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
6 Stockfish is free software: you can redistribute it and/or modify
7 it under the terms of the GNU General Public License as published by
8 the Free Software Foundation, either version 3 of the License, or
9 (at your option) any later version.
11 Stockfish is distributed in the hope that it will be useful,
12 but WITHOUT ANY WARRANTY; without even the implied warranty of
13 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
14 GNU General Public License for more details.
16 You should have received a copy of the GNU General Public License
17 along with this program. If not, see <http://www.gnu.org/licenses/>.
30 // There are 24 possible pawn squares: the first 4 files and ranks from 2 to 7
31 const unsigned MAX_INDEX = 2*24*64*64; // stm * psq * wksq * bksq = 196608
33 // Each uint32_t stores results of 32 positions, one per bit
34 uint32_t KPKBitbase[MAX_INDEX / 32];
36 // A KPK bitbase index is an integer in [0, IndexMax] range
38 // Information is mapped in a way that minimizes the number of iterations:
40 // bit 0- 5: white king square (from SQ_A1 to SQ_H8)
41 // bit 6-11: black king square (from SQ_A1 to SQ_H8)
42 // bit 12: side to move (WHITE or BLACK)
43 // bit 13-14: white pawn file (from FILE_A to FILE_D)
44 // bit 15-17: white pawn RANK_7 - rank (from RANK_7 - RANK_7 to RANK_7 - RANK_2)
45 unsigned index(Color us, Square bksq, Square wksq, Square psq) {
46 return wksq | (bksq << 6) | (us << 12) | (file_of(psq) << 13) | ((RANK_7 - rank_of(psq)) << 15);
56 inline Result& operator|=(Result& r, Result v) { return r = Result(r | v); }
59 KPKPosition() = default;
60 explicit KPKPosition(unsigned idx);
61 operator Result() const { return result; }
62 Result classify(const std::vector<KPKPosition>& db)
63 { return us == WHITE ? classify<WHITE>(db) : classify<BLACK>(db); }
65 template<Color Us> Result classify(const std::vector<KPKPosition>& db);
69 Square ksq[COLOR_NB], psq;
76 bool Bitbases::probe(Square wksq, Square wpsq, Square bksq, Color us) {
78 assert(file_of(wpsq) <= FILE_D);
80 unsigned idx = index(us, bksq, wksq, wpsq);
81 return KPKBitbase[idx / 32] & (1 << (idx & 0x1F));
85 void Bitbases::init() {
87 std::vector<KPKPosition> db(MAX_INDEX);
89 // Initialize db with known win / draw positions
90 std::generate(db.begin(), db.end(), [](){ static unsigned id; return KPKPosition(id++); });
92 // Iterate through the positions until none of the unknown positions can be
93 // changed to either wins or draws (15 cycles needed).
94 while (std::accumulate(db.begin(), db.end(), false, [&](bool repeat, KPKPosition& pos)
95 { return (pos == UNKNOWN && pos.classify(db) != UNKNOWN) || repeat; })){}
97 // Map 32 results into one KPKBitbase[] entry
100 KPKBitbase[pos.id / 32] |= 1 << (pos.id & 0x1F);
106 KPKPosition::KPKPosition(unsigned idx) {
109 ksq[WHITE] = Square((idx >> 0) & 0x3F);
110 ksq[BLACK] = Square((idx >> 6) & 0x3F);
111 us = Color ((idx >> 12) & 0x01);
112 psq = make_square(File((idx >> 13) & 0x3), RANK_7 - Rank((idx >> 15) & 0x7));
114 // Check if two pieces are on the same square or if a king can be captured
115 if ( distance(ksq[WHITE], ksq[BLACK]) <= 1
118 || (us == WHITE && (StepAttacksBB[PAWN][psq] & ksq[BLACK])))
121 // Immediate win if a pawn can be promoted without getting captured
122 else if ( us == WHITE
123 && rank_of(psq) == RANK_7
124 && ksq[us] != psq + DELTA_N
125 && ( distance(ksq[~us], psq + DELTA_N) > 1
126 || (StepAttacksBB[KING][ksq[us]] & (psq + DELTA_N))))
129 // Immediate draw if it is a stalemate or a king captures undefended pawn
130 else if ( us == BLACK
131 && ( !(StepAttacksBB[KING][ksq[us]] & ~(StepAttacksBB[KING][ksq[~us]] | StepAttacksBB[PAWN][psq]))
132 || (StepAttacksBB[KING][ksq[us]] & psq & ~StepAttacksBB[KING][ksq[~us]])))
135 // Position will be classified later
141 Result KPKPosition::classify(const std::vector<KPKPosition>& db) {
143 // White to move: If one move leads to a position classified as WIN, the result
144 // of the current position is WIN. If all moves lead to positions classified
145 // as DRAW, the current position is classified as DRAW, otherwise the current
146 // position is classified as UNKNOWN.
148 // Black to move: If one move leads to a position classified as DRAW, the result
149 // of the current position is DRAW. If all moves lead to positions classified
150 // as WIN, the position is classified as WIN, otherwise the current position is
151 // classified as UNKNOWN.
153 const Color Them = (Us == WHITE ? BLACK : WHITE);
154 const Result Good = (Us == WHITE ? WIN : DRAW);
155 const Result Bad = (Us == WHITE ? DRAW : WIN);
158 Bitboard b = StepAttacksBB[KING][ksq[Us]];
161 r |= Us == WHITE ? db[index(Them, ksq[Them] , pop_lsb(&b), psq)]
162 : db[index(Them, pop_lsb(&b), ksq[Them] , psq)];
166 if (rank_of(psq) < RANK_7) // Single push
167 r |= db[index(Them, ksq[Them], ksq[Us], psq + DELTA_N)];
169 if ( rank_of(psq) == RANK_2 // Double push
170 && psq + DELTA_N != ksq[Us]
171 && psq + DELTA_N != ksq[Them])
172 r |= db[index(Them, ksq[Them], ksq[Us], psq + DELTA_N + DELTA_N)];
175 return result = r & Good ? Good : r & UNKNOWN ? UNKNOWN : Bad;