/** Return a heuristic value for BOARD. This can be improved. */
private int StaticScore(Board board)
{
BoardStatus status = board.status;
if (status == BoardStatus.Checkmate)
{
if (board.whoseTurn == PieceColor.White)
{
return(WIN_VALUE);
}
else
{
return(-WIN_VALUE);
}
}
else if (status != BoardStatus.Playing) // AI shouldn't need to think about the promote state
{
return(0);
}
// TODO: quantify threats
int score = 0;
foreach (Square s in Square.squares)
{
PieceData?p = board.GetPiece(s);
if (p == null)
{
continue;
}
PieceType type = p.Value.type;
if (type == PieceType.King)
{
continue;
}
if (p.Value.color == PieceColor.White)
{
score += PIECE_VALUES[(int)type];
}
else
{
score -= PIECE_VALUES[(int)type];
}
}
return(score);
}
private IEnumerator <int?> FindMove(Board board, int depth, bool saveMove, bool max, int alpha, int beta)
{
BoardStatus status = board.status;
if (depth == 0 || (status != BoardStatus.Playing)) // AI shouldn't need to think about the promote state
{
yield return(StaticScore(board));
yield break;
}
List <Move> moves = new List <Move>(board.legalMoves);
if (saveMove)
{
Shuffle(moves);
}
if (max)
{
int bestVal = -INFINITY;
foreach (Move move in moves)
{
board.MakeMove(move);
int moveVal = 0;
IEnumerator <int?> enumerator = FindMove(board, depth - 1, false, false, alpha, beta);
yield return(null);
while (enumerator.MoveNext())
{
if (enumerator.Current != null)
{
moveVal = enumerator.Current.Value;
}
yield return(null);
}
bestVal = Math.Max(bestVal, moveVal);
board.Undo();
if (bestVal > beta) // prune; black would never let this happen
{
yield return(bestVal);
yield break;
}
if (bestVal > alpha) // this is the best option for white on the path to root (overall)
{
alpha = bestVal;
if (saveMove)
{
foundMove = move;
}
}
}
yield return(bestVal); // the best (maximal) value for white from this board position (not necessarily overall)
}
else
{
int bestVal = INFINITY;
foreach (Move move in moves)
{
board.MakeMove(move);
int moveVal = 0;
IEnumerator <int?> enumerator = FindMove(board, depth - 1, false, true, alpha, beta);
yield return(null);
while (enumerator.MoveNext())
{
if (enumerator.Current != null)
{
moveVal = enumerator.Current.Value;
}
yield return(null);
}
bestVal = Math.Min(bestVal, moveVal);
board.Undo();
if (bestVal < alpha) // prune; white would never let this happen
{
yield return(bestVal);
yield break;
}
if (bestVal < beta) // this is the best option for black on the path to root (overall)
{
beta = bestVal;
if (saveMove)
{
foundMove = move;
}
}
}
yield return(bestVal); // the best (minimal) value for black from this board position (not necessarily overall)
}
}