int AspirationSearch::alphaBeta(GameState& gameState, int depth, int alpha, int beta)
{
#ifdef GATHER_STATISTICS
++nodesVisited;
#endif // GATHER_STATISTICS
int originalAlpha = alpha;
uint64_t zobrist = gameState.getZobrist();
const TableData& tableData = transpositionTable.retrieve(zobrist);
// true iff relevant data was retrieved from the Transposition Table
bool tableDataValid = tableData.isValid();
#ifdef VERIFY_MOVE_LEGALITY
if(tableDataValid && !gameState.isMoveLegal(tableData.bestMove))
{
LOG_ERROR("ERROR: table data contains invalid move in AspirationSearch::alphaBetaTT")
tableDataValid = false;
}
#endif
if(tableDataValid)
{
if(tableData.depth >= depth) // ensure table stored in data resulted from a deep enough search
{
if(tableData.valueType == EValue::Type::REAL)
{
return tableData.value;
}
else if(tableData.valueType == EValue::Type::LOWER_BOUND)
{
alpha = std::max(alpha, tableData.value);
}
else if(tableData.valueType == EValue::Type::UPPER_BOUND)
{
beta = std::min(beta, tableData.value);
}
if(alpha >= beta)
{
return tableData.value;
}
}
}
EPlayerColors::Type winner = gameState.getWinner();
// stop search if we reached max depth or have found a winner
if(depth == 0 || winner != EPlayerColors::Type::NOTHING)
{
return evaluate(gameState, winner);
}
EPlayerColors::Type currentPlayer = gameState.getCurrentPlayer();
Move transpositionMove = (tableDataValid) ? tableData.bestMove : INVALID_MOVE;
Move killerMove1 = INVALID_MOVE;
Move killerMove2 = INVALID_MOVE;
if(killerMoves.size() > depth) // may have killer moves stored
{
std::vector<Move>& currentDepthKillerMoves = killerMoves[depth];
if(currentDepthKillerMoves.size() > 0)
{
killerMove1 = currentDepthKillerMoves[0];
if(currentDepthKillerMoves.size() > 1)
{
killerMove2 = currentDepthKillerMoves[1];
}
}
}
MoveGenerator moveGenerator(currentPlayer,
gameState.getBitboard(currentPlayer),
gameState.getBitboard(gameState.getOpponentColor(currentPlayer)),
transpositionMove, killerMove1, killerMove2);
int score = MathConstants::LOW_ENOUGH_INT;
Move m = moveGenerator.nextMove();
Move bestMove = m;
while(!(m == INVALID_MOVE))
{
gameState.applyMove(m); // apply move
int value = -alphaBeta(gameState, depth - 1, -beta, -alpha); // continue searching
gameState.undoMove(m); // finished searching this subtree, so undo the move
if(clock.getElapsedTimeInMilliSec() >= MIN_SEARCH_TIME_MS + MAX_EXTRA_SEARCH_TIME_MS) // exceeding time limit
{
return 0;
}
if(value > score) // new best move found
{
score = value;
bestMove = m;
}
if(score > alpha)
{
alpha = score;
}
if(score >= beta)
{
// pruning, store Killer Move
while(depth >= killerMoves.size()) // don't have a vector of killer moves yet for this depth
{
std::vector<Move> currentDepthKillerMoves;
currentDepthKillerMoves.reserve(2); // 2 slots of Killer Moves
killerMoves.push_back(currentDepthKillerMoves);
}
std::vector<Move>& currentDepthKillerMoves = killerMoves[depth]; // killer moves for this depth
if(currentDepthKillerMoves.size() > 0) // already have at least 1 killer move
{
if(currentDepthKillerMoves[0] == m) // this killer move already stored in first slot
{
break;
}
if(currentDepthKillerMoves.size() > 1) // already have 2 killer moves stored
{
if(currentDepthKillerMoves[1] == m) // this killer move already stored in second slot
{
break;
}
currentDepthKillerMoves[0] = currentDepthKillerMoves[1]; // move second kill move to first slot
currentDepthKillerMoves.pop_back(); // and then remove second (which is now also in first slot)
}
}
currentDepthKillerMoves.push_back(m); // and put the new kill move in second slot
break;
}
m = moveGenerator.nextMove();
}
// Store data in Transposition Table
if(score <= originalAlpha) // found upper bound
{
transpositionTable.storeData(bestMove, zobrist, score, EValue::Type::UPPER_BOUND, depth);
}
else if(score >= beta) // found lower bound
{
transpositionTable.storeData(bestMove, zobrist, score, EValue::Type::LOWER_BOUND, depth);
}
else // found exact value
{
transpositionTable.storeData(bestMove, zobrist, score, EValue::Type::REAL, depth);
}
return score;
}
Move AspirationSearch::startAspirationSearch(GameState& gameState)
{
clock.start();
EPlayerColors::Type winner = gameState.getWinner();
// stop search if we reached max depth or have found a winner
if(winner != EPlayerColors::Type::NOTHING)
{
return INVALID_MOVE; // can't return any normal move if game already ended
}
std::vector<Move> moves; // will store all the moves in the root node, necessary for move ordering based on scores found in previous searches
moves.reserve(16 * 4);
EPlayerColors::Type currentPlayer = gameState.getCurrentPlayer();
MoveGenerator moveGenerator(currentPlayer,
gameState.getBitboard(currentPlayer),
gameState.getBitboard(gameState.getOpponentColor(currentPlayer)));
Move rootMove = moveGenerator.nextMove();
while(!(rootMove == INVALID_MOVE))
{
moves.push_back(rootMove);
rootMove = moveGenerator.nextMove();
}
std::vector<int> moveScores; // will store the scores of the moves here, to use for sorting
moveScores.reserve(moves.size());
// best move found from a complete search (so not considering searches that were terminated early)
Move bestMoveCompleteSearch = moves[0];
int guess = lastRootEvaluation; // start guess with the final root evaluation of our previous search
int deltaGuess = 100;
// compensate guess for odd-even effect
if(searchDepth % 2 == 0) // last search ended at even depth
{
guess += 141;
}
// no need to compensate if previous search ended at odd depth, since we're also starting at odd depth now
searchDepth = 0;
while(true)
{
++searchDepth; // increment search depth for the new search
killerMoves.clear(); // clear table of killer moves
// ================= ALPHA BETA ALGORITHM STARTS HERE =================
int score = MathConstants::LOW_ENOUGH_INT;
int alpha = guess - deltaGuess;
int beta = guess + deltaGuess;
// best move for only this particular search
Move bestMove = moves[0];
for(int i = 0; i < moves.size(); ++i)
{
const Move& m = moves[i]; // select move
gameState.applyMove(m); // apply move
int value = -alphaBeta(gameState, searchDepth - 1, -beta, -alpha); // continue searching
gameState.undoMove(m); // finished searching this subtree, so undo the move
if(clock.getElapsedTimeInMilliSec() >= MIN_SEARCH_TIME_MS + MAX_EXTRA_SEARCH_TIME_MS) // exceeding time limit
{
bestMove = INVALID_MOVE;
break;
}
moveScores[i] = value;
if(value > score) // new best move found
{
score = value;
bestMove = m;
}
if(score > alpha)
{
alpha = score;
}
if(score >= beta)
{
break;
}
}
// ================= ALPHA BETA ALGORITHM RESTARTS IF ASPIRATION SEARCH GAVE INCORRECT RESULT =================
bool newSearchNeeded = false;
if(score >= (guess + deltaGuess) && !(bestMove == INVALID_MOVE))
{
newSearchNeeded = true;
alpha = score;
beta = MathConstants::LARGE_ENOUGH_INT;
}
else if(score <= (guess - deltaGuess) && !(bestMove == INVALID_MOVE))
{
newSearchNeeded = true;
alpha = MathConstants::LOW_ENOUGH_INT;
beta = score;
}
if(newSearchNeeded) // Aspiration Search failed us, re-start the entire thing
{
LOG_MESSAGE(StringBuilder() << ">>>>>>>>>>>>>>>> Aspiration Search required a new Search at depth = " << searchDepth << "! <<<<<<<<<<<<<<<<<<<")
LOG_MESSAGE(StringBuilder() << "Window = [" << (guess - deltaGuess) << ", " << (guess + deltaGuess) << "]")
score = MathConstants::LOW_ENOUGH_INT;
// best move for only this particular search
bestMove = moves[0];
for(int i = 0; i < moves.size(); ++i)
{
const Move& m = moves[i]; // select move
gameState.applyMove(m); // apply move
int value = -alphaBeta(gameState, searchDepth - 1, -beta, -alpha); // continue searching
gameState.undoMove(m); // finished searching this subtree, so undo the move
if(clock.getElapsedTimeInMilliSec() >= MIN_SEARCH_TIME_MS + MAX_EXTRA_SEARCH_TIME_MS) // exceeding time limit
{
bestMove = INVALID_MOVE;
break;
}
moveScores[i] = value;
if(value > score) // new best move found
{
score = value;
bestMove = m;
}
if(score > alpha)
{
alpha = score;
}
if(score >= beta)
{
break;
}
}
LOG_MESSAGE(StringBuilder() << "True score = " << score)
}
// ================= ALPHA BETA ALGORITHM ENDS HERE =================
if(!(bestMove == INVALID_MOVE)) // managed to complete the search within time
{
lastRootEvaluation = score;
if(score == WIN_EVALUATION) // the search was enough to prove a win for us, so return best move of this latest search
{
return bestMove;
}
else if(score == -WIN_EVALUATION) // the search proved a win for opponent, so return best move of the previous search
{
return bestMoveCompleteSearch;
}
// finished search, and didn't prove a win for either team, so save the new best result
bestMoveCompleteSearch = bestMove;
}
else
{
--searchDepth; // since last search was unsuccessful, decrement this so GUI doesn't lie to us
}
if(clock.getElapsedTimeInMilliSec() >= MIN_SEARCH_TIME_MS) // exceeding time limit
{
clock.stop();
return bestMoveCompleteSearch;
}
MoveOrdering::orderMoves(moves, moveScores); // order moves for the next search
// reset all scores to 0 before starting new search
for(int i = 0; i < moveScores.size(); ++i)
{
moveScores[i] = 0;
}
// set our new guess for the next depth
guess = score;
if(searchDepth % 2 == 0) // we've just done an even depth search, gonna do odd now, so compensate by increasing guess
{
guess += 141;
}
else // we've just done an odd depth search, gonna do even now, so compensate by decreasing guess
{
guess -= 141;
}
}
}