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 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 <cmath>
#include <cstring>

#include "tt.h"


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

/// Constructor

TranspositionTable::TranspositionTable(unsigned mbSize) {

  size = 0;
  generation = 0;
  writes = 0;
  entries = 0;
  set_size(mbSize);
}


/// Destructor

TranspositionTable::~TranspositionTable() {

  delete [] entries;
}


/// TranspositionTable::set_size sets the size of the transposition table,
/// measured in megabytes.

void TranspositionTable::set_size(unsigned mbSize) {

  assert(mbSize >= 4 && mbSize <= 1024);

  unsigned newSize = 1024;

  // We store a cluster of 4 TTEntry for each position and newSize is
  // the maximum number of storable positions
  for ( ; newSize * 4 * (sizeof(TTEntry)) <= (mbSize << 20); newSize *= 2);
  newSize /= 2;
  if (newSize != size)
  {
    size = newSize;
    delete [] entries;
    entries = new TTEntry[size * 4];
    if (!entries)
    {
      std::cerr << "Failed to allocate " << mbSize
                << " MB for transposition table."
                << std::endl;
      exit(EXIT_FAILURE);
    }
    clear();
  }
}


/// 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).
/// Perhaps we should also clear it when the "ucinewgame" command is recieved?

void TranspositionTable::clear() {

  memset(entries, 0, size * 4 * sizeof(TTEntry));
}


/// TranspositionTable::store writes a new entry containing a position,
/// a value, a value type, a search depth, and a best move to the
/// transposition table.  The transposition table is organized in clusters
/// of four TTEntry objects, and when a new entry is written, it replaces
/// the least valuable of the four entries in a cluster.  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 Position &pos, Value v, Depth d,
                               Move m, ValueType type) {
  TTEntry *tte, *replace;

  tte = replace = first_entry(pos);
  for (int i = 0; i < 4; i++, tte++)
  {
    if (!tte->key() || tte->key() == pos.get_key()) // empty or overwrite old
    {
        if (m == MOVE_NONE)
            m = tte->move();

        *tte = TTEntry(pos.get_key(), v, type, d, m, generation);
        return;
    }
    else if (i == 0)  // replace would be a no-op in this common case
        continue;

    int c1 = (replace->generation() == generation ?  2 : 0);
    int c2 = (tte->generation() == generation ? -2 : 0);
    int c3 = (tte->depth() < replace->depth() ?  1 : 0);

    if (c1 + c2 + c3 > 0)
        replace = tte;
  }
  *replace = TTEntry(pos.get_key(), v, type, d, m, generation);
  writes++;
}


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

TTEntry* TranspositionTable::retrieve(const Position &pos) const {

  TTEntry *tte = first_entry(pos);

  for (int i = 0; i < 4; i++, tte++)
  {
      if (tte->key() == pos.get_key())
          return tte;
  }
  return NULL;
}


/// TranspositionTable::first_entry returns a pointer to the first
/// entry of a cluster given a position.

inline TTEntry* TranspositionTable::first_entry(const Position &pos) const {

  return entries + (int(pos.get_key() & (size - 1)) << 2);
}

/// 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++;
  writes = 0;
}


/// TranspositionTable::insert_pv() is called at the end of a search
/// iteration, and inserts the PV back into the PV.  This makes sure the
/// old PV moves are searched first, even if the old TT entries have been
/// overwritten.

void TranspositionTable::insert_pv(const Position &pos, Move pv[]) {

  StateInfo st;
  Position p(pos);

  for (int i = 0; pv[i] != MOVE_NONE; i++)
  {
    store(p, VALUE_NONE, Depth(-127*OnePly), pv[i], VALUE_TYPE_NONE);
    p.do_move(pv[i], st);
  }
}


/// TranspositionTable::full() returns the permill of all transposition table
/// entries which have received at least one write during the current search.
/// It is used to display the "info hashfull ..." information in UCI.

int TranspositionTable::full() {

  double N = double(size) * 4.0;
  return int(1000 * (1 - exp(writes * log(1.0 - 1.0/N))));
}


/// Constructors

TTEntry::TTEntry() {
}

TTEntry::TTEntry(Key k, Value v, ValueType t, Depth d, Move m,
                 int generation) :
  key_ (k), data((m & 0x7FFFF) | (t << 20) | (generation << 23)),
  value_(int16_t(v)), depth_(int16_t(d)) {}