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 <cassert>
22#include <numeric>
23#include <vector>
24
25#include "bitboard.h"
26#include "types.h"
27
28namespace {
29
30 // There are 24 possible pawn squares: files A to D and ranks from 2 to 7.
31 // Positions with the pawn on files E to H will be mirrored before probing.
32 constexpr unsigned MAX_INDEX = 2*24*64*64; // stm * psq * wksq * bksq = 196608
33
34 // Each uint32_t stores results of 32 positions, one per bit
35 uint32_t KPKBitbase[MAX_INDEX / 32];
36
37 // A KPK bitbase index is an integer in [0, IndexMax] range
38 //
39 // Information is mapped in a way that minimizes the number of iterations:
40 //
41 // bit 0- 5: white king square (from SQ_A1 to SQ_H8)
42 // bit 6-11: black king square (from SQ_A1 to SQ_H8)
43 // bit 12: side to move (WHITE or BLACK)
44 // bit 13-14: white pawn file (from FILE_A to FILE_D)
45 // bit 15-17: white pawn RANK_7 - rank (from RANK_7 - RANK_7 to RANK_7 - RANK_2)
46 unsigned index(Color us, Square bksq, Square wksq, Square psq) {
47 return wksq | (bksq << 6) | (us << 12) | (file_of(psq) << 13) | ((RANK_7 - rank_of(psq)) << 15);
48 }
49
50 enum Result {
51 INVALID = 0,
52 UNKNOWN = 1,
53 DRAW = 2,
54 WIN = 4
55 };
56
57 Result& operator|=(Result& r, Result v) { return r = Result(r | v); }
58
59 struct KPKPosition {
60 KPKPosition() = default;
61 explicit KPKPosition(unsigned idx);
62 operator Result() const { return result; }
63 Result classify(const std::vector<KPKPosition>& db)
64 { return us == WHITE ? classify<WHITE>(db) : classify<BLACK>(db); }
65
66 template<Color Us> Result classify(const std::vector<KPKPosition>& db);
67
68 Color us;
69 Square ksq[COLOR_NB], psq;
70 Result result;
71 };
72
73} // namespace
74
75
76bool Bitbases::probe(Square wksq, Square wpsq, Square bksq, Color us) {
77
78 assert(file_of(wpsq) <= FILE_D);
79
80 unsigned idx = index(us, bksq, wksq, wpsq);
81 return KPKBitbase[idx / 32] & (1 << (idx & 0x1F));
82}
83
84
85void Bitbases::init() {
86
87 std::vector<KPKPosition> db(MAX_INDEX);
88 unsigned idx, repeat = 1;
89
90 // Initialize db with known win / draw positions
91 for (idx = 0; idx < MAX_INDEX; ++idx)
92 db[idx] = KPKPosition(idx);
93
94 // Iterate through the positions until none of the unknown positions can be
95 // changed to either wins or draws (15 cycles needed).
96 while (repeat)
97 for (repeat = idx = 0; idx < MAX_INDEX; ++idx)
98 repeat |= (db[idx] == UNKNOWN && db[idx].classify(db) != UNKNOWN);
99
100 // Map 32 results into one KPKBitbase[] entry
101 for (idx = 0; idx < MAX_INDEX; ++idx)
102 if (db[idx] == WIN)
103 KPKBitbase[idx / 32] |= 1 << (idx & 0x1F);
104}
105
106
107namespace {
108
109 KPKPosition::KPKPosition(unsigned idx) {
110
111 ksq[WHITE] = Square((idx >> 0) & 0x3F);
112 ksq[BLACK] = Square((idx >> 6) & 0x3F);
113 us = Color ((idx >> 12) & 0x01);
114 psq = make_square(File((idx >> 13) & 0x3), Rank(RANK_7 - ((idx >> 15) & 0x7)));
115
116 // Check if two pieces are on the same square or if a king can be captured
117 if ( distance(ksq[WHITE], ksq[BLACK]) <= 1
118 || ksq[WHITE] == psq
119 || ksq[BLACK] == psq
120 || (us == WHITE && (PawnAttacks[WHITE][psq] & ksq[BLACK])))
121 result = INVALID;
122
123 // Immediate win if a pawn can be promoted without getting captured
124 else if ( us == WHITE
125 && rank_of(psq) == RANK_7
126 && ksq[us] != psq + NORTH
127 && ( distance(ksq[~us], psq + NORTH) > 1
128 || (PseudoAttacks[KING][ksq[us]] & (psq + NORTH))))
129 result = WIN;
130
131 // Immediate draw if it is a stalemate or a king captures undefended pawn
132 else if ( us == BLACK
133 && ( !(PseudoAttacks[KING][ksq[us]] & ~(PseudoAttacks[KING][ksq[~us]] | PawnAttacks[~us][psq]))
134 || (PseudoAttacks[KING][ksq[us]] & psq & ~PseudoAttacks[KING][ksq[~us]])))
135 result = DRAW;
136
137 // Position will be classified later
138 else
139 result = UNKNOWN;
140 }
141
142 template<Color Us>
143 Result KPKPosition::classify(const std::vector<KPKPosition>& db) {
144
145 // White to move: If one move leads to a position classified as WIN, the result
146 // of the current position is WIN. If all moves lead to positions classified
147 // as DRAW, the current position is classified as DRAW, otherwise the current
148 // position is classified as UNKNOWN.
149 //
150 // Black to move: If one move leads to a position classified as DRAW, the result
151 // of the current position is DRAW. If all moves lead to positions classified
152 // as WIN, the position is classified as WIN, otherwise the current position is
153 // classified as UNKNOWN.
154
155 constexpr Color Them = (Us == WHITE ? BLACK : WHITE);
156 constexpr Result Good = (Us == WHITE ? WIN : DRAW);
157 constexpr Result Bad = (Us == WHITE ? DRAW : WIN);
158
159 Result r = INVALID;
160 Bitboard b = PseudoAttacks[KING][ksq[Us]];
161
162 while (b)
163 r |= Us == WHITE ? db[index(Them, ksq[Them] , pop_lsb(&b), psq)]
164 : db[index(Them, pop_lsb(&b), ksq[Them] , psq)];
165
166 if (Us == WHITE)
167 {
168 if (rank_of(psq) < RANK_7) // Single push
169 r |= db[index(Them, ksq[Them], ksq[Us], psq + NORTH)];
170
171 if ( rank_of(psq) == RANK_2 // Double push
172 && psq + NORTH != ksq[Us]
173 && psq + NORTH != ksq[Them])
174 r |= db[index(Them, ksq[Them], ksq[Us], psq + NORTH + NORTH)];
175 }
176
177 return result = r & Good ? Good : r & UNKNOWN ? UNKNOWN : Bad;
178 }
179
180} // namespace
181