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/*
Stockfish, a UCI chess playing engine derived from Glaurung 2.1
Copyright (C) 2004-2008 Tord Romstad (Glaurung author)
Copyright (C) 2008-2012 Marco Costalba, Joona Kiiski, Tord Romstad
Stockfish is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.
Stockfish is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program. If not, see <http://www.gnu.org/licenses/>.
*/
#include <cassert>
#include "bitboard.h"
#include "types.h"
namespace {
enum Result {
INVALID = 0,
UNKNOWN = 1,
DRAW = 2,
WIN = 4
};
inline Result& operator|=(Result& r, Result v) { return r = Result(r | v); }
struct KPKPosition {
Result classify_leaf(int idx);
Result classify(int idx, Result db[]);
private:
template<Color Us> Result classify(const Result db[]) const;
template<Color Us> Bitboard k_attacks() const {
return Us == WHITE ? StepAttacksBB[W_KING][wksq] : StepAttacksBB[B_KING][bksq];
}
Bitboard p_attacks() const { return StepAttacksBB[W_PAWN][psq]; }
void decode_index(int idx);
Square wksq, bksq, psq;
Color stm;
};
// The possible pawns squares are 24, the first 4 files and ranks from 2 to 7
const int IndexMax = 2 * 24 * 64 * 64; // stm * wp_sq * wk_sq * bk_sq = 196608
// Each uint32_t stores results of 32 positions, one per bit
uint32_t KPKBitbase[IndexMax / 32];
int index(Square wksq, Square bksq, Square psq, Color stm);
}
uint32_t Bitbases::probe_kpk(Square wksq, Square wpsq, Square bksq, Color stm) {
int idx = index(wksq, bksq, wpsq, stm);
return KPKBitbase[idx / 32] & (1 << (idx & 31));
}
void Bitbases::init_kpk() {
Result db[IndexMax];
KPKPosition pos;
int idx, bit, repeat = 1;
// Initialize table with known win / draw positions
for (idx = 0; idx < IndexMax; idx++)
db[idx] = pos.classify_leaf(idx);
// Iterate until all positions are classified (30 cycles needed)
while (repeat)
for (repeat = idx = 0; idx < IndexMax; idx++)
if (db[idx] == UNKNOWN && (db[idx] = pos.classify(idx, db)) != UNKNOWN)
repeat = 1;
// Map 32 position results into one KPKBitbase[] entry
for (idx = 0; idx < IndexMax / 32; idx++)
for (bit = 0; bit < 32; bit++)
if (db[32 * idx + bit] == WIN)
KPKBitbase[idx] |= 1 << bit;
}
namespace {
// A KPK bitbase index is an integer in [0, IndexMax] range
//
// Information is mapped in this way
//
// bit 0: side to move (WHITE or BLACK)
// bit 1- 6: black king square (from SQ_A1 to SQ_H8)
// bit 7-12: white king square (from SQ_A1 to SQ_H8)
// bit 13-14: white pawn file (from FILE_A to FILE_D)
// bit 15-17: white pawn rank - 1 (from RANK_2 - 1 to RANK_7 - 1)
int index(Square w, Square b, Square p, Color c) {
assert(file_of(p) <= FILE_D);
return c + (b << 1) + (w << 7) + (file_of(p) << 13) + ((rank_of(p) - 1) << 15);
}
void KPKPosition::decode_index(int idx) {
stm = Color(idx & 1);
bksq = Square((idx >> 1) & 63);
wksq = Square((idx >> 7) & 63);
psq = File((idx >> 13) & 3) | Rank((idx >> 15) + 1);
}
Result KPKPosition::classify_leaf(int idx) {
decode_index(idx);
// Check if two pieces are on the same square or if a king can be captured
if ( wksq == psq || wksq == bksq || bksq == psq
|| (k_attacks<WHITE>() & bksq)
|| (stm == WHITE && (p_attacks() & bksq)))
return INVALID;
// The position is an immediate win if it is white to move and the white
// pawn can be promoted without getting captured.
if ( rank_of(psq) == RANK_7
&& stm == WHITE
&& wksq != psq + DELTA_N
&& ( square_distance(bksq, psq + DELTA_N) > 1
||(k_attacks<WHITE>() & (psq + DELTA_N))))
return WIN;
// Check for known draw positions
//
// Case 1: Stalemate
if ( stm == BLACK
&& !(k_attacks<BLACK>() & ~(k_attacks<WHITE>() | p_attacks())))
return DRAW;
// Case 2: King can capture undefended pawn
if ( stm == BLACK
&& (k_attacks<BLACK>() & psq & ~k_attacks<WHITE>()))
return DRAW;
// Case 3: Black king in front of white pawn
if ( bksq == psq + DELTA_N
&& rank_of(psq) < RANK_7)
return DRAW;
// Case 4: White king in front of pawn and black has opposition
if ( stm == WHITE
&& wksq == psq + DELTA_N
&& bksq == wksq + DELTA_N + DELTA_N
&& rank_of(psq) < RANK_5)
return DRAW;
// Case 5: Stalemate with rook pawn
if ( bksq == SQ_A8
&& file_of(psq) == FILE_A)
return DRAW;
// Case 6: White king trapped on the rook file
if ( file_of(wksq) == FILE_A
&& file_of(psq) == FILE_A
&& rank_of(wksq) > rank_of(psq)
&& bksq == wksq + 2)
return DRAW;
return UNKNOWN;
}
template<Color Us>
Result KPKPosition::classify(const Result db[]) const {
// White to Move: If one move leads to a position classified as RESULT_WIN,
// the result of the current position is RESULT_WIN. If all moves lead to
// positions classified as RESULT_DRAW, the current position is classified
// RESULT_DRAW otherwise the current position is classified as RESULT_UNKNOWN.
//
// Black to Move: If one move leads to a position classified as RESULT_DRAW,
// the result of the current position is RESULT_DRAW. If all moves lead to
// positions classified as RESULT_WIN, the position is classified RESULT_WIN.
// Otherwise, the current position is classified as RESULT_UNKNOWN.
Result r = INVALID;
Bitboard b = k_attacks<Us>();
while (b)
{
r |= Us == WHITE ? db[index(pop_lsb(&b), bksq, psq, BLACK)]
: db[index(wksq, pop_lsb(&b), psq, WHITE)];
if (Us == WHITE && (r & WIN))
return WIN;
if (Us == BLACK && (r & DRAW))
return DRAW;
}
if (Us == WHITE && rank_of(psq) < RANK_7)
{
Square s = psq + DELTA_N;
r |= db[index(wksq, bksq, s, BLACK)]; // Single push
if (rank_of(s) == RANK_3 && s != wksq && s != bksq)
r |= db[index(wksq, bksq, s + DELTA_N, BLACK)]; // Double push
if (r & WIN)
return WIN;
}
return r & UNKNOWN ? UNKNOWN : Us == WHITE ? DRAW : WIN;
}
Result KPKPosition::classify(int idx, Result db[]) {
decode_index(idx);
return stm == WHITE ? classify<WHITE>(db) : classify<BLACK>(db);
}
}
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