int Min(ChessBoardSnapshot boardPosition, int depth)
{
if (depth <= 0 || boardPosition.IsEndGame())
{
return(GameManager.Instance.CalculateScore(boardPosition, playerType));
}
int lowestScore = int.MaxValue;
ChessBoardSnapshot[] nextBoardPositions = FindPossibleMoves(boardPosition, playerType.ToOpposite());
for (int i = 0; i < nextBoardPositions.Length; i++)
{
ChessBoardSnapshot board = nextBoardPositions[i];
int score = Max(board, depth - 1);
if (score < lowestScore)
{
lowestScore = score;
}
}
return(lowestScore);
}
int AlphaBeta(ChessBoardSnapshot boardPosition, int depth, int alpha, int beta, bool maximizingPlayer)
{
int origAlpha = alpha;
int origBeta = beta;
MinimaxNode node = new MinimaxNode();
// Transposition Table Lookup; node is the lookup key for ttEntry
ulong hash = boardPosition.board.ToZobristHash();
if (tTable.ContainsKey(hash))
{
node = tTable[hash];
if (node.depth >= depth)
{
switch (node.flag)
{
case MinimaxNodeFlag.Exact:
return(node.eval);
case MinimaxNodeFlag.LowerBound:
if (node.eval > alpha)
{
alpha = node.eval;
}
break;
case MinimaxNodeFlag.UpperBound:
if (node.eval < beta)
{
beta = node.eval;
}
break;
}
if (beta <= alpha)
{
return(node.eval);
}
}
}
// Minimax + Alpha Beta Pruning
if (depth <= 0 || boardPosition.IsEndGame())
{
return(GameManager.Instance.CalculateScore(boardPosition, playerType));
}
int val = 0;
if (maximizingPlayer)
{
val = int.MinValue;
ChessBoardSnapshot[] nextBoardPositions = FindPossibleMoves(boardPosition, playerType);
for (int i = 0; i < nextBoardPositions.Length; i++)
{
int newValue = AlphaBeta(nextBoardPositions[i], depth - 1, alpha, beta, false);
if (newValue > val)
{
val = newValue;
}
if (val > alpha)
{
alpha = val;
}
if (beta <= alpha)
{
break;
}
}
}
else
{
val = int.MaxValue;
ChessBoardSnapshot[] nextBoardPositions = FindPossibleMoves(boardPosition, playerType.ToOpposite());
for (int i = 0; i < nextBoardPositions.Length; i++)
{
int newValue = AlphaBeta(nextBoardPositions[i], depth - 1, alpha, beta, true);
if (newValue < val)
{
val = newValue;
}
if (val < beta)
{
beta = val;
}
if (beta <= alpha)
{
break;
}
}
}
// Transposition Table Store; node is the lookup key for ttEntry
node.hash = hash;
node.eval = val;
if (val <= origAlpha)
{
node.flag = MinimaxNodeFlag.UpperBound;
}
else if (val >= origBeta)
{
node.flag = MinimaxNodeFlag.LowerBound;
}
else
{
node.flag = MinimaxNodeFlag.Exact;
}
node.depth = depth;
if (tTable.ContainsKey(hash))
{
tTable[hash] = node;
}
else
{
tTable.Add(hash, node);
}
return(val);
}
