stockfish/src/movegen.cpp

/*
  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 <algorithm>

#include "bitcount.h"
#include "movegen.h"
#include "position.h"

// Simple macro to wrap a very common while loop, no facny, no flexibility,
// hardcoded list name 'mlist' and from square 'from'.
#define SERIALIZE_MOVES(b) while (b) (*mlist++).move = make_move(from, pop_1st_bit(&b))

// Version used for pawns, where the 'from' square is given as a delta from the 'to' square
#define SERIALIZE_MOVES_D(b, d) while (b) { to = pop_1st_bit(&b); (*mlist++).move = make_move(to + (d), to); }

namespace {

  enum CastlingSide {
    KING_SIDE,
    QUEEN_SIDE
  };

  template<CastlingSide>
  MoveStack* generate_castle_moves(const Position&, MoveStack*, Color us);

  template<Color, MoveType>
  MoveStack* generate_pawn_moves(const Position&, MoveStack*, Bitboard, Square);

  template<PieceType Pt>
  inline MoveStack* generate_discovered_checks(const Position& pos, MoveStack* mlist, Square from) {

    assert(Pt != QUEEN && Pt != PAWN);

    Bitboard b = pos.attacks_from<Pt>(from) & pos.empty_squares();

    if (Pt == KING)
        b &= ~QueenPseudoAttacks[pos.king_square(flip(pos.side_to_move()))];

    SERIALIZE_MOVES(b);
    return mlist;
  }

  template<PieceType Pt>
  inline MoveStack* generate_direct_checks(const Position& pos, MoveStack* mlist, Color us,
                                           Bitboard dc, Square ksq) {
    assert(Pt != KING && Pt != PAWN);

    Bitboard checkSqs, b;
    Square from;
    const Square* pl = pos.piece_list(us, Pt);

    if ((from = *pl++) == SQ_NONE)
        return mlist;

    checkSqs = pos.attacks_from<Pt>(ksq) & pos.empty_squares();

    do
    {
        if (   (Pt == QUEEN  && !(QueenPseudoAttacks[from]  & checkSqs))
            || (Pt == ROOK   && !(RookPseudoAttacks[from]   & checkSqs))
            || (Pt == BISHOP && !(BishopPseudoAttacks[from] & checkSqs)))
            continue;

        if (dc && bit_is_set(dc, from))
            continue;

        b = pos.attacks_from<Pt>(from) & checkSqs;
        SERIALIZE_MOVES(b);

    } while ((from = *pl++) != SQ_NONE);

    return mlist;
  }

  template<>
  FORCE_INLINE MoveStack* generate_direct_checks<PAWN>(const Position& p, MoveStack* m, Color us, Bitboard dc, Square ksq) {

    return (us == WHITE ? generate_pawn_moves<WHITE, MV_CHECK>(p, m, dc, ksq)
                        : generate_pawn_moves<BLACK, MV_CHECK>(p, m, dc, ksq));
  }

  template<PieceType Pt, MoveType Type>
  FORCE_INLINE MoveStack* generate_piece_moves(const Position& p, MoveStack* m, Color us, Bitboard t) {

    assert(Pt == PAWN);
    return (us == WHITE ? generate_pawn_moves<WHITE, Type>(p, m, t, SQ_NONE)
                        : generate_pawn_moves<BLACK, Type>(p, m, t, SQ_NONE));
  }

  template<PieceType Pt>
  FORCE_INLINE MoveStack* generate_piece_moves(const Position& pos, MoveStack* mlist, Color us, Bitboard target) {

    Bitboard b;
    Square from;
    const Square* pl = pos.piece_list(us, Pt);

    if (*pl != SQ_NONE)
    {
        do {
            from = *pl;
            b = pos.attacks_from<Pt>(from) & target;
            SERIALIZE_MOVES(b);
        } while (*++pl != SQ_NONE);
    }
    return mlist;
  }

  template<>
  FORCE_INLINE MoveStack* generate_piece_moves<KING>(const Position& pos, MoveStack* mlist, Color us, Bitboard target) {

    Bitboard b;
    Square from = pos.king_square(us);

    b = pos.attacks_from<KING>(from) & target;
    SERIALIZE_MOVES(b);
    return mlist;
  }

}


/// generate<MV_CAPTURE> generates all pseudo-legal captures and queen
/// promotions. Returns a pointer to the end of the move list.
///
/// generate<MV_NON_CAPTURE> generates all pseudo-legal non-captures and
/// underpromotions. Returns a pointer to the end of the move list.
///
/// generate<MV_NON_EVASION> generates all pseudo-legal captures and
/// non-captures. Returns a pointer to the end of the move list.

template<MoveType Type>
MoveStack* generate(const Position& pos, MoveStack* mlist) {

  assert(Type == MV_CAPTURE || Type == MV_NON_CAPTURE || Type == MV_NON_EVASION);
  assert(!pos.in_check());

  Color us = pos.side_to_move();
  Bitboard target;

  if (Type == MV_CAPTURE)
      target = pos.pieces(flip(us));

  else if (Type == MV_NON_CAPTURE)
      target = pos.empty_squares();

  else if (Type == MV_NON_EVASION)
      target = pos.pieces(flip(us)) | pos.empty_squares();

  mlist = generate_piece_moves<PAWN, Type>(pos, mlist, us, target);
  mlist = generate_piece_moves<KNIGHT>(pos, mlist, us, target);
  mlist = generate_piece_moves<BISHOP>(pos, mlist, us, target);
  mlist = generate_piece_moves<ROOK>(pos, mlist, us, target);
  mlist = generate_piece_moves<QUEEN>(pos, mlist, us, target);
  mlist = generate_piece_moves<KING>(pos, mlist, us, target);

  if (Type != MV_CAPTURE && pos.can_castle(us))
  {
      if (pos.can_castle(us == WHITE ? WHITE_OO : BLACK_OO))
          mlist = generate_castle_moves<KING_SIDE>(pos, mlist, us);

      if (pos.can_castle(us == WHITE ? WHITE_OOO : BLACK_OOO))
          mlist = generate_castle_moves<QUEEN_SIDE>(pos, mlist, us);
  }

  return mlist;
}

// Explicit template instantiations
template MoveStack* generate<MV_CAPTURE>(const Position& pos, MoveStack* mlist);
template MoveStack* generate<MV_NON_CAPTURE>(const Position& pos, MoveStack* mlist);
template MoveStack* generate<MV_NON_EVASION>(const Position& pos, MoveStack* mlist);


/// generate<MV_NON_CAPTURE_CHECK> generates all pseudo-legal non-captures and knight
/// underpromotions that give check. Returns a pointer to the end of the move list.
template<>
MoveStack* generate<MV_NON_CAPTURE_CHECK>(const Position& pos, MoveStack* mlist) {

  assert(!pos.in_check());

  Bitboard b, dc;
  Square from;
  Color us = pos.side_to_move();
  Square ksq = pos.king_square(flip(us));

  assert(pos.piece_on(ksq) == make_piece(flip(us), KING));

  // Discovered non-capture checks
  b = dc = pos.discovered_check_candidates();

  while (b)
  {
     from = pop_1st_bit(&b);
     switch (type_of(pos.piece_on(from)))
     {
      case PAWN:   /* Will be generated togheter with pawns direct checks */     break;
      case KNIGHT: mlist = generate_discovered_checks<KNIGHT>(pos, mlist, from); break;
      case BISHOP: mlist = generate_discovered_checks<BISHOP>(pos, mlist, from); break;
      case ROOK:   mlist = generate_discovered_checks<ROOK>(pos, mlist, from);   break;
      case KING:   mlist = generate_discovered_checks<KING>(pos, mlist, from);   break;
      default: assert(false); break;
     }
  }

  // Direct non-capture checks
  mlist = generate_direct_checks<PAWN>(pos, mlist, us, dc, ksq);
  mlist = generate_direct_checks<KNIGHT>(pos, mlist, us, dc, ksq);
  mlist = generate_direct_checks<BISHOP>(pos, mlist, us, dc, ksq);
  mlist = generate_direct_checks<ROOK>(pos, mlist, us, dc, ksq);
  return  generate_direct_checks<QUEEN>(pos, mlist, us, dc, ksq);
}


/// generate<MV_EVASION> generates all pseudo-legal check evasions when the side
/// to move is in check. Returns a pointer to the end of the move list.
template<>
MoveStack* generate<MV_EVASION>(const Position& pos, MoveStack* mlist) {

  assert(pos.in_check());

  Bitboard b, target;
  Square from, checksq;
  int checkersCnt = 0;
  Color us = pos.side_to_move();
  Square ksq = pos.king_square(us);
  Bitboard checkers = pos.checkers();
  Bitboard sliderAttacks = 0;

  assert(pos.piece_on(ksq) == make_piece(us, KING));
  assert(checkers);

  // Find squares attacked by slider checkers, we will remove
  // them from the king evasions set so to early skip known
  // illegal moves and avoid an useless legality check later.
  b = checkers;
  do
  {
      checkersCnt++;
      checksq = pop_1st_bit(&b);

      assert(color_of(pos.piece_on(checksq)) == flip(us));

      switch (type_of(pos.piece_on(checksq)))
      {
      case BISHOP: sliderAttacks |= BishopPseudoAttacks[checksq]; break;
      case ROOK:   sliderAttacks |= RookPseudoAttacks[checksq];   break;
      case QUEEN:
          // If queen and king are far we can safely remove all the squares attacked
          // in the other direction becuase are not reachable by the king anyway.
          if (squares_between(ksq, checksq) || (RookPseudoAttacks[checksq] & (1ULL << ksq)))
              sliderAttacks |= QueenPseudoAttacks[checksq];

          // Otherwise, if king and queen are adjacent and on a diagonal line, we need to
          // use real rook attacks to check if king is safe to move in the other direction.
          // For example: king in B2, queen in A1 a knight in B1, and we can safely move to C1.
          else
              sliderAttacks |= BishopPseudoAttacks[checksq] | pos.attacks_from<ROOK>(checksq);

      default:
          break;
      }
  } while (b);

  // Generate evasions for king, capture and non capture moves
  b = pos.attacks_from<KING>(ksq) & ~pos.pieces(us) & ~sliderAttacks;
  from = ksq;
  SERIALIZE_MOVES(b);

  // Generate evasions for other pieces only if not double check
  if (checkersCnt > 1)
      return mlist;

  // Find squares where a blocking evasion or a capture of the
  // checker piece is possible.
  target = squares_between(checksq, ksq) | checkers;

  mlist = generate_piece_moves<PAWN, MV_EVASION>(pos, mlist, us, target);
  mlist = generate_piece_moves<KNIGHT>(pos, mlist, us, target);
  mlist = generate_piece_moves<BISHOP>(pos, mlist, us, target);
  mlist = generate_piece_moves<ROOK>(pos, mlist, us, target);
  return  generate_piece_moves<QUEEN>(pos, mlist, us, target);
}


/// generate<MV_LEGAL> computes a complete list of legal moves in the current position

template<>
MoveStack* generate<MV_LEGAL>(const Position& pos, MoveStack* mlist) {

  MoveStack *last, *cur = mlist;
  Bitboard pinned = pos.pinned_pieces();

  last = pos.in_check() ? generate<MV_EVASION>(pos, mlist)
                        : generate<MV_NON_EVASION>(pos, mlist);

  // Remove illegal moves from the list
  while (cur != last)
      if (!pos.pl_move_is_legal(cur->move, pinned))
          cur->move = (--last)->move;
      else
          cur++;

  return last;
}


namespace {

  template<Square Delta>
  inline Bitboard move_pawns(Bitboard p) {

    return Delta == DELTA_N  ? p << 8 : Delta == DELTA_S  ? p >> 8 :
           Delta == DELTA_NE ? p << 9 : Delta == DELTA_SE ? p >> 7 :
           Delta == DELTA_NW ? p << 7 : Delta == DELTA_SW ? p >> 9 : p;
  }

  template<Square Delta>
  inline MoveStack* generate_pawn_captures(MoveStack* mlist, Bitboard pawns, Bitboard target) {

    const Bitboard TFileABB = (Delta == DELTA_NE || Delta == DELTA_SE ? FileABB : FileHBB);

    Bitboard b;
    Square to;

    // Captures in the a1-h8 (a8-h1 for black) diagonal or in the h1-a8 (h8-a1 for black)
    b = move_pawns<Delta>(pawns) & target & ~TFileABB;
    SERIALIZE_MOVES_D(b, -Delta);
    return mlist;
  }

  template<MoveType Type, Square Delta>
  inline MoveStack* generate_promotions(const Position& pos, MoveStack* mlist, Bitboard pawnsOn7, Bitboard target) {

    const Bitboard TFileABB = (Delta == DELTA_NE || Delta == DELTA_SE ? FileABB : FileHBB);

    Bitboard b;
    Square to;

    // Promotions and under-promotions, both captures and non-captures
    b = move_pawns<Delta>(pawnsOn7) & target;

    if (Delta != DELTA_N && Delta != DELTA_S)
        b &= ~TFileABB;

    while (b)
    {
        to = pop_1st_bit(&b);

        if (Type == MV_CAPTURE || Type == MV_EVASION || Type == MV_NON_EVASION)
            (*mlist++).move = make_promotion(to - Delta, to, QUEEN);

        if (Type == MV_NON_CAPTURE || Type == MV_EVASION || Type == MV_NON_EVASION)
        {
            (*mlist++).move = make_promotion(to - Delta, to, ROOK);
            (*mlist++).move = make_promotion(to - Delta, to, BISHOP);
            (*mlist++).move = make_promotion(to - Delta, to, KNIGHT);
        }

        // This is the only possible under promotion that can give a check
        // not already included in the queen-promotion.
        if (   Type == MV_CHECK
            && bit_is_set(pos.attacks_from<KNIGHT>(to), pos.king_square(Delta > 0 ? BLACK : WHITE)))
            (*mlist++).move = make_promotion(to - Delta, to, KNIGHT);
        else (void)pos; // Silence a warning under MSVC
    }
    return mlist;
  }

  template<Color Us, MoveType Type>
  MoveStack* generate_pawn_moves(const Position& pos, MoveStack* mlist, Bitboard target, Square ksq) {

    // Calculate our parametrized parameters at compile time, named
    // according to the point of view of white side.
    const Color    Them      = (Us == WHITE ? BLACK    : WHITE);
    const Bitboard TRank7BB  = (Us == WHITE ? Rank7BB  : Rank2BB);
    const Bitboard TRank3BB  = (Us == WHITE ? Rank3BB  : Rank6BB);
    const Square   UP        = (Us == WHITE ? DELTA_N  : DELTA_S);
    const Square   RIGHT_UP  = (Us == WHITE ? DELTA_NE : DELTA_SW);
    const Square   LEFT_UP   = (Us == WHITE ? DELTA_NW : DELTA_SE);

    Square to;
    Bitboard b1, b2, dc1, dc2, pawnPushes, emptySquares;
    Bitboard pawns = pos.pieces(PAWN, Us);
    Bitboard pawnsOn7 = pawns & TRank7BB;
    Bitboard enemyPieces = (Type == MV_CAPTURE ? target : pos.pieces(Them));

    // Pre-calculate pawn pushes before changing emptySquares definition
    if (Type != MV_CAPTURE)
    {
        emptySquares = (Type == MV_NON_CAPTURE ? target : pos.empty_squares());
        pawnPushes = move_pawns<UP>(pawns & ~TRank7BB) & emptySquares;
    }

    if (Type == MV_EVASION)
    {
        emptySquares &= target; // Only blocking squares
        enemyPieces  &= target; // Capture only the checker piece
    }

    // Promotions and underpromotions
    if (pawnsOn7)
    {
        if (Type == MV_CAPTURE)
            emptySquares = pos.empty_squares();

        pawns &= ~TRank7BB;
        mlist = generate_promotions<Type, RIGHT_UP>(pos, mlist, pawnsOn7, enemyPieces);
        mlist = generate_promotions<Type, LEFT_UP>(pos, mlist, pawnsOn7, enemyPieces);
        mlist = generate_promotions<Type, UP>(pos, mlist, pawnsOn7, emptySquares);
    }

    // Standard captures
    if (Type == MV_CAPTURE || Type == MV_EVASION || Type == MV_NON_EVASION)
    {
        mlist = generate_pawn_captures<RIGHT_UP>(mlist, pawns, enemyPieces);
        mlist = generate_pawn_captures<LEFT_UP>(mlist, pawns, enemyPieces);
    }

    // Single and double pawn pushes
    if (Type != MV_CAPTURE)
    {
        b1 = (Type != MV_EVASION ? pawnPushes : pawnPushes & emptySquares);
        b2 = move_pawns<UP>(pawnPushes & TRank3BB) & emptySquares;

        if (Type == MV_CHECK)
        {
            // Consider only pawn moves which give direct checks
            b1 &= pos.attacks_from<PAWN>(ksq, Them);
            b2 &= pos.attacks_from<PAWN>(ksq, Them);

            // Add pawn moves which gives discovered check. This is possible only
            // if the pawn is not on the same file as the enemy king, because we
            // don't generate captures.
            if (pawns & target) // For CHECK type target is dc bitboard
            {
                dc1 = move_pawns<UP>(pawns & target & ~file_bb(ksq)) & emptySquares;
                dc2 = move_pawns<UP>(dc1 & TRank3BB) & emptySquares;

                b1 |= dc1;
                b2 |= dc2;
            }
        }
        SERIALIZE_MOVES_D(b1, -UP);
        SERIALIZE_MOVES_D(b2, -UP -UP);
    }

    // En passant captures
    if (  (Type == MV_CAPTURE || Type == MV_EVASION || Type == MV_NON_EVASION)
        && pos.ep_square() != SQ_NONE)
    {
        assert(Us != WHITE || rank_of(pos.ep_square()) == RANK_6);
        assert(Us != BLACK || rank_of(pos.ep_square()) == RANK_3);

        // An en passant capture can be an evasion only if the checking piece
        // is the double pushed pawn and so is in the target. Otherwise this
        // is a discovery check and we are forced to do otherwise.
        if (Type == MV_EVASION && !bit_is_set(target, pos.ep_square() - UP))
            return mlist;

        b1 = pawns & pos.attacks_from<PAWN>(pos.ep_square(), Them);

        assert(b1);

        while (b1)
        {
            to = pop_1st_bit(&b1);
            (*mlist++).move = make_enpassant(to, pos.ep_square());
        }
    }
    return mlist;
  }

  template<CastlingSide Side>
  MoveStack* generate_castle_moves(const Position& pos, MoveStack* mlist, Color us) {

    CastleRight f = CastleRight((Side == KING_SIDE ? WHITE_OO : WHITE_OOO) << us);
    Color them = flip(us);

    // After castling, the rook and king's final positions are exactly the same
    // in Chess960 as they would be in standard chess.
    Square kfrom = pos.king_square(us);
    Square rfrom = pos.castle_rook_square(f);
    Square kto = relative_square(us, Side == KING_SIDE ? SQ_G1 : SQ_C1);
    Square rto = relative_square(us, Side == KING_SIDE ? SQ_F1 : SQ_D1);

    assert(!pos.in_check());
    assert(pos.piece_on(kfrom) == make_piece(us, KING));
    assert(pos.piece_on(rfrom) == make_piece(us, ROOK));

    // Unimpeded rule: All the squares between the king's initial and final squares
    // (including the final square), and all the squares between the rook's initial
    // and final squares (including the final square), must be vacant except for
    // the king and castling rook.
    for (Square s = std::min(kfrom, kto); s <= std::max(kfrom, kto); s++)
        if (  (s != kfrom && s != rfrom && !pos.square_is_empty(s))
            ||(pos.attackers_to(s) & pos.pieces(them)))
            return mlist;

    for (Square s = std::min(rfrom, rto); s <= std::max(rfrom, rto); s++)
        if (s != kfrom && s != rfrom && !pos.square_is_empty(s))
            return mlist;

    // Because we generate only legal castling moves we need to verify that
    // when moving the castling rook we do not discover some hidden checker.
    // For instance an enemy queen in SQ_A1 when castling rook is in SQ_B1.
    if (pos.is_chess960())
    {
        Bitboard occ = pos.occupied_squares();
        clear_bit(&occ, rfrom);
        if (pos.attackers_to(kto, occ) & pos.pieces(them))
            return mlist;
    }

    (*mlist++).move = make_castle(kfrom, rfrom);

    return mlist;
  }

} // namespace