pawns.cpp

/*
  Stockfish, a UCI chess playing engine derived from Glaurung 2.1
  Copyright (C) 2004-2008 Tord Romstad (Glaurung author)
  Copyright (C) 2008-2013 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 "bitcount.h"
#include "pawns.h"
#include "position.h"

namespace {

  #define V Value
  #define S(mg, eg) make_score(mg, eg)

  // Doubled pawn penalty by opposed flag and file
  const Score DoubledPawnPenalty[2][FILE_NB] = {
  { S(13, 43), S(20, 48), S(23, 48), S(23, 48),
    S(23, 48), S(23, 48), S(20, 48), S(13, 43) },
  { S(13, 43), S(20, 48), S(23, 48), S(23, 48),
    S(23, 48), S(23, 48), S(20, 48), S(13, 43) }};

  // Isolated pawn penalty by opposed flag and file
  const Score IsolatedPawnPenalty[2][FILE_NB] = {
  { S(37, 45), S(54, 52), S(60, 52), S(60, 52),
    S(60, 52), S(60, 52), S(54, 52), S(37, 45) },
  { S(25, 30), S(36, 35), S(40, 35), S(40, 35),
    S(40, 35), S(40, 35), S(36, 35), S(25, 30) }};

  // Backward pawn penalty by opposed flag and file
  const Score BackwardPawnPenalty[2][FILE_NB] = {
  { S(30, 42), S(43, 46), S(49, 46), S(49, 46),
    S(49, 46), S(49, 46), S(43, 46), S(30, 42) },
  { S(20, 28), S(29, 31), S(33, 31), S(33, 31),
    S(33, 31), S(33, 31), S(29, 31), S(20, 28) }};

  // Pawn chain membership bonus by file
  const Score ChainBonus[FILE_NB] = {
    S(11,-1), S(13,-1), S(13,-1), S(14,-1),
    S(14,-1), S(13,-1), S(13,-1), S(11,-1)
  };

  // Candidate passed pawn bonus by rank
  const Score CandidateBonus[RANK_NB] = {
    S( 0, 0), S( 6, 13), S(6,13), S(14,29),
    S(34,68), S(83,166), S(0, 0), S( 0, 0)
  };

  // Weakness of our pawn shelter in front of the king indexed by [king pawn][rank]
  const Value ShelterWeakness[2][RANK_NB] =
  { { V(141), V(0), V(38), V(102), V(128), V(141), V(141) },
    { V( 61), V(0), V(16), V( 44), V( 56), V( 61), V( 61) } };

  // Danger of enemy pawns moving toward our king indexed by [pawn blocked][rank]
  const Value StormDanger[2][RANK_NB] =
  { { V(26), V(0), V(128), V(51), V(26) },
    { V(13), V(0), V( 64), V(25), V(13) } };

  // Max bonus for king safety. Corresponds to start position with all the pawns
  // in front of the king and no enemy pawn on the horizont.
  const Value MaxSafetyBonus = V(263);

  #undef S
  #undef V

  template<Color Us>
  Score evaluate_pawns(const Position& pos, Bitboard ourPawns,
                       Bitboard theirPawns, Pawns::Entry* e) {

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

    Bitboard b;
    Square s;
    File f;
    Rank r;
    bool passed, isolated, doubled, opposed, chain, backward, candidate;
    Score value = SCORE_ZERO;
    const Square* pl = pos.piece_list(Us, PAWN);

    // Loop through all pawns of the current color and score each pawn
    while ((s = *pl++) != SQ_NONE)
    {
        assert(pos.piece_on(s) == make_piece(Us, PAWN));

        f = file_of(s);
        r = rank_of(s);

        // This file cannot be half open
        e->halfOpenFiles[Us] &= ~(1 << f);

        // Our rank plus previous one. Used for chain detection
        b = rank_bb(r) | rank_bb(Us == WHITE ? r - Rank(1) : r + Rank(1));

        // Flag the pawn as passed, isolated, doubled or member of a pawn
        // chain (but not the backward one).
        chain    =   ourPawns   & adjacent_files_bb(f) & b;
        isolated = !(ourPawns   & adjacent_files_bb(f));
        doubled  =   ourPawns   & forward_bb(Us, s);
        opposed  =   theirPawns & forward_bb(Us, s);
        passed   = !(theirPawns & passed_pawn_mask(Us, s));

        // Test for backward pawn
        backward = false;

        // If the pawn is passed, isolated, or member of a pawn chain it cannot
        // be backward. If there are friendly pawns behind on adjacent files
        // or if can capture an enemy pawn it cannot be backward either.
        if (   !(passed | isolated | chain)
            && !(ourPawns & attack_span_mask(Them, s))
            && !(pos.attacks_from<PAWN>(s, Us) & theirPawns))
        {
            // We now know that there are no friendly pawns beside or behind this
            // pawn on adjacent files. We now check whether the pawn is
            // backward by looking in the forward direction on the adjacent
            // files, and seeing whether we meet a friendly or an enemy pawn first.
            b = pos.attacks_from<PAWN>(s, Us);

            // Note that we are sure to find something because pawn is not passed
            // nor isolated, so loop is potentially infinite, but it isn't.
            while (!(b & (ourPawns | theirPawns)))
                Us == WHITE ? b <<= 8 : b >>= 8;

            // The friendly pawn needs to be at least two ranks closer than the
            // enemy pawn in order to help the potentially backward pawn advance.
            backward = (b | (Us == WHITE ? b << 8 : b >> 8)) & theirPawns;
        }

        assert(opposed | passed | (attack_span_mask(Us, s) & theirPawns));

        // A not passed pawn is a candidate to become passed if it is free to
        // advance and if the number of friendly pawns beside or behind this
        // pawn on adjacent files is higher or equal than the number of
        // enemy pawns in the forward direction on the adjacent files.
        candidate =   !(opposed | passed | backward | isolated)
                   && (b = attack_span_mask(Them, s + pawn_push(Us)) & ourPawns) != 0
                   &&  popcount<Max15>(b) >= popcount<Max15>(attack_span_mask(Us, s) & theirPawns);

        // Passed pawns will be properly scored in evaluation because we need
        // full attack info to evaluate passed pawns. Only the frontmost passed
        // pawn on each file is considered a true passed pawn.
        if (passed && !doubled)
            e->passedPawns[Us] |= s;

        // Score this pawn
        if (isolated)
            value -= IsolatedPawnPenalty[opposed][f];

        if (doubled)
            value -= DoubledPawnPenalty[opposed][f];

        if (backward)
            value -= BackwardPawnPenalty[opposed][f];

        if (chain)
            value += ChainBonus[f];

        if (candidate)
            value += CandidateBonus[relative_rank(Us, s)];
    }

    e->pawnsOnSquares[Us][BLACK] = popcount<Max15>(ourPawns & BlackSquares);
    e->pawnsOnSquares[Us][WHITE] = pos.piece_count(Us, PAWN) - e->pawnsOnSquares[Us][BLACK];

    e->pawnsOnSquares[Them][BLACK] = popcount<Max15>(theirPawns & BlackSquares);
    e->pawnsOnSquares[Them][WHITE] = pos.piece_count(Them, PAWN) - e->pawnsOnSquares[Them][BLACK];

    return value;
  }
}

namespace Pawns {

/// probe() takes a position object as input, computes a Entry object, and returns
/// a pointer to it. The result is also stored in a hash table, so we don't have
/// to recompute everything when the same pawn structure occurs again.

Entry* probe(const Position& pos, Table& entries) {

  Key key = pos.pawn_key();
  Entry* e = entries[key];

  // If e->key matches the position's pawn hash key, it means that we
  // have analysed this pawn structure before, and we can simply return
  // the information we found the last time instead of recomputing it.
  if (e->key == key)
      return e;

  e->key = key;
  e->passedPawns[WHITE] = e->passedPawns[BLACK] = 0;
  e->kingSquares[WHITE] = e->kingSquares[BLACK] = SQ_NONE;
  e->halfOpenFiles[WHITE] = e->halfOpenFiles[BLACK] = 0xFF;

  Bitboard wPawns = pos.pieces(WHITE, PAWN);
  Bitboard bPawns = pos.pieces(BLACK, PAWN);
  e->pawnAttacks[WHITE] = ((wPawns & ~FileHBB) << 9) | ((wPawns & ~FileABB) << 7);
  e->pawnAttacks[BLACK] = ((bPawns & ~FileHBB) >> 7) | ((bPawns & ~FileABB) >> 9);

  e->value =  evaluate_pawns<WHITE>(pos, wPawns, bPawns, e)
            - evaluate_pawns<BLACK>(pos, bPawns, wPawns, e);
  return e;
}


/// Entry::shelter_storm() calculates shelter and storm penalties for the file
/// the king is on, as well as the two adjacent files.

template<Color Us>
Value Entry::shelter_storm(const Position& pos, Square ksq) {

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

  Value safety = MaxSafetyBonus;
  Bitboard b = pos.pieces(PAWN) & (in_front_bb(Us, ksq) | rank_bb(ksq));
  Bitboard ourPawns = b & pos.pieces(Us) & ~rank_bb(ksq);
  Bitboard theirPawns = b & pos.pieces(Them);
  Rank rkUs, rkThem;
  File kf = file_of(ksq);

  kf = (kf == FILE_A) ? FILE_B : (kf == FILE_H) ? FILE_G : kf;

  for (int f = kf - 1; f <= kf + 1; f++)
  {
      // Shelter penalty is higher for the pawn in front of the king
      b = ourPawns & FileBB[f];
      rkUs = b ? rank_of(Us == WHITE ? lsb(b) : ~msb(b)) : RANK_1;
      safety -= ShelterWeakness[f != kf][rkUs];

      // Storm danger is smaller if enemy pawn is blocked
      b  = theirPawns & FileBB[f];
      rkThem = b ? rank_of(Us == WHITE ? lsb(b) : ~msb(b)) : RANK_1;
      safety -= StormDanger[rkThem == rkUs + 1][rkThem];
  }

  return safety;
}


/// Entry::update_safety() calculates and caches a bonus for king safety. It is
/// called only when king square changes, about 20% of total king_safety() calls.

template<Color Us>
Score Entry::update_safety(const Position& pos, Square ksq) {

  kingSquares[Us] = ksq;
  castleRights[Us] = pos.can_castle(Us);
  minKPdistance[Us] = 0;

  Bitboard pawns = pos.pieces(Us, PAWN);
  if (pawns)
      while (!(DistanceRingsBB[ksq][minKPdistance[Us]++] & pawns)) {}

  if (relative_rank(Us, ksq) > RANK_4)
      return kingSafety[Us] = make_score(0, -16 * minKPdistance[Us]);

  Value bonus = shelter_storm<Us>(pos, ksq);

  // If we can castle use the bonus after the castle if is bigger
  if (pos.can_castle(make_castle_right(Us, KING_SIDE)))
      bonus = std::max(bonus, shelter_storm<Us>(pos, relative_square(Us, SQ_G1)));

  if (pos.can_castle(make_castle_right(Us, QUEEN_SIDE)))
      bonus = std::max(bonus, shelter_storm<Us>(pos, relative_square(Us, SQ_C1)));

  return kingSafety[Us] = make_score(bonus, -16 * minKPdistance[Us]);
}

// Explicit template instantiation
template Score Entry::update_safety<WHITE>(const Position& pos, Square ksq);
template Score Entry::update_safety<BLACK>(const Position& pos, Square ksq);

} // namespace Pawns