1/*
2 Stockfish, a UCI chess playing engine derived from Glaurung 2.1
3 Copyright (C) 2004-2008 Tord Romstad (Glaurung author)
4 Copyright (C) 2008-2015 Marco Costalba, Joona Kiiski, Tord Romstad
5 Copyright (C) 2015-2019 Marco Costalba, Joona Kiiski, Gary Linscott, Tord Romstad
6
7 Stockfish is free software: you can redistribute it and/or modify
8 it under the terms of the GNU General Public License as published by
9 the Free Software Foundation, either version 3 of the License, or
10 (at your option) any later version.
11
12 Stockfish is distributed in the hope that it will be useful,
13 but WITHOUT ANY WARRANTY; without even the implied warranty of
14 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
15 GNU General Public License for more details.
16
17 You should have received a copy of the GNU General Public License
18 along with this program. If not, see <http://www.gnu.org/licenses/>.
19*/
20
21#include <algorithm>
22#include <cassert>
23
24#include "bitboard.h"
25#include "pawns.h"
26#include "position.h"
27#include "thread.h"
28
29namespace {
30
31 #define V Value
32 #define S(mg, eg) make_score(mg, eg)
33
34 // Pawn penalties
35 constexpr Score Backward = S( 9, 24);
36 constexpr Score Doubled = S(11, 56);
37 constexpr Score Isolated = S( 5, 15);
38 constexpr Score WeakLever = S( 0, 56);
39 constexpr Score WeakUnopposed = S(13, 27);
40
41 // Connected pawn bonus
42 constexpr int Connected[RANK_NB] = { 0, 7, 8, 12, 29, 48, 86 };
43
44 // Strength of pawn shelter for our king by [distance from edge][rank].
45 // RANK_1 = 0 is used for files where we have no pawn, or pawn is behind our king.
46 constexpr Value ShelterStrength[int(FILE_NB) / 2][RANK_NB] = {
47 { V( -6), V( 81), V( 93), V( 58), V( 39), V( 18), V( 25) },
48 { V(-43), V( 61), V( 35), V(-49), V(-29), V(-11), V( -63) },
49 { V(-10), V( 75), V( 23), V( -2), V( 32), V( 3), V( -45) },
50 { V(-39), V(-13), V(-29), V(-52), V(-48), V(-67), V(-166) }
51 };
52
53 // Danger of enemy pawns moving toward our king by [distance from edge][rank].
54 // RANK_1 = 0 is used for files where the enemy has no pawn, or their pawn
55 // is behind our king. Note that UnblockedStorm[0][1-2] accommodate opponent pawn
56 // on edge, likely blocked by our king.
57 constexpr Value UnblockedStorm[int(FILE_NB) / 2][RANK_NB] = {
58 { V( 89), V(-285), V(-185), V(93), V(57), V( 45), V( 51) },
59 { V( 44), V( -18), V( 123), V(46), V(39), V( -7), V( 23) },
60 { V( 4), V( 52), V( 162), V(37), V( 7), V(-14), V( -2) },
61 { V(-10), V( -14), V( 90), V(15), V( 2), V( -7), V(-16) }
62 };
63
64 #undef S
65 #undef V
66
67 template<Color Us>
68 Score evaluate(const Position& pos, Pawns::Entry* e) {
69
70 constexpr Color Them = (Us == WHITE ? BLACK : WHITE);
71 constexpr Direction Up = (Us == WHITE ? NORTH : SOUTH);
72
73 Bitboard neighbours, stoppers, doubled, support, phalanx;
74 Bitboard lever, leverPush;
75 Square s;
76 bool opposed, backward, passed;
77 Score score = SCORE_ZERO;
78 const Square* pl = pos.squares<PAWN>(Us);
79
80 Bitboard ourPawns = pos.pieces( Us, PAWN);
81 Bitboard theirPawns = pos.pieces(Them, PAWN);
82
83 Bitboard doubleAttackThem = pawn_double_attacks_bb<Them>(theirPawns);
84
85 e->passedPawns[Us] = e->pawnAttacksSpan[Us] = 0;
86 e->kingSquares[Us] = SQ_NONE;
87 e->pawnAttacks[Us] = pawn_attacks_bb<Us>(ourPawns);
88
89 // Loop through all pawns of the current color and score each pawn
90 while ((s = *pl++) != SQ_NONE)
91 {
92 assert(pos.piece_on(s) == make_piece(Us, PAWN));
93
94 Rank r = relative_rank(Us, s);
95
96 e->pawnAttacksSpan[Us] |= pawn_attack_span(Us, s);
97
98 // Flag the pawn
99 opposed = theirPawns & forward_file_bb(Us, s);
100 stoppers = theirPawns & passed_pawn_span(Us, s);
101 lever = theirPawns & PawnAttacks[Us][s];
102 leverPush = theirPawns & PawnAttacks[Us][s + Up];
103 doubled = ourPawns & (s - Up);
104 neighbours = ourPawns & adjacent_files_bb(s);
105 phalanx = neighbours & rank_bb(s);
106 support = neighbours & rank_bb(s - Up);
107
108 // A pawn is backward when it is behind all pawns of the same color on
109 // the adjacent files and cannot safely advance. Phalanx and isolated
110 // pawns will be excluded when the pawn is scored.
111 backward = !(neighbours & forward_ranks_bb(Them, s))
112 && (stoppers & (leverPush | (s + Up)));
113
114 // A pawn is passed if one of the three following conditions is true:
115 // (a) there is no stoppers except some levers
116 // (b) the only stoppers are the leverPush, but we outnumber them
117 // (c) there is only one front stopper which can be levered.
118 passed = !(stoppers ^ lever)
119 || ( !(stoppers ^ leverPush)
120 && popcount(phalanx) >= popcount(leverPush))
121 || ( stoppers == square_bb(s + Up) && r >= RANK_5
122 && (shift<Up>(support) & ~(theirPawns | doubleAttackThem)));
123
124 // Passed pawns will be properly scored later in evaluation when we have
125 // full attack info.
126 if (passed)
127 e->passedPawns[Us] |= s;
128
129 // Score this pawn
130 if (support | phalanx)
131 {
132 int v = Connected[r] * (phalanx ? 3 : 2) / (opposed ? 2 : 1)
133 + 17 * popcount(support);
134
135 score += make_score(v, v * (r - 2) / 4);
136 }
137
138 else if (!neighbours)
139 score -= Isolated + WeakUnopposed * int(!opposed);
140
141 else if (backward)
142 score -= Backward + WeakUnopposed * int(!opposed);
143
144 if (doubled && !support)
145 score -= Doubled;
146 }
147
148 // Penalize our unsupported pawns attacked twice by enemy pawns
149 score -= WeakLever * popcount( ourPawns
150 & doubleAttackThem
151 & ~e->pawnAttacks[Us]);
152
153 return score;
154 }
155
156} // namespace
157
158namespace Pawns {
159
160/// Pawns::probe() looks up the current position's pawns configuration in
161/// the pawns hash table. It returns a pointer to the Entry if the position
162/// is found. Otherwise a new Entry is computed and stored there, so we don't
163/// have to recompute all when the same pawns configuration occurs again.
164
165Entry* probe(const Position& pos) {
166
167 Key key = pos.pawn_key();
168 Entry* e = pos.this_thread()->pawnsTable[key];
169
170 if (e->key == key)
171 return e;
172
173 e->key = key;
174 e->scores[WHITE] = evaluate<WHITE>(pos, e);
175 e->scores[BLACK] = evaluate<BLACK>(pos, e);
176
177 return e;
178}
179
180
181/// Entry::evaluate_shelter() calculates the shelter bonus and the storm
182/// penalty for a king, looking at the king file and the two closest files.
183
184template<Color Us>
185void Entry::evaluate_shelter(const Position& pos, Square ksq, Score& shelter) {
186
187 constexpr Color Them = (Us == WHITE ? BLACK : WHITE);
188
189 Bitboard b = pos.pieces(PAWN) & ~forward_ranks_bb(Them, ksq);
190 Bitboard ourPawns = b & pos.pieces(Us);
191 Bitboard theirPawns = b & pos.pieces(Them);
192
193 Score bonus = make_score(5, 5);
194
195 File center = clamp(file_of(ksq), FILE_B, FILE_G);
196 for (File f = File(center - 1); f <= File(center + 1); ++f)
197 {
198 b = ourPawns & file_bb(f);
199 int ourRank = b ? relative_rank(Us, frontmost_sq(Them, b)) : 0;
200
201 b = theirPawns & file_bb(f);
202 int theirRank = b ? relative_rank(Us, frontmost_sq(Them, b)) : 0;
203
204 int d = std::min(f, ~f);
205 bonus += make_score(ShelterStrength[d][ourRank], 0);
206
207 if (ourRank && (ourRank == theirRank - 1))
208 bonus -= make_score(82 * (theirRank == RANK_3), 82 * (theirRank == RANK_3));
209 else
210 bonus -= make_score(UnblockedStorm[d][theirRank], 0);
211 }
212
213 if (mg_value(bonus) > mg_value(shelter))
214 shelter = bonus;
215}
216
217
218/// Entry::do_king_safety() calculates a bonus for king safety. It is called only
219/// when king square changes, which is about 20% of total king_safety() calls.
220
221template<Color Us>
222Score Entry::do_king_safety(const Position& pos) {
223
224 Square ksq = pos.square<KING>(Us);
225 kingSquares[Us] = ksq;
226 castlingRights[Us] = pos.castling_rights(Us);
227
228 Bitboard pawns = pos.pieces(Us, PAWN);
229 int minPawnDist = pawns ? 8 : 0;
230
231 if (pawns & PseudoAttacks[KING][ksq])
232 minPawnDist = 1;
233
234 else while (pawns)
235 minPawnDist = std::min(minPawnDist, distance(ksq, pop_lsb(&pawns)));
236
237 Score shelter = make_score(-VALUE_INFINITE, 0);
238 evaluate_shelter<Us>(pos, ksq, shelter);
239
240 // If we can castle use the bonus after the castling if it is bigger
241 if (pos.can_castle(Us | KING_SIDE))
242 evaluate_shelter<Us>(pos, relative_square(Us, SQ_G1), shelter);
243
244 if (pos.can_castle(Us | QUEEN_SIDE))
245 evaluate_shelter<Us>(pos, relative_square(Us, SQ_C1), shelter);
246
247 return shelter - make_score(0, 16 * minPawnDist);
248}
249
250// Explicit template instantiation
251template Score Entry::do_king_safety<WHITE>(const Position& pos);
252template Score Entry::do_king_safety<BLACK>(const Position& pos);
253
254} // namespace Pawns
255