movepick.cpp

/*
  Stockfish, a UCI chess playing engine derived from Glaurung 2.1
  Copyright (C) 2004-2008 Tord Romstad (Glaurung author)
  Copyright (C) 2008-2010 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/>.
*/


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

#include <cassert>

#include "history.h"
#include "movegen.h"
#include "movepick.h"
#include "search.h"
#include "value.h"


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

namespace {

  enum MovegenPhase {
    PH_TT_MOVES,       // Transposition table move and mate killer
    PH_GOOD_CAPTURES,  // Queen promotions and captures with SEE values >= 0
    PH_KILLERS,        // Killer moves from the current ply
    PH_NONCAPTURES,    // Non-captures and underpromotions
    PH_BAD_CAPTURES,   // Queen promotions and captures with SEE values < 0
    PH_EVASIONS,       // Check evasions
    PH_QCAPTURES,      // Captures in quiescence search
    PH_QCHECKS,        // Non-capture checks in quiescence search
    PH_STOP
  };

  CACHE_LINE_ALIGNMENT
  const uint8_t MainSearchPhaseTable[] = { PH_TT_MOVES, PH_GOOD_CAPTURES, PH_KILLERS, PH_NONCAPTURES, PH_BAD_CAPTURES, PH_STOP};
  const uint8_t EvasionsPhaseTable[] = { PH_TT_MOVES, PH_EVASIONS, PH_STOP};
  const uint8_t QsearchWithChecksPhaseTable[] = { PH_TT_MOVES, PH_QCAPTURES, PH_QCHECKS, PH_STOP};
  const uint8_t QsearchWithoutChecksPhaseTable[] = { PH_TT_MOVES, PH_QCAPTURES, PH_STOP};
}


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


/// Constructor for the MovePicker class. Apart from the position for which
/// it is asked to pick legal moves, MovePicker also wants some information
/// to help it to return the presumably good moves first, to decide which
/// moves to return (in the quiescence search, for instance, we only want to
/// search captures, promotions and some checks) and about how important good
/// move ordering is at the current node.

MovePicker::MovePicker(const Position& p, Move ttm, Depth d, const History& h,
                       SearchStack* ss, Value beta) : pos(p), H(h) {
  int searchTT = ttm;
  ttMoves[0].move = ttm;
  badCaptureThreshold = 0;
  badCaptures = moves + MOVES_MAX;

  pinned = p.pinned_pieces(pos.side_to_move());

  if (ss && !p.is_check())
  {
      ttMoves[1].move = (ss->mateKiller == ttm) ? MOVE_NONE : ss->mateKiller;
      searchTT |= ttMoves[1].move;
      killers[0].move = ss->killers[0];
      killers[1].move = ss->killers[1];
  } else
      ttMoves[1].move = killers[0].move = killers[1].move = MOVE_NONE;

  if (p.is_check())
      phasePtr = EvasionsPhaseTable;
  else if (d > DEPTH_ZERO)
  {
      // Consider sligtly negative captures as good if at low
      // depth and far from beta.
      if (ss && ss->eval < beta - PawnValueMidgame && d < 3 * ONE_PLY)
          badCaptureThreshold = -PawnValueMidgame;

      phasePtr = MainSearchPhaseTable;
  }
  else if (d >= DEPTH_QS_CHECKS)
      phasePtr = QsearchWithChecksPhaseTable;
  else
  {
      phasePtr = QsearchWithoutChecksPhaseTable;

      // Skip TT move if is not a capture or a promotion, this avoids
      // qsearch tree explosion due to a possible perpetual check or
      // similar rare cases when TT table is full.
      if (ttm != MOVE_NONE && !pos.move_is_capture_or_promotion(ttm))
          searchTT = ttMoves[0].move = MOVE_NONE;
  }

  phasePtr += int(!searchTT) - 1;
  go_next_phase();
}


/// MovePicker::go_next_phase() generates, scores and sorts the next bunch
/// of moves when there are no more moves to try for the current phase.

void MovePicker::go_next_phase() {

  curMove = moves;
  phase = *(++phasePtr);
  switch (phase) {

  case PH_TT_MOVES:
      curMove = ttMoves;
      lastMove = curMove + 2;
      return;

  case PH_GOOD_CAPTURES:
      lastMove = generate_captures(pos, moves);
      score_captures();
      return;

  case PH_KILLERS:
      curMove = killers;
      lastMove = curMove + 2;
      return;

  case PH_NONCAPTURES:
      lastMove = generate_noncaptures(pos, moves);
      score_noncaptures();
      sort_moves(moves, lastMove, &lastGoodNonCapture);
      return;

  case PH_BAD_CAPTURES:
      // Bad captures SEE value is already calculated so just pick
      // them in order to get SEE move ordering.
      curMove = badCaptures;
      lastMove = moves + MOVES_MAX;
      return;

  case PH_EVASIONS:
      assert(pos.is_check());
      lastMove = generate_evasions(pos, moves);
      score_evasions();
      return;

  case PH_QCAPTURES:
      lastMove = generate_captures(pos, moves);
      score_captures();
      return;

  case PH_QCHECKS:
      lastMove = generate_non_capture_checks(pos, moves);
      return;

  case PH_STOP:
      lastMove = curMove + 1; // Avoids another go_next_phase() call
      return;

  default:
      assert(false);
      return;
  }
}


/// MovePicker::score_captures(), MovePicker::score_noncaptures() and
/// MovePicker::score_evasions() assign a numerical move ordering score
/// to each move in a move list.  The moves with highest scores will be
/// picked first by get_next_move().

void MovePicker::score_captures() {
  // Winning and equal captures in the main search are ordered by MVV/LVA.
  // Suprisingly, this appears to perform slightly better than SEE based
  // move ordering. The reason is probably that in a position with a winning
  // capture, capturing a more valuable (but sufficiently defended) piece
  // first usually doesn't hurt. The opponent will have to recapture, and
  // the hanging piece will still be hanging (except in the unusual cases
  // where it is possible to recapture with the hanging piece). Exchanging
  // big pieces before capturing a hanging piece probably helps to reduce
  // the subtree size.
  // In main search we want to push captures with negative SEE values to
  // badCaptures[] array, but instead of doing it now we delay till when
  // the move has been picked up in pick_move_from_list(), this way we save
  // some SEE calls in case we get a cutoff (idea from Pablo Vazquez).
  Move m;

  // Use MVV/LVA ordering
  for (MoveStack* cur = moves; cur != lastMove; cur++)
  {
      m = cur->move;
      if (move_is_promotion(m))
          cur->score = QueenValueMidgame;
      else
          cur->score =  pos.midgame_value_of_piece_on(move_to(m))
                      - pos.type_of_piece_on(move_from(m));
  }
}

void MovePicker::score_noncaptures() {
  // First score by history, when no history is available then use
  // piece/square tables values. This seems to be better then a
  // random choice when we don't have an history for any move.
  Move m;
  Piece piece;
  Square from, to;

  for (MoveStack* cur = moves; cur != lastMove; cur++)
  {
      m = cur->move;
      from = move_from(m);
      to = move_to(m);
      piece = pos.piece_on(from);
      cur->score = H.value(piece, to) + H.gain(piece, to);
  }
}

void MovePicker::score_evasions() {
  // Try good captures ordered by MVV/LVA, then non-captures if
  // destination square is not under attack, ordered by history
  // value, and at the end bad-captures and non-captures with a
  // negative SEE. This last group is ordered by the SEE score.
  Move m;
  int seeScore;

  // Skip if we don't have at least two moves to order
  if (lastMove < moves + 2)
      return;

  for (MoveStack* cur = moves; cur != lastMove; cur++)
  {
      m = cur->move;
      if ((seeScore = pos.see_sign(m)) < 0)
          cur->score = seeScore - HistoryMax; // Be sure are at the bottom
      else if (pos.move_is_capture(m))
          cur->score =  pos.midgame_value_of_piece_on(move_to(m))
                      - pos.type_of_piece_on(move_from(m)) + HistoryMax;
      else
          cur->score = H.value(pos.piece_on(move_from(m)), move_to(m));
  }
}

/// MovePicker::get_next_move() is the most important method of the MovePicker
/// class. It returns a new legal move every time it is called, until there
/// are no more moves left.
/// It picks the move with the biggest score from a list of generated moves taking
/// care not to return the tt move if has already been searched previously.
/// Note that this function is not thread safe so should be lock protected by
/// caller when accessed through a shared MovePicker object.

Move MovePicker::get_next_move() {

  Move move;

  while (true)
  {
      while (curMove != lastMove)
      {
          switch (phase) {

          case PH_TT_MOVES:
              move = (curMove++)->move;
              if (   move != MOVE_NONE
                  && move_is_legal(pos, move, pinned))
                  return move;
              break;

          case PH_GOOD_CAPTURES:
              move = pick_best(curMove++, lastMove).move;
              if (   move != ttMoves[0].move
                  && move != ttMoves[1].move
                  && pos.pl_move_is_legal(move, pinned))
              {
                  // Check for a non negative SEE now
                  int seeValue = pos.see_sign(move);
                  if (seeValue >= badCaptureThreshold)
                      return move;

                  // Losing capture, move it to the tail of the array, note
                  // that move has now been already checked for legality.
                  (--badCaptures)->move = move;
                  badCaptures->score = seeValue;
              }
              break;

          case PH_KILLERS:
              move = (curMove++)->move;
              if (   move != MOVE_NONE
                  && move_is_legal(pos, move, pinned)
                  && move != ttMoves[0].move
                  && move != ttMoves[1].move
                  && !pos.move_is_capture(move))
                  return move;
              break;

          case PH_NONCAPTURES:

              // Sort negative scored moves only when we get there
              if (curMove == lastGoodNonCapture)
                  insertion_sort<MoveStack>(lastGoodNonCapture, lastMove);

              move = (curMove++)->move;
              if (   move != ttMoves[0].move
                  && move != ttMoves[1].move
                  && move != killers[0].move
                  && move != killers[1].move
                  && pos.pl_move_is_legal(move, pinned))
                  return move;
              break;

          case PH_BAD_CAPTURES:
              move = pick_best(curMove++, lastMove).move;
              return move;

          case PH_EVASIONS:
          case PH_QCAPTURES:
              move = pick_best(curMove++, lastMove).move;
              if (   move != ttMoves[0].move
                  && pos.pl_move_is_legal(move, pinned))
                  return move;
              break;

          case PH_QCHECKS:
              move = (curMove++)->move;
              if (   move != ttMoves[0].move
                  && pos.pl_move_is_legal(move, pinned))
                  return move;
              break;

          case PH_STOP:
              return MOVE_NONE;

          default:
              assert(false);
              break;
          }
      }
      go_next_phase();
  }
}