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317 lines
11 KiB
C++
317 lines
11 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) 2008-2014 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 <algorithm>
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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 file
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const Score Doubled[FILE_NB] = {
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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 Isolated[2][FILE_NB] = {
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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 Backward[2][FILE_NB] = {
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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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// Connected pawn bonus by file and rank (initialized by formula)
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Score Connected[FILE_NB][RANK_NB];
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// Candidate passed pawn bonus by rank
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const Score CandidatePassed[RANK_NB] = {
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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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// Bonus for file distance of the two outermost pawns
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const Score PawnsFileSpan = S(0, 15);
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// Unsupported pawn penalty
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const Score UnsupportedPawnPenalty = S(20, 10);
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// Weakness of our pawn shelter in front of the king indexed by [rank]
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const Value ShelterWeakness[RANK_NB] =
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{ V(100), V(0), V(27), V(73), V(92), V(101), V(101) };
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// Danger of enemy pawns moving toward our king indexed by
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// [no friendly pawn | pawn unblocked | pawn blocked][rank of enemy pawn]
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const Value StormDanger[3][RANK_NB] = {
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{ V( 0), V(64), V(128), V(51), V(26) },
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{ V(26), V(32), V( 96), V(38), V(20) },
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{ V( 0), V( 0), V(160), 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 horizon.
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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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template<Color Us>
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Score evaluate(const Position& pos, Pawns::Entry* e) {
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const Color Them = (Us == WHITE ? BLACK : WHITE);
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const Square Up = (Us == WHITE ? DELTA_N : DELTA_S);
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const Square Right = (Us == WHITE ? DELTA_NE : DELTA_SW);
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const Square Left = (Us == WHITE ? DELTA_NW : DELTA_SE);
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Bitboard b, p, doubled;
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Square s;
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File f;
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bool passed, isolated, opposed, connected, backward, candidate, unsupported;
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Score value = SCORE_ZERO;
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const Square* pl = pos.list<PAWN>(Us);
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Bitboard ourPawns = pos.pieces(Us, PAWN);
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Bitboard theirPawns = pos.pieces(Them, PAWN);
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e->passedPawns[Us] = e->candidatePawns[Us] = 0;
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e->kingSquares[Us] = SQ_NONE;
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e->semiopenFiles[Us] = 0xFF;
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e->pawnAttacks[Us] = shift_bb<Right>(ourPawns) | shift_bb<Left>(ourPawns);
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e->pawnsOnSquares[Us][BLACK] = popcount<Max15>(ourPawns & DarkSquares);
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e->pawnsOnSquares[Us][WHITE] = pos.count<PAWN>(Us) - e->pawnsOnSquares[Us][BLACK];
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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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// This file cannot be semi-open
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e->semiopenFiles[Us] &= ~(1 << f);
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// Previous rank
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p = rank_bb(s - pawn_push(Us));
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// Our rank plus previous one
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b = rank_bb(s) | p;
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// Flag the pawn as passed, isolated, doubled,
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// unsupported or connected (but not the backward one).
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connected = ourPawns & adjacent_files_bb(f) & b;
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unsupported = !(ourPawns & adjacent_files_bb(f) & p);
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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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// If the pawn is passed, isolated, or connected it cannot be
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// backward. If there are friendly pawns behind on adjacent files
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// or if it can capture an enemy pawn it cannot be backward either.
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if ( (passed | isolated | connected)
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|| (ourPawns & pawn_attack_span(Them, s))
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|| (pos.attacks_from<PAWN>(s, Us) & theirPawns))
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backward = false;
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else
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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 picking the closest pawn there.
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b = pawn_attack_span(Us, s) & (ourPawns | theirPawns);
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b = pawn_attack_span(Us, s) & rank_bb(backmost_sq(Us, b));
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// If we have an enemy pawn in the same or next rank, the pawn is
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// backward because it cannot advance without being captured.
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backward = (b | shift_bb<Up>(b)) & theirPawns;
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}
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assert(opposed | passed | (pawn_attack_span(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 than or equal to 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 = pawn_attack_span(Them, s + pawn_push(Us)) & ourPawns) != 0
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&& popcount<Max15>(b) >= popcount<Max15>(pawn_attack_span(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 -= Isolated[opposed][f];
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if (unsupported && !isolated)
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value -= UnsupportedPawnPenalty;
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if (doubled)
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value -= Doubled[f] / rank_distance(s, lsb(doubled));
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if (backward)
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value -= Backward[opposed][f];
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if (connected)
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value += Connected[f][relative_rank(Us, s)];
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if (candidate)
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{
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value += CandidatePassed[relative_rank(Us, s)];
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if (!doubled)
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e->candidatePawns[Us] |= s;
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}
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}
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// In endgame it's better to have pawns on both wings. So give a bonus according
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// to file distance between left and right outermost pawns.
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if (pos.count<PAWN>(Us) > 1)
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{
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b = e->semiopenFiles[Us] ^ 0xFF;
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value += PawnsFileSpan * int(msb(b) - lsb(b));
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}
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return value;
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}
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} // namespace
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namespace Pawns {
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/// init() initializes some tables by formula instead of hard-coding their values
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void init() {
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const int bonusesByFile[8] = { 1, 3, 3, 4, 4, 3, 3, 1 };
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int bonus;
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for (Rank r = RANK_1; r < RANK_8; ++r)
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for (File f = FILE_A; f <= FILE_H; ++f)
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{
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bonus = r * (r-1) * (r-2) + bonusesByFile[f] * (r/2 + 1);
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Connected[f][r] = make_score(bonus, bonus);
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}
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}
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/// probe() takes a position object as input, computes a Entry object, and returns
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/// a pointer to it. The result is also stored in a hash table, so we don't have
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/// to recompute everything when the same pawn structure occurs again.
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Entry* probe(const Position& pos, Table& entries) {
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Key key = pos.pawn_key();
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Entry* e = entries[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->value = evaluate<WHITE>(pos, e) - evaluate<BLACK>(pos, e);
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return e;
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}
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/// Entry::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 Entry::shelter_storm(const Position& pos, Square ksq) {
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const Color Them = (Us == WHITE ? BLACK : WHITE);
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static const Bitboard MiddleEdges = (FileABB | FileHBB) & (Rank2BB | Rank3BB);
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Value safety = MaxSafetyBonus;
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Bitboard b = pos.pieces(PAWN) & (in_front_bb(Us, rank_of(ksq)) | rank_bb(ksq));
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Bitboard ourPawns = b & pos.pieces(Us);
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Bitboard theirPawns = b & pos.pieces(Them);
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Rank rkUs, rkThem;
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File kf = std::max(FILE_B, std::min(FILE_G, file_of(ksq)));
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for (File f = kf - File(1); f <= kf + File(1); ++f)
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{
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b = ourPawns & file_bb(f);
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rkUs = b ? relative_rank(Us, backmost_sq(Us, b)) : RANK_1;
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b = theirPawns & file_bb(f);
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rkThem = b ? relative_rank(Us, frontmost_sq(Them, b)) : RANK_1;
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if ( (MiddleEdges & make_square(f, rkThem))
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&& file_of(ksq) == f
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&& relative_rank(Us, ksq) == rkThem - 1)
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safety += 200;
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else
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safety -= ShelterWeakness[rkUs]
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+ StormDanger[rkUs == RANK_1 ? 0 : rkThem == rkUs + 1 ? 2 : 1][rkThem];
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}
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return safety;
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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, Square ksq) {
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kingSquares[Us] = ksq;
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castlingRights[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 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 castling if it is bigger
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if (pos.can_castle(MakeCastling<Us, KING_SIDE>::right))
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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(MakeCastling<Us, QUEEN_SIDE>::right))
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bonus = std::max(bonus, shelter_storm<Us>(pos, relative_square(Us, SQ_C1)));
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return make_score(bonus, -16 * minKPdistance[Us]);
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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, Square ksq);
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template Score Entry::do_king_safety<BLACK>(const Position& pos, Square ksq);
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} // namespace Pawns
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