Stockfish/src/tt.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 <cstring>
#include <iostream>

#include "bitboard.h"
#include "tt.h"

TranspositionTable TT; // Our global transposition table

TranspositionTable::TranspositionTable() {

  size = generation = 0;
  entries = NULL;
}

TranspositionTable::~TranspositionTable() {

  delete [] entries;
}


/// TranspositionTable::set_size() sets the size of the transposition table,
/// measured in megabytes. Transposition table consists of a power of 2 number of
/// TTCluster and each cluster consists of ClusterSize number of TTEntries. Each
/// non-empty entry contains information of exactly one position.

void TranspositionTable::set_size(size_t mbSize) {

  size_t newSize = 1ULL << msb((mbSize << 20) / sizeof(TTCluster));

  if (newSize == size)
      return;

  size = newSize;
  delete [] entries;
  entries = new (std::nothrow) TTCluster[size];

  if (!entries)
  {
      std::cerr << "Failed to allocate " << mbSize
                << "MB for transposition table." << std::endl;
      exit(EXIT_FAILURE);
  }

  clear(); // Operator new is not guaranteed to initialize memory to zero
}


/// TranspositionTable::clear() overwrites the entire transposition table
/// with zeroes. It is called whenever the table is resized, or when the
/// user asks the program to clear the table (from the UCI interface).

void TranspositionTable::clear() {

  memset(entries, 0, size * sizeof(TTCluster));
}


/// TranspositionTable::store() writes a new entry containing position key and
/// valuable information of current position. The lowest order bits of position
/// key are used to decide on which cluster the position will be placed.
/// When a new entry is written and there are no empty entries available in cluster,
/// it replaces the least valuable of entries. A TTEntry t1 is considered to be
/// more valuable than a TTEntry t2 if t1 is from the current search and t2 is from
/// a previous search, or if the depth of t1 is bigger than the depth of t2.

void TranspositionTable::store(const Key posKey, Value v, Bound t, Depth d, Move m, Value statV, Value kingD) {

  int c1, c2, c3;
  TTEntry *tte, *replace;
  uint32_t posKey32 = posKey >> 32; // Use the high 32 bits as key inside the cluster

  tte = replace = first_entry(posKey);

  for (int i = 0; i < ClusterSize; i++, tte++)
  {
      if (!tte->key() || tte->key() == posKey32) // Empty or overwrite old
      {
          // Preserve any existing ttMove
          if (m == MOVE_NONE)
              m = tte->move();

          tte->save(posKey32, v, t, d, m, generation, statV, kingD);
          return;
      }

      // Implement replace strategy
      c1 = (replace->generation() == generation ?  2 : 0);
      c2 = (tte->generation() == generation || tte->type() == BOUND_EXACT ? -2 : 0);
      c3 = (tte->depth() < replace->depth() ?  1 : 0);

      if (c1 + c2 + c3 > 0)
          replace = tte;
  }
  replace->save(posKey32, v, t, d, m, generation, statV, kingD);
}


/// TranspositionTable::probe() looks up the current position in the
/// transposition table. Returns a pointer to the TTEntry or NULL if
/// position is not found.

TTEntry* TranspositionTable::probe(const Key posKey) const {

  uint32_t posKey32 = posKey >> 32;
  TTEntry* tte = first_entry(posKey);

  for (int i = 0; i < ClusterSize; i++, tte++)
      if (tte->key() == posKey32)
          return tte;

  return NULL;
}


/// TranspositionTable::new_search() is called at the beginning of every new
/// search. It increments the "generation" variable, which is used to
/// distinguish transposition table entries from previous searches from
/// entries from the current search.

void TranspositionTable::new_search() {
  generation++;
}