stockfish/src/pawns.cpp

/*
  Stockfish, a UCI chess playing engine derived from Glaurung 2.1
  Copyright (C) 2004-2008 Tord Romstad (Glaurung author)
  Copyright (C) 2008 Marco Costalba

  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/>.
*/


////
//// Includes
////

#include <cassert>
#include <cstring>

#include "pawns.h"


////
//// Local definitions
////

namespace {

  /// Constants and variables

  // Doubled pawn penalty by file, middle game.
  const Value DoubledPawnMidgamePenalty[8] = {
    Value(20), Value(30), Value(34), Value(34),
    Value(34), Value(34), Value(30), Value(20)
  };

  // Doubled pawn penalty by file, endgame.
  const Value DoubledPawnEndgamePenalty[8] = {
    Value(35), Value(40), Value(40), Value(40),
    Value(40), Value(40), Value(40), Value(35)
  };

  // Isolated pawn penalty by file, middle game.
  const Value IsolatedPawnMidgamePenalty[8] = {
    Value(20), Value(30), Value(34), Value(34),
    Value(34), Value(34), Value(30), Value(20)
  };

  // Isolated pawn penalty by file, endgame.
  const Value IsolatedPawnEndgamePenalty[8] = {
    Value(35), Value(40), Value(40), Value(40),
    Value(40), Value(40), Value(40), Value(35)
  };

  // Backward pawn penalty by file, middle game.
  const Value BackwardPawnMidgamePenalty[8] = {
    Value(16), Value(24), Value(27), Value(27),
    Value(27), Value(27), Value(24), Value(16)
  };

  // Backward pawn penalty by file, endgame.
  const Value BackwardPawnEndgamePenalty[8] = {
    Value(28), Value(32), Value(32), Value(32),
    Value(32), Value(32), Value(32), Value(28)
  };

  // Pawn chain membership bonus by file, middle game.
  const Value ChainMidgameBonus[8] = {
    Value(14), Value(16), Value(17), Value(18),
    Value(18), Value(17), Value(16), Value(14)
  };

  // Pawn chain membership bonus by file, endgame.
  const Value ChainEndgameBonus[8] = {
    Value(16), Value(16), Value(16), Value(16),
    Value(16), Value(16), Value(16), Value(16)
  };

  // Candidate passed pawn bonus by rank, middle game.
  const Value CandidateMidgameBonus[8] = {
    Value( 0), Value(12), Value(12), Value(20),
    Value(40), Value(90), Value( 0), Value( 0)
  };

  // Candidate passed pawn bonus by rank, endgame.
  const Value CandidateEndgameBonus[8] = {
    Value( 0), Value(24), Value(24), Value(40),
    Value(80), Value(180), Value(0), Value( 0)
  };

  // Pawn storm tables for positions with opposite castling:
  const int QStormTable[64] = {
    0,  0,  0,  0, 0, 0, 0, 0,
  -22,-22,-22,-13,-4, 0, 0, 0,
   -4, -9, -9, -9,-4, 0, 0, 0,
    9, 18, 22, 18, 9, 0, 0, 0,
   22, 31, 31, 22, 0, 0, 0, 0,
   31, 40, 40, 31, 0, 0, 0, 0,
   31, 40, 40, 31, 0, 0, 0, 0,
    0,  0,  0,  0, 0, 0, 0, 0
  };

  const int KStormTable[64] = {
    0, 0, 0, 0,  0,  0,  0,  0,
    0, 0, 0,-4,-13,-22,-27,-27,
    0, 0, 0,-4, -9,-13,-18,-18,
    0, 0, 0, 0,  9,  9,  9,  9,
    0, 0, 0, 0,  9, 18, 27, 27,
    0, 0, 0, 0,  9, 27, 40, 36,
    0, 0, 0, 0,  0, 31, 40, 31,
    0, 0, 0, 0,  0,  0,  0,  0
  };

  // Pawn storm open file bonuses by file:
  const int KStormOpenFileBonus[8] = {
    45, 45, 30, 0, 0, 0, 0, 0
  };

  const int QStormOpenFileBonus[8] = {
    0, 0, 0, 0, 0, 30, 45, 30
  };

}


////
//// Functions
////

/// Constructor

PawnInfoTable::PawnInfoTable(unsigned numOfEntries) {

  size = numOfEntries;
  entries = new PawnInfo[size];
  if (entries == NULL)
  {
      std::cerr << "Failed to allocate " << (numOfEntries * sizeof(PawnInfo))
                << " bytes for pawn hash table." << std::endl;
      exit(EXIT_FAILURE);
  }
  clear();
}


/// Destructor

PawnInfoTable::~PawnInfoTable() {
  delete [] entries;
}


/// PawnInfoTable::clear() clears the pawn hash table by setting all
/// entries to 0.

void PawnInfoTable::clear() {
  memset(entries, 0, size * sizeof(PawnInfo));
}


/// PawnInfoTable::get_pawn_info() takes a position object as input, computes
/// a PawnInfo 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.

PawnInfo *PawnInfoTable::get_pawn_info(const Position &pos) {

  assert(pos.is_ok());

  Key key = pos.get_pawn_key();
  int index = int(key & (size - 1));
  PawnInfo *pi = entries + index;

  // If pi->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 (pi->key == key)
      return pi;

  // Clear the PawnInfo object, and set the key
  pi->clear();
  pi->key = key;

  Value mgValue[2] = {Value(0), Value(0)};
  Value egValue[2] = {Value(0), Value(0)};

  // Loop through the pawns for both colors
  for (Color us = WHITE; us <= BLACK; us++)
  {
    Color them = opposite_color(us);
    Bitboard ourPawns = pos.pawns(us);
    Bitboard theirPawns = pos.pawns(them);
    Bitboard pawns = ourPawns;

    // Initialize pawn storm scores by giving bonuses for open files
    for (File f = FILE_A; f <= FILE_H; f++)
        if (pos.file_is_half_open(us, f))
        {
            pi->ksStormValue[us] += KStormOpenFileBonus[f];
            pi->qsStormValue[us] += QStormOpenFileBonus[f];
        }

    // Loop through all pawns of the current color and score each pawn
    while (pawns)
    {
        bool passed, doubled, isolated, backward, chain, candidate;
        Square s = pop_1st_bit(&pawns);
        File f = square_file(s);
        Rank r = square_rank(s);

        assert(pos.piece_on(s) == piece_of_color_and_type(us, PAWN));

        // The file containing the pawn is not half open
        pi->halfOpenFiles[us] &= ~(1 << f);

        // Passed, isolated or doubled pawn?
        passed = pos.pawn_is_passed(us, s);
        isolated = pos.pawn_is_isolated(us, s);
        doubled = pos.pawn_is_doubled(us, s);

        // We calculate kingside and queenside pawn storm scores
        // for both colors. These are used when evaluating middle
        // game positions with opposite side castling.
        //
        // Each pawn is given a base score given by a piece square table
        // (KStormTable[] or QStormTable[]). This score is increased if
        // there are enemy pawns on adjacent files in front of the pawn.
        // This is because we want to be able to open files against the
        // enemy king, and to avoid blocking the pawn structure (e.g. white
        // pawns on h6, g5, black pawns on h7, g6, f7).

        // Kingside and queenside pawn storms
        int KBonus = KStormTable[relative_square(us, s)];
        int QBonus = QStormTable[relative_square(us, s)];
        bool outPostFlag = (KBonus > 0 && (outpost_mask(us, s) & theirPawns));
        bool passedFlag = (QBonus > 0 && (passed_pawn_mask(us, s) & theirPawns));

        switch (f) {

        case FILE_A:
            QBonus += passedFlag * QBonus / 2;
            break;

        case FILE_B:
            QBonus += passedFlag * (QBonus / 2 + QBonus / 4);
            break;

        case FILE_C:
            QBonus += passedFlag * QBonus / 2;
            break;

        case FILE_F:
            KBonus += outPostFlag * KBonus / 4;
            break;

        case FILE_G:
            KBonus += outPostFlag * (KBonus / 2 + KBonus / 4);
            break;

        case FILE_H:
            KBonus += outPostFlag * KBonus / 2;
            break;

        default:
            break;
        }
        pi->ksStormValue[us] += KBonus;
        pi->qsStormValue[us] += QBonus;

        // Member of a pawn chain (but not the backward one)? We could speed up
        // the test a little by introducing an array of masks indexed by color
        // and square for doing the test, but because everything is hashed,
        // it probably won't make any noticable difference.
        chain =  ourPawns
               & neighboring_files_bb(f)
               & (rank_bb(r) | rank_bb(r - (us == WHITE ? 1 : -1)));

        // Test for backward pawn
        //
        // If the pawn is passed, isolated, or member of a pawn chain
        // it cannot be backward. If can capture an enemy pawn or if
        // there are friendly pawns behind on neighboring files it cannot
        // be backward either.
        if (   passed
            || isolated
            || chain
            || (pos.pawn_attacks(us, s) & theirPawns)
            || (ourPawns & behind_bb(us, r) & neighboring_files_bb(f)))
            backward = false;
        else
        {
            // We now know that there are no friendly pawns beside or behind this
            // pawn on neighboring files. We now check whether the pawn is
            // backward by looking in the forward direction on the neighboring
            // files, and seeing whether we meet a friendly or an enemy pawn first.
            Bitboard b;
            if (us == WHITE)
            {
                for (b = pos.pawn_attacks(us, s); !(b & (ourPawns | theirPawns)); b <<= 8);
                backward = (b | (b << 8)) & theirPawns;
            }
            else
            {
                for (b = pos.pawn_attacks(us, s); !(b & (ourPawns | theirPawns)); b >>= 8);
                backward = (b | (b >> 8)) & theirPawns;
            }
        }

        // Test for candidate passed pawn
        candidate =    !passed
                     && pos.file_is_half_open(them, f)
                     && (  count_1s_max_15(neighboring_files_bb(f) & (behind_bb(us, r) | rank_bb(r)) & ourPawns)
                         - count_1s_max_15(neighboring_files_bb(f) & in_front_bb(us, r)              & theirPawns)
                         >= 0);

        // In order to prevent doubled passed pawns from receiving a too big
        // bonus, only the frontmost passed pawn on each file is considered as
        // a true passed pawn.
        if (passed && (ourPawns & squares_in_front_of(us, s)))
        {
            // candidate = true;
            passed = false;
        }

        // Score this pawn
        Value mv = Value(0), ev = Value(0);
        if (isolated)
        {
            mv -= IsolatedPawnMidgamePenalty[f];
            ev -= IsolatedPawnEndgamePenalty[f];
            if (pos.file_is_half_open(them, f))
            {
                mv -= IsolatedPawnMidgamePenalty[f] / 2;
                ev -= IsolatedPawnEndgamePenalty[f] / 2;
            }
        }
        if (doubled)
        {
            mv -= DoubledPawnMidgamePenalty[f];
            ev -= DoubledPawnEndgamePenalty[f];
        }
        if (backward)
        {
            mv -= BackwardPawnMidgamePenalty[f];
            ev -= BackwardPawnEndgamePenalty[f];
            if (pos.file_is_half_open(them, f))
            {
                mv -= BackwardPawnMidgamePenalty[f] / 2;
                ev -= BackwardPawnEndgamePenalty[f] / 2;
            }
        }
        if (chain)
        {
            mv += ChainMidgameBonus[f];
            ev += ChainEndgameBonus[f];
        }
        if (candidate)
        {
            mv += CandidateMidgameBonus[relative_rank(us, s)];
            ev += CandidateEndgameBonus[relative_rank(us, s)];
        }

        mgValue[us] += mv;
        egValue[us] += ev;

        // If the pawn is passed, set the square of the pawn in the passedPawns
        // bitboard
        if (passed)
            set_bit(&(pi->passedPawns), s);
    } // while(pawns)
  } // for(colors)

  pi->mgValue = int16_t(mgValue[WHITE] - mgValue[BLACK]);
  pi->egValue = int16_t(egValue[WHITE] - egValue[BLACK]);
  return pi;
}