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Stockfish, a UCI chess playing engine derived from Glaurung 2.1
Copyright (C) 2004-2026 The Stockfish developers (see AUTHORS file)
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/>.
*/
#ifndef TT_H_INCLUDED
#define TT_H_INCLUDED
#include <cstddef>
#include <cstdint>
#include <tuple>
#include "memory.h"
#include "types.h"
namespace Stockfish {
class ThreadPool;
struct TTEntry;
struct Cluster;
// There is only one global hash table for the engine and all its threads. For chess in particular, we even allow racy
// updates between threads to and from the TT, as taking the time to synchronize access would cost thinking time and
// thus elo. As a hash table, collisions are possible and may cause chess playing issues (bizarre blunders, faulty mate
// reports, etc). Fixing these also loses elo; however such risk decreases quickly with larger TT size.
//
// `probe` is the primary method: given a board position, we lookup its entry in the table, and return a tuple of:
// 1) whether the entry already has this position
// 2) a copy of the prior data (if any) (may be inconsistent due to read races)
// 3) a writer object to this entry
// The copied data and the writer are separated to maintain clear boundaries between local vs global objects.
// A copy of the data already in the entry (possibly collided). `probe` may be racy, resulting in inconsistent data.
struct TTData {
Move move;
Value value, eval;
Depth depth;
Bound bound;
bool is_pv;
TTData() = delete;
// clang-format off
TTData(Move m, Value v, Value ev, Depth d, Bound b, bool pv) :
move(m),
value(v),
eval(ev),
depth(d),
bound(b),
is_pv(pv) {};
// clang-format on
};
// This is used to make racy writes to the global TT.
struct TTWriter {
public:
void write(Key k, Value v, bool pv, Bound b, Depth d, Move m, Value ev, uint8_t generation8);
private:
friend class TranspositionTable;
TTEntry* entry;
TTWriter(TTEntry* tte);
};
class TranspositionTable {
public:
~TranspositionTable() { aligned_large_pages_free(table); }
void resize(size_t mbSize, ThreadPool& threads); // Set TT size
void clear(ThreadPool& threads); // Re-initialize memory, multithreaded
int hashfull(int maxAge = 0)
const; // Approximate what fraction of entries (permille) have been written to during this root search
void
new_search(); // This must be called at the beginning of each root search to track entry aging
uint8_t generation() const; // The current age, used when writing new data to the TT
std::tuple<bool, TTData, TTWriter>
probe(const Key key) const; // The main method, whose retvals separate local vs global objects
TTEntry* first_entry(const Key key)
const; // This is the hash function; its only external use is memory prefetching.
private:
friend struct TTEntry;
size_t clusterCount;
Cluster* table = nullptr;
uint8_t generation8 = 0; // Size must be not bigger than TTEntry::genBound8
};
} // namespace Stockfish
#endif // #ifndef TT_H_INCLUDED
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