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LibChess: Only save hash of board state for repetition checking

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This more than doubles the number of nodes that MCTS can search for a
given time limit, and greatly reduces memory usage.
This commit is contained in:
Peter Elliott 2021-06-19 14:49:53 -06:00 • committed by Andreas Kling
parent 675b8ba995
commit 57bb4d1aec
2 changed files with 9 additions and 10 deletions
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12
Userland/Libraries/LibChess/Chess.cpp
View file

@ -579,14 +579,12 @@ bool Board::apply_move(const Move& move, Color color)
bool Board::apply_illegal_move(const Move& move, Color color)
{
Board clone = *this;
clone.m_previous_states = {};
clone.m_moves = {};
auto state = Traits<Board>::hash(*this);
auto state_count = 0;
if (m_previous_states.contains(clone))
state_count = m_previous_states.get(clone).value();
if (m_previous_states.contains(state))
state_count = m_previous_states.get(state).value();
m_previous_states.set(clone, state_count + 1);
m_previous_states.set(state, state_count + 1);
m_moves.append(move);
m_turn = opposing_color(color);
@ -758,7 +756,7 @@ Board::Result Board::game_result() const
if (m_moves_since_capture == 50 * 2)
return Result::FiftyMoveRule;
auto repeats = m_previous_states.get(*this);
auto repeats = m_previous_states.get(Traits<Board>::hash(*this));
if (repeats.has_value()) {
if (repeats.value() == 3)
return Result::ThreeFoldRepetition;

7
Userland/Libraries/LibChess/Chess.h
View file

@ -172,7 +172,8 @@ private:
bool m_black_can_castle_kingside { true };
bool m_black_can_castle_queenside { true };
HashMap<Board, int> m_previous_states;
// We trust that hash collisions will not happen to save lots of memory and time.
HashMap<unsigned, int> m_previous_states;
Vector<Move> m_moves;
friend struct Traits<Board>;
};
@ -276,7 +277,7 @@ void Board::generate_moves(Callback callback, Color color) const
template<>
struct AK::Traits<Chess::Piece> : public GenericTraits<Chess::Piece> {
static unsigned hash(Chess::Piece piece)
static unsigned hash(const Chess::Piece& piece)
{
return pair_int_hash(static_cast<u32>(piece.color), static_cast<u32>(piece.type));
}
@ -284,7 +285,7 @@ struct AK::Traits<Chess::Piece> : public GenericTraits<Chess::Piece> {
template<>
struct AK::Traits<Chess::Board> : public GenericTraits<Chess::Board> {
static unsigned hash(Chess::Board chess)
static unsigned hash(const Chess::Board chess)
{
unsigned hash = 0;
hash = pair_int_hash(hash, static_cast<u32>(chess.m_white_can_castle_queenside));