = b & pos.pieces(Us) & ~pawnAttacks[Them];
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Bitboard theirPawns = b & pos.pieces(Them);
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Score bonus = make_score(5, 5);
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File center = std::clamp(file_of(ksq), FILE_B, FILE_G);
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for (File f = File(center - 1); f <= File(center + 1); ++f)
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{
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b = ourPawns & file_bb(f);
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int ourRank = b ? relative_rank(Us, frontmost_sq(Them, b)) : 0;
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b = theirPawns & file_bb(f);
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int theirRank = b ? relative_rank(Us, frontmost_sq(Them, b)) : 0;
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int d = edge_distance(f);
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bonus += make_score(ShelterStrength[d][ourRank], 0);
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if (ourRank && (ourRank == theirRank - 1))
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bonus -= BlockedStorm[theirRank];
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else
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bonus -= make_score(UnblockedStorm[d][theirRank], 0);
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}
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// King On File
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bonus -= KingOnFile[pos.is_on_semiopen_file(Us, ksq)][pos.is_on_semiopen_file(Them, ksq)];
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return bonus;
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}
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/// Entry::do_king_safety() calculates a bonus for king safety. It is called only
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/// when king square changes, which is about 20% of total king_safety() calls.
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template<Color Us>
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Score Entry::do_king_safety(const Position& pos) {
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Square ksq = pos.square<KING>(Us);
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kingSquares[Us] = ksq;
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castlingRights[Us] = pos.castling_rights(Us);
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auto compare = [](Score a, Score b) { return mg_value(a) < mg_value(b); };
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Score shelter = evaluate_shelter<Us>(pos, ksq);
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// If we can castle use the bonus after castling if it is bigger
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if (pos.can_castle(Us & KING_SIDE))
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shelter = std::max(shelter, evaluate_shelter<Us>(pos, relative_square(Us, SQ_G1)), compare);
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if (pos.can_castle(Us & QUEEN_SIDE))
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shelter = std::max(shelter, evaluate_shelter<Us>(pos, relative_square(Us, SQ_C1)), compare);
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// In endgame we like to bring our king near our closest pawn
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Bitboard pawns = pos.pieces(Us, PAWN);
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int minPawnDist = 6;
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if (pawns & attacks_bb<KING>(ksq))
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minPawnDist = 1;
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else while (pawns)
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minPawnDist = std::min(minPawnDist, distance(ksq, pop_lsb(pawns)));
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return shelter - make_score(0, 16 * minPawnDist);
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}
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// Explicit template instantiation
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template Score Entry::do_king_safety<WHITE>(const Position& pos);
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template Score Entry::do_king_safety<BLACK>(const Position& pos);
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} // namespace Pawns
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} // namespace Stockfish
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