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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) 2008-2015 Marco Costalba, Joona Kiiski, Tord Romstad

  Copyright (C) 2015-2018 Marco Costalba, Joona Kiiski, Gary Linscott, 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 <algorithm>

#include <cassert>

#include <numeric>

#include <vector>

#include "bitboard.h"

#include "types.h"

namespace {

  // There are 24 possible pawn squares: the first 4 files and ranks from 2 to 7

  const unsigned MAX_INDEX = 2*24*64*64; // stm * psq * wksq * bksq = 196608

  // Each uint32_t stores results of 32 positions, one per bit

  uint32_t KPKBitbase[MAX_INDEX / 32];

  // A KPK bitbase index is an integer in [0, IndexMax] range

  //

  // Information is mapped in a way that minimizes the number of iterations:

  //

  // bit  0- 5: white king square (from SQ_A1 to SQ_H8)

  // bit  6-11: black king square (from SQ_A1 to SQ_H8)

  // bit    12: side to move (WHITE or BLACK)

  // bit 13-14: white pawn file (from FILE_A to FILE_D)

  // bit 15-17: white pawn RANK_7 - rank (from RANK_7 - RANK_7 to RANK_7 - RANK_2)

  unsigned index(Color us, Square bksq, Square wksq, Square psq) {

    return wksq | (bksq << 6) | (us << 12) | (file_of(psq) << 13) | ((RANK_7 - rank_of(psq)) << 15);

  }

  enum Result {

    INVALID = 0,

    UNKNOWN = 1,

    DRAW    = 2,

    WIN     = 4

  };

  Result& operator|=(Result& r, Result v) { return r = Result(r | v); }

  struct KPKPosition {

    KPKPosition() = default;

    explicit KPKPosition(unsigned idx);

    operator Result() const { return result; }

    Result classify(const std::vector<KPKPosition>& db)

    { return us == WHITE ? classify<WHITE>(db) : classify<BLACK>(db); }

    template<Color Us> Result classify(const std::vector<KPKPosition>& db);

    Color us;

    Square ksq[COLOR_NB], psq;

    Result result;

  };

} // namespace

bool Bitbases::probe(Square wksq, Square wpsq, Square bksq, Color us) {

  assert(file_of(wpsq) <= FILE_D);

  unsigned idx = index(us, bksq, wksq, wpsq);

  return KPKBitbase[idx / 32] & (1 << (idx & 0x1F));

}

void Bitbases::init() {

  std::vector<KPKPosition> db(MAX_INDEX);

  unsigned idx, repeat = 1;

  // Initialize db with known win / draw positions

  for (idx = 0; idx < MAX_INDEX; ++idx)

      db[idx] = KPKPosition(idx);

  // Iterate through the positions until none of the unknown positions can be

  // changed to either wins or draws (15 cycles needed).

  while (repeat)

      for (repeat = idx = 0; idx < MAX_INDEX; ++idx)

          repeat |= (db[idx] == UNKNOWN && db[idx].classify(db) != UNKNOWN);

  // Map 32 results into one KPKBitbase[] entry

  for (idx = 0; idx < MAX_INDEX; ++idx)

      if (db[idx] == WIN)

          KPKBitbase[idx / 32] |= 1 << (idx & 0x1F);

}

namespace {

  KPKPosition::KPKPosition(unsigned idx) {

    ksq[WHITE] = Square((idx >>  0) & 0x3F);

    ksq[BLACK] = Square((idx >>  6) & 0x3F);

    us         = Color ((idx >> 12) & 0x01);

    psq        = make_square(File((idx >> 13) & 0x3), Rank(RANK_7 - ((idx >> 15) & 0x7)));

    // Check if two pieces are on the same square or if a king can be captured

    if (   distance(ksq[WHITE], ksq[BLACK]) <= 1

        || ksq[WHITE] == psq

        || ksq[BLACK] == psq

        || (us == WHITE && (PawnAttacks[WHITE][psq] & ksq[BLACK])))

        result = INVALID;

    // Immediate win if a pawn can be promoted without getting captured

    else if (   us == WHITE

             && rank_of(psq) == RANK_7

             && ksq[us] != psq + NORTH

             && (    distance(ksq[~us], psq + NORTH) > 1

                 || (PseudoAttacks[KING][ksq[us]] & (psq + NORTH))))

        result = WIN;

    // Immediate draw if it is a stalemate or a king captures undefended pawn

    else if (   us == BLACK

             && (  !(PseudoAttacks[KING][ksq[us]] & ~(PseudoAttacks[KING][ksq[~us]] | PawnAttacks[~us][psq]))

                 || (PseudoAttacks[KING][ksq[us]] & psq & ~PseudoAttacks[KING][ksq[~us]])))

        result = DRAW;

    // Position will be classified later

    else

        result = UNKNOWN;

  }

  template<Color Us>

  Result KPKPosition::classify(const std::vector<KPKPosition>& db) {

    // White to move: If one move leads to a position classified as WIN, the result

    // of the current position is WIN. If all moves lead to positions classified

    // as DRAW, the current position is classified as DRAW, otherwise the current

    // position is classified as UNKNOWN.

    //

    // Black to move: If one move leads to a position classified as DRAW, the result

    // of the current position is DRAW. If all moves lead to positions classified

    // as WIN, the position is classified as WIN, otherwise the current position is

    // classified as UNKNOWN.

    const Color  Them = (Us == WHITE ? BLACK : WHITE);

    const Result Good = (Us == WHITE ? WIN   : DRAW);

    const Result Bad  = (Us == WHITE ? DRAW  : WIN);

    Result r = INVALID;

    Bitboard b = PseudoAttacks[KING][ksq[Us]];

    while (b)

        r |= Us == WHITE ? db[index(Them, ksq[Them]  , pop_lsb(&b), psq)]

                         : db[index(Them, pop_lsb(&b), ksq[Them]  , psq)];

    if (Us == WHITE)

    {

        if (rank_of(psq) < RANK_7)      // Single push

            r |= db[index(Them, ksq[Them], ksq[Us], psq + NORTH)];

        if (   rank_of(psq) == RANK_2   // Double push

            && psq + NORTH != ksq[Us]

            && psq + NORTH != ksq[Them])

            r |= db[index(Them, ksq[Them], ksq[Us], psq + NORTH + NORTH)];

    }

    return result = r & Good  ? Good  : r & UNKNOWN ? UNKNOWN : Bad;

  }

} // namespace