public static int AlphaBetaTT(Board board, int alpha, int beta, byte depthLeft, sbyte side, UInt16[] killers, int level)
{
int score;
TranspositionTableEntry entry = transpositionTable.Retrieve(board.ZobristHash);
if (entry.IsValid() && (TranspositionTableEntryHelper.GetDepthSearched(entry.Data) >= depthLeft))
{
if (TranspositionTableEntryHelper.GetEntryType(entry.Data) == TranspositionTableEntryHelper.EntryTypeExactValue)
{
return TranspositionTableEntryHelper.GetScore(entry.Data);
}
if ((TranspositionTableEntryHelper.GetEntryType(entry.Data) == TranspositionTableEntryHelper.EntryTypeLowerBound) &&
(TranspositionTableEntryHelper.GetScore(entry.Data) > alpha))
{
alpha = TranspositionTableEntryHelper.GetScore(entry.Data);
}
else if ((TranspositionTableEntryHelper.GetEntryType(entry.Data) == TranspositionTableEntryHelper.EntryTypeUpperBound) &&
(TranspositionTableEntryHelper.GetScore(entry.Data) < beta))
{
beta = TranspositionTableEntryHelper.GetScore(entry.Data);
}
if (alpha >= beta)
{
return TranspositionTableEntryHelper.GetScore(entry.Data);
}
}
if (depthLeft == 0)
{
score = Quiescence_Limited(board, side, alpha, beta, (level + 1) % 2); // the last arg ensures that quiescence evaluates opponent's move as the last one.
// Quiescence(board, side, alpha, beta);
// Evaluation2.EvaluateFromPerspectiveOf(board, side);
if (score <= alpha)
{
transpositionTable.Add(new TranspositionTableEntry(board.ZobristHash,
TranspositionTableEntryHelper.ComposeData(score, TranspositionTableEntryHelper.InvalidMove,
depthLeft, TranspositionTableEntryHelper.EntryTypeLowerBound)));
}
else if (score >= beta)
{
transpositionTable.Add(new TranspositionTableEntry(board.ZobristHash,
TranspositionTableEntryHelper.ComposeData(score, TranspositionTableEntryHelper.InvalidMove,
depthLeft, TranspositionTableEntryHelper.EntryTypeUpperBound)));
}
else
{
transpositionTable.Add(new TranspositionTableEntry(board.ZobristHash,
TranspositionTableEntryHelper.ComposeData(score, TranspositionTableEntryHelper.InvalidMove,
depthLeft, TranspositionTableEntryHelper.EntryTypeExactValue)));
}
return score;
}
int movesCount;
UInt16[] moves = board.GenerateMoves(out movesCount);
#region Move Ordering
// this is the index with which a good move will be swapped with...
int indexToSwapWith = 0;
/*************** Transposition Table Reordering **************/
// NOTE (17 Jul 2013): There appears to be no gain from transposition table re-ordering, therefore
// commenting it out.
//if (entry.IsValid())
//{
// UInt16 bestMoveFromTable = TranspositionTableEntryHelper.GetBestMove(entry.Data);
// if (bestMoveFromTable != TranspositionTableEntryHelper.InvalidMove)
// {
// for (int i = 0; i < movesCount; i++)
// {
// if (moves[i] == bestMoveFromTable)
// {
// // swap...
// UInt16 temp = moves[i];
// moves[i] = moves[indexToSwapWith];
// moves[indexToSwapWith] = temp;
// indexToSwapWith++;
// break;
// }
// }
// }
//}
/*************** End of Transposition Table Reordering **************/
/*************** Killer Heuristic Reordering *****************/
if (killers[level] != 0)
{
for (int i = 0; i < movesCount; i++)
{
if (killers[level] == moves[i])
{
// if killer move found then swap that with the first move.
UInt16 temp = moves[i];
moves[i] = moves[indexToSwapWith];
moves[indexToSwapWith] = temp;
indexToSwapWith++;
break;
}
}
}
// set killer for next level to 0
killers[level + 1] = 0;
/*************** End of Killer Heuristic Reordering *****************/
#endregion
// NOTE: diluting negative infinity by level so that the farther away the checkmate is from current
// board position the lower will be the score. this is to get the engine to checkmate opponent
// as soon as possible rather than wasting time in capture or other moves which ultimately lead to
// checkmate but are not the fastest route to checkmakte.
int bestScore = Constants.NegativeInfinity + level;
UInt16 bestMove = TranspositionTableEntryHelper.InvalidMove;
BoardState state;
sbyte oppositeSide = (sbyte)(-1 * side);
for (int i = 0; i < movesCount; i++)
{
state = board.GetBoardState();
if (!board.MakeMove(moves[i]))
{
continue;
}
score = -AlphaBetaTT(board, -beta, -alpha, (byte)(depthLeft - 1), oppositeSide, killers, level + 1);
board.RestoreState(state);
if (score > bestScore)
{
bestScore = score;
bestMove = moves[i];
}
if (bestScore > alpha)
{
alpha = bestScore;
}
if (bestScore >= beta)
{
// beta-cutoff, therefore update killer for this level
killers[level] = moves[i];
break;
}
}
// following nested ifs check for stalemate.
if (bestScore == (Constants.NegativeInfinity + level))
{
if (!board.IsInCheck(side))
{
bestScore = Constants.DrawScore;
}
}
if (bestScore <= alpha) // lower bound value
{
transpositionTable.Add(new TranspositionTableEntry(board.ZobristHash,
TranspositionTableEntryHelper.ComposeData(bestScore, bestMove,
depthLeft, TranspositionTableEntryHelper.EntryTypeLowerBound)));
}
else if (bestScore >= beta) // upper bound value
{
transpositionTable.Add(new TranspositionTableEntry(board.ZobristHash,
TranspositionTableEntryHelper.ComposeData(bestScore, bestMove,
depthLeft, TranspositionTableEntryHelper.EntryTypeUpperBound)));
}
else // true minimax value
{
transpositionTable.Add(new TranspositionTableEntry(board.ZobristHash,
TranspositionTableEntryHelper.ComposeData(bestScore, bestMove,
depthLeft, TranspositionTableEntryHelper.EntryTypeExactValue)));
}
return bestScore;
}
